Pixie Scheme III Help File

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Pixie Scheme III Help File

For Pixie Scheme III, version 1.12

Copyright © 1988-1991, and 2006-2011 Jay Reynolds Freeman, all rights reserved.
Personal Web Site: http://web.mac.com/Jay_Reynolds_Freeman
EMail: Jay_Reynolds_Freeman@mac.com.

Run mad as often as you chuse, but do not faint. -- Jane Austen

Pixie Scheme III is open-source software under the GNU General Public License.
Please see the Pixie Scheme III Warranty for precise information.


Table of Contents:

Pixie III Face


Introduction:

Welcome to the help file for Pixie Scheme III, an implementation of the Scheme programming language for the Apple iPad™. Pixie Scheme III was written by me, Jay Reynolds Freeman. By all means EMail me about it if you wish.

Pixie Scheme III is an implementation of the "R5" dialect of Scheme (major Revision 5 of Scheme), that is missing several features.

Here are two other ways to explain what Pixie Scheme III is:

This document is not a complete Scheme language description or programming manual. For a complete and authoritative manual, I strongly recommend that you obtain and peruse a copy of the 1998 Revised5 Report on the Algorithmic Language Scheme, edited by Richard Kelsey, William Clinger and Jonathan Rees. That report is available on several Internet sites, such as http://www.schemers.org.

You might also consider reading some of the Scheme References and Lisp References listed later herein.

All of Pixie Scheme III's procedures and special forms are described in detail, with examples and discussion, in the Pixie Scheme III Dictionary, which accompanies Pixie Scheme III. You can find it by using the Pixie Scheme III Help Panel.

The original Pixie Scheme was named after one of my cats -- "Pixie". If you don't think that naming a computer program after a cat makes sense, remember that it ran on a computer named after a raincoat.


Pixie Scheme III Startup Actions:

On startup, Pixie Scheme III

If you would prefer a different-sized font, there are buttons in the Features Panel (which appears when you press the Features button, at lower left) to increase and decrease font size.


Saving Your Work Between Sessions:

Pixie Scheme III will automatically save all text in its window, as well as other details of your environment, whenever you leave the application, and will restore them when you return, but it cannot save information that you have put in Scheme main memory -- function definitions, data, and the like -- unless you give it a hand. You have to push a button -- the "Save World" button in the Pixie Scheme III Features Panel -- to make it happen. (Users of hardware keyboards may press "option-s" instead.)

I suggest that the way to use this mechanism is to get in the habit of saving frequently: Any time you are staring at the screen, wondering what to do next, let your fingers find the "Save World" button and press it. Furthermore, even though Pixie Scheme III will save the text in the main window when you leave the app, I recommend that you get in the habit of making a copy of the text in the main window occasionally, and pasting it somewhere else, in another application, for safekeeping. Don't think of the Pixie Scheme III window as a full-fledged program editor; its main purpose is to make it easy for you to get text into and out of Pixie Scheme III by cut-and-paste.

You don't need to know how the "Save World" mechanism works in order to use it -- just try it and see -- but if you are curious about what is going on, read on.

Pixie Scheme III is able to write a special kind of file, called a "world file", into the area of iPad memory that is reserved for its use. That file contains the entire content of Pixie Scheme III's main Scheme memory at the time -- all your Scheme definitions and Scheme data are in it. The world file also contains a lot of other information about what you were doing, including all the text that was in the main window, the font size in use, and a handful of other things about how you had Pixie Scheme III set up and what you were doing with it.

Every saved world overwrites the previous one. Thus the act of saving a world creates an up-to-date version of your Scheme "world" -- environment and data -- that will persist, even when you are not running Pixie Scheme III. Every time you start Pixie Scheme III, it looks in its reserved memory area for a saved world file. If it finds one, it loads it -- thereby restoring your Scheme procedures and data -- and also uses the additional information to re-create your working environment, so you can pick up from where you left off. If it cannot find a world file of yours, Pixie Scheme III will just load one of its own, that is stored in the application itself. (The only reason it might not find a world file of your own would be if you had never yet saved a world file, or if you had deliberately decided to restart the application in its original condition, as described immediately below.)

Pixie Scheme III's mechanism for saving only the text from the window works just like "Save World", but it is fast enough that I can make it happen automatically and reliably.

If for any reason you should become dissatisfied with the worlds you have been saving, you might consider using the "Reload Original Scheme World" button, that appears when you press the "Features" button at the lower left corner of the application. It will cause Pixie Scheme III to load the original world file -- which does not contain your personal saved Scheme data! When you use this button, Pixie Scheme III will not override any of the other settings and information in the environment you are using: For example, the font size will remain as you last set it. Furthermore, no text will be removed from the main window.

When you have reloaded the original world file, there may still be a saved world file around, that you had previously created by pressing the "Save World" button in the Pixie Scheme III Features Panel. Pixie Scheme III will try to start up using that saved world the next time you restart it. If you do not wish to use the saved world again, press the "Save World" button sometime after you have reloaded the original world, and before you next quit the application. That will overwrite the old saved world file with a new one.


Restoring Pixie Scheme III to Original Settings:

To restore Pixie Scheme III to the way it was when you originally obtained it, use iTunes™: Connect your iPad to the computer you use to back up and maintain your iPad. Open iTunes and go to the iTunes window for your iPad, and press the "Apps" button at the top of that window. Uncheck Pixie Scheme III from the list of installed apps at the left side of that window, and then press the "Sync" button at lower right. After the sync has completed, recheck Pixie Scheme III in the list of installed apps, and press the "Sync" button again. The first sync will remove Pixie Scheme III and all its data -- including any saved world that you may have created -- from your iPad. The second sync will reinstall Pixie Scheme, which will use its own internal saved world the next time you launch it. You may then save new worlds to suit yourself.

I must note in passing that Apple updates iTunes frequently, so it is possible that small details of the layout and organization of the iTunes window for the iPad will have changed between the time I wrote these instructions and the time you read them. Notwithstanding, I expect that the general procedure of removing Pixie Scheme III from your iPad and then reinstalling it will still work, and that it will be accomplished by means very similar to what I have just described.


Interacting with Pixie Scheme III:

While you are reading this section, remember that there is also a section called Common Problems and Solutions.

The main part of the Pixie Scheme III Window is where you do almost everything with the Pixie Scheme III program -- things like entering Scheme source code, running it, and looking at the results. In addition to the usual iPad interface stuff, however, Pixie Scheme III provides a handful of buttons and a couple of status indicators. They allow you to do things to the Pixie Scheme III program -- things like setting options and taking control of Scheme programs that are misbehaving. They also allow you to keep track of what Pixie Scheme III is doing, and to access the program's internal help files and other documentation.

Note that Pixie Scheme III requires ASCII characters for everything it does. (ASCII characters are pretty much the letters of the English alphabet and the standard punctuation marks -- what you get from the keyboard of an old-fashioned typewriter.)

Let's discuss the window and buttons in more detail.


The Pixie Scheme III Window:

The Pixie Scheme III window contains a large work area for text -- think of it as a blackboard or a slate where both you and the Pixie Scheme III program can write things. It also contains several buttons, pop-up panels, and information displays.


How To Get Pixie Scheme III To Do Something:

Main Window

The Pixie Scheme III Main Window, at reduced size.

The idea is to type Scheme expressions into the Pixie Scheme III main window and evaluate them, whereupon Pixie Scheme III will type out the results in the main window, for you to look at. The next few paragraphs describe how to do that on a plain iPad. If you are using an add-on keyboard, there are some additional ways to do things that are described in the next section.

To evaluate a typed expression, select it in any of the usual ways (and there is one unusual one -- we will get to that in the next paragraph); then use a button which will appear at the top right corner of the main window, labeled "Evaluate": Press it to tell Pixie Scheme III to evaluate the selected expression.

Most of the time, when you are typing something into the main window, you are intending to select and evaluate what you have typed. Pixie Scheme III can help: The application remembers where you started your most recent batch of typing, and is prepared to select what you have typed at the push of a button. In more detail, it works like this. In the image above, note that the user has not typed any text yet, and that the button at the top right of the main window is invisible. In the image just below, the user has typed "(+ 2 2)", and now the top right button has appeared, with its label reading "Select That?".

Note that the matching parentheses of the typed expression are highlighted in green. Pixie Scheme III highlights matching parentheses this way whenever it finds them, but it ignores parentheses in strings (that is, between double-quotes) and in comments (that is, to the right of a semicolon). If you type a parenthesis for which there is no match, Pixie Scheme III will highlight it in a different color.

Main Window with Typed Text

The Pixie Scheme III Main Window, with typed text, at reduced size.

In the next image, the user has pressed the "Select That?" button, the expression "(+ 2 2)" has been selected, and the top right button has changed its label, to "Evaluate".

Main Window with Typed Text Selected

The Pixie Scheme III Main Window, with selected text, at reduced size.

When the user presses the "Evaluate" button, Pixie Scheme III evaluates the text selected, and prints out whatever may result, in this case the number "4". The last step would have happened the same way if you had selected the text by touching the screen and dragging the ends of the selection bar.

Main Window with Typed Text Evaluated

The Pixie Scheme III Main Window, showing an evaluation, at reduced size.

The flow of interaction with Pixie Scheme III this way should be very natural: You type an expression to evaluate, press the "Select That?" button to make sure what you are about to evaluate is what you think it is, then press the same button -- now labeled "Evaluate" -- again, to pass the expression to Pixie Scheme III.

The iPad's "cut-and-paste" mechanism is very useful for entering Scheme expressions: Pixie Scheme III will notice when you have pasted some text, and will enable the top right button as "Select That?", to make it easy for you to evaluate what you have put in.

By the way, I expect you noticed that Pixie Scheme III's "glass keyboard" has an enhancement: There is an extra row of smaller keys at the top. That row contains a selection of the most-used keys that you would have to shift to get if you were using the unenhanced glass keyboard. They work like the regular keys. They are even set up to make the same "click" sound that the regular keys do, provided you have enabled "Keyboard Clicks" in the first place. (To find that preference, go to the "Settings" app of your iPad, press the "General" tab and then press the "Sounds" button.)

There is one more thing to note about the extra keys: They are a little small. For that reason, I decided not to shrink the extra keys when you rotate your iPad to portrait orientation. Instead, the extra keys all stay the same size, but fewer of them are shown. There are twenty extra keys in landscape orientation, but only fifteen in portrait orientation.

Main Window in portrait orientation

The Pixie Scheme III Main Window in portrait orientation.

The image above is at the same overall scale as the previous pictures that showed the keyboard in landscape orientation; that is, the dimensions of the whole screen are the same. As you can see, the regular keyboard keys are considerably smaller -- that's the way the iPad works. The extra keys are the same size they were in landscape orientation, but fewer of them are shown. I hope I have made a good tradeoff between having enough extra keys so that you don't have to shift too often, and having them too small to use.


Using A Hardware Keyboard:

If you are using a hardware keyboard with your iPad, there are a couple of shortcuts that use the "option" key, to help with common commands. (I would have liked to use the command key rather than the option key for keyboard shortcuts, but there seemed to be no way for an iPad app to tell that the command key has been pressed.)

Things may become confused if you switch your hardware keyboard on and off, or if it stops working, while you are in the middle of using Pixie Scheme III. Pixie Scheme III may not be able to figure out for sure whether a hardware keyboard is there until you have done a few keyboard operations. To get things back to normal, try rotating the iPad a few times, so that the orientation of the screen switches back and forth between landscape and portrait. If the hardware keyboard is not on, you might also try moving the glass keyboard up and down by the usual taps and key presses. As a last resort, press the "Home" button to leave Pixie Scheme III for a moment, then start it back up again. Caution: You might want to save your work -- that is, save a world -- before you do.

It may seem a bit odd to use "real" characters, like "option-&rsquo" and "option-s", for keyboard shortcuts. That is possible because the R5 Scheme specification allows only ASCII characters for input to Scheme itself. Thus Pixie Scheme III is free to intercept non-ASCII characters and treat them as keyboard shortcuts. When you type "option-&rsquo", what gets sent to Pixie Scheme III -- and intercepted -- is actually the non-ASCII character '', and when you type "option-s", what gets sent is ''.


No Tabs Allowed:

Pixie Scheme III will not allow "tab" characters as inputs. If you type a tab, or paste in some text containing tabs, Pixie Scheme III will silently convert each tab to a single blank space.

If you are looking at a standard iPad, whose keyboard has no "tab" key in the first place, you may be wondering how you could possibly get a tab into Pixie Scheme III. The answer is, that some of the accessory hardware keyboards available for the iPad do have "tab" keys, or that you may have pasted in some text that was originally created in an application where tabs were available.

Incidentally, if you wish to use tabs in the output of Pixie Scheme III, there are ways to do that. See the sections on #\tab and #\return and on slashification.


The Dialog Panel:

Dialog Panel

Pixie Scheme III's Dialog Panel.

The Pixie Scheme III Dialog Panel occasionally drops down from the top of the Pixie Scheme III window, when the program needs some kind of text input from the user.

You don't actually need to type a response: You may also use cut-and-paste, as long as your response is just one line long.

In a typical dialog, Pixie Scheme III might ask you for the content of a string.


Buttons:

In the paragraphs above, I have already mentioned the button at the top right of the main window. It appears, with label "Select That?", when Pixie Scheme III knows about a range of text in the main window that you might wish to select. It appears, with label "Evaluate", when a selected range of text is marked in the main window. At other times, the button is not visible.

There is also a button at the top left of the main window, labeled "Reset Scheme". It appears only when Pixie Scheme III is evaluating something. You may press it then to make evaluation stop. That would be useful, for example, if the code that was running was in an infinite loop.

Reset Button

The Reset Scheme button, at top left of the Pixie Scheme III Main Window.

If for some reason Pixie Scheme III is running very slowly while it is processing text to be evaluated, then you may temporarily see this same button with the title "Forget Typing". Pressing the button at that time will cause Pixie Scheme III to stop processing that text.


Information Displays:

The Pixie Scheme III Main Window contains several items that display information to the user.


Parenthesis Matching:

Pixie Scheme III attempts to show matching parentheses as you type and select text, by highlighting them in color. When the cursor is just to the right of a parenthesis that has a match, both parentheses will be highlighted in green. When the cursor is just to the right of a left parenthesis that does not have a match, that parenthesis will be highlighted in orange. When the cursor is just to the right of a right parenthesis that does not have a match, that parenthesis will be highlighted in red.

Parenthesis matching ignores parentheses that are in comments, in strings, or in the form of Scheme characters. That is, Pixie Scheme III's parenthesis-matching algorithm will ignore the following parentheses:

Strictly speaking, the parenthesis matcher can only hope to work on text that is syntactically correct Scheme code -- something that a Scheme interpreter can understand and evaluate. If you enter some incorrect Scheme code -- something that will cause an error when Pixie Scheme III tries to parse it -- and that code contains parentheses, the parenthesis matcher may become confused until you have fixed the problem: It is not smart enough to guess what you meant to do.

If for any reason the parenthesis matcher becomes confused and seems inclined to stay that way, just save a world, put the Pixie Scheme III application in background by pressing the "Home" button, and then return to it. Pixie Scheme III will reanalyze all the parentheses when it returns from background. If the problem persists, you might also want to send a bug report to Jay_Reynolds_Freeman@mac.com. You don't actually need to save a world if you don't mind accepting some chance of losing recent work, in case iOS decides to shut down Pixie Scheme III while it is in the background.


Pretty-Printing:

Pixie Scheme III has simple pretty-printing available. When you select some text and tap so that the edit menu comes up, if the selected text is a parenthesized expression, the edit menu will have an extra item on it, "Pretty-Print". Tap that item, and Pixie Scheme III will pretty-print the selected expression; that is, it will reformat the expression using indentation to show how deeply parentheses are nested. The next three pictures show how it works.

Pretty Print 1

A parenthesized expression, ready to tap and bring up the edit menu.


Pretty Print 2

The edit menu has a new item, "Pretty-Print", at the right end.


Pretty Print 3

The resulting pretty-printed expression.

The pretty-print is very simple: The extra leading white space for each line is composed of exactly twice as many blank-space characters as there are unbalanced left parentheses between the start of the expression and the start of the line. There are much fancier algorithms, but different programmers like different pretty-print styles, and there is no pleasing everyone. Yet the real purpose of pretty-printing is not to make code look good, but to make it easy for programmers to see the structure of what they have written, and I believe this minimal routine will do the job.


Pop-Up Panels:

BottomBar

Pixie Scheme III's complete bottom bar, at reduced size.

Pixie Scheme III has two pop-up panels -- Apple sometimes calls these "popovers" -- that provide access to special features and information. They pop up when you press the "Features" button or the "Help" button, which are located respectively at the left and right ends of the bar at the bottom of the Pixie Scheme III window.


The Features Panel:

Features Panel

The Features Panel.

The Features Panel pops up when you press the "Features" button, at the lower left corner of the Pixie Scheme III main window.


The Help Panel:

Help Panel

The Help Panel.

The Help Panel pops up when you press the "Help" button, at the lower right corner of the Pixie Scheme III main window. It provides access to some HTML files and text files that tell how to use Pixie Scheme III, and provide other information.


Another View of Interacting with Pixie Scheme III:

There is another way to think about how you may interact with Pixie Scheme III:

There are four ways for Pixie Scheme III to provide information to you:

There are three ways for you to provide information to Pixie Scheme III:


Error Handling:

When Pixie Scheme III encounters an error from which it can recover, its general strategy is to print an error message -- I hope a useful one -- in the Pixie Scheme III Window, then abort whatever Scheme processing is going on and return control to you at "top level" in the Pixie Scheme III window. For example, suppose you tried to add using a Scheme object that is not a number. You might type:

whereupon Pixie Scheme III would print

A similar error message, and a similar return to the "top level" of control of Pixie Scheme III, would occur even if the problem occurred deep in some elaborate Scheme procedure.

