39.14 SRFI-17 - Generalized set!

This is an implementation of SRFI-17: Generalized set!

It exports the Guile procedure make-procedure-with-setter under the SRFI name getter-with-setter and exports the standard procedures car, cdr, ..., cddddr, string-ref and vector-ref as procedures with setters, as required by the SRFI.

SRFI-17 was heavily criticized during its discussion period but it was finalized anyway. One issue was its concept of globally associating setter properties with (procedure) values, which is non-Schemy. For this reason, this implementation chooses not to provide a way to set the setter of a procedure. In fact, (set! (setter proc) setter) signals an error. The only way to attach a setter to a procedure is to create a new object (a procedure with setter) via the getter-with-setter procedure. This procedure is also specified in the SRFI. Using it avoids the described problems.

Here is a simple example:

     (define car (getter-with-setter car set-car!))
     (define cdr (getter-with-setter cdr set-cdr!))

Here is an example that makes define-record-type (see SRFI-9) accessor and mutator procs easier to handle:

     (use-modules (srfi srfi-9))
     (define-record-type sprite
       (new-sprite x y)
       sprite?
       (x sprite-x sprite-x!)
       (y sprite-y sprite-y!))
     
     (use-modules (srfi srfi-17))
     (define sprite-x (getter-with-setter sprite-x sprite-x!))
     (define sprite-y (getter-with-setter sprite-y sprite-y!))