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Re: Independent optimizing compilation



Sam Tobin-Hochstadt <samth@xxxxxxxxxxx> writes:

> On Mon, 2005-12-05 at 19:32 +0100, Michael Sperber wrote:
>
>> Some of these goals conflict, at least among the module systems and
>> designs we've looked at, which is why we (eventually) narrowed down
>> the list of requirements.  I'll be happy to elaborate, but it's a long
>> tortuous story ... :-)
>
> I think this story, and a comparison with existing Scheme module
> systems, would be very helpful for understanding the proposal.  At least
> the latter belongs in the SRFI document as well.

I'll try.  It gets long and ugly, though.

DISCLAIMER: THE VIEWS EXPRESSED OR IMPLIED IN THIS MESSAGE ARE MY OWN
ONLY; EVEN THOUGH I AM AN R6RS EDITOR, I CAN'T SPEAK FOR THE OTHER
R6RS COMMITTEES.  I TAKE NO RESPONSIBILITY FOR ANY BRAIN DAMAGE
INCURRED AS A RESULT OF READING THIS MESSAGE.

I listed a couple of possible requirements, and everyone can go and
look for module systems that satisfy any given set of them.  One is
enough to create a conflict, however.  So, for the purposes of this
message, take just one: namespace management.

What's essential to namespace management?  Two things: exporting names
from one environment, and importing them into another.  Suppose you
have forms:

(export <identifier> ...)
(import <environment spec>)

where EXPORT somehow makes identifiers from the current environment
available for use by an IMPORT form in another place.  Supposedly,
there's some way to reference the EXPORT form or its environment via
the <environment spec> thing.  (Now, there's a whole set of tricky
issues surrounding the possible natures of <environment spec>---around
the issue of "linking"---but I'll ignore these here.)

A symptomatic question for issue of "namespace management" is: where
can EXPORT and IMPORT forms occur?  Consider three different module
systems, those of Scheme 48, PLT Scheme (its MODULE form, to be
precise), and Chez Scheme.  Here are links explaining those module
systems, if you're interested:

The "Module system" chapter of the
The Incomplete Scheme 48 Reference Manual
http://www.s48.org/1.3/manual/manual.html
Matthew Flatt: Composable and Compilable Macros
http://www.cs.utah.edu/plt/publications/macromod.pdf
Oscar Waddell and R. Kent Dybvig: Extending the Scope of Syntactic Abstraction
http://www.cs.indiana.edu/~dyb/pubs/popl99.pdf

Anyway, the answers to the question are:

Scheme 48: in the separate module language
PLT Scheme: at the toplevel of a MODULE body
Chez Scheme: inside a MODULE form, which is just DEFINE and can occur
  anywhere DEFINE occurs

(Andre could add a fourth point along this spectrum with the previous
incarnation of his module system.)

Why do these three Schemes take such different positions, with Scheme
48 and Chez at the extremes?  Chez's position is well-supported in the
preamble of the R5RS:

> Scheme demonstrates that a very small number of rules for forming
> expressions, with no restrictions on how they are composed [...]

... and this clearly applies to Chez's module system: Its MODULE form
(along with its IMPORT form) is just like a definition, and can be
combined with other definitions.  Moreover, MODULE and IMPORT forms
can result from macro expansion, and this allows expressing a number
of module-system-related problems to be solved by writing a bunch of
macros on top of the primitives of the module system.  This is very
nice.

But this generality also has a price.  The root of the problem can be
cast in terms of a seemingly simple extension to R5RS: internal
DEFINE-SYNTAX.  Consider the following code fragment:

(let-syntax ((foo (syntax-rules ()
                    ((foo) 'outer))))
  (let ()
    (define a (foo))
    (define-syntax foo
      (syntax-rules ()
        ((foo) 'inner)))
    a))

What should the return values be?  Regular internal definitions are
mutually recursive, and, if the same holds true for internal syntax
definitions, well, the return value surely should be 'inner, right?

