;; Cut and paste this code into your favorite editor, or
;; perhaps into the Wraith Scheme Input Panel.


;;                           FUNCTIONAL PROGRAMMING

;;        Some people like to write code in a style known as "functional
;;        programming", in which the values assigned to variables are never
;;        changed, or perhaps changed only at top-level in the program's
;;        environment.

;;        This code disables the R5 Scheme procedures that end with a "!"
;;        (and shows by example how to disable other procedures as well).
;;        Disabling these functions goes a long way toward making Wraith
;;        Scheme a functional-programming language.  The procedures disabled
;;        are:

;;           set-car!
;;           set-cdr!
;;           string-fill!
;;           string-set!
;;           vector-fill!
;;           vector-set!
;;           set!

;;        Once the procedures are disabled, they are gone for the remainder
;;        of your Wraith Scheme session.  You will have to quit Wraith Scheme
;;        and restart to get them back again.

;;        The code requires that "set!" be the last procedure disabled: It is
;;        used to disable all the rest of them.  Bear that in mind if you
;;        wish to disable additional procedures.

;;        Wraith Scheme does have other procedures that change state.
;;        Most of them involve I/O in some way.  They include:

;;           e::clear-permanent!
;;           e::load-world!
;;           e::set-current-directory!
;;           e::set-input-port!
;;           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!

;;        The procedure "define" also may change program state, in that you may
;;        re-"define" a variable that already has a value.  I didn't have an easy
;;        fix for that one, so "define" is still there.  For that matter, the
;;        standard Scheme "load" procedure may change state, as will other 
;;        procedures that bring in code from a file.

;;        Yet what's here may help keep would-be functional programmers honest.

;;        A note of WARNING, however:  If you use this stuff, disable the
;;        Wraith Scheme compiler -- it uses "set!" internally, and will become
;;        confused by its absence ...

;;        To use this code, copy it directly into the Wraith Scheme Input
;;        panel, or cut-and-paste it into another file and work from there.

;;        Here comes the actual code ...

(e::clear-permanent! 'set-car!)
(set! set-car! '<set-car!-is-disabled>)

(e::clear-permanent! 'set-cdr!)
(set! set-cdr! '<set-cdr!-is-disabled>)

(e::clear-permanent! 'string-fill!)
(set! string-fill! '<string-fill!-is-disabled>)

(e::clear-permanent! 'string-set!)
(set! string-set! '<string-set!-is-disabled>)

(e::clear-permanent! 'vector-fill!)
(set! vector-fill! '<vector-fill!-is-disabled>)

(e::clear-permanent! 'vector-set!)
(set! vector-set! '<vector-set!-is-disabled>)

(e::clear-permanent! 'set!)
(set! set! '<set!-is-disabled>)