[r6rs-discuss] Proposed features for small Scheme, part 9: modules

[Peter Bex]

On Mon, Oct 12, 2009 at 12:23:33AM -0400, John Cowan wrote:
> A small Scheme module takes the general form:
> 
> 	(module module-name
> 	  [(export name ...)]
> 	  [(import importspec ...)]
> 	  [(feature-groups name ...)]
> 	  form ...)

[snip]

> The feature-groups list specifies the feature groups which this
> implementation must provide in order for the module to work.  This is
> an extension to R6RS, which does not have feature groups.  If the
> implementation cannot load the feature groups dynamically, it must
> fail to define the module.  (Why aren't feature groups just modules?
> Because they don't define separate namespaces, and because they mostly
> reflect the limitations of particular small Scheme systems rather than
> components that can be loaded into them.)

What added benefit do feature groups have?  If the module cannot be
defined because a feature is not implemented, what happens?  Is that
an error?  I'm asking because I suppose it would also be an error to
define a module containing references to undefined identifiers.

In other words, if a module requires the feature group "regular
expressions" (just an example), the implementation would give an error
saying regular expressions are not supported.  However, if there were
no "feature-groups", wouldn't the module declaration give an error
saying something like "undefined identifier: regex-match"?  In essence,
feature groups don't seem to add anything here.

Regards,
Peter
-- 
http://sjamaan.ath.cx
--
"The process of preparing programs for a digital computer
 is especially attractive, not only because it can be economically
 and scientifically rewarding, but also because it can be an aesthetic
 experience much like composing poetry or music."
							-- Donald Knuth
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