Pixie Scheme III may also encounter errors from which no recovery is possible, in which case its general strategy is to open a special panel to present an error message, and then exit. If you should ever see a fatal error message whose cause is not obviously due to something you understand and can control yourself, I would like to hear about it: Send bug reports to Jay_Reynolds_Freeman@mac.com.


Controlling Pixie Scheme III:


The Garbage Collectors:

Reclaiming memory that is no longer in use is an important part of any computer application, and Lisp systems of all kinds have long been equipped with "garbage collectors" to perform that task. Pixie Scheme III has two garbage collectors, so it is important to tell you a little about what they are and how they work together.

Pixie Scheme III has both a full garbage collector and a generational garbage collector. These two systems are not completely independent, as we shall soon see. The reason for having two systems is that the full garbage collector sometimes takes an objectionable amount of time to do its job: Pixie Scheme III would just sit there, with the red "GC" light turned on, for long enough to bother you.

The generational garbage collector addresses this problem by doing frequent garbage-collection operations that each only collect a little bit of garbage. Since the amount of garbage collected per collection is small, the operations are fast, and Pixie Scheme III is more responsive. On the other hand, the combined time to set up and do many small collections turns out to be more than the time required for one full collection that collects the same amount of garbage, so Pixie Scheme III's overall speed is lower when the generational garbage collector is running, than when it is not.

Furthermore, the generational garbage collector is not as efficient as the full one: Some garbage sneaks through it without being collected. Therefore, even when the generational garbage collector is running, Pixie Scheme III has to do a full garbage collection occasionally.

So both garbage-collection systems have pros and cons. Notwithstanding, for simplicity, I decided to make Pixie Scheme III always use the generational garbage collector. I mention the full garbage collector here only because the generational one uses it from time to time, and because you yourself may request a full garbage collection by evaluating "(e::full-gc)".


Suggestions for Using Pixie Scheme III:

This section includes some hints based on my own personal experience in using Pixie Scheme III. What's here is in no sense required practice, or even best practice, it is just a list of miscellaneous tips that have made my own life easier when I myself use Pixie Scheme III to develop and run programs.

Some of these matters are complicated, in the sense that you may need more than just a little knowledge of programming and the Unix / iOS operating system in order to understand what I am talking about. If that happens, I apologize, and it won't do any harm to ignore that suggestion entirely.


Differences from R5 Scheme:

Herein I describe how Pixie Scheme III differs from "R5" Scheme by going through the Revised5 Report on the Algorithmic Language Scheme, section by section, and listing differences between Pixie Scheme III and that standard. It is my intention that any essential or non-essential feature of Scheme that is mentioned in the R5 report, is either provided as described in the R5 report, or mentioned here with an indication of how Pixie Scheme III differs from the R5 report.

One major difference is that Pixie Scheme III provides numerous enhancements and special features, in great part in the form of extra built-in procedures and constants whose identifiers generally begin with "e::" or "c::". I mention some of these features in passing in this section, keeping the descriptions brief for clarity of presentation. I have described them in more detail in a separate Enhancements section.

I have summarized the R5 features that Pixie Scheme III lacks, in the Optional Features Omitted section. Each missing feature is also mentioned separately in the discussion of the appropriate section of the R5 report, below.

Note that all of Pixie Scheme III's procedures and special forms are described in detail, with examples and discussion, in the Pixie Scheme III Dictionary, which accompanies Pixie Scheme III and may be reached via the Pixie Scheme III Help Panel.


Run-Time Text Input:

Some procedures that take string arguments will ask you to type in the string to be used when they are called without the argument that normally provides that information. These procedures use the Pixie Scheme III dialog panel, that drops down from the top of the Pixie Scheme III main window.

For example, in straight "R5" Scheme,

is not a legal expression -- "make-string" requires an argument. In Pixie Scheme III, however,

will display the dialog panel with a place for you to type in the intended content of the string. The full list of standard Scheme procedures that are modified to work this way in Pixie Scheme III is:

Several non-standard procedures, described in the "Enhancements" section, also work this way.


R5 Section 1:


R5 Section 2:


R5 Section 3:


R5 Section 4:


R5 Section 5:


R5 Section 6:


R5 Section 7:


Optional Features Omitted:

In the section-by-section discussions above, of how Pixie Scheme III differs from the R5 standard, I have listed the optional R5 features that Pixie Scheme III lacks. These features are "optional" in that the R5 report mentions them, perhaps with such language as "some implementations support ...". Here they are once again, all in one place, in summary:


Enhancements:

This section describes Pixie Scheme III's non-standard features, procedures and syntax. Many are rather conventional: Most implementations of Scheme will have them in some form, but they are sufficiently dependent on the particular computer and operating system in use that the R5 report cannot specify them. Others deal with matters of controversy in the Scheme community. Possibly some version of them will be incorporated into the standard at a future date. Meanwhile, I have provided what seems best to me.

There are a moderate number of enhancements that are specific to the look and feel of the Apple iPad, or that allow you to access special features of the iPad itself. And finally, there are some enhancements that allow you to inspect or modify parts of Pixie Scheme III at a rather low level. Some of these may be useful in debugging programs, or in satisfying your curiosity about what's going on. Others are there because I needed them for some purpose related to developing Pixie Scheme III, or for some personal project using Pixie Scheme III.

Some of these enhancements are identified by naming conventions. In particular, Pixie Scheme III uses symbols which begin with the characters "e::" for enhancements that in my opinion will probably be of broad interest to users, and "c::" for more specialized enhancements that will probably be less generally useful.

By and large, enhancements whose names begin with "e::" will be reasonably well-documented and will have reasonable handling of errors. They should be no risker to use -- with "risk" in the sense of unexpected crashes and mysterious behaviors -- than standard Scheme procedures, special forms, and macros. Furthermore, these enhancements are more likely to continue to exist in the same form in future releases of Pixie Scheme III, or at the very least, I will make a big to-do in documentation if there are changes.

Enhancements whose names begin with "c::" will be less-well documented, riskier, and more likely to change in future releases. Most of the "c::" enhancements are provided for my own use in developing Pixie Scheme III, or are auxiliaries used by standard Scheme procedures or by "e::" enhancements. I have left them in the release and documented some of them.

You might remember the distinction this way:

e:: for enhancement, c:: for caution

A cynic might have said:

e:: for enhancement, c:: for catastrophe

Note that all of Pixie Scheme III's procedures and special forms are described in detail, with examples and discussion, in the Pixie Scheme III Dictionary, which accompanies Pixie Scheme III and may be reached via the Pixie Scheme III Help Panel.


Applicative Programming:

The term "applicative programming" means different things to different people. As an enhancement, Pixie Scheme III has means to make it easy to do a pretty good job at one of the kinds of programming that is commonly called "applicative". Specifically, there is means provided to do programming that is for the most part referentially transparent.

Referential transparency encapsulates the condition that every time you do the same thing, you get the same result. In the context of programming, the idea is that every time you call the same procedure or special form, with the same arguments, you should get the same answer, and furthermore, the procedure should have no observable side effects.

For example, the arithmetic procedure "+" is referentially transparent: Every time you call

    (+ 2 2)
    

you can reasonably expect to get back the same answer -- 4. On the other hand, consider the following procedure, "foo":

    (define (foo n)
      (let ((return-value a))
        (set! a n)
        return-value))
    

Procedure "foo" is not referentially transparent, because it depends on the initial value of "a" (and I have assumed that "a" will be defined before "foo" is executed). For example, suppose that a starts out as 3, and that you call

    (foo 42)
    

twice in a row. The value returned from the first call will be 3, but the value returned from the second call will be a different value, 42.

It is easy to see that the problem with "foo" stems from the use of "set!": It changes state, and that is the difficulty. After the "set!", there is a value in Scheme main memory, that is not garbage, that is different from what it was before. That is an observable side-effect: Any procedure that uses that value to determine its own returned value will not be referentially transparent.

    Technical Note: The wording about "non-garbage" is necessary because Lisp-class languages may well create temporary data structures in main memory while performing a procedure; these will remain -- for a while, at least -- as garbage, after the procedure has returned, but since they are garbage they will be inaccessible by any subsequent operation.

It follows that in order for programs to run in a referentially transparent manner, nothing must alter non-garbage state. The enhancement described here provides exactly that feature. In essence, it provides a way to run a given chunk of Pixie Scheme III code with the provision that any attempt by that chunk to change non-garbage state will fail -- it will report an error.

Before getting on with the details of what that means and how it works, I must point out that there are a few problems with this simple notion of referential transparency:

  • First, code that does input or output cannot be guaranteed to be referentially transparent: For example, if you write a program that asks what time it is, or that returns the next character the user types at the keyboard, there is certainly no expectation of getting the same result every time. If your program displays output to the screen, the screen will look different after it runs.

  • Second, since the new feature does not make a permanent change in Pixie Scheme III, you can run a chunk of code referentially transparently twice, and change some state your chunk depends upon in between those two runs.

The bottom line is, that although this new enhancement is probably useful if you wish to write referentially transparent code, it is certainly not perfect.

With all that said, the actual details of the enhancement are simple. There is just one new procedure:

(e::perform-applicative <thunk>)

    (In which <thunk> is a procedure of no arguments or a lambda expression of no arguments.)

    This procedure performs <thunk> with Pixie Scheme III's ability to change state turned off, and returns whatever <thunk> did. Specifically, within the code represented by <thunk>, any attempt to use any of the following procedures and special forms will be interpreted as an error:

      set!
      set-car!
      set-cdr!
      string-fill!
      string-set!
      vector-fill!
      vector-set!
      e::clear-permanent!
      e::set-list-print-depth!
      e::set-list-print-length!
      e::set-permanent!
      e::set-tag!
      e::set-vector-print-depth!
      e::set-vector-print-length!
      

    If <thunk> should create a continuation which is subsequently returned to (as by "call-with-current-continuation"), then that continuation will inherit the applicative restrictions.

    By way of illustration:

      (e::perform-applicative (lambda () (+ 2 2)))  ;; ==> 4
      

    but

      (define a 3)
      (e::perform-applicative (lambda () (set! a 42)))
      

    produces the error message:

      In built-in routine "set!":
      Problem: Attempt to use a non-applicative procedure or special form during applicative programming.
        Last lambda called (which may have returned) was recently named:
          thunk
        Recent names of non-tail-recursive stacked lambda expressions:
          e::perform-applicative
        (Resetting)
      Top-level loop ...
      

    Technical Note: In contrast to almost everything else in Scheme, the scope of these "applicative restrictions" is dynamic, not lexical. A purist would almost certainly say, "Horrors!". I say, "Have fun!".


Class System:

Pixie Scheme III comes with a simple class system for creating objects with encapsulated data and procedures. That system is generally similar in intent and in operation to the class systems in C++, Java, and other programming languages: It facilitates object-oriented programming in a straightforward way. Notwithstanding, the Pixie Scheme III class system is somewhat more crudely implemented than either of those other class systems.

It is beyond the scope of this document to describe what class mechanisms are for, or what object-oriented programming is all about, or how to think in terms of object-oriented design. There are books on those subjects -- hundreds of them. If you are not familiar with these concepts, this section of the Pixie Scheme III help file won't help you much: You should probably forget about the Pixie Scheme III class system until you have learned more about classes in general. I will proceed on the assumption that you have some familiarity with other class systems and their uses.


Class System Procedure:

The procedure which provides access to the Pixie Scheme III class system is

    (e::make-simple-class
      <symbol which the class takes to be its name>
      <list of classes from which the new class inherits>
      <list of lists like (<symbol> <object>) for class variables>
      <list of lists like (<symbol> <object>) for instance variables>
      )
    

The first list of lists provides the names of the class variables and their default bindings. The second list of lists provides the names of the class's instance variables and their default bindings. The procedure returns a class in the Pixie Scheme III class system.

Many examples follow shortly.


Important Notes:

  • The interface to the Pixie Scheme III class system is entirely procedural: There are no "keyword positions", in the sense that there is no place in the Pixie Scheme III class system where you can use an unquoted symbol without very likely getting into problems because of Pixie Scheme III trying to evaluate it. In practical terms, that means that when you use the instance variable names of a class, you have to quote them, and there are other things you have to quote as well. Examples will follow.

  • The Pixie Scheme III class system allows multiple inheritance to an arbitrary depth.

  • There is no notion of public, private, or protected variables: They are all public.

  • There is no specific notion of "superclass" in the Pixie Scheme III class system. If that bothers you, keep reading.

  • The Pixie Scheme III class system achieves operation roughly equivalent to calls to "super" -- but a little more crudely -- by "flattening" the combined namespaces of a class's instance variables and class variables, together with the instance variables and class variables of all the classes it inherits from. That is, each class has a single namespace that contains all the instance variables and class variables it ever heard about, from its own definition or from any of the classes it inherits from.

    As long as all of the names therein are distinct, the system will do just fine; that is, if there is only one instance variable named "frob" among all those names, Pixie Scheme III doesn't need to know which class it was inherited from in order to set it, get it, or apply it as a method.

    Pixie Scheme III will warn you if a flattened namespace contains any duplicated names, but it will not prevent you from creating classes with such duplications. Just be sure you know what you are doing, if you do.

      Technical Note: Flattening the namespace was a deliberate design decision, and was rather a consequence of multiple inheritance. The problem is, that if a class inherits from an inheritance tree that is, say, seven deep and contains 297 classes, then it can be rather difficult to decide and to specify which super-super-super-superduperclass it is that you want to send a message to. Even if you have a way to accomplish that feat, the syntax for doing it is likely to be bewildering beyond belief. Flattening the namespace was a straightforward way out. I suspect it will not be a problem if you bear in mind, while you are building class hierarchies, that their namespaces will all be flattened.

      For example, you could start the names of variables in each class with the name of that class. Thus in a class named "foo", you might have variables "foo-x", "foo-y", and so on. That will work as long as a class has "foo" in at most one place in its inheritance.

  • The Pixie Scheme III class system does not make automatic use of "init" methods or of specialized "constructor" methods. Notwithstanding, the system requires that every class variable and instance variable have a default value, and provides a way to override the default values of instance variables when creating an instance of a class.

    You may of course create any specialized methods you wish to run after you have created an instance of a class, but you will have to run them yourself. One way to do that is to create a specialized procedure that creates the class instance, then runs your method, and then returns the modified class instance.


A Simple Class Example -- Right Triangles:

Let's go through a simple example. Suppose we want to have a class of right triangles. It takes two sides to define one -- say, side-1 and side-2. Here is the Pixie Scheme III code to create a minimal version of such a class. (You can cut and paste this code into Pixie Scheme III to try it out.)

    (define right-triangle
      (e::make-simple-class
        'right-triangle
        '()
        '()
        '((side-1 0) (side-2 0))
        )
      )
    

In that code, "e::make-simple-class" is the procedure to make a class, whose full syntax I have not yet described. The first argument, "'right-triangle", -- note the quote -- is what the class will understand its name to be. It will be used, for example, in constructing error messages, things like "While trying to create a class of type 'right-triangle', the world suddenly came to an end. Sorry about that." Note that in this example, that argument happens to be the same as the first argument to "define", here:

    (define right-triangle
      ;; ...
      )
    

There is no requirement that those two symbols be the same, but it is good practice to make them so.

The second argument, which is '(), would be a list of classes that "right-triangle" inherited from, except there aren't any.

The third argument, which is also '(), would be a list of class variable names and default values, except that there aren't any yet. (Class variables are shared by all instances of the given class.)

The fourth argument is list of instance-variable names and default values that will be used by each instance of the given class. Again, note the quote. I hope the syntax is obvious, and you do have to provide a default value for each instance variable. There are other ways you could have built up this list. For example, instead of writing

    '((side-1 0) (side-2 0)),
    

you might alternatively have written

    (list (list 'side-1 0) (list 'side-2 0))
    

Now let's make a triangle:

    (define first-triangle (right-triangle '()))
    

The argument to "right-triangle" is a list of overrides to the default instance-variable values, except there aren't any.

We now have created a right triangle whose sides are both zero. You get the instance-variable values like this. Note the quotes.

    (first-triangle 'side-1)  ;; ==> 0
    (first-triangle 'side-2)  ;; ==> 0
    

A triangle with zero-length sides is not very interesting. Let's fix that. Watch those quotes.

    (first-triangle 'set! 'side-1 3)  ;; ==> 0
    (first-triangle 'set! 'side-2 4)  ;; ==> 0
    (first-triangle 'side-1)  ;; ==> 3
    (first-triangle 'side-2)  ;; ==> 4
    

We could have made a triangle with non-zero side lengths by passing in a list of initializers when we made it:

    (define first-triangle (right-triangle '((side-1 5)(side-2 12))))
    (second-triangle 'side-1)  ;; ==> 5
    (second-triangle 'side-2)  ;; ==> 12
    


Creating and Using Methods:

One thing we might want to know about a right triangle is the length of its hypotenuse. Let us redefine the right-triangle class to include a class method that calculates the hypotenuse. For clarity, I will define the method first, then reconstruct the class and the instances.

    (define (hypotenuse-code this)
      (let
        ((a (this 'side-1))
         (b (this 'side-2))
        )
      (sqrt (+ (* a a) (* b b)))))
    

What's this about "this"? Wait and see ...

In the new class definition immediately below, note the use of back-quote and comma in providing the binding for the class variable "hypotenuse". I could also have just used "list" explicitly, as in (list (list 'hypotenuse hypotenuse-code)).