Consider this code fragment, then:

(let-syntax ((foo (syntax-rules ()
                    ((foo ?x) (define ?x 'outer)))))
  (let ()
    (foo a)
    (define-syntax foo
      (syntax-rules ()
        ((foo ?x) (define ?x 'inner))))
    a))

If, in the first example, the macro invocation of FOO references the
subsequent definition, surely the same should be the case here, right?
After all, macro definitions and invocations are in the same order in
both examples.  Yet, in PLT Scheme and Chez Scheme, you get 'outer,
proving that the opposite holds true.

The explanation is that PLT and Chez do two expansion passes, one to
figure out the set of active definitions, and then another to do the
full expansion.  Details have, until recently, varied.  Here's some
indication of the potential impact of such algorithmic variations:

http://list.cs.brown.edu/pipermail/plt-scheme/2005-June/009213.html

So, with internal DEFINE-SYNTAX, you get more generality
(DEFINE-SYNTAX can occur wherever DEFINE occurs), but you also have to
explain the expansion algorithm, and, worse, syntax definitions don't
behave like regular ones: macros can expand into definitions, and
macro expansion depends on what macros are defined, so you get a
conceptual recursion that has to be resolved using some kind of
sequential aspect of the macro-expansion algorithm.  (Because of this,
the problem can actually be explained in terms of plain R5RS, but the
examples get more obscure.)

Now, Chez's MODULE and IMPORT forms are definitions on steroids, so
the problems explained above are exacerbated in the sense that your
language definition gets more complex, and gets an operational flavor.
Looking in the R5RS for guidance, you get:

> "[Scheme] was designed to have an exceptionally clear and simple
> semantics [...]"

... and arguably this kind of change gets us away from "exceptional
clarity."  At this point, you have to choose between the
"compositionality" and the "simplicity" requirements of R5RS.  Among
the R6RS editors, we couldn't agree which one was more serious or less
important or whatever.

Consider another aspect, hygiene.

Scheme 48 takes the stance that the module definitions live in a
separate language (Taylor Campbell has done a pretty good job of
explaining some of the philosophical issues behind its design), and
this means that I can read the module structure of a program
independently from the code implementing the modules.  Also, I can
figure out a module's exports by just looking through the module
definition.  This restricts what you can do, but it also makes it very
clear where to look for your definitions.  In PLT Scheme or Chez
Scheme, macro expansion can be employed to construct the module
dependency graph or the exports of a module, and do fancy stuff.

Now, 

(import <module spec>)

is (some people disagree on my use of this word) an unhygienic
construct: it binds names not mentioned in the form.  Moreover, with
macro expansion, you can obscure the fact that an import is
happening.  With Chez's construct (as explained in the paper cited
above; I hear there've been changes since), you additionally get this
in local environments:

(module m (foo bar)
  (define foo 'm))

(define-syntax baz
  (syntax-rules ()
    ((baz) (import m))))

(let ((foo 'bar))
  (baz)
  foo) 

In the last expression, you don't see the lexically enclosing FOO, but
rather one that's imported.  Whether you're considering this a problem
or not depends on the kinds of programs and programmers you deal
with, and what your brain considers "simple." 

You might also argue that, with the introduction of SYNTAX-CASE (as
we're planning to do), you can write non-hygienic macros anyway, and
therefore, the cat's out of the bag, we might as well go down the
whole route, ladidadida.  Or you might argue the opposite
dumdidumdidum.

... and on and on and on ...

Get the picture?  This kind of argument occurs over almost any aspect
of the module system, and it is always impossible to resolve in a
"best" manner, or has been so far, anyway.  (I sometimes wish this
were ML: none of the hard problems there. :-) ) Moreover, a bunch of
people are doing exciting and fresh research on the subject, none of
which will be ready for R6RS.

Which is why the R6RS editors re-focused on meeting a minimal set of
requirements, in an almost-minimal fashion, in the context of a
not-really-a-module-system.  For now, we're just concentrating on
letting you give your code to someone else, and use someone else's
code, while letting implementors provide and invent all kinds of
exciting module systems, just so long as they implement the library
standard, too.  Maybe, in a few years, we'll have a better answer for
the "module-system question" where it'll be easier to agree.  Right
now, the issues are genuinely too hard.

Hope this helps clear things up a little bit.

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla