[gambit-list] Thoughts on Scheme

Álvaro Castro-Castilla alvaro.castro.castilla at gmail.com
Sun Jan 6 05:38:23 EST 2013

I agree with Meng.
I see syntax-rules as a DSL for hygienic macros. It is completely "schemey"
in the same way libraries like Kanren for logic programming, or FrTime for
reactive programming are. The only difference is that when using hygienic
macros, your code becomes data as well, to be processed before it actually
turns into code.
Actually, when you take into account a couple of pitfalls regarding lexical
scoping and shadowing and use a variety of techniques, including
continuation-passing-style, writing syntax-rules macros are extremely
powerful and similar to regular recursive scheme. Unhigienic macros are
well-known timebombs that are waiting to explode as soon as client code
does something the library didn't think of. They are useful for
self-sufficient systems, as Mikael said.

Meng, I wasn't aware of this "riaxpander", it seems chicken also has it. Is
your riaxpander implementation open source?

I would add these comments to the original post:

- CAR and CDR are shorter than FIRST and REST (why not HEAD-TAIL?), so
besides the historical meaning, I prefer them for this reason. However, you
can always define your own first and rest (the former is actually defined
in SRFI-1). About making them generic, next point:
- I think the procedure specialization for types (char=, *-lenght, etc...)
is good as it favors performance. If you want the generic ones, it is
straightforward to define. For instance, that's what the author of SRFI-47
does: replace array=? with an array-augmented version of R5RS equal?. While
you can do your own specialization, you couldn't do it the other way
around: given a generic procedure in R5RS, specialize it for your types.
- Mikael's point about symmetry is absolutely beautiful. And indeed I
defend the usefulness of values, which are of special interest in
functional programming where you avoid side-effects. Also, the points about
C/C++ (and assembly I may add) are completely true, but that's one of the
reasons I find Gambit a particularly powerful system.
- Generics are defined in several libraries, there are many implementations.
- Promise and force are re-defined in R5RS terms in SRFI-45. I agree with
this point, but I don't know the deeper reasons why they are included,
because even opening the possibility to  implementations-defined optimized
representation of this primitives could have been done with an SRFI.

Thanks to everyone for your comments.

Best regards

On Sat, Jan 5, 2013 at 1:16 PM, Meng Zhang <wsxiaoys.lh at gmail.com> wrote:

> Great post Mikael! So happy to read your thoughts.
> On Sat, Jan 5, 2013 at 6:37 PM, Mikael <mikael.rcv at gmail.com> wrote:
>> The syntactic-closures based macro systems (syntax-rules/syntax case
>> including define-syntax, syntax-rules, let-syntax, letrec-syntax) bring
>> incredible complexity and with that low debuggability through a very
>> complex identifier concept based a kind of duality of the identifier symbol
>> in itself and the syntactic environment in which it is used in a particular
>> instance, and brings a complex API for handling macros with this, that by
>> nature is not Schemy and not suited for debugging.
>> Per came with the suggestions above based on having spent approx 6 months
>> fulltime on developing the Black Hole module system, which does hybrid
>> define-macro and syntactic closures expansion.
>> There might be some caveat I didn't get, but, I'd guess you could
>> actually make a macro expander that supports both an alias macro system as
>> per above and a syntactic-closures for compatibility with code that uses
>> it, possibly by splitting expansion into two expansion phases, thus
>> isolating all the identifier-related complexity.
> While talking about the syntactic-closures, If we ignore syntax-case, I'll
> disagree that it brings "incredible" complexity. I've been uncertain
> on syntactic-closures for years, while this Holiday I finally got spare
> time reading its implementation in riaxpander/chibi-scheme, I found
> the concept of it is quite straight forward. Though I deadly missed an
> easy-to-understand, concrete implementation with document for it during
> the learning process.
> I've cloned such a system in gambit, and keep digging on how to
> integrate hygiene with module system and gambit's compiling process.
> Thanks
> Meng
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