    (define right-triangle
      (e::make-simple-class
        'right-triangle
        '()
        `((hypotenuse ,hypotenuse-code))
        '((side-1 0) (side-2 0))
        )
      )
    
    (define first-triangle (right-triangle '()))
    (first-triangle 'set! 'side-1 3)  ;; ==> 0
    (first-triangle 'set! 'side-2 4)  ;; ==> 0
    (define second-triangle (right-triangle '((side-1 5) (side-2 12))))
    
    (first-triangle 'method 'hypotenuse)   ;; ==> 5
    (second-triangle 'method 'hypotenuse)  ;; ==> 13
    

The point about "this" is that when you send a "method" message to a class instance, the method you name -- in this case "hypotenuse" -- is called with the class instance itself prepended to any other method arguments you may have provided. The effect in the two cases just given is as if "hypotenuse-code" were called as follows:

    (hypotenuse-code first-triangle)
    (hypotenuse-code second-triangle)
    

Note that you do not have to use the name "this" for the corresponding name in the argument list of "hypotenuse-code". You can call that argument anything you like. Names like "this" or "self" are conventional, though.

Methods may have other arguments than "this". For example, you might have wanted a method for right triangles that returned a boolean value, #t or #f, depending on whether the hypotenuse of the triangle was greater than a passed value. The code for that method might be written like this:

    (define (hypotenuse-greater-than-code this x)
      (> (this 'method 'hypotenuse) x))
    

Then you could have yet another redefinition of the right triangle class:

    (define right-triangle-again
      (e::make-simple-class
        'right-triangle-again
        '()
        `((hypotenuse ,hypotenuse-code)
          (hypotenuse> ,hypotenuse-greater-than-code)
          )
        '((side-1 0) (side-2 0))
        )
      )
    
    (define first-triangle-again (right-triangle-again '()))
    (first-triangle-again 'set! 'side-1 3)  ;; ==> 0
    (first-triangle-again 'set! 'side-2 4)  ;; ==> 0
    (define second-triangle-again (right-triangle-again '((side-1 5) (side-2 12))))
    
    (first-triangle-again 'method 'hypotenuse)      ;; ==> 5
    (second-triangle-again 'method 'hypotenuse)     ;; ==> 13
    (first-triangle-again 'method 'hypotenuse> 8)   ;; ==> #f
    (second-triangle-again 'method 'hypotenuse> 8)  ;; ==> #t
    

In the last two calls, the effect is as if "hypotenuse-greater-than-code" were called as follows:

    (hypotenuse-greater-than-code first-triangle-again 8)
    (hypotenuse-greater-than-code second-triangle-again 8)
    


Inheritance:

Let's see about inheritance. First let's define a simple class for colored objects:

    (define colored-object
      (e::make-simple-class
        'colored-object
        '()
        '()
        '((color chartreuse))
        )
      )
    
    (define test-colored-object
      (colored-object '((color vermilion))))
    (test-colored-object 'color)  ;; ==> vermilion
    

Now let's define a class for a colored right triangle

    (define colored-right-triangle
      (e::make-simple-class
        'colored-right-triangle
        (list right-triangle colored-object)
        '()
        '()
        )
      )
    

The new class inherits from "right-triangle" and from "colored-object", and does not add any more instance variables of its own. Let's make and test some colored triangles.

    (define first-triangle (colored-right-triangle '()))
    (first-triangle 'set! 'side-1 3)  ;; ==> 0
    (first-triangle 'set! 'side-2 4)  ;; ==> 0
    (first-triangle 'set! 'color 'burgundy)
    (define second-triangle
      (colored-right-triangle
        '((side-1 5) (side-2 12) (color puce))))
    (first-triangle 'method 'hypotenuse)   ;; ==> 5
    (first-triangle 'color)                ;; ==> burgundy
    (second-triangle 'method 'hypotenuse)  ;; ==> 13
    (second-triangle 'color)               ;; ==> puce
    

If you would like to investigate some of the inner details of a class, there are a few extra variable names that all classes will have, and all class instances will respond to.

    (first-triangle 'class-name)
      ;; ==> colored-right-triangle
    
    (first-triangle 'class-variable-names)
      ;; ==> (hypotenuse)
    
    (first-triangle 'all-variable-names)
      ;; ==> (side-1 side-2 color all-variable-names inheritance class-variable-names class-name this hypotenuse)
    
    (first-triangle 'inheritance)
      ;; ==> (colored-right-triangle (right-triangle) (colored-object))
    

That last says that "colored-right triangle" inherits both from "right-triangle" and all its ancestors (except there aren't any), and from "colored-object" and all its ancestors (except there aren't any of those, either).

    (first-triangle 'this)
      ;; ==> #<Interpreted lambda expression, possibly named ">*<">
    

It would probably be a bad idea to "set!" "this". Don't say I didn't warn you.


Inheritance -- A Familiar Example:

Here is an example of multiple inheritance based on the genetic inheritance of a child from the two previous generations.

    (define maternal-grandmother (e::make-simple-class 'maternal-grandmother '() '() '()))
    (define maternal-grandfather (e::make-simple-class 'maternal-grandfather '() '() '()))
    (define paternal-grandmother (e::make-simple-class 'paternal-grandmother '() '() '()))
    (define paternal-grandfather (e::make-simple-class 'paternal-grandfather '() '() '()))
    
    (define mother (e::make-simple-class 'mother `(,maternal-grandmother ,maternal-grandfather) '() '()))
    (define father (e::make-simple-class 'father `(,paternal-grandmother ,paternal-grandfather) '() '()))
    
    (define child (e::make-simple-class 'child `(,mother ,father) '() '()))
    
    (define a-child (child '()))
    
    (a-child 'inheritance)
      ;; ==> (child (mother (maternal-grandmother) (maternal-grandfather)) (father (paternal-grandmother) (paternal-grandfather)))
      ;;
      ;; ... or, reformatted for clarity ...
      ;;
      ;; ==> (child
      ;;       (mother
      ;;         (maternal-grandmother)
      ;;         (maternal-grandfather)
      ;;         )
      ;;       (father
      ;;         (paternal-grandmother)
      ;;         (paternal-grandfather)
      ;;         )
      ;;       )
    


Class Variables:

Class variables are shared by all instances of the given class. They are commonly used to hold methods associated with the class, but there are other possible uses. Perhaps you wish to implement a counter of some sort that is shared by all instances of a class, or maybe an inter-instance signaling mechanism. Here is an example that shows the use of a class variable as a counter.

    (define (increment-counter-code this)
      (this 'set! 'counter (+ (this 'counter) 1)))
    
    (define class-variable-example
      (e::make-simple-class
        'class-variable-example
        '()
        `((counter 0)
          (increment-counter ,increment-counter-code)
          )
        '()
        )
      )
    
    (define instance-1 (class-variable-example '()))
    (define instance-2 (class-variable-example '()))
    
    (instance-1 'counter)  ;; ==> 0
    (instance-2 'counter)  ;; ==> 0
    
    (instance-1 'method 'increment-counter)
    (instance-1 'counter)  ;; ==> 1
    (instance-2 'counter)  ;; ==> 1
    
    (instance-2 'method 'increment-counter)
    (instance-1 'counter)  ;; ==> 2
    (instance-2 'counter)  ;; ==> 2
    

As you see, the new value of the class variable is seen by both instances, no matter which instance has changed it.

Class variable values are only shared by instances of a particular class. There is no sharing of class variables between different classes, even if one inherits a class variable from another, or if both inherit the same class variable from a common ancestor.

For example, suppose that class "root" has a class variable named "root-var". Suppose that classes "branch-one" and "branch-two" both inherit from "root". Then all instances of "branch-one" will share one value of "root-var", all instances of "branch-two" will share another, independent value of "root-var", and all instances of "root" itself will share yet a third independent value of "root-var".


Methods and Procedures as Variables:

It is important to note that you may, if you wish, store a procedure which is not intended to be used as a method, as the value of a class variable or an instance variable. Recall that when you invoke a class instance with "'method", the value of the variable you name is treated as a procedure, and that the actual class instance in question is prepended to whatever other arguments you supply, for you to use as the value of "this" or of some other convenient variable name.

Alternatively, suppose we had a class whose sole purpose was to store functions to do trigonometric or geometric calculations. Such a class might very well store a hypotenuse procedure, which we could get out and apply to any two numbers that we considered to be the sides of a right triangle. That is not as contrived an example as it seems: Perhaps we are doing calculations in Euclidean and non-Euclidean geometries, and we want to see what the properties of shapes are in several such instances of geometries. Anyway, let's define a simple class:

    (define (hypotenuse-procedure a b)
      (sqrt (+ (* a a) (* b b))))
    
    (define geometry
      (e::make-simple-class
        'geometry
        '()
        `((hypotenuse ,hypotenuse-procedure))
        '()
        )
      )
    
    (define my-geometry (geometry '()))
    

To use the hypotenuse procedure, we just pull out the value of the variable "'hypotenuse" and use it as a procedure.

    ((my-geometry 'hypotenuse) 8 15)  ;; ==> 17
    

I hope you see the difference. The symbol "'method" appeared nowhere in the use of the "geometry" class. We are in effect just storing ordinary procedures, which do not need a "this" variable, as the values of variables of the class.


Class Summary:

  • To make a new class, use the procedure:

      (e::make-simple-class
        <symbol which the class takes to be its name>
        <list of classes from which the new class inherits>
        <list of lists like (<symbol> <object>) for class variables>
        <list of lists like (<symbol> <object>) for instance variables>
        )
      

    For example, to create a class with one instance variable, "a", and a class variable "add-to-a", which will be a method that will add a number to "a":

      (define (add-to-a-code this n) (+ (this 'a) n))
      (define foo-class
        (e::make-simple-class
          'foo
          '()
          `((add-to-a ,add-to-a-code))
          '((a 0))
          )
        )
      

  • To make a new instance of a particular class, call the class with a list of overrides for its variables:

      (define my-foo (foo-class '()))
      (define my-foo (foo-class '((a 42))))
      

  • The class variables of a given class are common to to all instances of the class. The instance variables of a given class are separate for each instance of the class.

  • In addition to any variables that you may provide, every class has the following five instance variables built in:

      class-name             ;; Symbol naming the class.
      class-variable-names   ;; List of symbols which name
                             ;;   the class variables.
      inheritance            ;; A tree, in list form, showing
                             ;;   the (multiple) inheritance of
                             ;;   the given class.
      all-variable-names     ;; List of symbols which name
                             ;;   all the variables of the given
                             ;;   instance of the class -- both
                             ;;   class and instance variables.
      this                   ;; The instance of the class.
      

  • The Pixie Scheme III class system handles multiple inheritance by flattening the namespace of variables. It will warn you about duplicate names (which override their predecessors), but won't prevent you from using them.

  • A class instance is itself a procedure, which will use the first argument with which it is called to decide what the call is supposed to do:

      (my-foo 'a)                    ;; Returns my-foo's value of a.
      (my-foo 'set! 'a 3)            ;; Sets my-foo's value of a to 3.
      (my-foo 'method 'add-to-a 42)  ;; Adds 42 to my-foo's value of a.
      

  • When 'method is used to invoke a method of a class, the actual call to the method code has the value of the instance of the class prepended to any other arguments that may be passed with 'method. That is why the "this" was used in the Scheme source above for "add-to-a-code".

  • There are no "init" methods that run automatically.

  • There is no built-in notion of superclass.

  • There are no private or protected variables or methods: They are all public.

  • The interface to the Pixie Scheme III class system is entirely procedural -- no macros -- which means that many things have to be quoted.


Compiler:

Pixie Scheme III contains a simple compiler: What it produces is not native code for the iPad microprocessor, but merely a somewhat more optimized form of Scheme, which the interpreter can evaluate more quickly.

    Technical Note: The compiled code is of a form that is sometimes called "threaded", with the word used in the sense of "threaded interpreted languages" -- like Forth -- rather than in the sense of "process threads".

Pixie Scheme III has an internal "compile defines" flag that causes each "define" automatically to compile the quantity being defined, provided that "define" is used with the syntax

    (define (<name> <argument> ...) ... )
    

that is, with parentheses around the function name and argument list.

That flag is turned on and off by checking and unchecking the "compile defines" item of the Features Panel, or by using the procedures described in the Enhancements section under State Flags. With the flag on,

    (define (inc x) (+ x 1))
    

will create a compiled procedure called "inc", that adds one to its argument.

The idea of compilation is to do as much of the work of a program as possible just once, at compile-time, instead of many times, every time the program is run. Thus the compiler ...

  • Expands macros,
  • Precalculates the positions within the environment of variables (including the arguments) referenced by compiled procedures,

  • Inserts the actual values of variables that are "permanent", within the bodies of compiled procedures.

Code that has had these things done runs much faster than code that has not.

Even if the "compile defines" menu item is not checked, the compiler will expand macros in procedures that are "define"d using the syntax

    (define (<name> <argument ...>) ... )
    

If the "compile defines" menu item is not checked, there is still a way to compile things, using the procedure "e::compile-form":

    (e::compile-form <form>)
    

in which <form> evaluates to what you want to compile. "<Form>" will typically be either a lambda expression or a symbol to which a lambda expression is bound. You will probably wish to bind the value returned by "e::compile-form" to a symbol, for subsequent use. For example, you might first enter

    (define inc (lambda ( x) (+ x 1)))
    

followed by

    (define inc (e::compile-form inc))
    

The result would be a compiled procedure called "inc", that adds one to its argument.

The compiler attempts to do something sensible with every <form>, but compilation has no effect on <form>s which do not evaluate to lambda expressions: Thus for example

    (e::compile-form '(1 2 3)) ;; ==> (1 2 3).
    

If you define a lambda expression with "compile defines" flag off, then turn it on, you can compile the expression easily, as shown here:

With "compile defines" off:

    (define foo (lambda ...)).
    

Then, after turning "compile defines" on:

    (define foo (e::compile-form foo))
    

The compiler includes some other procedures of interest:

    (e::define-no-compile <symbol> <form>)
    (e::define-no-compile (<symbol> ... ) <form> ... )
    

"e::define-no-compile" acts as "define" does when the "compile defines" flag is unchecked: That is, even if "compile defines" is checked, and even if quantity being defined evaluates to a lambda expression, nevertheless "e::define-no-compile" will merely bind the evaluated quantity to the <symbol>, without compiling it. Use "e::define-no-compile" to make sure something never gets compiled.

"e::define-no-compile" recursively searches its second argument for macro "calls", and expands any that it finds, before binding the result to the <symbol>.


Compiling and Permanence:

Effective use of the compiler requires understanding how it uses the "permanence" mechanism. To make a symbol permanent is to make a promise to the compiler, that whatever value is associated with the symbol at compile-time will continue to be associated with it at run-time. Thus the compiler can substitute the value for the symbol once and for all in compiled code, and save the time required to look up the symbol time after time, at run-time. The down side of permanence is that if you ever decide to make a permanent symbol un-permanent, and change its value, you will have to recompile all the procedures that use it, that were compiled while it had the first permanent value. Otherwise, those procedures will continue to use the old value.

The typical symbols you will probably wish to make permanent are the names of procedures, and there is a special procedure to help a bunch of compiled, permanent procedures all know that they are all permanent. That function is "e::substitute-new-permanent-symbols"; you use it on a procedure that has already been compiled, to let that procedure know about symbols that it contains, that have been made permanent after it was compiled:

    (e::substitute-new-permanent-symbols <form>)
    

Note that you do not have to bind the result of "e::substitute-new-permanent-symbols" to anything; that procedure modifies its argument: It looks through the function body for symbols that have become permanent, and substitutes in their values.

Here is some boiler-plate for how to use the "permanence" mechanism effectively in compiled code. Suppose that the "compile defines" menu item is checked, and that you wish to compile three functions, "foo", "bar" and "baz", that call each other, or that call themselves recursively, or both. The idea is to define them all first, then make them all permanent, then apply "e::substitute-new-permanent-symbols" to all of them. You do all this with the "compile defines" flag set:

    (define my-function-names '(foo bar baz))
    
    (define (foo ...) ...)
    (define (bar ...) ...)
    (define (baz ...) ...)
    
    (map e::set-permanent! my-function-names)
    (map e::substitute-new-permanent-symbols my-function-names)
    


Debugger:

The debugging mechanism provided with Pixie Scheme III is very incomplete, though there is a hook for making it better. Pixie Scheme III's mechanism for handling recoverable errors -- the kind that eventually return control to top level --knows to look for a symbol called "e::debug". If that symbol is bound to a procedure, the error-handling message will call it with a single argument. Thus you may create a debugger merely by defining "e::debug", like this:

    (define e::debug (lambda (x) ...))
    
Pixie Scheme III will then call "e::debug" as part of its error handling, after the error message has been printed, instead of returning control to the top-level environment.

The argument to "e::debug" will be a non-empty list of all the environments in the lexical scope of the place where the error happened. The last member of that list will be the top-level environment: It will be a vector, each of whose elements is a list of symbols. Every symbol bound in the top-level environment will appear in exactly one vector element. If the list that is passed to "e::debug" has more members, its first member will be a list of the symbols bound in the lexical scope most local to the error, its second will be a list of the symbols bound in the next lexical scope outward, and so on till the top level. Only the last member will be a vector; all the others will be lists.

You may have your "e::debug" do whatever you like with this information. You might display the symbols. It will be hard to obtain their values, because the environment list passed to "e::debug" as an argument is not the one Pixie Scheme III searches automatically from within "e::debug", when looking for variable bindings. What's more, the environment list is not a very interesting data structure for debugging. Much more useful would be access to Pixie Scheme III's run-time stack (which is different from the central processor's "machine stack"). You can always show the contents of this stack, whether from the debugger or not, by calling the procedure "e::show-stack", but there is presently no reasonable way to learn what procedures are associated with specific items on the stack, or to investigate any particular stacked item in more detail.

Your "e::debug" probably ought to terminate with a call to "e::reset", so that Pixie Scheme III will return to top-level for further input after "e::debug" has run.

Be careful that "e::debug" is correct code. It is difficult to recover from errors in the debugger. Recursive errors are particularly nasty: The handling of an error in the debugger invokes another copy of the faulty debugger, which causes another error, and so on.

