The docs
(http://docs.racket-lang.org/syntax/stxparse-patterns.html#%28def._%28%28lib._syntax%2Fparse..rkt%29._pattern-expander%29%29)
give an example, ~maybe, that doesn't actually work:
> (define-syntax ~maybe
(pattern-expander
(syntax-rules ()
[(~maybe pat ...)
(~optional (~seq pat ...))])))
> (define-syntax-class blah
(pattern (hi (~maybe two three) ...)))
. ~optional: cannot use identifier tainted by macro transformation in: ~optional
I noticed this because I wanted to do something analogous, which fails with the
same error (identifier tainted by macro transformation):
(define-syntax ~bdatum
(pattern-expander
(syntax-rules ()
[(~bdatum id)
(~and id (~datum id))]
[(~bdatum bid id)
(~and bid (~datum id))])))
Any insight into either correct usage of pattern-expander, or a more efficient
shorthand to achieve the same result I was aiming for with ~bdatum (being able
to match with ~datum while still having a pattern variable that preserves
source information).
Thanks!
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