On 2011-08-10 00:17:34 -0400, Harry Spier wrote:
> I get this error:
> syntax-rules: variable used twice in pattern in: unquote
The problem here is that "," is a special character used by the Reader
for unquoting. You can't put it in a macro keyword list because the
reader tries to read ",)" as unquoting ")".
You *can* put "unquote" in the keyword list though.
(define-syntax array-ref
(syntax-rules (unquote)
[(_ v [d1]) (vector-ref v d1)]
[(_ v [d1 , d2 ])
(array-ref (array-ref v [d1]) [ d2 ])]
[(_ v [d1 , d2 , d3])
(array-ref (array-ref (array-ref v [d1]) [d2]) [d3])]))
However, this macro is a bit dubious because it's abusing unquote. It'd
be better to avoid using "," as a keyword in your macro since it's
confusing to people who are used to reading Racket code.
Cheers,
Asumu
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