In addition to whatever the debugger may do, Pixie Scheme III handles non-fatal errors simply: It prints out some information about the error and what caused it, before invoking any debugger that may be present. If no debugger is present, Pixie Scheme III then returns control to the top-level loop. Different things might happen when a debugger is loaded -- depending on what the debugger is supposed to do.

The printed information will include the name of the last lambda expression entered before the error was encountered, whether or not that lambda expression had returned. It will also include the names of all lambda expressions whose activation records are stacked in Pixie Scheme III's internal stack; however, this list will not include the names of lambda expressions that were departed from by a tail-recursive call: Their activation records will by then no longer be stacked.

If you are wondering what I mean by the "name" of a lambda expression, wait for a few paragraphs.

By the way, fatal errors are handled differently, depending on what they are and when they occur: Usually, Pixie Scheme III will print out some kind of message and then exit. Any debugger present will not be called.

    Technical Note: A fatal error is one so serious that Pixie Scheme III can no longer run, and since any debugger of the kind described necessarily runs in Pixie Scheme III, it cannot run, either.

If the you ever see an with a message like

    "Implementation error ..."
    

or

    "Implementation: ..."
    

then I would very much appreciate a bug report with as many details as you can provide. (In particular, include the entire message.) Such messages indicate that I have inadequately guarded against some particular problem: It's my fault; I will be eager to do better.

  • Lambda Expression "Names":

    The internal representation of a Pixie Scheme III lambda expression has a place where Pixie Scheme III can record an identifier, which I will somewhat incorrectly refer to as the lambda expression's name. This information can be of great use in a stack trace, for error reporting or debugging. That isn't quite as simple as storing the identifier to which the lambda expression is bound, because a lambda expression may be bound to more than one identifier, as in

      (define (dessert-topping) (lambda () ...)
        ;; It's a dessert topping!
      (define floor-wax dessert-topping)
        ;; No!  It's a floor wax!
      

    Or a lambda may have no name at all, as in

      ((lambda (n) (+ n 1)) 3)
      

    What Pixie Scheme III does is initialize each lambda expression's "internal" name to (). If a lambda is subsequently bound or assigned to a symbol, Pixie Scheme III records that symbol within the lambda expression, as its name. Thereafter, if that lambda expression should subsequently be rebound or reassigned by "define", by "set!", by "e::define-no-compile", Pixie Scheme III overwrites the old internal name with the new one. Those are the only ways to cause an internal name to be overwritten: Pixie Scheme III will not give a new internal name to a lambda which already has one, when that lambda is rebound or reassigned by any other means.

    In particular, Pixie Scheme III will not set the internal name of a lambda expression to one of the top-level loop variables: I personally find that way too confusing when I am staring at debugger output.

    Thus a lambda expression which has been named at top-level will not be renamed if it gets passed to a procedure as a parameter, or if it gets rebound locally by "let" or some similar mechanism. On the other hand, lambda expressions which do not have top-level names will be named at first opportunity, even if only by being bound to a local variable or to a formal parameter.

    You might say that lambda expressions want names so badly that they will grab one at the first opportunity, and will give it up only when forced to do so.

    I hope these conventions will correspond reasonably to your notions of what a lambda expression ought to think its name is, and that they will help with debugging in any case.

    In case it is not yet clear, this business of storing a lambda expression's name inside the lambda expression is an addition to, not a substitute for or alteration of, the normal Scheme operations of binding and assignment: It is merely a debugging aid. If you didn't know about the "internal" names (and if you never made any mistakes that caused a stack trace to be printed), you'd never notice they were there. What's more, you don't need to know about them to use Pixie Scheme III.


Forgettable Objects:

Pixie Scheme III has an enhancement that allows creation and use of data structures that are set up automatically to be forgotten -- purged from Scheme main memory -- after some time has passed, even if they are not garbage in the formal sense of the word. The interface to forgettable objects lets you ask whether an object still exists before trying to use it, and provides you with knowledge and control of the precise details of when and if it disappears.

So what good is that?

As with most of the enhancements to Pixie Scheme III, I put this one in because I thought I might use it in personal projects. I am intending to provide a facility usable by artificial intelligence projects to mimic the "short-term memory" of biological minds. Stored data can go away, and instead of having a program crash when it tries to use an invalid reference, the interface provides a way to tell that "I forgot", which the program can then deal with, perhaps by rebuilding a data structure or by going out and making a new, current, observation of relevant real-world data. (That is sort of like what you and I do when we forget things, though perhaps without the whining.)

Note the difference in behavior from standard Lisp garbage collection of unreferenced items. With forgettable objects, a program can have what it thinks is a valid reference to something, and it won't find out that the reference no longer works till it tries to use it.

But wait, there's more! I tried to create a mechanism that would also allow several other useful behaviors related to forgetting things, such as:

  • Caches in which an object is guaranteed to be remembered for a certain time, and then will be forgotten when nothing else has a reference to it.

  • Use-count mechanisms for deciding what to do with an object based on how much it has been used. (The forgettable-object interface provides primitives for creating such a mechanism.)

  • Time-of-last-reference mechanisms for deciding what to do with an object based on when it was last used. (The forgettable-object interface provides primitives for creating such a mechanism.)

  • Means for a user to prepare in advance to delete large, infrequently-used data structures from Scheme memory, at times when memory is getting tight or garbage collection is taking too long.


Forgettable Object Content:

A forgettable object is in essence a new kind of Scheme object, which is a wrapper around any Scheme object and two additional items:

  • An expiration time, after which the object might get forgotten (details shortly).

      Technical Note: Expiration times are "Unix times" -- unsigned integers which are interpreted as seconds since the beginning of calendar year 1970 (common era). In Apple Macintosh 64-bit applications, such as Pixie Scheme III, these integers are 64-bit, which means that Pixie Scheme III's mechanisms for dealing with time will cease to operate correctly approximately 6000000000000 years from now.

      If you are still using Pixie Scheme III then, and you would like a better mechanism to deal with time, send me some EMail and I will see what I can do ...

  • A probability of remembering -- a number in the range [ 0, 1 ] -- which affects what happens at the object's expiration time.

      Technical Note: Pixie Scheme III's particular implementation of forgettable objects actually involves three new kinds of Scheme objects: forgettable-unexpired, forgettable-forgotten, and forgettable-remembered.


Forgetting Behavior:

  • Before the object's expiration time, the object is remembered, unless the forgettable object -- the "wrapper" -- has itself become garbage, in which case it and its contents will be subject to garbage collection like any other Scheme object.

      Technical Note: If the forgettable object itself becomes garbage, then what happens to the Scheme object within it depends on whether that inside object is itself garbage. It may not be garbage in its own right; for example, it will not be garbage if you have defined at top level an object that is not an atom, bound it to an identifier, and then put that bound object into a forgettable object. The bound object will remain non-garbage as long as the top-level binding exists, without regard to the garbage status of the forgettable object into which it has been placed.

      It does not make much sense to make atoms forgettable.

      As an enhancement, Pixie Scheme III provides the procedure "e::atom?" to determine what kinds of objects are atoms. See the section about enhancements which are miscellaneous procedures.

  • All the rest of this discussion only applies to forgettable objects at times past their expiration times.

  • After expiration time, changes in "forgotten" status only take place during full garbage collection. Generational garbage collection does not cause any changes in "forgotten" status.

  • At full-garbage-collection time, the garbage collector will first figure out what is and is not garbage, as it usually does, but *without* going from the forgettable object "wrapper" to the things inside it.

      Technical Note: At this stage, the garbage collector treats the pointer from the forgettable object to the things which it contains as a "weak reference", in the sense that the term is used elsewhere in computer science; for example, in Java, C#, Python, or Perl, or in Apple's Objective-C with garbage collection turned on. (Be cautious, the term "weak reference" means somewhat different things to different people.)

    The garbage collector will then deal with unexpired forgettable objects that are not garbage themselves.

  • If the object wrapped within a non-garbage unexpired forgettable object is not garbage, then the garbage collector will not garbage-collect any of the content of the forgettable object, and it will not change the status of the forgettable object -- it will leave it unexpired.

  • If the object wrapped within a non-garbage unexpired forgettable object is garbage, then the garbage collector will decide what to do based on the object's probability of remembering.

  • If the probability of remembering is one, the status of the forgettable object will be changed from "unexpired" to "remembered", and the forgettable object and its contents will no longer be subject to "forgetting".

      Technical Note: In effect, the the pointer from the forgettable object to the things which it contains is no longer a weak reference.

    A forgettable object which is remembered is of course subject to normal garbage collection, if the forgettable object itself should become garbage. (Note once again, that just because the forgettable object is garbage does not necessarily mean that the object contained within it is also garbage.)

  • If the probability of remembering is zero, the status of the forgettable object will be changed from "unexpired" to "forgotten", the content of the forgettable object will be garbage-collected -- and will disappear -- and further attempts to access that content will report failure, as discussed below.

    A forgettable object which is "forgotten" is of course subject to normal garbage collection, if the forgettable object itself should become garbage.

  • If the probability of remembering is greater than zero and less than one, Pixie Scheme III will select a random number in the interval [ 0, 1 ] and will compare that random number with the probability of remembering. If the random number is less than the given probability of remembering, Pixie Scheme III will change the status of the forgettable object to "remembered"; if not, it will change the status of the forgettable object to "forgotten", and will act as described above in either case.

    For example, a probability of remembering of 0.75 corresponds to a 75 percent chance of being remembered, and a 25 percent chance of being forgotten, once the forgettable object is past its expiration time and the object within it is otherwise garbage.

  • Remember once again, that the status of an unexpired forgettable object can only change if the object within it is garbage, and that the statuses of forgettable objects which are either forgotten or remembered are not changeable at all.


Programming Interface to Forgettable Objects:

    (e::make-forgettable <given object> <a Unix time> <probability of remembering> )
    

      Create and return an unexpired forgettable object containing the given object, with the given Unix time as its expiration time, and with the given probability of remembering.

    (e::forgettable? <object>)
    

      Is the given object a forgettable object?

    (e::forgettable-forgotten? <object>)
    

      Is the given object a forgettable object that has been forgotten?

    (e::forgettable-remembered? <object>)
    

      Is the given object a forgettable object that has been remembered?

    (e::forgettable-unexpired? <object>)
    

      Is the given object a forgettable object that is neither forgotten nor remembered?

    (e::forgettable-object <forgettable-object>)
    

      Returns multiple values: The first is #t if the forgettable object has not been forgotten and #f if it has been forgotten; the second is the stored object if the forgettable object has not been forgotten and is undefined if that object has been forgotten.

    (e::forgettable-expiration-time <forgettable-object>)
    

      Returns multiple values: The first is #t if the forgettable object has not been forgotten and #f if it has been forgotten; the second is the expiration time, which is still remembered by the forgettable object even if the object stored within it has itself been forgotten.

    (e::forgettable-remember-probability <forgettable-object>)
    

      Returns multiple values: The first is #t if the forgettable object has not been forgotten and #f if it has been forgotten; the second is the probability of remembering if the forgettable object has not been forgotten and is undefined if that object has been forgotten.

    (e::set-forgettable-expiration-time! <forgettable-object> <a Unix time>)
    

      Change the expiration time of the forgettable object to the new value provided. This operation will succeed if, and only if, the forgettable object is unexpired. Return #t if the operation was successful and return #f if not.

    (e::set-forgettable-remember-probability! <forgettable-object> <probability>)
    

      Change the probability of remembering of the given forgettable object to the new value provided. This operation will succeed if, and only if, the forgettable object is unexpired. Return #t if the operation was successful and return #f if not.

    e::entropy-death
    

      This symbol is bound to the largest value that Pixie Scheme III can store as a signed fixnum; if taken as a Unix time, it corresponds to approximately 3000000000000 years in the future.

        Technical Note: It is presumptuous of me to label an epoch so near with a symbol that suggests that the Universe will have achieved entropy death by then. I beg forgiveness from inhabitants of solar systems whose suns are faint red dwarves.


Some Uses of Forgettable Objects:

  • A forgettable object whose expiration date is in the future and whose probability of remembering is zero is cached until that time, and for as long after as anyone is using the object.

  • To use other mechanisms of "expiring" objects, wrap "e::forgettable-object" inside a specifically written accessor, which might keep track of a use count or time of last use, and might change the object's expiration time to one in the past when it decides to allow the object to expire.

Logic Programming:

Pixie Scheme III provides some support for logic programming in Scheme along the lines of Friedman, Byrd and Kiselyov, 2005). Pixie Scheme III does not include any of the fancy software from this work; that's copyrighted, but I hope there are enough primitives to get you going.

  • Pixie Scheme III provides two specialized "logic constants", "#u" and "#s", for use in logic programming. You couldn't even get Pixie Scheme III to read these identifiers unless I built them in, because R5 identifiers are not generally allowed to start with '#'. So here they are, for anyone who wishes to use them.

    Each of "#u" and "#s" evaluates to itself.

  • Procedures for Logic Programming:

    (e::logic-constant? <object>)
    

      Returns #t if, and only if, its argument is either #u or #s; otherwise, returns #f.


Non-Printing Objects:

Pixie Scheme III provides a specialized non-printing object, "#n", whose printed representation is an empty string and which furthermore will cause Pixie Scheme III's top-level read-eval-print loop to omit printing a newline when #n is the result of the "eval" part of the loop.

"#n" evaluates to itself. To see what "non-printing" means, compare the following examples, in which I have added comments to distinguish what you type at the keyboard from what Pixie Scheme III prints at the end of evaluation:

    #t  ;;  You type #t, and then a newline.
    #t  ;;  Pixie Scheme III evaluates #t as #t, which it prints, followed by a newline.
    #n  ;;  You type #n, and then a newline, but Pixie Scheme III prints nothing.
    
        ;;  In that last example, Pixie Scheme III actually evaluated #n to #n, but
        ;;    that result was not displayed, nor was a newline.
    
    (define x 3)   ;;  You type this definition, and then a newline.
    x              ;;    Pixie Scheme III prints "x", and then a newline
    x              ;;  You type "x", and then a newline.
    3              ;;    Pixie Scheme III prints its value, 3, and then a newline.
    x              ;;  You type "x", and then a newline (again).
    3              ;;    Pixie Scheme III prints its value, 3, and then a newline.
    (define y #n)  ;;  You type this definition, and then a newline.
    y              ;;    Pixie Scheme III prints "y", and then a newline.
    y              ;;  You type "y" and a newline, and Pixie Scheme III prints nothing.
    y              ;;  You type "y" and a newline again, and Pixie Scheme III prints nothing.
    

I added "#n" primarily as a value to return from interrupt handlers. I had some interrupts that were called frequently but mostly did nothing; I wanted to have them print nothing at all so as not to clutter up log files with repetitive meaningless output. An interrupt handler has to return something to the top-level read-eval-print loop of the kitten which handles it, so a non-printing object was just the thing to use for a returned value.

Note that you can inspect the non-printing object and get a printable result. Thus in the previous example, the input:

    (e::inspect y)
    

produces the output:

    0000000000000025-0000000000000004: Peculiar Leaf -- Not a Car
    #<Non-printing object>
    


  • Procedures for Non-Printing Objects:

    (e::non-printing-object? <object>)
    

      Returns #t if, and only if, its argument is #n; otherwise, returns #f.

You might consider the 'n' in "#n" to stand for "non-printing" or for "nothing".


Procedures and Forms:

This subsection describes some of the procedures and special forms that are present in Pixie Scheme III, that are not part of the R5 standard. I have by no means described all such extras: If you explore the system with the various low-level inspection tools described herein, you will find many procedures I do not mention. I am not trying to keep secrets: Rather, I have described only those enhancements that seem stable enough to use in your own code. The ones not described may change in detail or disappear entirely in future releases of Pixie Scheme III.

All the global symbols that I have used in constructing these enhancements start with either "e::" ('e' for enhancement) or with "c::" ('c' for compiler). To avoid naming conflicts with present and future enhancements, please do not create global symbols that start with either of these trios of characters.

    Technical Note: The double-colon in "e::" and "c::" may suggest some kind of "package" system, such as is widely used in other Lisps. There is no package system in Pixie Scheme III. The colon is an ordinary character. Symbols that contain a double colon are in no way different from symbols that do not. I use the double-colon in this way merely as a naming convention.

And please be wary about using any "c::..." procedures in your own code. Some will cause a crash -- Pixie Scheme III will cease to operate -- if misused.

  • Bit Operations:

    There are several procedures which allow direct manipulation of the individual bits of a 64-bit fixnum.

    (e::bit-and <integer> <integer>)
    

      Returns the bit-by-bit Boolean "and" of the arguments.

    (e::bit-not <integer>)
    

      Returns the bit-by-bit Boolean "not" of the arguments.

    (e::bit-or <integer> <integer>)
    

      Returns the bit-by-bit Boolean "or" of the arguments.

    (e::bit-xor <integer> <integer>)
    

      Returns the bit-by-bit Boolean exclusive "or" of the arguments.

    (e::bit-shift-left <integer> <integer>)
    

      Returns the first argument shifted left by the number of bits which is the second argument, inserting zeros at the right as necessary.

    (e::bit-shift-right-arithmetic <integer> <integer>)
    

      Returns the first argument shifted right by the number of bits which is the second argument, replicating the most-significant bit at the left as necessary.

    (e::bit-shift-right-logical <integer> <integer>)
    

      Returns the first argument shifted right by the number of bits which is the second argument, inserting zeros at the left as necessary.

  • Continued Fractions:

    See Long Ratnums and Continued Fractions.

  • Evaluation:

    (e::cons-with-continuation <object>)
    

      This procedure is left over from older releases of Pixie Scheme III, that did not implement the "R5" Scheme standard. That standard has an "eval" procedure, which Pixie Scheme III now supports. Yet "e::cons-with-continuation" is still there, in case anyone wants to use it.

