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Re: fundamental design issues (long message)



   Date: Fri, 2 Dec 2005 22:04:03 -0700
   From: Matthew Flatt <mflatt@xxxxxxxxxxx>

   Thanks for your thoughts, and for taking the time to put together an
   alternative proposal.

   The editors looked closely at Scheme48's module design. I'll try to
   report the editors' consensus (or at least dominant opinion) on the
   issues that you bring up:

Thanks for the speedy response.  I hope that the individual editors
will elaborate on their own opinions, but in the meantime I'll briefly
respond to these points:

   1. On the issue of storing source, there is firm opposition to mixing
      any notion of files into the standard. The current proposal can be
      seen as requiring portable code to use an implicit `code' form.

I'm a bit confused, if this is the case, about how libraries can use
other libraries in practice if there is no way to refer to them other
than by some unspecified URI convention.  Surely some notion of
resources containing Scheme source is implied by the relative URI
naming mechanism for other libraries; why couldn't this be applied
equally well to what I (& Scheme48) call FILES clauses?

   2. The `<language>' part of the library form is present so in the hope
      that we can refine the `import', `export', and library body syntax
      in the future without breaking R6RS-based code. There are other ways
      to achieve the same goal, but declaring the library's language seems
      to be the simplest.

Do you expect to be changing the module system dramatically in the
near future after finalization?  I think that this use of the language
device conflates two separate issues, the module language and the
source code language, which should be managed in different ways.  A
SCHEME (or SCHEME-R6RS, or (SCHEME R6RS), or whatever) module/library
is a simple way to manage the latter; if you really want to include
information about the module language revision, why not include
something at the top of the file, like (R6RS) or something?  Is it
useful to be able to use dramatically different module languages
within a single set of modules?

One useful property of having the Scheme language module hold regular
status among other modules is that its interface can be transformed as
with any other modules.  Aubrey Jaffer's concern about shadowing
standard Scheme identifiers would be easily answered in this way; for
example, a module that uses Jaffer's commutative ring library might be
defined like this:

 (define-module my-module my-module-interface
   (import (modify r6rs-scheme (hide + * / - zero?))
           commutative-rings
           ...)
   ...)

   3. On the issue of using `for-syntax' and nested `for-syntax', the
      editors have a bit more work to do. There is a rationale for the
      current syntax, however: it's easier to state --- and maybe easier
      to check --- the constraint that an identifier can be imported from
      only one source, even across phases. (This constraint doesn't exist
      in Scheme48 or MzScheme, but it's a crucial compromise in the
      current SRFI.)

I'm a bit unclear on this constraint you mention; could you explain it
in a bit more detail?

                     Meanwhile, there is significant opposition to the
      idea that a library can be instantiated once for its use in all
      phases. (Compiling a library multiple times is not necessary.)

This was the point I had in mind when I mentioned that the SRFI
authors would probably disagree with me.  I've read the paper, and I
see in section 3.3 (Phase Separation) *that* the MzScheme module
system executes every module for every phase at which another module
uses it, but I can't find any explanation of *why* that is.  The need
for the separation of the lexical environment is obvious, but why
could not two phases' lexical environments refer to identical modules
(i.e. the same, single instantiation of one module)?

I'll agree, of course, that in any actual software system modules will
be instantiated more than once for their existence in the universe,
but I'd like to understand why they are mandated to be instantiated
for every phase at which every other module's instantiation uses them.

   4. On indirect exports, I'm actually not certain whether it matters to
      have a separate `indirect-export' form instead of bundling the
      information with `define-syntax'. I'll have to get back to you on
      this suggestion.

OK.  I'm not sure whether I mentioned this already, but Scheme48 gets
by with the auxiliary name list (or indirect export list, whichever
term is preferred) being specified in each macro's definition, which
makes it possible for the SYNTAX-RULES macro transformer to generate
the list itself and not need it specified elsewhere.  The need to
specify the auxiliary names explicitly in psyntax's module system has
always bugged me.

   5. On the issue of separating interface from implementation, there was
      considerable opposition to requiring a separation. Optional
      separation, then, was omitted in the interest of simplicity. The
      current proposal does, however, include a relevant compromise:
      macros cannot expand to `import' or `export', and all imports and
      exports must appear at the beginning of the library, so the
      library's interface is textually apparent.

I'm curious to know why there was opposition to separation.  One of
the important properties of my proposal, though, is stronger than the
'textually apparent' nature of libraries' exports in the current SRFI.
Not only is it textually apparent separately from the library's body,
but you don't need to look at the expression or file containing the
body at all: all you need are the module descriptions.  bear's point
here is relevant, as is the point that code embedded in module
definitions is fundamentally incompatible with reader extensions,
which I'd like the module system not to preclude.