Another macro/module question related to implementing srfis.

[Rob Browning <rlb at cs dot utexas dot edu>]

I'd like to be able to use srfi-0 (cond-expand) to implement as many
of the srfis as possible (when possible and desirable) using common
code and "cond-expanding" the implementation specific bits.  This,
however, is turning out to be more problematic than I initially
suspected[1].

The problem I've run into now is that I'd like to be able to say
things like this (presume that cond-expand is already defined):

  (cond-expand
   (guile 
    (define-module (srfi srfi-8)
      :use-module (ice-9 slib))
    (require 'macro-by-example)
    (require 'values))

   (else))                                ; Nothing

The natural expansion of this (when you're in guile) would be:

  (begin
    (define-module (srfi srfi-8)
      :use-module (ice-9 slib))
    (require (quote macro-by-example))
    (require (quote values)))

But when you try to load this via (use-modules (srfi srfi-8)), you get
an error that I don't fully understand, but I suspect has something to
do with "begin" interfering with the module system:

  $ (cd build/guile && guile)
  guile>   (begin
      (define-module (srfi srfi-8)
        :use-module (ice-9 slib))
      (require (quote macro-by-example))
      (require (quote values)))
  standard input:4:5: In expression (require (quote macro-by-example)):
  standard input:4:5: Unbound variable: require
  ABORT: (misc-error)

  Type "(backtrace)" to get more information.
  guile>

Am I doing something wrong here, or is this just a problem with the
current implementation?

Thanks

[1] One uncomfortable problem is that cond-expand isn't specified to
work in anything other than at the "top-level", so you can't do things
like this:

  (define (foo bar)

    ;; Implement fast helper functions for each implementation...
    (cond-expand
      (guile
        (define (optimized-helper baz) ...))
      (rscheme
        (define (optimized-helper baz) ...))
      ;; RXRS default helper
      (else
        (define (optimized-helper baz) ...)))

      ...)

-- 
Rob Browning <rlb@cs.utexas.edu> PGP=E80E0D04F521A094 532B97F5D64E3930