      Many Lisps would call this procedure "eval". It causes its argument to be evaluated in the same environment as that in which "e::cons-with-continuation" itself was called.

        Technical Note: In case the name does not convince you that "e::cons-with-continuation" simply conses its argument onto the next-to-be-evaluated end of the current continuation, that is indeed what it does.

      Thus:

        (e::cons-with-continuation '(+ 2 2))    ;; ==> 4
        (define b 3)                            ;; ==> 3
        (define a 'b)                           ;; ==> a
        (e::cons-with-continuation a)           ;; ==> 3
        

      What is going on in that last example is that when you type

        (e::cons-with-continuation a)           ;; ==> 3
        

      the reader gets 'a, and the usual practice of evaluating the arguments to a procedure before calling it means that what is passed to "e::cons-with-continuation" is b. Procedure "e::cons-with-continuation" thus pushes b onto the next-to-be-evaluated end of the continuation, whereupon it is immediately evaluated to 3, which is what the top-level read-eval-print loop prints out.

      The main use of "eval" and similar features is a bit more complicated, however: One typically builds up the argument to be passed to eval a piece at a time, often in quite distinct and changing environments, then evals it all at once:

        (define to-be-evaled '())
        (set! to-be-evaled (cons 'operator to-be-evaled))
        (define x 42)
        (define y 88)
        (set-cdr! to-be-evaled (list 'x 'y))
        (define x 2)
        (define y 2)
        (define operator +)
        to-be-evaled                              ;; ==> (operator x y)
        (e::cons-with-continuation to-be-evaled)  ;; ==> 4
        

      In contrast, suppose we had slightly changed the line containing "set-cdr!" ...

        (define to-be-evaled '())
        (set! to-be-evaled (cons 'operator to-be-evaled))
        (define x 42)
        (define y 88)
        (set-cdr! to-be-evaled (list x y))        ;; NOTE: No quotes.
        (define x 2)
        (define y 2)
        (define operator +)
        to-be-evaled                              ;; ==> (operator 42 88)
        (e::cons-with-continuation to-be-evaled)  ;; ==> 130
        

      Here is another example: In it, note that even though "to-be-evaled" is defined in an environment in which x, y, and z are all zero, what happens when "to-be-evaled" is consed with the continuation depends on what x, y, and z were in the environment where that happened.

        (define x 0)
        (define y 0)
        (define z 0)
        
        ;; At this point x, y and z are all zero.
        
        (define to-be-evaled '(list x y z))
        
        (let ((y 1)(z 1))            ;; Now y and z are one.
          (let ((z 2))               ;; Now z is two.
            (e::cons-with-continuation to-be-evaled)))
        
        ;; ==> (0 1 2)
        

    In the past, there has been considerable controversy in the Scheme community concerning what "eval" ought to do, and whether it even ought to be part of the Scheme language at all. It took a long time for people to agree on the definition of "eval" used in R5 Scheme.

  • Files and Directories:

    (e::error-port)
    

      Returns the port to which Pixie Scheme II| sends error messages. In Pixie Scheme III, that port is always the Pixie Scheme III window.

      This procedure allows you to write your own error handlers that send messages to the same port that Pixie Scheme II uses.

  • Infinities and Nans:

    (e::nan? <object>)
    

      Returns #t if the <object> is an IEEE "nan" (which stands for "not a number"), otherwise returns #f.

    (e::inf? <object>)
    

      Returns #t if the <object> is an IEEE infinity (a special bit pattern used when floating-point operations have calculated numbers too large to be represented as ordinary IEEE floating-point numbers), otherwise returns #f.

  • Long Ratnums and Continued Fractions:

    (e::coerce-to-long-ratnum-if-possible <real>)
    

      If the <real> can be coerced to a long ratnum, returns it; otherwise returns #f

    (e::coercible-to-fixnum? <object>)
    

      Returns #t if the <object> is an integer in the range of 64-bit signed integers, and returns #f if not.

    (e::coercible-to-long-ratnum? <object>)
    

      Returns #t if the <object> is a number that can be expressed as a rational using 64-bit signed integers, and returns #f if not.

    (e::continued-fraction-list->real <list of integers>)
    

      Accepts a <list of integers> in the usual form for expressing a continued fraction, and returns the continued fraction as a long ratnum.

    (e::derationalize <real number>)
    

      If the argument is a rational number whose numerator and denominator are stored separately, divides the numerator by the denominator and returns the result; otherwise, does nothing.

    (e::long-ratnum? <object>)
    

      Returns #t if the <object> is a rational number whose numerator and denominator are stored separately, and returns #f if not.

    (e::make-long-ratnum <integer> <integer>)
    

      Creates a rational number whose numerator and denominator are stored separately, in which the numerator is the first argument and the denominator is the second argument. The stored numerator will bear the sign of the result. The result will be exact if, and only if, both arguments are exact.

    (e::real->continued-fraction-list <real>)
    

      Accepts a <real> in the range where conversion to a long ratnum is possible, and return the list of integers that expresses it as a continued fraction in the usual form.

  • Inspecting Scheme Objects:

    (e::inspect <object>)
    

      Displays some useful information about the <object> and returns the <object>. Just what is displayed depends on what the <object> is. The fact that "e::inspect" returns its argument means that you can generally wrap a call to "e::inspect" around any expression in a Scheme program, to see what is going on, without otherwise interfering with the operation of the program.

  • Macros -- An Alternate Implementation:

    In addition to the "hygienic" macro implementation described in the R5 report, Pixie Scheme III provides a "lower-level" macro implementation, that is more like the macro implementations in older forms of Lisp.

      Technical Note: This older implementation of macros descends from Pixie Scheme III's predecessor, Pixie Scheme, which I wrote before the R5 report was released. The older form is "non-hygienic", but is in some ways more versatile than what the R5 report describes, so I left it in as an enhancement.

      Besides, several key elements of Pixie Scheme III are written as Scheme macros using the older macro implementation.

    (e::expand-macro <macro>)
    

      Expands a macro. Details follow in an example, a few paragraphs down.

    (e::macro? <object>)
    

      Returns #t if <object> is a macro, and returns #f if not.

    (e::macro <symbol> <expression>)
    

      Binds a macro to <symbol>. Other Lisp implementations might have used the name "defmacro" for this operation.

    (e::macro-body <expression>)
    

      Creates a macro from the <expression>, but does not bind it to a symbol. That is,

        (e::macro <symbol> <expression>)
        

      is equivalent to

        (define <symbol> (e::macro-body <expression>)
        

    Pixie Scheme III implements this older form of macros in a rather conventional way. I will first describe this facility simply, glossing over some technical points, and will then fill in the details.

    To begin, "e::macro" resembles "define": The action of

      (e::macro <symbol> <expression>)
      

    is to evaluate the <expression>, to declare that whatever the evaluation returns is a macro, and finally to bind that macro to the <symbol>.

    The action of "e::macro-body" more nearly resembles the evaluation of a lambda expression: That is, the action of

      (e::macro-body <expression>)
      

    is merely to evaluate the <expression> and declare that whatever the evaluation returns is a macro.

    There is one catch: The final action of the <expression> must be to evaluate a lambda expression of one argument, so that the procedure thereby created is the value that will become the macro. Pixie Scheme III reports an error if the value of the <expression> is not a procedure that was derived from a lambda expression of one argument.

    A macro is invoked, or "called", by evaluating a pair whose car evaluates to a macro. That is, if you have defined

      (e::macro my-macro ...)
      

    then you may invoke my-macro as

      (my-macro <stuff>)
      

    The action of such a call is (1) to pass the entire pair -- in this case "(my-macro <stuff>)" -- to the procedure associated with the macro; whereupon (2) that procedure does whatever it is supposed to do, to the pair; and (3) the result returned by the procedure is itself evaluated.

    Here is a simple example. Suppose you are tired of writing assignment statements in the form "(set! foo bar)", and would rather write them as "(assign bar to foo)" Define the macro:

      (e::macro assign (lambda (form) `(set! ,(cadddr form) ,(cadr form))))
      

    You can then write

      (define foo #f)      ;; ==> foo
      (define bar 3)       ;; ==> bar
      (assign bar to foo)  ;; ==> foo
      

    The last expression assigns bar, which evaluates to 3, to foo. Now

      foo                  ;; ==> 3
      

    Let's look closely at how that works. The macro call "(assign 3 to foo)", causes the entire expression, "(assign 3 to foo)" to be passed as the argument "form" to "(lambda (form) `(set! ,(cadddr form) ,(cadr form))))". That procedure returns a list whose first element is "set!", whose second element is the cadddr of the form (namely the symbol "foo"), and whose third element is the cadr of the form, namely the number 3. That list, "(set! foo 3)" is in turn evaluated, causing 3 to become the value of foo.

    You can see what is happening by evaluating

      (e::expand-macro '(assign 3 to foo)) ;; ==> (set! foo 3)
      

    This expression causes only the first two parts of the macro call "(assign 3 to foo)" to take place: It returns the unevaluated result from the procedure, namely the list "(set! foo 3)".

    If you want to see the lambda expression itself, instead of what happens when you apply it, use "e::inspect". When you enter

      (e::inspect assign)
      

    Pixie Scheme III prints some information about where in Scheme memory the macro is located, then prints the lambda expression itself, though not as nicely pretty-printed as I have provided it here.

      (lambda (form)
        (quasiquote (set! (unquote (cadddr form))
                          (unquote (cadr form)))))
      

    Note in passing that "assign" is not a very well-behaved macro. For simplicity, it does not include various tests for syntax errors. A better version would have a lambda expression that checked that "form" was a list of exactly four elements, that the third element was the symbol "to", and that the fourth element was a symbol.

    The matter I have glossed over is what environments the various evaluations take place in. (Many people find the subject of environments confusing: You need not worry too much about this matter at first, though it may come back to haunt you if you become a serious Scheme enthusiast.)

    Briefly, the expansion of a macro -- that is, the evaluation of the procedure of one argument that is assigned to the macro name -- takes place in the lexical scope of the macro definition; but the second evaluation, of whatever expression the expansion returns, takes place in the lexical scope of the macro call.

    Here is an example that may make this issue clearer. Maybe ...

    First, let's define a global variable, named "my-variable", whose value is the symbol "global-value".

      (define my-variable 'global-value) ;; ==> my-variable
      my-variable                        ;; ==> global-value
      

    Now let's create a macro named "foo", whose body contains several instances of the symbol "my-variable".

      (e::macro foo
        (display "Defining foo -- my-variable is ")
        (display my-variable)
        (newline)
        (lambda ( the-argument-is-not-used )
          (display "Expanding foo -- my-variable was ")
          (display my-variable)
          (display " in the environment where foo was defined.")
          (newline)
          (display "In the present environment my-variable is ")
          'my-variable))
      

    As "foo" is being defined, the first two "display"s and the subsequent "newline" are evaluated, so that what appears in the Pixie Scheme III window is

      Defining foo - my-variable is global-value
      foo
      

    The "display"s printed the value of "my-variable" in the environment where foo was defined; that value was still "global-value". The lambda expression was then converted into a procedure, which became the value of "foo", which was returned and displayed at top level. The procedure uses "my-variable" in the fifth-from-last line of its definition, in "display". The value of "my-variable" so referenced is again from the environment in which "foo" was defined. So when "foo" is called, it will print (reformatted here to make the lines shorter):

      Expanding foo -- my-variable was global-value
        in the environment where foo was defined.
      

    before the lambda expression returns any value.

    The value returned by the lambda expression is a quoted symbol, "'my-variable", and that leading quote is a key to understanding what happens next. This expression will be evaluated in the environment in which the call to "foo" took place. In that environment, "my-variable" will not necessarily have the same value that it did in the environment where "foo" was defined. Thus after the return value has itself been evaluated, it will become

      "In the present environment my-variable is <?>"
      

    We don't now know for sure what the "<?>" will be.

    To continue the example, let's create a local environment in which "my-variable" has as value the symbol "local-value", and call "foo" in that environment. We might evaluate

      (let ((my-variable 'local-value)) (foo))
      

    What gets printed is, after reformatting it to make the lines shorter:

      Expanding foo -- my-variable was global-value
        in the environment where foo was defined.
      In the present environment my-variable is local-value
      

  • Miscellaneous Predicates:

    (e::all? <list>)
    

      Returns #t if no object in the <list> is #f, and returns #f otherwise. Returns #t if the <list> is the empty list.

    (e::any? <list>)
    

      Returns #t if at least one object in the <list> is not #f, and returns #f otherwise. Returns #f if the <list> is the empty list.

    (e::atom? <object>)
    

      Returns #t if <object> is an atom, and returns #f otherwise.

        Technical Note: An atom is a Scheme object that does not contain any pointers or other references to any other Scheme objects. Just what objects are atoms is implementation-dependent.

    (e::closed-port? <object>)
    

      Returns #t if <object> is an input port or an output port which has been closed, and returns #f otherwise.

    (e::fixnum? <object>)
    

      Returns #t if the <object> is a number stored as a 64-bit integer, otherwise returns #f. All numbers so stored are integers.

    (e::float? <object>)
    

      Returns #t if the <object> is stored in an IEEE floating-point format, otherwise returns #f.

      These formats can represent integers, rationals which are not integers, IEEE nans and IEEE infinities. Pixie Scheme III finds occasion to use IEEE formats for all of these entities.

    (e::forced? <object>)
    

      Returns #t if <object> is a promise which has been forced, and returns #f otherwise.

    (e::long-complex? <object>)
    

      Returns #t if the <object> is a number stored in an implementation-dependent format that separately stores the real and imaginary part of a complex number, otherwise returns #f.

    (e::promise? <object>)
    

      Returns #t if <object> is a promise, and returns #f otherwise.

  • Miscellaneous Procedures:

    (e::deep-copy <object>)
    

      Constructs a copy of its argument which is "equal?" to the argument, but in which copied vectors, lists and strings in corresponding positions in the structure are not themselves "eq?" to one another.

    (e::error <string>)
    

      Prints a string to the default error port, and resets Pixie Scheme III to the top-level loop.

    (e::warning <string>)
    

      Prints a string to the default error port, and then continues; does not reset Pixie Scheme III to the top-level loop.

    (e::gensym)
    

      Returns a symbol guaranteed to be different from any symbol already encountered by Pixie Scheme III. Symbols created by "e::gensym" print as strings of the form "gXXXXX...", where "XXXXX..." are the digits of a positive decimal integer that is greater than or equal to 23000.

        Technical Note: The choice of "23000" is in memory of Ratfor. "Ratfor" is not the name of one of my cats; at least, not yet.

    (c::load-from-string <string>)
    

      Causes the literal text of the string argument to be read at top level by the Pixie Scheme III read-eval-print loop, which will presumably evaluate it and print the result.

        Technical Note: This procedure acts very simply, by "pushing back" the text of its argument into the queue of characters from which Pixie Scheme III obtains input.

    (e::make-integer-range <integer> <integer> <nonzero integer>)
    

      Returns a list of integers that starts at the first argument and advances toward the second argument, but not to or past it, in steps of the third argument. Thus for example:

        (e::make-integer-range 0 10 1)
        

      returns (0 1 2 3 4 5 6 7 8 9).

    (e::original-cwcc <object>)
    

      Pixie Scheme III implements "dynamic-wind" generally along the lines of the implementation demonstrated by Jonathan Rees (1992) in "The Scheme of Things: The June Meeting", published in Lisp Pointers V(4), October-December 1992. That implementation involves modifying "call-with-current-continuation". The unmodified version of "call-with-current-continuation" is available as "e::original-cwcc".

    (e::read-string-with-prompt <string>)
    

      Uses the Pixie Scheme III dialog panel, that drops down from the top of the main window, to display the string that is its argument, and to accept a different string, which is returned. The string you type into the dialog panel does not require enclosing double-quotes.

    (e::reduce <binary-operation> <list> [<start-value>])
    

      Applies the given operation to the first two elements of the list, then applies the given operation to the result and the next element of the list, and so on. If the optional start value is given, starts out by applying the given operation to the start value and the first element of the list. For example:

        (e::reduce + (list 2 3 4 5) 42)
        

      calculates ((((42 + 2) + 3) + 4) + 5), which is 56.

    (e::select <predicate> <list> [<key>])
    

      With no <key>, makes up a new list comprising all elements of the given list for which the <predicate> is true. If the optional <key> is present, the <predicate> is applied to whatever results from applying the <key> to the list elements. Thus:

        (e::select positive? '(1 -1 0 2 -2 3 -3))
          ;; ==> (1 2 3)
        

      but

        (e::select positive? '((a 1) (b -1) (c 0) (d 2) (e -2) (f 3) (g -3) ) cadr)
          ;; ==> ((a 1) (d 2) (f 3))
        

      Thus for example, the list elements may be some sort of structure, and the <key> gives you the chance to pull out a particular part of the structure for testing by the <predicate>: In the case just shown, the structure is a list of two items, and the particular part is the second item.

      You could always do the same thing by using a specialized lambda expression for the <predicate>, but sometimes using the <key> makes things clearer.

    (e::select <predicate> <list> [<key>])
    

      With no <key>, makes up a new list comprising all elements of the given list for which the predicate is true. If the optional <key> is present, the predicate is applied to whatever results from applying the key to the list elements. Thus:

        (e::select positive? '(1 -1 0 2 -2 3 -3))
          ;; ==> (1 2 3)
        

      but

        (e::select positive? '((a 1) (b -1) (c 0) (d 2) (e -2) (f 3) (g -3) ) cadr)
          ;; ==> ((a 1) (d 2) (f 3))
        

      Thus for example, the list elements may be some sort of structure, and the key gives you the chance to pull out a particular part of the structure for testing by the predicate: In the case just shown, the structure is a list of two items, and the particular part is the second item. You could always do the same thing by using a specialized lambda expression for the predicate, but sometimes using the key makes things clearer.

