Re: [PATCH] psyntax: custom ellipses using 'with-ellipsis' or R7RS syntax-rules

[Mark H Weaver <mhw@netris.org>]

ludo@gnu.org (Ludovic Courtès) writes:

> Mark H Weaver <mhw@netris.org> skribis:
>
>> ludo@gnu.org (Ludovic Courtès) writes:
>>
>>> Does something like this work:
>>>
>>>   (define-syntax define-inline
>>>     (with-ellipsis ---
>>>       (syntax-rules ()
>>>         ((_ (name parms ---) exp ---)
>>>          (define-syntax name
>>>             (syntax-rules ()
>>>               ((_ args (--- ---))
>>>                ((lambda (parms ---) exp ---)
>>>                 args (--- ---)))))))))
>>
>> No, because as noted in the docs, the custom ellipsis does not propagate
>> to the generated code.
>
> OK, right; it’d work with ‘with-ellipsis’ repeated after the inner
> ‘define-syntax’ I suppose.

Yes.

> Actually my question was more about the ellipsis escaping form
> (... ...).  It is affected by ‘with-ellipsis’, right?

Yes, so the following works:

  (define-syntax define-inline
    (with-ellipsis ---
      (syntax-rules ()
        ((_ (name parms ---) exp ---)
         (define-syntax name
           (with-ellipsis (--- ---)
             (syntax-rules ()
               ((_ args (--- ---))
                ((lambda (parms ---) exp ---)
                 args (--- ---))))))))))

Needless to say, the whole point of custom ellipses is to avoid having
to ever escape ellipses, but you can still do it.

>> Note that as currently implemented, the effect of 'with-ellipsis'
>> also does not propagate into nested syntax definition forms such as
>> 'let-syntax', 'letrec-syntax', and 'define-syntax'.  We could go either
>> way on this.
>
> Well, I think it’s fine this way, but then again I’ve been living in
> world without that feature.  ;-)
>
> How does R7RS syntax-rules behave in that respect?  I guess we should
> just follow suit.

In R7RS, custom ellipsis is not part of the lexical environment, and
thus does not propagate at all.  A custom ellipsis affects only the
patterns and templates of the 'syntax-rules' macro it is passed to.

I ended up making the effect of 'with-ellipsis' propagate into syntax
definition forms, since the semantics seem simpler to me.

I pushed this and the 'syntax-error' patch to stable-2.0.

     Thanks!
       Mark