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Documentation:Special form define-record-type - Gambit wiki

Documentation:Special form define-record-type

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User contributed comments, clarifications and examples

Mark Feeley explains Gambit-C Records on r6rs-editors


Content from define-type on main wiki page

define-type / define-structure usage

Here is a traduction of some of Marc's slides from an introduction to scheme course. I believe they might be usefull since define-type / define-record are very lightly documented in the official doc.

Syntax

(define-type type-name <attribute or field>...)

Attribute is a keyword. An attribute may have arguments?

Field is a symbol or a list. If a list, the first element is the field name, optionally followed by getter and setter, then field attributes

The simplest form of syntax specifies only the type name and the name of each fields

  • ex:
C: typedef struct { int x; int y; } point2d;
Gambit : (define-type point2d x y)
  • Such type definition are equivalent to multiple definitions:
(define (make-point2d x y)...)   ;constructor
(define (point2d? obj)... )      ;predicate
(define (point2d-x p)...)        ;access to x field
(define (point2d-x-set! p x)...) ;mutation of x field
(define (point2d-y p)...)        ;access to y field
(define (point2d-y-set! p y)...) ;mutation of y field
  • ex:
> (define-type point2d x y)
> (define p (make-point2d 11 22))
> p
#<point2d #2 x: 11 y: 22>
> (point2d? p)
#t
> (point2d-x p)
11
> (point2d-x-set! p 33)
> p
#<point2d #2 x: 33 y: 22>

define-type attributes

  • These structures also supports inheritance
  • The parameter extender: name specifies the name of the definition form to use to define a subtype:
> (define-type point2d
    extender: define-type-of-point2d
    x
    y)
> (define-type-of-point2d point3d z)
> (define p3 (make-point3d 11 22 33))
> p3
#<point3d #2 x: 11 y: 22 z: 33>
> (point2d? p3)
#t
> (point3d? p3)
#t
> (point2d-x p3)
11
> (point3d-z p3)
33


TODO other define-type's parameters:

  • id:
  • constructor:
  • constant-constructor:
  • predicate:
  • implementer:
  • type-exhibitor:
  • prefix:
  • opaque:
  • macros:

Field attributes

  • Field attributes:
    • read-write:
    • read-only: non mutable field
    • equality-test:
    • equality-skip: the equal? function will ignore this field
    • printable:
    • unprintable: the write function will ignore it
    • init: cst initial value
  • ex:
> (define-type noeud
    (a unprintable:)
    (b read-only: init: 9)
    (c equality-skip:))
> (define x (make-noeud 1 2))
> (noeud-a-set! x x)
> x
#<noeud #2 b: 9 c: 2>
> (equal? x (make-noeud x 3))
#t
> (noeud-b-set! x 999)
*** ERROR -- Unbound variable: noeud-b-set!

All field attributes except init: can be specified at define-type level and will affect all fields. A field can specify its own field attributes to override default attributes.

  • ex:
> (define-type foo unprintable: a (b printable:))
> (make-foo 1 2)
#<foo #3 b: 2>
  • Field may have non-conventional getter and setter names, specified after field name in that order
  • ex:
> (define-type foo (a get-a) (b get-b set-b))
> (define x (make-foo 1 2))
> (get-a x)
1
> (set-a x 2)
*** ERROR IN (console)@14.2 -- Unbound variable: set-a
> (get-b x)
2
> (set-b x 3)
> (get-b x)
3
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