    (e::usleep <integer>)
    

      Calls the Unix/C++ "usleep" function with the given integer as argument. Nominally, this function makes the Pixie Scheme III interpreter sleep -- ignore input and do nothing -- for a number of microseconds which is equal to the given integer. Because of delays in starting up the procedure, the granularity of the period of sleep is much coarser than one microsecond; the calling interface merely provides consistency with the Unix/C++ function.

      Only the part of Pixie Scheme III that reads, evaluates, and prints Scheme expressions is put to sleep by this command. Pixie Scheme III will respond to many user actions while sleeping; for example, changes in window size. Actions that would cause the processing of Scheme expressions will not take place until Pixie Scheme III has waked up, and may block other user actions until wake-up has taken place.

      In principle, calling "e::usleep" with a sufficiently large argument could cause Pixie Scheme III to sleep for an objectionable length of time. If that happens, you might have to push the iPad "Home" button to force Pixie Scheme III to terminate.

      There is a special hazard to using the "e::usleep" procedure on the iPad. Invoking that procedure with a sleep time that is too long may make the iPad's operating system think that Pixie Scheme III has become unresponsive because of a bug or some other problem with the application; in that case, the iPad operating system may terminate Pixie Scheme III without warning. I have used "e::usleep" on the iPad with sleep times as long as 3000000 microseconds -- three seconds -- with no problem, but your mileage may vary. I decided to leave the "e::usleep" procedure installed in Pixie Scheme III, instead of taking it out entirely, because sleeping for short amounts of time is often useful when controlling the speed of interactive Scheme programs.

    (e::version)
    

      Returns a string that shows the date and time of creation of the version of the Pixie Scheme III program that is running.

  • Multiple-Values Objects and Operations:

    Pixie Scheme III implements the procedures "values" and "call-with-values", as described in the "R5" report. The implementation features an additional kind of Scheme object, which I have unimaginatively labeled a "multiple values return". The intended use of these objects is to pass data between "values" and "call-with-values", but if you should happen to call "values" in some context other than providing input to "call-with-values", you might encounter one. Thus, for example, at top level:

      (values 2 3 4)  ;;  ==>
      
      #<Multiple Values Return>
      List of values: (2 3 4)
      

    Furthermore:

    (e::multiple-values? <object>)
    

      Returns #t if the <object> is a multiple-values return, otherwise returns #f.

  • Numeric Formatting:

    Straight R5 Scheme provides no convenient way to format numbers in varying ways without writing a lot of code on your own. Pixie Scheme III has a few enhancements to do some of the common formatting tasks, and perhaps to make it easier to create your own code to do more such tasks.

    The interface is built on a single low-level procedure, which uses the numeric formatting capabilities of the underlying C++ implementation in which Pixie Scheme III is written. (I do not propose to tell you how to do numeric formatting in C in this document, since there are plenty of widely available sources that do a better job of that than I could. For example, try typing "man printf" in a Unix shell, such as the Macintosh "Terminal" application.)

    The low-level procedure is:

    (c::number->string-with-c-format <number> <string>)
    

      This procedure accepts a number as first argument, coerces it to a C++ "double", and converts it to a number using the second argument as a C++ "format" string.

        Technical Note: The underlying C++ call ends up as

          snprintf( scratch, 256, <your number coerced to double>, <your format string>)

        in which "scratch" is a pointer to a buffer containing 256 characters. Following this C++ call, the content of "scratch" is again copied into a newly-allocated string in Pixie Scheme III's main memory, so there is no need to worry about saving "scratch" or overwriting it.

      Note that "c::number->string-with-c-format" does no checking whatsoever of whether its "string" argument is appropriate for use as a C++ "format" string. Thus there is a noteworthy probability that ill-considered use of this procedure may cause Pixie Scheme III to crash. This procedure is perhaps best used as a primitive for procedures which are themselves well tested and debugged.

      Thus for example:

        (c::number->string-with-c-format 1 "%.3f") ;; ==> "1.000" (c::number->string-with-c-format 29.95 "For the low, low price of only $%.2f!!") ;; ==> "For the low, low price of only $29.95!!"

    There are two additional procedures which operate at slightly higher level:

    (e::number->string-with-n-decimals <number> <number>)
    

      This procedure returns a string containing the first argument printed in decimal format with n digits to the right of the decimal point, where n is the second argument. The value of n must be in the range [0 .. 10].

      For example:

        (e::number->string-with-n-decimals 1/3 3) ;; ==> "0.333" (e::number->string-with-n-decimals (- (/ 1000 3)) 6) ;; ==> "333.333333"

    (e::money->string <number>)
    

      Prints the number with two digits to the right of the decimal point, as in:

        (e::money->string 29.95) ;; ==> "29.95"

  • Permanence:

    If only one value will ever be bound or assigned to a particular symbol, you can speed up the operation of both interpreted and compiled code by making that symbol "permanent". You can reverse the process later, if necessary; but if you do, be sure to recompile any compiled expressions that use the symbol: Otherwise, strange things may happen. It is possible -- though perhaps not useful -- to make permanent a symbol that already has different values bound to it in different scopes.

    Pixie Scheme III will report an error if you attempt to bind or to assign a value to a permanent symbol.

    Most of the symbols that denote the built-in primitive operations of Pixie Scheme III are permanent. These symbols include "+", "car", "if", "call-with-current-continuation" and so on.

    (e::set-permanent! <symbol>)
    

      Makes the value assigned to <symbol> be permanent; that is, declares to Pixie Scheme III that the value presently assigned to <symbol> will never change. Does not change that value.

    (e::clear-permanent! <symbol>)
    

      Makes the value assigned to <symbol> be non-permanent; that is, declares to Pixie Scheme III that the value presently assigned to <symbol> may change. Does not change that value.

    (e::permanent? <symbol>)
    

      Returns #t if the <symbol> is permanent, otherwise returns #f.

        Technical Note: I added the "permanent" feature because in another Lisp system, I once inadvertently and unknowingly redefined "T" -- its "true Boolean". That system kept a master value of T somewhere, to copy into code when necessary. Somehow, I changed it. Code that already had the "right" value of T continued to work correctly, but new stuff did not. The effect was that I had a weird bug, and whenever I changed a procedure, in the act of debugging, the bug spread. Unreal ...

        The folks at MIT's project MAC, who wrote the original "MACLisp" (a 1970's-vintage Lisp that ran on mainframe computers, not -- as you might suppose from the name alone -- a Lisp for the Macintosh) issued special error messages if you tried to alter the values of T and nil in that system. They were:

          VERITAS AETERNA -- do not setq T!
          NIHIL EX NIHIL -- do not setq nil!
          

        They had the right idea. (Welcome to the twilight zone -- a CalTech graduate -- me -- just said something complimentary about MIT.)

        Perhaps you can figure out how I know about those error messages ...

        Elsewhere herein I tell how, in the early stages of development of Pixie Scheme -- Pixie Scheme III's predecessor -- I accidentally managed to create nearly nine billion separate and operationally inequivalent kinds of truth and falsehood.

        Non-Technical Note: According to artificial-intelligence historian Pamela McCorduck, what "MAC" stood for was either "Man And Computers" or "Machine-Aided Cognition", and the ambiguity was deliberate. See the footnote on p. 287 of Machines Who Think, which is listed in the Other References section herein.

        I have occasionally wondered whether Jeff Raskin deliberately spelled "Macintosh" the way he did in order to include a subtle reference to project MAC. Or maybe he just thought somebody was all wet and needed a raincoat.

  • Print Length and Depth:

    Eight procedures deal with parameters that control how much of a complicated list or vector gets printed, and thereby incidentally prevent Pixie Scheme III from entering an infinite loop when asked to print a circular data structure. There are four such parameters, but their values are not directly available: You must use the functions to manipulate them. One parameter limits the number of elements of a list that will be printed. If this parameter is 3, then '(1 2 3 4 5) will print as

      (1 2 3 ...)
      

    The second parameter similarly limits the number of elements of a vector that will be printed.

    The third parameter limits the depth of nested lists that will be printed. If that parameter is 3, then '(((((foo))))) will be printed as

      (((...)))
      

    The fourth parameter similarly limits the depth of nested vectors that will be printed.

    (e::list-print-depth)
    

      Returns the current maximum depth to which nested lists will be printed.

    (e::list-print-length)
    

      Returns the current maximum number of elements of a list that will be printed.

    (e::vector-print-depth)
    

      Returns the current maximum depth to which nested vectors will be printed.

    (e::vector-print-length)
    

      Returns the current maximum number of elements of a vector that will be printed.

    (e::set-list-print-depth! <positive integer>)
    

      Sets the current maximum depth to which nested lists will be printed. The <positive integer> must exceed two.

    (e::set-list-print-length! <positive integer>)
    

      Sets the current maximum number of list elements that will be printed. The <positive integer> must exceed two.

    (e::set-vector-print-depth!  <positive integer>)
    

      Sets the current maximum depth to which nested vectors will be printed. The <positive integer> must exceed two.

    (e::set-vector-print-length! <positive integer>)
    

      Sets the current maximum number of vector elements that will be printed. The <positive integer> must exceed two.

  • Random Numbers:

    Two procedures provide access to a quite good Unix random number implementation:

    (e::random)
    

      Returns a random integer generated by the Unix/C++ function "random" (which should not be confused with the earlier random-number implementation, "rand").

    (e::srandom <integer>)
    

      Seeds the Unix/C++ "random" implementation with the given integer.

    Pixie Scheme III itself seeds the "random" implementation with a different number every time it starts up. If you wish to generate a repeatable string of random numbers -- perhaps for debugging a program that uses "e::random", re-seed the implementation, via "e::srandom", with the same integer, before each sequence of calls to "e::random".

    See the Unix documentation for details of "random" and "srandom".

  • Sorting and Merging:

    Five procedures deal with sorting and merging. These provide similar functionality to that in Scheme SRFI 95, which is on line at http://srfi.schemers.org/srfi-95/srfi-95.html as I write these words. (SRFI means "Scheme Request For Implementation", and refers to a large body of code specifications which the Scheme community has decided would be useful if generally available.) I cannot implement the SRFI precisely because it depends on another SRFI which I have not implemented, but I chose to provide similar functionality.

    The merge and sort procedures are stable when used with comparison predicates that return #f when applied to identical arguments.

    See the source code file "SortMerge.s", which is provided as part of the open-source distribution of Wraith Scheme and Pixie Scheme III, for details of the origin and copyright of the source code for these functions. In essence, much of that code derives from the work of Aubrey Jaffer, and is in public domain.

    (e::merge <list> <list> <comparison procedure> [<key>])
    

      Merges two sorted lists into sorted order, in order from "least" to "greatest" in the sense described below. The result is a new list. Neither of the original lists is changed by this procedure.

      Without the optional <key>, merges in sorted order, two lists which have already been sorted using the <comparison procedure>. That procedure might descriptively be named "less?", and is presumed to accept two arguments and to return a boolean value indicating whether the first argument is "less" than the second. For example:

        (e::merge (list 1 3 5) (list 2 4) <)  ;;==> (1 2 3 4 5)
        

      If the optional <key> is present, then the procedure for deciding which of two elements is less, during both the initial sorts of the separate lists and the merge itself, is that the <key> is first applied to the elements being compared, and then the <comparison procedure> is applied to the results thereby obtained. A common use of the <key> is if the elements being compared are some kind of structure, such as a list or vector, and the "less?" procedure is intended to apply to only one element of the structure. For example, if the lists are themselves composed of two-element lists, and the desired comparison is based on the second elements of those lists, then the <key> "cadr" could be used to extract the second elements for comparison. For example:

        (e::merge (list '(a 1) '(b 3) '(c 5)) (list '(d 2) '(e 4)) < cadr)
          ;;==> ((a 1) (d 2) (b 3) (e 4) (c 5))
        

      You could always do the same thing by using a specialized lambda expression for the <predicate>, but sometimes using the <key> makes things clearer.

    (e::merge! <list> <list> <comparison procedure> [<key>])
    

      Like "e::merge" (immediately above), except that it destroys the structure of both of the lists that are its arguments, and that the returned list will be composed of some of their components.

    (e::sort <list or vector> <comparison procedure> [<key>])
    

      Sorts either a list or a vector -- your choice -- into a new instance of the same kind of entity. The original list or vector is not changed by this procedure. The sorted result will be in order from "least" to "greatest" in the sense described below.

      Without the optional <key>, uses the <comparison procedure> to decide how to sort. That procedure might descriptively be named "less?", and is presumed to accept two arguments and to return a boolean value indicating whether the first argument is "less" than the second. For example:

        (e::sort (vector 2 3 1) <)  ;;==> #(1 2 3)
        

      If the optional <key> is present, then the procedure for deciding which of two elements is less is that the <key> is first applied to the elements being compared, and then the <comparison procedure> is applied to the results thereby obtained. A common use of the <key> is if the elements being compared are some kind of structure, such as a list or vector, and the "less?" procedure is intended to apply to only one element of the structure. For example, if the lists are themselves composed of two-element lists, and the desired comparison is based on the second elements of those lists, then the <key> "cadr" could be used to extract the second elements for comparison. For example:

        (e::sort (list '(a 2) '(b 3) '(c 1)) < cadr)
          ;;==> ((c 1) (a 2) (b 3))
        

      You could always do the same thing by using a specialized lambda expression for the <predicate>, but sometimes using the <key> makes things clearer.

    (e::sort! <vector> <comparison procedure> [<key>])
    

      Like "e::sort" (immediately above), except that it only sorts vectors, and it sorts the given vector by permuting the elements within it, so that the returned vector is the same as (in the sense of "eq?") the original vector, but has had its elements sorted.

    (e::sorted? <list or vector> <comparison procedure> [<key>])
    

      Determines whether the given list or vector is sorted, in order from "least" to "greatest" in the sense described below.

      Without the optional <key>, uses the <comparison procedure> as the basis for deciding whether one element is less than another. That procedure might descriptively be named "less?", and is presumed to accept two arguments and to return a boolean value indicating whether the first argument is "less" than the second. For example:

        (e::sorted? (vector 1 2 3) <)  ;;==> #t
        

      If the optional <key> is present, then the procedure for deciding which of two elements is less is that the <key> is first applied to the elements being compared, and then the <comparison procedure> is applied to the results thereby obtained. A common use of the <key> is if the elements being compared are some kind of structure, such as a list or vector, and the "less?" procedure is intended to apply to only one element of the structure. For example, if the lists are themselves composed of two-element lists, and the desired comparison is based on the second elements of those lists, then the <key> "cadr" could be used to extract the second elements for comparison. For example:

        (e::sorted? (list (c 1) (a 2) (b 3))  ;;==> #t
        

      You could always do the same thing by using a specialized lambda expression for the <predicate>, but sometimes using the <key> makes things clearer.

  • State Flags:

    Several procedures allow Scheme programs to examine and modify the internal flags that control whether defines are compiled automatically, and whether the display routines for numbers use the full precision available.

    (e::set-compiler-on!)
    (e::clear-compiler-on!)
    (e::compiler-on?)
    

      The preceding procedures respectively set, clear, and return the value of the internal flag that controls whether Pixie Scheme III compiles define automatically. The procedures that change the flag take effect at once, but the corresponding displays in the Features Panel may not update immediately. Be patient: They will catch up when Pixie Scheme III isn't busy.

    (e::set-show-full-precision!)
    (e::clear-show-full-precision!)
    (e::show-full-precision?)
    

      The preceding procedures respectively set, clear, and return the value of the internal flag that controls whether Pixie Scheme III display routines use the full precision available. The procedures that change the flag take effect at once, but the corresponding displays in the Features Panel may not update immediately. Be patient: They will catch up when Pixie Scheme III isn't busy.

  • Storage Management:

    (e::active-room)
    

      Returns an integer, which is the number of bytes of storage space available in the current Pixie Scheme III active generation, but yet unused. When this number becomes too small, a generational garbage collection must take place -- and will take place automatically -- as an attempt to free up some space.

    (e::active-store-size)
    

      Returns an integer, which is the total number of bytes in the current Pixie Scheme III active generation. This number is necessarily the sum of the number of bytes used there, and the number that remain available.

    (e::aging-room)
    

      Returns an integer, which is the number of bytes of storage space available in Pixie Scheme III aging generation, that are unused. When generational garbage collection is in use, this number will typically be either zero or very close to it, since the current aging generation is merely the previous active generation after it has become so large that generational garbage collection was necessary.

    (e::full-gc)
    

      Performs garbage collection. Pixie Scheme III will also automatically invoke this routine when necessary.

        Technical Note: The garbage-collection algorithm used by Pixie Scheme III compacts lists quite efficiently, using "cdr coding" wherever possible.

    (e::ninety-percent-used?)
    

      Returns a boolean, which is true if the amount of storage space in Pixie Scheme III main memory, that is in use by Pixie Scheme III, exceeds ninety percent of the total amount of storage space there available.

    (e::room)
    

      Returns an integer, which is the number of bytes of storage space available in Pixie Scheme III main memory, but yet unused. When this number becomes too small, a full garbage collection must take place -- and will take place automatically -- as an attempt to free up some space.

    (e::show-room)
    

      Shows some information about how much storage space has been used, and about how much remains available. Returns #t.

    (e::store-size)
    

      Returns an integer, which is the total number of bytes in either one of the two storage spaces used by Pixie Scheme III. This number is necessarily the sum of the number of bytes used and the number that remain available. It is also one of the numbers printed out at the top of the Pixie Scheme III window when the program begins.

  • System Information:

    This section deals with procedures that allow examination or modification of some of the internal data structures used by Pixie Scheme III. Abuse of some of these functions will cause fatal errors, crashes and other disasters. Detailed description of these data structures is beyond the scope of this help file; however, experienced Lisp programmers will know what I am talking about.

