Re: MzScheme for Embedding

[Matthias Felleisen <matthias@cs.rice.edu>]

  The biggest problem is that units can't export macros.  I understand
  the rationale for this, but that choice does mean that units don't
  meet a pretty widespread need.  I don't currently understand how
  macros are scoped in MzScheme.  It looks like they have a totally
  separate, kind of ad-hoc mechanism for dealing with that.

Let me calrify the rationale behind this, just so that we're on the same
page. 

Units (aka modules) must satisfy three criteria for us: 

 1. they must be compiled (analyzed) separately. 
 2. they must be first-class. 
 3. they must be dynamically loadable. 

So they are full-fledged values. 

Macros on the other hand are things that live at compile-time. They don't
have a value. It makes no sense to speak of passing "begin" to a function 
f as in
  
  (f begin)

This means macros must be elaborated the very moment we compile a module.
Macros can only be elaborated if we know their rewriting function (let's
not argue how it is expressed, via ` and , or via pattern-matching). That
implies that while a module's imports and exports can only be determined at
run-time, a macro must be imported/exported at compile time.  Furthermore,
because modules are first-class, we can only determine at run-time which
module exchange things via import-export. All of this says: 

 macro export and import can't be related to value export and import. 

[Anyone who claims anything else weakens one of the assumptions.]

;; --- 

Our systematic conclusion is that we separate out the mechanism for
importing values into a module from the mechanism for importing macros. 
We call an imported collection of macros a <language> and use unit/lang 
to specify modules whose macros are imported: 

 (unit/lang <language> <value-imports> <value-exports> <definitions>)

A language is specified as a pair: a collection of traditional macros and a
run-time system, which is a set of bindings for the free variables
generated by macros. At the moment we use a collection path for putting
languages in. Using this pair, this expression is then elaborated into a
classical unit.

The new unit system isn't finished yet and won't come with 102, because the
person who implements this system, well, isn't finished for all kinds of
reasons ... 

-- Matthias