[racket] Generics and gen:custom-write

Thanks for your answer.
Too bad -- I guess I'll have to rewrite the custom print then.

Laurent


On Mon, Dec 2, 2013 at 9:16 PM, Vincent St-Amour <stamourv at ccs.neu.edu>wrote:

> `define/generic' binds the specified generic function to a new
> identifier, to allow methods to call the generic function recursively.
> In your case, your `write-proc' calls `super-write' which, after
> dispatch, calls your `write-proc' again.
>
> What you want would be some a bit like CLOS's `call-next-method', which
> Racket's generics don't have.
>
> I can't think of a way to do what you want, short of reimplementing the
> behavior for `print' in your method.
>
> The interface for `gen:custom-write' (which it inherited from
> `prop:custom-write') is not great. IMO, it would be nicer to have
> `write', `display' and `print' be separate methods (which would support
> your use case), but I don't know how to retrofit that on top of the
> existing custom-write interface provided by runtime system.
>
> Vincent
>
>
>
> At Mon, 2 Dec 2013 18:05:59 +0100,
> Laurent wrote:
> >
> > [1  <multipart/alternative (7bit)>]
> > [1.1  <text/plain; ISO-8859-1 (7bit)>]
> > Anyone has an answer to this?
> >
> > Thanks,
> > Laurent
> >
> >
> > On Tue, Nov 12, 2013 at 1:58 PM, Laurent <laurent.orseau at gmail.com>
> wrote:
> >
> > > I'm trying to implement a custom writer for a struct with generics,
> but I
> > > think I'm misunderstanding something.
> > >
> > > What I want to do is specialize only the behavior for `write' and
> > > `display', but leave the behavior for `print' untouched. So I thought
> that
> > > is what define/generic was for, but the following loops indefinitely:
> > >
> > > #lang racket
> > > (require racket/generic)
> > >
> > > (struct A (x y)
> > >   #:transparent
> > >   #:methods gen:custom-write
> > >   [(define/generic super-write write-proc)
> > >    (define (write-proc a port mode)
> > >      (case mode
> > >        [(#t) (write (list (A-x a) (A-y a)) port)]
> > >        [(#f) (display (list (A-x a) (A-y a)) port)]
> > >        [else (super-write a port mode)]))])
> > >
> > > (define a (A 1 'b))
> > > (displayln a) ; ok
> > > (write a)(newline) ; ok
> > > (print a)(newline) ; infinite loop
> > >
> > > What's the correct way to do that then?
> > >
> > > Thanks,
> > > Laurent
> > >
> > [1.2  <text/html; ISO-8859-1 (quoted-printable)>]
> >
> > [2  <text/plain; us-ascii (7bit)>]
> > ____________________
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