I think I've asked a question like this in the past, but I still haven't
found a good solution. I'm going to give a concrete example:
Let's say I have a `date` struct like this:
(struct date (y m d))
And I'm perfectly happy with the default match expander:
(match d
[(date y m d) ...])
But I want a custom constructor:
(define (date* y [m 1] [d 1])
(date y m d))
And, of course, I want to provide the custom constructor as `date`, not
`date*`:
(define make-date
(procedure-rename
(λ (y [m 1] [d 1])
(date y m d))
'date))
(define-match-expander $date
(syntax-rules ()
[(_ y m d) (date y m d)])
(λ (stx)
(syntax-case stx ()
[(_ xs ...) #'(make-date xs ...)]
[_ #'make-date])))
(provide (rename-out [$date date]))
But I also want to provide the constructor with a contract:
(provide/contract
[rename
$date date
(->i ([year exact-integer?])
([month (integer-in 1 12)]
[day (year month) (day-of-month/c year month)])
[d date?])])
Unfortunately, although this provides `date` as a constructor function, it
does not allow it to be used as a match expander.
What's the best way of getting both?
I think it might work if I defined `make-date` above with `define/contract`
(thus narrowing the contract boundary) and then doing the normal `(provide
(rename-out ...))`, as in the second-to-last example, above.
But is there a painless way to accomplish this without narrowing the
contract boundary?
-Jon
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