2012/11/15 Álvaro Castro-Castilla alvaro.castro.castilla@gmail.com

Hi,

In the past I've used Blackhole, the bundled syntax-case implementation and Alexpander. Each one has its own advantages and disadvantages:

blackhole

• A: it works well, and not only syntax-rules, but also

sc-macro-transformer and er-macro-transformer

• D: forces you to use its module system. I think this is a mistake, since

is not standard, but anyway it has some current limitations for my work that I tried to fix in my own branch but stem from deep parts of blackhole that would require a deep understanding and rewrite. I tried that already and spent a fair amount of time working with Per these issues and decided to drop it.

..

The point is: is there any stable, good implementation of R5RS syntax-rules that is fully-compatible and nice to all of Gambit's functionality?

Beyond Black Hole, nope.

I believe this should be a basic feature of the Gambit ecosystem, but maybe I'm missing something. Any help, ideas or opinions?

There's space for an even better working Black Hole and perhaps for some more hooks in Gambit for module system functionality.

Best regards,

Álvaro Castro-Castilla

Perhaps it can be looked with benefit on like this:

While the great majority of Scheme's language spec is "The Answer"-grade (you get what I mean - the grade where it's like wow that must really be the way to do it; obviously there's others too, some documented and others yet to be found),

The macro systems developed, well, ever, for Scheme, don't come close to that grade of quality.

Basically define-macro is nice and effective but has a narrow limit for how much you can abstract on it. Syntax-rules comes with power from the fact that it's purely functional, but it's a limited purpose tool and it's obvious there's more to be made. Syntactic closures and syntax-case are extremely powerful but come with API:s that are completely unsuitable for debugging and increase the complexity of the Scheme language and the complexity of the Scheme environment many-fold, and this way *completely* and fundamentally non-Schemy. It's not simple anywhere, at the level of concept it's like biting yourself in your own tail.

Luckily though, Scheme doesn't stand and fall with the perfection of its macro system, practically the macros are only used for limited rewriting purposes, so given its perfection in Scheme's other areas, it's fine there's diversity and limits about the macros.

Gambit nicely takes the position of kind of not caring about any of this stuff at all, focusing on executing Scheme with clarity and excellence, and implements a define-macro expander as it provides an enormous benefit at almost no cost, 10 lines of sourcecode perhaps, and if not else, it's used all across Gambit's own sources.

Black Hole is the best attempt for Gambit at uniting the algorithms and designs of define-macro, syntax-rules and syntactic closures, which are contradictory with each others as well as in themselves imperfect.

If the macro expansion you're looking for cannot be made with the current version of Black Hole and you feel it would be of a general value to have the support you have on your mind, probably the easiest thing for both you and everyone, would be that you decipher Black Hole from bottom up, and reimplement it from scratch (now that you concluded there's no easier way), perhaps with a twist, with the particular aspects you want now in it.

This way you'd have the opportunity to get really experienced in the more and less subtle aspects of different types of Scheme macro expansion, and into an excellent position for documenting and commenting your module system/Black Hole, and spreading the knowledge of how this stuff works and fits together.

Per put approx six man months into Black Hole. This was a very giving adventure in computer science for him, and brought a depth and specificity of understanding he could not have anticipated.

With him as conclusions he got an understanding of algorithmic limits within syntactic closures that I still feel I have not really understood from him, I'd be highly interested in getting this someday.

Probably given that his code is there and you also can ask him some things, you could get to where he got within approx six weeks.

It will very most probably be more than worth it, and you'll have the full satisfaction of having done it yourself!

Handling of modularized code is essential for incremental development in Gambit, so it is well spent time in all cases, that the understanding, software and conversation on this topic is expanded.

(Regarding Black Hole I feel its source could be better commented, and its concepts and details and their limits much better documented, I mean currently there's almost nil of that. Being aware that it does deliver for all the purposes I have though, I'll work with making the 'new version' (syntactictower version) spin better generally in the time to come; over the last ~5 years I used the legacy branch and it works extremely well. Matters unrelated to macro expansion are what is pulling me to upgrade.)

If not else I hope this email encouraged good conversation. :-)

Brgds, Mikael

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