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quoted data in macro expansion

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/