> On Jun 25, 2017, at 12:25 PM, Matthew Butterick <m...@mbtype.com> wrote:
>
> How does one attach a generic interface to a class?
>
> `class*` allows one to attach an `interface` [2] (in the class sense of that
> term, which I understand is separate from that of "generic interface").
>
> Then the docs for `interface*` show an example attaching the
> `prop:custom-write` property. [3] But the docs for `prop:custom-write` say
> that it's deprecated, and one should "use `gen:custom-write`, instead" [4]
> (that is, the generic-interface version this concept).
>
> Yet I when I put `gen:custom-write` into `interface*`, it triggers an
> "illegal use of syntax" error.
>
> #lang racket
>
> (define i<%> (interface* () ([gen:custom-write
> (lambda (obj port mode) (void))])
> method1 method2 method3))
>
> So I'm not clear whether I'm doing it wrong, or if `interface*` only works
> with the (deprecated) property-based technique.
It's possible to do that with the private internal macros in
"racket/private/generic-methods.rkt". If you don't mind using something that's
supposed to be private and unstable, and if you don't mind things breaking if
anything ever changes, you can use `generic-property` and
`generic-method-table` like this:
#lang racket
(require racket/private/generic-methods)
(define i<%>
(interface* ()
([(generic-property gen:custom-write)
(generic-method-table gen:custom-write
(define (write-proc obj port mode)
(void)))])))
However, it might be better for someone to make a pull request to update the
`interface*` form to allow generic interfaces properly without the user needing
these internal macros.
Alex Knauth
> [2]
> http://docs.racket-lang.org/reference/createinterface.html?q=class*#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._interface%29%29
>
> <http://docs.racket-lang.org/reference/createinterface.html?q=class*#(form._((lib._racket/private/class-internal..rkt)._interface))>
>
> [3]
> http://docs.racket-lang.org/reference/createinterface.html?q=class*#%28form._%28%28lib._racket%2Fprivate%2Fclass-internal..rkt%29._interface%2A%29%29
>
> <http://docs.racket-lang.org/reference/createinterface.html?q=class*#(form._((lib._racket/private/class-internal..rkt)._interface*))>
>
> [4]
> http://docs.racket-lang.org/reference/Printer_Extension.html?q=class*#%28def._%28%28quote._~23~25kernel%29._prop~3acustom-write%29%29
>
> <http://docs.racket-lang.org/reference/Printer_Extension.html?q=class*#(def._((quote._~23~25kernel)._prop~3acustom-write))>
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