quoted data in macro expansion

[Per Bothner <per@xxxxxxxxxxx>]

Am I correct in understanding that any quoted S-expressions, in either a 
macro "parameter" or in a template, is copied?  This is of course needed 
for any quote expression in the template if it spliced in.  However, in 
many (most? all?) existing syntax-case and syntax-rules implementations 
a quoted form in macro parameter that is not "destructured" is used 
verbatim, and likewise for a quoted form in a template if it contains no 
pattern variables.

I don't know if this is a big deal, but it breaks pair identity in cases 
where it is preserved in existing implementations.  This seems to make 
SRFI-72 essentially incompatible with SRFI-38, in the sense that you 
can't use of SRFI-38 shared structure in programs (though it is still 
available for external data).  And of course cycles are prohibited.

A solution might be something like the following: when creating a syntax 
object from a pair or vector, enter both the original pair/vector and 
the corresponding syntax pair/vector in a table. (If the original 
pair/vector exists in the table, re-use the syntax pair/vector.)  Then 
quote and related forms (e.g. syntax->datum) does the inverse mapping.

Of course for this to be efficient, you probably need a (non-portable) 
object hash table, though various tricks are possible.
--
	--Per Bothner
per@xxxxxxxxxxx   http://per.bothner.com/