Re: Macro for replacing a placeholder in an expression

[Maxime Devos]

On 28-07-2022 10:39, Zelphir Kaltstahl wrote:
> I aimed to do everything with syntax-rules, as the simplest means, but 
> when writing the code I have, I hit the snag, that one could not have 
> multiple ellipses at the same level of nesting in the patterns.

IIUC, you mean:

(syntax-rules ()
  ((foo x ... y ...) [the replacement]))

? If so, such a construct is ambigious.

> After some thinking I found the solution to build up a temporary list, 
> which then is of course 1 deeper level of nesting, where I could then 
> use ellipses again. I felt quite clever doing that trick. Maybe I 
> could implement a syntax-map using that trick and then use syntax-map 
> in my macro instead. 
> I have a question regarding syntax-case:
>
> If I use it, does my code become less portable to other Schemes?
>
> And regarding syntax-rules:
>
> How portable are macros, which exclusively use syntax-rules?
Everything that supports syntax-case most likely supports syntax-rules 
too, as syntax-rules can easily be defined in terms of syntax-case, so 
syntax-rules is at least as portable as syntax-case.

syntax-rules and syntax-case are pretty standard (it's in the R6RS), so 
I expect them to be available in recent-ish non-minimalistic Schemes.

However, some Schemes (likely Schemes that predate the R6RS, or in 
Schemes that try new things out), use other syntax systems.

Anyway, I've found a table:

> https://docs.scheme.org/surveys/syntax-definitions/

Looks like syntax-rules is supported by all the tested systems. 
syntax-case is less supported, but among the 'low-level' macro systems 
it has the highest support.  It's a bit out-of-date though, syntax-case 
is supported for Chicken: 
http://code.call-cc.org/legacy-eggs/3/syntax-case.html,

Decide for yourself of course, but I don't think going for maximal 
portability is worth the effort here of something making things work 
with syntax-rules.

> > Greetings,
> > Maxime
>
> Thank you again for your help and explanations! Things are much 
> clearer now!
>
> Best regards,
> Zelphir

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