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Re: Update to SRFI 72 (Sorry I'm lost)



> From: Andre van Tonder <andre@xxxxxxxxxxxxxxxxx>
> 
> An update has been posted to the SRFI-72 draft.
> 
> * Added section on reflective tower:
> 
>    "The proposed macro system has an infinite reflective tower. In
>    contrast with comparable macro systems, the hygiene algorithm
>    implements a refinement of lexical scoping that takes into
>    account the level of use of an identifier in determining its
>    meaning."

Although the above quotation occurs three times almost verbatim,
in the abstract, the introduction, and the specification, it does
not get any clearer with repetition.  As Brian Smith used the term
"reflective tower" it referred to an interpreter for a language
that could express modifications to its own interpreter.  I don't
think that's what you are after, is it?

The second mention of the "infinite reflective tower" is followed
by a reference [11] to Richard Kelsey and Jonathan Rees
 - The Scheme 48 implementation  http://s48.org/
but a couple hours searching the Scheme 48 manual at that
web site turned up no mention of reflective towers, and not much
about macro expansion.

Perhaps a better reference would be
<a href="http://www2.parc.com/csl/groups/sda/projects/reflection96/abstracts/queinnec.html";>
Queinnec - Macroexpansion Reflective Tower</a>
but I don't know whether the reference would say "our proposed system
is just like the one described by Queinecc, but with |syntax| instead
of |quote|" or whether it would say "the work of Queinecc is totally
obsolete, we have improved it in the following twelve ways...".

A couple of examples of odd scope rules just isn't enough for me
to figure out the plan.  This seems to be a very big change from
the previous proposal, and not like anything that I understand well.

> * Added BEGIN-FOR-SYNTAX.

I understood this when it was contrasted with |begin-for-import|,
but now I am lost.  What does it mean to evaluate "at one reflective
level higher"?

Do all macros that expand to macro definitions now have to appear
inside two nested |begin-for-syntax|?

> * Added SYNTACTIC-WIND.

Oy.

> * Removed EXPAND.

Why?  ...and if it is gone, why is this code in the section
on the reflective tower?

     (for-each (lambda (form) (eval (expand form))) sequence)
       == (for-each eval (map-in-order expand sequence))

The phrase: 
> ... the hygiene algorithm of this SRFI implements a refinement of
> lexical scoping that takes into account the level of use of an
> identifier in determining its meaning.

might be better worded as: the algorithm of this SRFI totally ignores
lexical scoping by making the meaning of an identifier depend upon
an invisible property called its "level" rather than its position
in the source code. ...but I have already stated my objections
when you first brought up this idea.

I thought the contribution of srfi-72 would be to standardize
some conventions for writing procedural macros while preserving
hygiene.  I am not sure whether this "infinite reflective tower"
is a minor variation, an incompatible new plan, or perhaps a
refinement that could be put into a different srfi leaving
srfi-72 alone.  In other words, would the old srfi be compatible
with the reflective tower, except that it would give errors
(identifer used outside its scope of definition) in some cases
where the tower would find and use a binding occurrence that
would appear to be shadowed?

-- 
     -- Keith Wright

Programmer in Chief, Free Computer Shop
 ---  Food, Shelter, Source code.  ---