Am Di., 25. Aug. 2020 um 09:58 Uhr schrieb Alex Shinn <xxxxxx@gmail.com>:

> Oh, sorry, yes this appears to work for match-letrec now as well.
> Cycles inside complex pattern-walking macros still make me nervous though.
>
> I suggest adding "it is an error to use any cyclic literals in match patterns, quoted or otherwise" to the SRFI.

I agree that cycles outside quotes in match patterns should be an
error. But banning them inside quotes would make no sense when the
underlying implementation is a conforming R7RS one.

>
>> > Cyclic literals in source are asking for trouble.  They are likely to break in many implementations in many situations.
>>
>> If I understand you correctly this argument sounds a bit like using
>> hygienic macros is asking for trouble because there are
>> implementations that do not implement them properly.
>
>
> There is a long history of well supported hygienic macros, with documentation
> and examples of their implementation and edge cases.  The same is not true
> of cyclic literals in Scheme source.  Arguing to support cycles at all in basic
> operations like equal? is somewhat recent.  I've never heard anyone argue
> seriously about encouraging cyclic literals in source.

The can appear naturally when you embed Scheme data literally in
source files, which is not too far fetched.

That they specifically appear as part of patterns is, admittedly, more
theoretical than what would probably happen in practice.

> Note your own syntax-case implementation for Chibi currently doesn't support cyclic literals.

It should as far as the language allows this. Can you give me an
example, which I would have to repair?

> But I would not object to simply removing match-letrec from the SRFI.

I'd rather remove match-letrec than banning literals that contain
cyclic structure.

>> > And I don't want to spend time rewriting (well, wiring through an extra parameter into every line of the file) match.scm,
>> > in order to support match-letrec, at least until I first see a real-world example where someone actually wants it.
>>
>> In some sense, this shows that, while possible, syntax-rules are
>> inadequate for writing complex macros like match when you later want
>> to extend them.
>
>
> No, it just shows that I'm a very busy person, starting a new job next week, with no time to spend on this.

Congratulations to your new job! :)

> There is nothing difficult about the rewrite.

Sorry, I didn't mean that it was difficult, least of all for you. Just
tedious and error-prone. My point about the relative inadequacy of
syntax-rules as is was: An addition of some local things (like
match-letrec) shouldn't lead to a global rewrite.