    (e::bound-instance? <symbol>)
    

      Returns #t if the <symbol> has a value in the environment in which "e::bound-instance?" is called, otherwise returns #f. Many Lisp systems would call this function "boundp".

    (e::get-tag <object>)
    

      Returns a positive integer that indicates the type of <object> and conveys information about how <object> is cdr-coded.

    (e::set-tag! <object> <integer>)
    

      Changes Pixie Scheme III's understanding of what kind of thing <object> is. Returns <object>, with its new tag. Warning: Misuse of this function will cause Pixie Scheme III to crash. (Indeed, almost any use of this function will cause Pixie Scheme III to crash.)

    (e::show-active-memory)
    

      Shows an extremely lengthy display of all objects in that part of the active generation that is presently in use by Pixie Scheme III. Returns #t.

      Fair warning: This procedure can generate enormous amounts of output.

    (e::show-aging-memory)
    

      Shows an extremely lengthy display of all objects in that part of the aging generation that is presently in use by Pixie Scheme III. Returns #t.

      Fair warning: This procedure can generate enormous amounts of output.

    (e::show-dynamic-environment-list)
    

      Shows all symbols, and their values (if any), that are visible in the lexical scope from which this procedure was called, except for those in the top-level environment. Returns #t. The display will consist of a sequence of several similar displays. The first one will show all symbols in the innermost lexical scope visible to the caller, then the next innermost lexical scope, and so on out to, but not including, the top-level, "global" environment.

    (e::show-environment)
    

      Shows all symbols, and their values (if any), that are visible in the innermost lexical scope from which this procedure was called. Returns #t.

    (e::show-environment-list)
    

      Shows all symbols and their values (if any) that are visible in the lexical scope from which this procedure was called. Returns #t. The display will consist of a sequence of several similar displays. The first one will show all symbols in the innermost lexical scope visible to the caller, then the next innermost lexical scope, and so on to the top-level, or "global" environment. This last display will be extremely lengthy, for it will include all the Pixie Scheme III primitives, like "+", "car" and "cons".

    (e::show-memory)
    

      Shows an extremely lengthy display of all objects in the part of main memory that is presently in use by Pixie Scheme III. Returns #t.

      Fair warning: If you call this procedure when Pixie Scheme III is using a large main memory -- say a Gigabyte -- and if that memory is full, or nearly so, the procedure may well attempt to generate a hundred million lines of output.

    (e::show-stack)
    

      Shows the contents of the internal stack used by Pixie Scheme III. Returns #t. This stack is different from the "machine stack", and has nothing to do with any microprocessor "stack" register. Also see the procedure "e::write-stack".

    (e::stack-depth)
    

      Returns an integer, which is the number of objects on Pixie Scheme III's internal stack.

    (e::write-continuation)
    

      Writes out the current continuation as a list. Returns #t. I use "continuation" here in the sense of the 'C' register of an SECD machine.

    (e::write-stack)
    

      Writes out the contents of Pixie Scheme III's internal stack as a list, with the most recent object pushed as the car. Returns #t. Also see the procedure "e::show-stack".

  • Top-Level Control:

    Several procedures allow top-level control of the operation of the Pixie Scheme III application:

    (e::exit)
    

      Causes Pixie Scheme III to terminate. Does not save a world before exiting.

    (e::reset)
    

      Stops whatever Pixie Scheme III is doing, and returns control to you. You can also invoke the "e::reset" procedure from the "Reset Scheme" button.

    One procedure causes Pixie Scheme III to crash:

    (c::down-in-flames!!!)
    

      DANGER!! This procedure causes Pixie Scheme III to fail. (If not, it's a bug.)

      I created this procedure so that I could easily test Pixie Scheme III's mechanism for reporting fatal errors.


Sense Lights:

Pixie Scheme III provides some "sense lights" -- in effect, light bulbs at the bottom of the main window -- that are under user control. There are eight of them. They are normally invisible and will remain so unless Pixie Scheme III procedures are used to make them show up. When they are visible, they are lined up in a row near the lower left corner of the Pixie Scheme III Window, as shown in the next figure.

Sense Light Position.

Pixie Scheme III's sense lights, glowing various colors.

The sense lights are numbered 0 through 7, from left to right. Each sense light may have one of eight illumination states, and those states are also numbered. In illumination state zero, the sense light is turned "off" -- it is visible, but resembles an unilluminated panel light, as in the leftmost light in the preceding figure. Illumination states 1 through 6 correspond respectively to the light being lit with the color red, orange, yellow, green, cyan, or magenta. In illumination state 7, the light is emitting darkness -- it looks like it is glowing black.

The visibility of each sense light has nothing to do with its illumination state. If you make a visible sense light invisible, it retains whatever illumination state it previously had, and will reappear looking just the same after if you should make it visible again. Similarly, if you change the illumination state of a sense light that is invisible, the sense light does not show up -- it won't do that till you make it visible, but when it does it will have whatever illumination state you have just provided.

I did not create names for the separate colors or illumination states; that is, there is no built-in Pixie Scheme III object named "e::red" or anything like that. Feel free to define such auxiliaries if you wish.

Procedures for working with sense lights include:

    (e::hide-sense-lights!)
    

      Makes all the sense lights invisible. Does not change the illumination state of any sense light.

    (e::rotate-sense-lights! <integer>)
    

      Circularly rotate the pattern of sense lights -- both illumination and visibility -- by the given integer. Positive integers rotate to the left. The rotation is done modulo the number of sense lights.

    (e::sense-light-number? <object>)
    

      Returns #t if <object> is an exact integer in the range legal for a sense light number, and returns #f otherwise.

    (e::sense-light-illumination-number? <object>)
    

      Returns #t if <object> is an exact integer in the range legal for a sense light illumination number, and returns #f otherwise.

    (e::set-sense-light-illuminations! <sense-light-illumination-state-number>)
    

      Sets the illumination state of all sense lights to the indicated state. Does not change the visibility of any sense light.

    (e::set-sense-light-n-illumination! <sense-light-number> <sense-light-illumination-state-number>)
    

      Sets the illumination state of the given sense light to the indicated state. Does not change the visibility of the given sense light.

    (e::set-sense-light-n-visibility! <sense-light-number> <boolean>)
    

      If the boolean is true (#t), makes the given sense light visible; if the boolean is false (#f), makes the given sense light invisible. Does not change the illumination state of the given sense light.

    (e::set-sense-lights-off!)
    

      Sets the illumination state of all sense lights to "off". Does not change the visibility of any sense light.

    (e::show-sense-lights!)
    

      Makes all the sense lights visible. Does not change the illumination state of any sense light.

There is no way to read the illumination state or visibility of sense lights from Pixie Scheme III.

Note that "e::set-sense-light-n-illumination!" and "e::set-sense-light-n-visibility!" provide full control of sense light operation. The other procedures that affect illumination and visibility are for convenience.

Procedures which change the illumination state or visibility of sense lights are relatively slow to execute. When speed is particularly important, make minimal use of sense lights.


Top-Level Loop Variables:

Like Common Lisp, Pixie Scheme III maintains several variables that may prove handy should you forget to save an interesting input expression or output result. Their names are closely related to the names of similar variables used in Common Lisp.

    Common Lisp    Pixie Scheme III
    ===========    =============
        -              >-<
        +              >+<
        ++             >++<
        +++            >+++<
        *              >*<
        **             >**<
        ***            >***<
    

Pixie Scheme III cannot use the exact same variable names as Common Lisp, because there is only one namespace in Scheme, and the symbols "+", "-" and "*" are already bound to the procedures for addition, subtraction and multiplication.

When Pixie Scheme III starts running, these variables are all initialized to the empty list.

    >+<
    >++<
    >+++<
    

      While a Scheme S-expression is being evaluated by the top-level loop, the variable >+< is bound to the previous form that was read by the loop. The variable >++< is bound to the previous value of >+< (that is, the form evaluated two interactions ago), and >+++< is bound to the previous value of >++<.
    >-<
    

      While a Scheme S-expression is being evaluated by the top-level loop, the variable >-< is bound to the S-expression itself; that is, to the value that will be bound to >+< once the current interaction is complete.
    >*<
    >**<
    >***<
    

      While a Scheme S-expression is being evaluated by the top-level loop, the variable >*< is bound to the result that was printed at the end of the last time through the loop; that is, to the value that was produced by evaluating the S-expression in >+< The variable >**< is bound to the previous value of >*< (that is, to the result from the form evaluated two interactions ago), and >***< is bound to the previous value of >**<.

If the evaluation of >-< is aborted, then the values bound to >*<, >**<, and >***< are not updated; they are updated only if the printing of the value, returned from evaluating >-<, has begun.


Bugs, Flaws, Limitations, and Dealing with Them:

I am sure Pixie Scheme III contains bugs, and I want to hear about them so that I can fix them.

I am particularly worried about the possibility of bugs that are platform dependent, in that they occur on some combinations of iPad hardware and operating system and not on others. I do not have a large collection of different iPads to test Pixie Scheme III on, so it is logically conceivable that I might release a version of Pixie Scheme III containing a bug that I am not equipped to discover.

So by all means, do send in bug reports.

My EMail address is Jay_Reynolds_Freeman@mac.com.

If you cannot reach me by EMail, try paper mail to

    Jay Reynolds Freeman
    Post Office Box 60628
    Palo Alto, CA, 94306-0628
    U. S. A.

If you ever encounter a fatal or non-fatal error with a message like

    "Implementation error ..."
    

or

    "Implementation: ..."
    

then I would much appreciate a bug report with as many details as you can provide. (In particular, include the entire message.) Such errors indicate that I have inadequately guarded against some anticipatable problem: It's my fault; I will be eager to do better.


Known Bugs and Flaws:

The only reason I will release a distribution of Pixie Scheme III containing a known bug or a glaring flaw is that there is some reason I cannot fix the problem. At present I have none to report.


Limitations:

  • Read and Print Overflow:

    Although Pixie Scheme III can store large and complicated data structures in its main memory, it cannot necessarily read them in all at once: Pixie Scheme III can read lists that are very long. In testing, I have loaded files whose text included lists of 100 000 items, something like:

      '(item-1 item-2 ... item-99999 item-100000)
      

    However, lists that are too deeply nested will cause Pixie Scheme III to crash. Such a list might be

      '((( ... ((( some-list-item ))) ... )))
      

    in which the "..." stands for tens of thousands of parentheses of the appropriate kind

      Technical Note: The problem is stack overflow in the underlying routines, which are written in C. When the reader reads lists, it iterates in the "cdr" direction, but does recursive (and non-tail-recursive) calls in the "car" direction.

    There is a similar problem in the routines that print long or deeply-nested lists, but I have not tested to determine its extent.

  • Numerical Exactness:

    Pixie Scheme III relies in part upon built-in features of iOS and of iPad hardware to determine whether or not the result of a floating-point arithmetic calculation is exact; this term refers among other things to what the standard Scheme procedures "exact?" and "inexact?" return. These built-in features may vary in capability from one version of iOS to the next, and from one kind of iPad hardware to another.

    The main consequence here is that sometimes you might expect that a floating-point calculation would return an exact result, but the procedure "exact?" doesn't say that it did. Furthermore, what "exact?" has to say about the result of any given calculation may vary depending on what kind of hardware and which version of iOS you are running.

    This mildly inconsistent behavior is not technically a Pixie Scheme III bug, since the R5 standard is rather lenient about requiring Scheme implementations to return exact results whenever possible, but it bugs me, so I thought I would report it here.

  • Numerical Accuracy:

    The floating-point algorithms used by the iPad may differ from one version of iOS to the next and also from one kind of processor hardware to the next. Thus the numerical results returned by Pixie Scheme III floating-point calculations may vary depending on which version of iOS you are running and on what kind of hardware you have, even if you perform the same calculation, using the same input data. The differences are typically very small -- down in the least significant digits of the result -- but they are there. The trigonometric and inverse-trigonometric routines are where I have most often noticed such differences.


Common Problems and Solutions:

Here are a few hints about problems that may happen when you try to run Pixie Scheme III:

  • Pixie Scheme III fails to open.

    If Pixie Scheme III fails to open, it may be that the app itself has somehow become damaged, or it may be that you have previously saved a world for Pixie Scheme III, by pressing the "Save World" button in the Features Panel, and that the world has somehow become damaged. Try restoring Pixie Scheme III to original settings. That will delete the saved world and reinstall the application from your computer. Failing all else, contact me, Jay_Reynolds_Freeman@mac.com, for support.

  • Pixie Scheme III runs very slowly.

    It is of course possible that you are running a Scheme program that simply takes a long time to do its job, but there is at least one other possible cause of sloth that you might want to know about. The computer chip that runs Pixie Scheme III must also update the Pixie Scheme III window for you to look at, and the more text there is in that window -- including any text that has scrolled off screen -- the longer it takes. If there is any text in the window, or scrolled off the top or bottom, that you no longer need, it may help to delete it: Just select the text and then use the "Cut" command to remove what you have selected.

  • You aren't quite sure you know enough Lisp or Scheme to operate Pixie Scheme III properly.

    See the Scheme References and Lisp References sections. In order from simplest to most complicated, I recommend (1) Friedman and Felleisen, (2) Springer and Friedman, and (3) Abelson, Sussman and Sussman. And do get a copy of the "R5" report: You will need it sooner or later.

    Or, try an Internet search.

    Don't forget the help files available via the Help Panel.

  • The fonts are too small to read comfortably, particularly with that silly yellow background.

    Look in the Features Panel for commands to change font size. And hey, I like the yellow background.

  • Compiling takes forever.

    Pixie Scheme III's internal compiler is indeed fairly slow, and I have no present fix for that problem. Remember that you can turn the compiler off, via the "Compile Defines" menu item in the Features Panel, or by means of procedures described in the State Flags section. Also, be advised that the compiler is much happier dealing with a large number of small procedures than with a small number of large ones.

  • You can't keep all these stupid parentheses balanced!

    Welcome to the club. Many programming editors have parenthesis-matching features; if you are developing code on a Macintosh, to run later on an iPad, you might try the version of Emacs that comes with Apple's "Xcode" development environment -- it runs in a Terminal shell. As I write these words, Xcode is available free from Apple.

    I do not presently know of any editors that run on the iPad, that provide any assistance with balancing parentheses, except for the rather minimal support that Pixie Scheme III itself provides via the simple editor built into its main window.

    Pixie Scheme III itself will also provide cues about missing parentheses and missing double-quotes when you try to evaluate expressions that are unbalanced in that respect.


Elementary Debugging:

If you encounter a problem while running Pixie Scheme III, you might want to do a few things on your own before contacting me, both to save your time if the problem turns out not to be Pixie Scheme III's fault, and to help me identify and fix any bug that may be present.

Try to pin down what it takes to make the problem happen, as accurately and completely as you can. Unreproducible bugs are almost impossible to fix. They are like the rattle in your car that goes away when you take it to a mechanic, or the things that go bump in the night but are not there when the sun rises.

Here are some things to do, that might help pin down a problem:

  • If the problem happened when you were running code that you had compiled, try again without compiling the code. That is, turn off the "Compile Defines" option, reload your code without applying "e::compile-form" to any of it, and try again. If the problem does not recur, or recurs in a different way, it may be that the difficulty is with the compiler itself.

  • Turn off your iPad and start it up again.

  • Try restoring Pixie Scheme III to original settings.

  • Carefully consider the possibility of a computer virus or similar software abomination. There are no general rules to detect these: No two are alike. Magazine articles, Internet searches, user groups and dealers can help you learn about the latest viruses. As I write these words, there are few viruses or other software threats to the iPad, and I hope that remains the case, but by all means, seek out the latest information on the matter.

  • See if you can duplicate the problem on another iPad. If so, then you will know that your own iPad's hardware and software are not at fault.

If you do send me a bug report, give your best description of the problem and of what I must do to make it recur for investigation. State what kind of iPad you used, how much memory it had, what version of iOS you used, and whether you were using the Pixie Scheme III compiler. If you suspect a problem with Pixie Scheme III's interaction with some other software, tell me about that, too.

It will help me a lot if you do these things and describe what happened, and you may be able to save yourself some time if the problem turns out to be one you can fix.


Timeline:

What's New:

This section summarizes what is new in Pixie Scheme III since the previous release. I omit cosmetic improvements, minor changes in documentation, and minor changes in displays.

Items are listed in more or less the order in which I dealt with them.

The current release is 1.12, the fifth release of Pixie Scheme III.

  • Added support for rational numbers in which the numerator and denominator are stored separately, plus some related auxiliary procedures as enhancements. See R5 Section 6 for details. Also see the section on Long Ratnums and Continued Fractions.


What Used to be New:

This section summarizes changes in Pixie Scheme III between its first release and the last one before what is now current. I may not bother to mention minor bug fixes, cosmetic improvements, minor changes in documentation, and minor changes in displays.

Release 1.11 was the fourth release of Pixie Scheme III.

  • Added new procedures e::select, e::reduce, and e::make-integer-range, all described in the section on Miscellaneous Procedures.

  • Added new procedures related to sorting and merging.

  • Bug Fix: Internal definitions would not nest; that is, if an internal definition contained another internal definition, the code would not work correctly. Fixed. (See the R5 report, section 5.2.2, for "internal definitions".)

Version 1.10 was the third release of Pixie Scheme III.

  • Added complex numbers with non-zero imaginary part, implemented according to the R5 report.

  • Added new predicate "e::long-complex?", for identifying numbers stored in a new, implementation-dependent format that supports complex numbers with non-zero imaginary part.

  • Added a row of extra keys to the top of the main "glass keyboard", so that less use of the shift key is required.

  • Added pretty-printing.

  • Arranged to convert any "tab" character entered into Pixie Scheme III to a single "space" character. This change was in essence a workaround for the problem that iOS 4.2.1 provides insufficient information about the position and expansion of tabs for Pixie Scheme III to deal with them properly.

