How do I provide a name as both a match expander and contracted function?

[Jack Firth]

If I'm making my own data types, one common thing I want to do is make smart constructors that double as match expanders. I could use `define-match-expander` with two transformers, but then I can't add a contract to the smart constructor with `contract-out`. If I try to write this:

(provide
 (contract-out
  [foo (->* (widget?) (#:name symbol?) foo?)]))

(define (make-foo widget #:name [name #f])
  ...)

(define-match-expander foo
  (syntax-parser [(_ widget) #'(? foo? (app foo-widget widget))])
  (syntax-parser [(_ arg ...) #'(make-foo arg ...)]))

...then the use of `contract-out` hides the fact that `foo` is a match expander. What should I do?

[Sorawee Porncharoenwase]

IIUC, this is what Alex Knauth did in https://github.com/mbutterick/txexpr/pull/8.

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[Eric Griffis]

Hi Jack,

It can be done with make-rename-transformer.

(define (foo? obj) (equal? obj foo-impl))
(define foo-widget list)

(define/contract (foo-impl _) (-> number? symbol?) 'a-foo)

(define-match-expander foo
  (syntax-parser [(_ widget) #'(? foo? (app foo-widget widget))])

  (make-rename-transformer #'foo-impl))

; (foo #t) ; contract violation

(foo 123)

Eric

On Mon, Dec 30, 2019 at 7:07 PM Jack Firth <jackh...@gmail.com> wrote:

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