On 2011-08-24 10:31:00 -0400, Alexander Kasiukov wrote:
>
> What is the best way to make a class in Racket abstract? I would
> like to have a class which
In addition to all the approaches so far, you can also use contracts to
keep a class from being instantiated even when you export it.
#lang racket/load
;; cannot instantiate base% outside of this module
(module a racket
(define base%
(class object%
(super-new)
(init abstract)
(define/public (common)
(displayln "noop"))))
(define sub%
(class base%
;; this works here
(super-new [abstract "dummy"])))
(provide/contract
[sub% (class/c)]
[base% (class/c (init [abstract none/c]))]))
(require 'a)
(send (new sub%) common) ; ok
(new base% [abstract 0]) ; contract error
(new (class base% (super-new [abstract 0]))) ; contract error
This doesn't let you subclass outside of the module though because of
the contract. You can still use the class value for is-a? or subclass?
checks, though it may be better to use interfaces if you need that
anyway.
Cheers,
Asumu
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