Re: [Scheme-reports] auxiliary syntax

On Mon, Jan 7, 2013 at 8:28 PM, Andy Wingo <wingo@x> wrote:

On Sun 06 Jan 2013 19:56, Peter Bex <Peter.Bex@x> writes:

> The second form in the following returns 1 in Chibi, Chicken, Gambit,
> MIT Scheme and Gauche:
>
> (define else #f)
> (cond (else 1))

Q: Why is this? A: Because all identifiers are present in the initial
environment in these implementations, so "define" is really "set!"
here. Therefore the "else" that is here is the same as the "else" that
the "cond".

This is not true of Chibi. Also, the test is broken - it only works in

Chibi (and Chez) in the repl. Separate modules preserve hygiene.

Also, I don't think people realize how broken the whole situation is,

so I'm going to elaborate.

There are basically two things you may be trying to do when matching

an identifier - match a binding or match that identifier by name.

For an example of matching bindings hygienically, the (outdated)

http://synthcode.com/scheme/fast-math.scm uses a term optimization

library to rewrite arithmetic expressions more efficiently at compile time.

It does this by matching on patterns of `+', `-', `*' and `/'. It would in

general be broken to attempt to rewrite these if they weren't the right

bindings. Hygienic matching is also necessary for the more fancy

syntax-rules tricks like checking if a pattern is a symbol or testing set

membership.

Almost all DSLs, however, fall into the latter case. I have an

unpublished macro that lets you match alternate SREs in a

cond-like pattern, and traces the statically named submatches within

the SRE, binding them as local variables in the corresponding

clauses. SchemeQL, SCSH's run syntax, the portable match.scm,

and define-record-type to name a few fall into this category. There's

no ambiguity and no reason to rename or worry about hygiene for

these identifiers.

So in the upcoming R7RS community, how am I to distribute

my portable SRE macro? I'm being told to bind every SRE

identifier to something and export it from the library. Many of these

are already in (scheme base) like `or' and `*', including some new

"shared auxiliary syntax ones" like `=>', while others I'll need to

define specially like `:' and `?'. Hmmm... those are very common

identifiers in DSLs - do I want them to belong exclusively to the

SRE library? A local solution I'm seriously considering is to create

a (chibi auxiliary-syntax) library, and whenever I need to create a

new auxiliary syntax for some library to add it to that. Obviously

this doesn't scale to actually sharing code with other people, which

was sort of the whole purpose of this standard.

The solution some suggest is just to using renaming. Thus

usually SREs use `:' and `?', but when I also happen to be using

the (foo bar) library which uses those identifiers I have to rename

them to `::' and `??'. That also doesn't scale, making it impossible

to remember. I could try to prefix the SRE identifiers consistently,

e.g. `rx:' and `rx?', except oops, the underlying SRE engine doesn't

know these names! Now we're at an impasse - it's simply impossible

to reconcile the macro extensions to the DSL with the underlying

unhygienic symbol language.

Really, when you want to wrap macros on top of a non-macro-based

DSL, the only solution is to use unhygienic matching. But there's

no way to do this with syntax-rules, which is all we have in both R5RS

and R7RS small.

This is a SERIOUS problem. I'm still unaware of any halfway

decent solution, and although I don't think we're without hope, I'd

appreciate it if people stop putting their head in the sand and

pretending there isn't a problem here.

[Now, as a footnote, the example people always use when discussing

this is `cond', which is unfortunate because cond is a special case.

Cond really, really wants to just match `else' unhygienically but

it can't because it has an ambiguous syntax. So it's a sort of rare

hybrid case, which for lack of a better alternative we need to treat

hygienically, but it's unproductive to dwell too much on it.]

Alex