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Re: [PATCH] psyntax: custom ellipses using 'with-ellipsis' or R7RS synta
From: |
Ludovic Courtès |
Subject: |
Re: [PATCH] psyntax: custom ellipses using 'with-ellipsis' or R7RS syntax-rules |
Date: |
Wed, 08 Jan 2014 21:53:12 +0100 |
User-agent: |
Gnus/5.130007 (Ma Gnus v0.7) Emacs/24.3 (gnu/linux) |
Mark H Weaver <address@hidden> skribis:
> address@hidden (Ludovic Courtès) writes:
>
>> Mark H Weaver <address@hidden> 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’.