[R6RS] strawman module syntax

[Matthew Flatt]

Here's a strawman proposal for the syntax of a module system.
Naturally, it's based on the MzScheme system, so you may want to start
with

 http://www.htus.org/Book/Staging/how-to-use-modules/

The idea is that a program X corresponds to a file "X.scm" (which
contains a single `X' module declaration) plus all of the module files
in the transitive closure of "require"s starting from "X.scm".

I make no attempt to provide a semantics at this point, because I
expect it's obvious.

I've stuck to MzScheme's syntax (a restricted version), rather than
trying to anticipate objections and fix them. It's just a strawman,
after all. There are some exceptions, because Richard, Mike, and I
discussed this a couple of years ago, and I've deviated from MzScheme
in a few ways based on that discussion.

At this point, I'm ignoring the possibility of macros other than those
implemented with `syntax-rules'.

Matthew

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<module> = (module <identifier>   ; Matches the filename?
                   <require>      ; Language designation
             <clause> ...)

 ; The syntax for <clause> is determined by the language,
 ;  but the standard-language <clause> is as follows
<clause> = (require <require-spec> ...)
         | (provide <provide-spec> ...)
         | (begin <statement> ...)
                                 ; Core R6RS statements
         | (include <path-spec> ...)
                                 ; Each files that contains
                                 ;  a sequnce of core R6RS
                                 ;  statements

<require> = <path-spec>          ; A path to a module "file"
          | <identifier>         ; A pre-defined module 

<provide> = <identifier>         ; Possibly defined as syntax
          | (all-from <require>) ; Re-export imports using
                                 ;  local names

For _X_ as either <require> or <provide>:

 <_X_-spec> = _X_                ; Everything from _X_
            | (only <_X_-spec> <identifier> ...)
                                 ; Restrict to specific ids
            | (except <_X_-spec> <identifier> ...)
                                 ; Remove specific ids
            | (add-prefix <_X_-spec> <identifier>)
                                 ; Add a uniform prefix
            | (drop-prefix <_X_-spec> <identifier>)
                                 ; Remove a uniform prefix
            | (rename <_X_-spec> (<identifier> <identifier>) ...)
                                 ; Rename
            | (alias <_X_-spec> (<identifier> <identifier>) ...)
                                 ; Add additional name

<path-spec> = <string>           ; A Unix-style relative path
            | (file <string>)    ; An OS-specific absolute path
            | (lib <string> ...) ; A path in a designated
                                 ;  library area, much like
                                 ;  CLASSPATH