From: Matthew Flatt <mflatt@cs.utah.edu>
To: plt-scheme@tux.cs.brown.edu
Date: Mon, 9 Sep 2002 06:31:39 -0600 (MDT)
Subject: [plt-scheme] 202.2

The exp-tagged code for MzScheme and MrEd in CVS is now version 202.2.

202.2 supports structure types whose instances act as procedures.

There is no high-level syntax for procedureness, currently. Use the
primitive `make-struct-type’ function to declare procedure information
for a new structure type.

Procedureness can be added to a structure in either of two ways:

1. A field within the structure is designated as containing a
   procedure. This is useful for wrapping procedures with information.

  Example (from the documentation):

   (define-values (struct:ap make-annotated-proc
                             annotated-proc? ap-ref ap-set!)
       (make-struct-type ’anotated-proc #f 2 0 #f null #f 0))
       ;; field 0 is the procedure-container--------------^
   (define (proc-annotation p) (ap-ref p 1))

   (define plus1 (make-annotated-proc
                   (lambda (x) (+ x 1))
                   "adds 1 to its argument"))

   (procedure? plus1) ; => #t
   (annotated-proc? plus1) ; => #t
   (plus1 10) ; => 11
   (proc-annotation plus1) ; => "adds 1 to its argument"

2. A method-like procedure can be attached to the structure type,
   instead of individual instances. This is useful for making
   applications of instances act like a particular function on the
   instances (which is what Lauri Alanko wants).

  Example (from the documentation):

   (define-values (struct:fish make-fish fish? fish-ref fish-set!)
      (make-struct-type ’fish #f 2 0 #f null #f
                        ;; Here’s the procedure for app. of an instance:
                        (lambda (f n) (fish-set! f 0 (+ n (fish-ref f 0))))))
   (define (fish-weight f) (fish-ref f 0))
   (define (fish-color f) (fish-ref f 1))

   (define wanda (make-fish 12 ’red))
   (fish? wanda) ; => #t
   (procedure? wanda) ; => #t
   (fish-weight wanda) ; => 12
   (for-each wanda ’(1 2 3))
   (fish-weight wanda) ; => 18

Matthew