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

[ludo@gnu.org (Ludovic Courtès)]

Mark H Weaver <mhw@netris.org> skribis:

> ludo@gnu.org (Ludovic Courtès) writes:
>
>> Mark H Weaver <mhw@netris.org> skribis:
>>
>>> In the end, here's how this works: 'with-ellipsis' binds a special
>>> identifier named #{ $sc-ellipsis }# using a new 'ellipsis' binding type.
>>> The new ellipsis identifier is stored within the binding.  In order to
>>> determine whether an identifier X is an ellipsis, the binding for
>>> #{ $sc-ellipsis }# is looked up in the lexical environment of X.  If the
>>> binding is found and has binding-type 'ellipsis', then X is compared to
>>> the identifier stored in the binding using 'bound-id=?'.  Otherwise, X
>>> is compared to '...' using 'free-id=?' as was done before.
>>
>> This looks nice!  Thanks for providing the detailed reasoning, that’s
>> insightful.
>>
>> 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.

Actually my question was more about the ellipsis escaping form
(... ...).  It is affected by ‘with-ellipsis’, right?  (It may be a
obvious question, but I’m not familiar with the implementation.)

[...]

> It is important that the custom ellipsis does not propagate to the
> generated code, so that we can use 'with-ellipsis' to implement R7RS
> 'syntax-rules', which allows a custom ellipsis as its first operand,
> before the literals list.  In R7RS 'syntax-rules', the custom ellipsis
> does not propagate to generated code.

Yes, that make sense.

> 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.

>> Could you wrap lines to 80 columns in psyntax.scm?
>
> Ordinarily I try to keep lines to 80 columns, but psyntax.scm already
> has a great deal of code that violates that rule.  Fixing that would be
> a rather large commit, and I'm not sure it would be an improvement.

OK (I do find it hard to read long lines, FWIW.)

Thanks!

Ludo’.