On this page:
define-interface/ dynamic
send/ i
send*/ i
send/ apply/ i
define/ i
init/ i
init-field/ i
init-private/ i
Version: 5.0.2

34 Interface-Oriented Programming for Classes

Ryan Culpepper <ryanc@racket-lang.org>

 (require unstable/class-iop)

This library is unstable; compatibility will not be maintained. See Unstable for more information.

(define-interface name-id (super-ifc-id ...) (method-id ...))
Defines name-id as a static interface extending the interfaces named by the super-ifc-ids and containing the methods specified by the method-ids.

A static interface name is used by the checked method call variants (send/i, send*/i, and send/apply/i). When used as an expression, a static interface name evaluates to an interface value.


  > (define-interface stack<%> () (empty? push pop))
  > stack<%>


  > (define stack%
      (class* object% (stack<%>)
        (define items null)
        (define/public (empty?) (null? items))
        (define/public (push x) (set! items (cons x items)))
        (define/public (pop) (begin (car items) (set! items (cdr items))))

(define-interface/dynamic name-id ifc-expr (method-id ...))
Defines name-id as a static interface with dynamic counterpart ifc-expr, which must evaluate to an interface value. The static interface contains the methods named by the method-ids. A run-time error is raised if any method-id is not a member of the dynamic interface ifc-expr.

Use define-interface/dynamic to wrap interfaces from other sources.


  > (define-interface/dynamic object<%> (class->interface object%) ())
  > object<%>


(send/i obj-exp static-ifc-id method-id arg-expr ...)
Checked variant of send.

The argument static-ifc-id must be defined as a static interface. The method method-id must be a member of the static interface static-ifc-id; otherwise a compile-time error is raised.

The value of obj-expr must be an instance of the interface static-ifc-id; otherwise, a run-time error is raised.


  > (define s (new stack%))
  > (send/i s stack<%> push 1)
  > (send/i s stack<%> popp)

  eval:9:0: send/i: method not in static interface in: popp

  > (send/i (new object%) stack<%> push 2)

  send/i: interface check failed on: (object)

(send*/i obj-expr static-ifc-id (method-id arg-expr ...) ...)
Checked variant of send*.


  > (send*/i s stack<%>
      (push 2)

(send/apply/i obj-expr static-ifc-id method-id arg-expr ... list-arg-expr)
Checked variant of send/apply.


  > (send/apply/i s stack<%> push (list 5))

(define/i id static-ifc-id expr)
Checks that expr evaluates to an instance of static-ifc-id before binding it to id. If id is subsequently changed (with set!), the check is performed again.

No dynamic object check is performed when calling a method (using send/i, etc) on a name defined via define/i.

(init/i (id static-ifc-id maybe-default-expr) ...)
(init-field/i (id static-ifc-id maybe-default-expr) ...)
(init-private/i (id static-ifc-id maybe-default-expr) ...)
maybe-default-expr = ()
  | default-expr
Checked versions of init and init-field. The value attached to each id is checked against the given interface.

No dynamic object check is performed when calling a method (using send/i, etc) on a name bound via one of these forms. Note that in the case of init-field/i this check omission is unsound in the presence of mutation from outside the class. This should be fixed.