When using syntax/parse, is there a good way to do something like this?
(define-splicing-syntax-class options
(pattern (~seq (~or (~optional (~seq (~and #:a a?)))
(~optional (~seq (~and #:b b?)))
(~optional (~seq (~and #:c c?)))
(~optional (~seq (~and #:d d?))))
...)
#:attr a/b #'(a? b?)
#:attr c/d #'(c? d?)))
When using the above syntax class to parse #'(#:a #:d #:b), then the a/b
attribute should be #'(#:a #:b) and the c/d attribute should be #'(#:d).
However, this doesn't work, of course, because if one of the options isn't
defined, the attribute will be #f, and the attribute binding will fail.
I can get around that by doing something like this:
(define-splicing-syntax-class options
(pattern (~seq (~or (~optional (~seq (~and #:a a?)))
(~optional (~seq (~and #:b b?)))
(~optional (~seq (~and #:c c?)))
(~optional (~seq (~and #:d d?))))
...)
#:attr a/b #`(#,@(if (attribute a?) #'(a?) #'())
#,@(if (attribute b?) #'(b?) #'()))
#:attr c/d #`(#,@(if (attribute c?) #'(c?) #'())
#,@(if (attribute d?) #'(d?) #'()))))
But that's rather long-winded and verbose. Even better would be a way to group
the clauses within the pattern, something like this:
(define-splicing-syntax-class options
(pattern (~seq (~or (~and (~seq (~optional (~seq (~and #:a a?)))
(~optional (~seq (~and #:b b?))))
a/b)
(~and (~seq (~optional (~seq (~and #:c c?)))
(~optional (~seq (~and #:d d?))))
c/d))
...)))
But that obviously doesn't work, and I'm not sure what it would do even if it
compiled.
Anyway, is there a more concise way to do this? I know it's relatively easy
using `template` from syntax/parse/experimental/template, but this is going
into the Typed Racket code, so we're intentionally avoiding a dependency on
that.
Alexis
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