  • Improved recognition of when a hardware keyboard is present.

  • Bug Fix: Required R5 procedure "string" was missing. Added it.

Version 1.01 was the second release of Pixie Scheme III.

  • Added support for use of add-on physical keyboards.

  • Added parenthesis-matching features for text typed in the main window.

  • Added automatic saving of text in the main window, whenever the user leaves the application; that text is restored when the user returns. This feature also saves the font size in use and a few other user-interface details, but does not save the content of Scheme main memory -- that takes more time than iOS allows on entering background.

  • Bug fix: Fixed a slew of bugs dealing with reporting that floating-point calculations have produced inexact results. (The detection of floating-point inexactness is not well supported on the iPad platform.) You would have only seen these bugs if you were using the Scheme "exact" bit, accessed for example via procedures "exact?" and "inexact?". In consequence of these fixes, Pixie Scheme III is very cautious about asserting that the result of a floating-point calculation is exact, and rarely does so.

Version 1.00 was the first release of Pixie Scheme III, so everything was new.


What Might be New in the Future:

Besides fixing bugs and making internal improvements, my wish list for the future of Pixie Scheme III includes the following items, which are not necessarily in the order in which I might get around to doing them:

  • More support for logic programming.

  • At least partial "R7" compliance: To elaborate, as I write these words the R7 report is in draft, available at http://www.scheme-reports.org/2011/working-group-1.html. (Look on that page for the most recent version of the R7RS report.) It is not clear what portion of the draft will finally be approved, and there is a possibility that the entire report will be rejected by the Scheme community, so it is premature to speak specifically of moving Pixie Scheme III to R7. Notwithstanding, I suspect that some of the changes in the draft report are likely to show up in "standard" Scheme in the future, and I expect that I will be adding at least some of them piecemeal, in an order affected by ease of implementation, utility of the new material, coolness, and my best guess about what features are really going to be real. Expect further details -- and new features -- in future releases of Pixie Scheme III.

The future probably does not hold:

  • "R6" compliance.

    R6 is a much more complicated standard than R5. I find most of its new features neither interesting nor useful to me personally, and there are other aspects of Scheme that I would prefer to work on. In that context, I simply don't have time enough to develop and maintain an R6 implementation of the quality that I would like to present as an iPad application.

    There was a great deal of controversy about the R6 standard in the Scheme community. I hope no one will misconstrue lack of R6 support in Pixie Scheme III for me taking a position against the R6 standard and being too cowardly to say so. The plain truth is that I find the R6 standard overwhelming: It is too complicated for me to support all by myself.

  • A "bignum" package.


Miscellaneous Information:

Numbers Revisited:

Many people think the different kinds of numbers are separate, so that no number can be of more than one kind. They think that integers and reals are different kinds of numbers, so that no integer can be real, and so on.

That's false. They overlap. Specifically, every integer is also rational, real and complex; every rational is real and complex; every real is complex; and they're all numbers.

For example, 1 is an integer. It is also a rational (being equal to the quotient of two integers, namely 1/1, or 2/2, or...), it is on the real line, and it is also in the complex plane -- we could write it as 1 + 0i. Typographic choices, such as the inclusion of a decimal point, an exponent, or trailing zeros, do not alter the mathematics. Thus "1", "1.", "1.0", "1.0e0" and "1.000000000000000" are all ways to write the same mathematical number; namely, the integer "one".

Some wag once said, "A computer scientist is a person who refuses to believe that 1.0 is an integer." Don't you make that mistake.

Much of this confusion is because many computer languages use different kinds of machine storage for numbers given with a decimal point than for numbers given without one. A language might store "1.0" as an IEEE 64-bit floating-point number, yet store "1" as a 32-bit word with just the least-significant bit turned on. It is the integer "one" in either case. Some people think floating-point numbers cannot be integers. That's wrong. What counts is the value, not how it's stored.

If you want to know how Pixie Scheme III is storing a particular number, there are some special predicates you can use to find out.

More confusion stems from the fact that typographic conventions are widely used to convey information about the precision of a number when that number is imperfectly known. Thus in science or engineering, use of the string "1.000" to represent a number often means "the number I am talking about is between 0.9995 and 1.0005, but I don't know for sure what it is, so I will use the representative exact value 1 to stand in its place, and will give the value to four significant digits to hint at the level of my ignorance." This convention obscures the mathematics of numeric types: Thus in the example given, the integer 1 might actually be standing for the integer 1, for the rational number 10001/10000, for the irrational number (1 + pi/10000), or for the complex number (1 + i * pi/10000).

The built-in features of Scheme are in any case not powerful enough to support such conventions: You can specify that a number is exact or inexact by means of the "#e" and "#i" prefixes, but Scheme records only that one bit of information. There is no built-in means to tell Scheme how many digits of precision the number has. As far as input to Scheme is concerned, the strings "1.50000" and "1.5" indicate the same number and the same status of the "exact bit". (In the case of Pixie Scheme III, that number is the inexact rational 1.5.)

Just for fun, ask yourself (A) Which of the following numbers are complex? (B) Which are real? (C) Rational? (D) Integer?

    42
    1.
    -2.000000
    345.62e2
    9.6149604775e23
    

    (Hint: Does 9.6149604775e23 have a fractional part?)

Answers: (A) All of them. (B) All of them. (C) All of them. (D) All of them.


Scheme References:

These items are all from my bookshelves or download directories. Many have more recent editions.

  • Harold Abelson, Gerald Jay Sussman and Julie Sussman, 1985. Structure and Interpretation of Computer Programs, MIT. This is an introductory book on computer science for very bright college students. It both teaches and uses an earlier version of Scheme than that described in the R3 report, but the differences are minor. It features many excellent examples of programming in Lisp in general and in Scheme in particular. The early sections are a good introduction to the language, the later ones show well just how powerful it is.

  • William Clinger, "The Semantics of Scheme", in Byte 13, no. 2, 221-227 (February 1988). One of several introductory articles on Lisp-class languages in this issue of Byte.

  • William Clinger and Jonathan A. Rees (editors), 1991. "The revised 4 report on the algorithmic language Scheme", Lisp Pointers 4(3), ACM. The RN reports (N = 3, 4, 5 ...) define Scheme. They are manuals, not tutorials -- readable once you have some background in any Lisp, and invaluable for specific details, but likely to intimidate a beginner. I strongly recommend that you get copies anyway.

  • R. Kent Dybvig, 1987, and subsequent editions. The Scheme Programming Language, Prentice-Hall. The third edition is MIT Press, 2003. An outstanding work describing Scheme. Later editions describe the "R6" variety of Scheme; thus they are perhaps less directly appropriate for learning Pixie Scheme III, which is an "R5" Scheme.

  • Daniel P. Friedman, William E. Byrd and Oleg Kiselyov, 2005. The Reasoned Schemer, MIT. Scheme integrated with logic programming.

  • Daniel P. Friedman and Matthias Felleisen, 1974, 1986, 1987, and subsequent editions. The Little Lisper, MIT. An introductory book on programming in a style appropriate to Lisp, as a programmed text. The more recent editions use a dialect of Scheme as the language in which the examples and discussion are presented.

  • Richard Kelsey, William Clinger and Jonathan Rees (editors), 20 February 1998. "Revised5 Report on the Algorithmic Language Scheme". Available on several Internet sites, such as http://www.schemers.org.

  • Jonathan Rees, "The Scheme of Things: The June Meeting", Lisp Pointers V(4) (October-December 1992).

  • J. Rees and W. Clinger (editors), "Revised3 Report on the Algorithmic Language Scheme", ACM SIGPLAN Notices 21, no. 12, 37-79 (December 1986); also an MIT Technical Report.

  • George Springer and Daniel P. Friedman, 1989. Scheme and the Art of Programming, McGraw-Hill. A good introduction to programming and to Scheme. More elementary than Ableson, Sussman and Sussman.


Lisp References:

These items are all from my bookshelves. Many have more recent editions.

  • Harold Abelson and Gerald Jay Sussman, "Lisp: A Language for Stratified Design", in Byte (magazine) 13, no. 2, 207-218 (February 1988). One of several introductory articles on Lisp-class languages in this issue of Byte.

  • John Allen, 1978. Anatomy of Lisp, McGraw-Hill. An oldie but a goodie. This rather advanced text describes Lisp mechanisms and data structures in considerable detail, with attention both to theory and to practice. Not for the faint of heart.

  • Eugene Charniak, Christopher K. Riesbeck and Drew V. McDermott, 1980. Artificial Intelligence Programming, Lawrence Erlbaum Associates, Publishers. An oldie but a goodie. The first section is titled "Advanced Lisp Programming", but after that it gets harder.

  • Peter Henderson, 1980. Functional Programming Application and Implementation, Prentice-Hall. An outstanding book on the theory, design and implementation of a class of Lisp-like functional programming languages. Pixie Scheme III was originally based on the implementation of an SECD machine described herein. I found the book to be somewhat a "sleeper": I almost stopped reading it because the material in the first few chapters seemed elementary and familiar. I am glad I continued.

  • Samuel N. Kamin, 1990. Programming Languages: An Interpreter-Based Approach, Addison-Wesley. How to implement any language you like as long as it's Lisp. Well, not really: This book gives miniature, subset implementations of a representative selection of modern programming languages, all of whose syntaxes have been modified to be Lisp-like (prefix notation with lots of parentheses). The common syntax highlights the differences in semantics. The languages covered include a rather standard simple Lisp as well as simple subset of Scheme.

  • Guy L. Steele Jr., 1984. Common Lisp, Digital Press. The definitive guide to that dialect of Lisp which has the greatest claim to being a standard. As a manual, the book is excellent, but it is neither a tutorial nor an introduction. There is a revision (1990), even thicker than the original, that goes into more detail on such special features as the Common Lisp Object-Oriented Programming System.

  • David S. Touretzky, "How Lisp Has Changed", in Byte (magazine) 13, no 2, 229-234 (February 1988). One of several introductory articles on Lisp-class languages in this issue of Byte.

  • Patrick Henry Winston and Berthold Klaus Paul Horn, 1981, and subsequent editions. Lisp, Addison-Wesley. This rather venerable tome is still a good introduction to Lisp and to what you can do with the language. The first edition uses the "MACLisp" dialect, which is no longer widely used (unless you have in your garage a DEC-20 mainframe or a Symbolics 36XX with down-rev software), but I believe that subsequent editions use Common Lisp. I learned Lisp with this book and a DEC-20. No, not in my garage. (Though I do have a friend who used to have a DEC-20 in her garage.)


Other References:

  • Douglas R. Hofstadter, 1979. Goedel, Escher, Bach, Basic Books. A thoughtful and deep book that deals (among other things) with some interesting and profound concepts that are part of computer science, and with other interesting and profound concepts that would be part of computer science if anyone could figure out how to work them in.

  • Richard Jones and Rafael Lins, 1996. Garbage Collection: Algorithms for Automatic Dynamic Memory Management, Wiley. An oldie but a goodie.

  • Tracy Kidder, 1981. The Soul of a New Machine, Little, Brown and Company. Everyone contemplating a career in the computer industry should read this book, the better to recognize job offers from places like this. What you do when you do get such an offer is up to you: I recommend you accept. But remember, misery loves company.

  • Pamela McCorduck, 2004. Machines Who Think (25th-anniversary update), A. K. Peters. Well-informed and thoroughly readable history of artificial intelligence.

  • David A. Mindell, 2008. Digital Apollo: Human and Machine in Spaceflight, MIT. Long ago, in a galaxy far, far away, the first digital computer used in an aerospace vehicle that carried a crew flew on board the first spacecraft to visit another world. The whole Apollo flight computer was rather less capable than most of today's five-dollar microcontrollers, but the interface design for it was challenging, and figuring out what it would do and how the crew would interact with it was more so. It was also robust: Could your laptop take a lightning strike and reboot without a blink in a fraction of a second?

  • Dr. Seuss, 1955. On Beyond Zebra, Random House. As this classic children's book so clearly demonstrates, a language should allow you to express not only things you can think of, but also things you could not possibly have thought of if you hadn't learned it.

  • P. W. Singer, 2009. Wired For War: The Robotics Revolution and Conflict in the Twenty-first Century, Penguin. A thought-provoking read about some of the scarier aspects of artificial intelligence. Military robots are widespread, effective, sometimes already have autonomous authority to shoot, and are in general specifically designed to disobey Asimov's three laws of robotics.

  • Guy Steele, 1983. The Hacker's Dictionary, Harper and Row. A funny and quite accurate guide to the jargon of the best of the best of computer scientists. This dialect is to programmers what Chuck Yeager's drawl is to test pilots. I thought the book was a joke until I joined an artificial intelligence lab and found everybody really did talk that way. In this context, incidentally, "hacker" has a much broader and more respectable meaning than its popular use as a synonym for "computer criminal". Virus weenies and digital peeping Toms do not deserve so honorable a label.

  • Edward R. Tufte, 1983. The Visual Display of Quantitative Information, Graphics Press.
  • Edward R. Tufte, 1990. Envisioning Information, Graphics Press.
    These books are the main reason I laugh uproariously whenever anyone tells me that some computer or program has decent graphics or a decent user interface. Few books on computer graphics or computer interfaces mention any of Tufte's works; perhaps the authors are too embarrassed.

  • David Michael Ungar, 1987. The Design and Evaluation of a High Performance Smalltalk System, MIT. If you read the table of contents and the back cover casually, you might think that this book is about implementing Smalltalk on a chip. Actually, the lion's share of the text deals with good and better garbage collection algorithms.


Whimsy:

On Dialectic and History:

Letter sent in 1988 to the editors of "The California Tech", California Institute of Technology, Pasadena, California:

    Dear Editors:

            This June it will be twenty years since I graduated from CalTech. The anniversary has made me think: Possibly my thoughts may be relevant to the present student body.

            After CalTech I went to graduate school at the University of California's Berkeley campus. It was like going from a monastery to a madhouse. From the late sixties through the mid seventies, Berkeley was a tumultuous place, full of people who were bound and determined to change the world. What's more, each of them seemed to have a different idea of how to go about it. I could scarcely pass through the campus gates without being accosted by handbill-distributors from every imaginable kind of political, social and economic action group, all bent on modifying civilization to suit themselves. There were libertarians and communists, republicans and democrats, churches both bizarre and familiar, ecological advocates, draft resisters and military recruiters. The campus seemed overflowing with charismatic, impressive leaders, each with a separate agenda for social change.

            Often there were demonstrations: I rapidly learned to cut through the back alleys to get to class. Sometimes there was real violence: Once I walked out of the student cafeteria after lunch, only to find a National Guard helicopter spraying the plaza with riot-control gas. I decided I had better go back inside and have dessert.

            I was too bewildered to have anything to do with all this. I gravitated toward a small, ever-changing circle of science buffs and technology enthusiasts, people much like myself. We had no charisma and impressed no one. We would go out for pizza and talk. The discussion topics changed as our informal membership varied -- many liked space exploration, one was enthusiastic about computers, and so forth. If pressed, most of us would cautiously admit to some faith that scientific and technical developments were also important agents of social change, but in the midst of widespread turmoil of a different kind, I found such a belief ever less tenable. As the years passed, it seemed increasingly that the things I was interested in could scarcely matter in comparison to those other, more powerful forces.

            Yet in retrospect, it is remarkable how little came from the social and political movements of that time and place. I no longer remember the names of any of the Berkeley people who were out to change the world. I scarcely recall what they wanted to do. After a decade or two I must look hard to find our planet any different for their efforts. It seems that in the long run, they mattered all but naught.

            To my embarrassment, I have even forgotten most of the people in our group of pizza-eaters. Only recently did someone remind me of the name of the fellow who came to a few meetings and talked with such zeal about the prospect of small computers and their likely capabilities.

            His name was Steve Wozniak.

            Keep the faith.

                                                                    Jay Reynolds Freeman

                                                                    BS (Physics) '68
                                                                    May 1, 1988


Excuses:

The following excuses may help explain flaws or omissions in Pixie Scheme III:

  • I do not possess your level of wisdom, insight or experience.

  • I didn't think of that.

  • I was interested in writing a Lisp system, not a (choose one or more)

    • text editor.

    • graphics interface.

    • NS/UI object interface.

    • whizzy graphics package.

    • ...

  • I forgot.
  • I didn't know how.

  • I thought I knew how, but I was wrong.

  • I was too lazy to do it right.

  • I did it the the way the R5 report says it should be done.

  • I did it my way.


Thanks To:

    (Pretty much in alphabetical order, except for the cats, who of course have the last word.)

  • Apple Incorporated, for providing excellent software development environments and numerous examples of code, all for free.

  • The members of the Silicon Valley "Cocoaheads" group, for advice about the Pixie Scheme III user interface.

  • The creators of the various "Rn" Scheme reports, for providing much clear explanatory text and numerous examples -- both simple and subtle -- of how the language is supposed to work. I am particularly grateful for the example implementations of derived expression types in section 7.3 of the R5 report, which formed the basis of the implementation of most of those types in Pixie Scheme III.

  • Several users of earlier versions of Pixie Scheme III, and other members of the on-line Lisp and Scheme communities, for suggestions, advice, and sympathy.

  • Mike Deering, for lengthy discussions of tagged-pointer Lisps.

  • Libby Hudson, for moral and immoral support.

  • Sandy Lerner, for encouragement in development and advice on commercial prospects.

  • Tim May, for humor, and for the chance to play with his computer, software and other toys.

  • Charles Smith, for hiring me into a major artificial-intelligence laboratory.

  • Pixie, Wraith, and numerous other cats, for love and purrs, and for the sharing of their ineffable names.

    Notwithstanding the honest acknowledgments and grateful thanks given above, I am confident that all errors and misfeatures in Pixie Scheme III are nobody's fault but mine.


-- Jay Reynolds Freeman, May 2011

Pixie III Face