Node:Procedures with Setters, Next:, Previous:Procedure Properties, Up:Procedures and Macros

23.4 Procedures with Setters

A procedure with setter is a special kind of procedure which normally behaves like any accessor procedure, that is a procedure which accesses a data structure. The difference is that this kind of procedure has a so-called setter attached, which is a procedure for storing something into a data structure.

Procedures with setters are treated specially when the procedure appears in the special form set! (REFFIXME). How it works is best shown by example.

Suppose we have a procedure called foo-ref, which accepts two arguments, a value of type foo and an integer. The procedure returns the value stored at the given index in the foo object. Let f be a variable containing such a foo data structure.1

(foo-ref f 0)       => bar
(foo-ref f 1)       => braz

Also suppose that a corresponding setter procedure called foo-set! does exist.

(foo-set! f 0 'bla)
(foo-ref f 0)       => bla

Now we could create a new procedure called foo, which is a procedure with setter, by calling make-procedure-with-setter with the accessor and setter procedures foo-ref and foo-set!. Let us call this new procedure foo.

(define foo (make-procedure-with-setter foo-ref foo-set!))

foo can from now an be used to either read from the data structure stored in f, or to write into the structure.

(set! (foo f 0) 'dum)
(foo f 0)          => dum

make-procedure-with-setter procedure setter Scheme Procedure
scm_make_procedure_with_setter (procedure, setter) C Function
Create a new procedure which behaves like procedure, but with the associated setter setter.

procedure-with-setter? obj Scheme Procedure
scm_procedure_with_setter_p (obj) C Function
Return #t if obj is a procedure with an associated setter procedure.

procedure proc Scheme Procedure
scm_procedure (proc) C Function
Return the procedure of proc, which must be either a procedure with setter, or an operator struct.

setter proc Scheme Procedure
Return the setter of proc, which must be either a procedure with setter or an operator struct.


  1. Working definitions would be:

    (define foo-ref vector-ref)
    (define foo-set! vector-set!)
    (define f (make-vector 2 #f))