Checking for fallbacks in generic contracts
Sage Gerard
Playing with this program. Its output is in the comments.
; /tmp/gen.rkt:9:10: assertion violation
; expected: a procedure
; given: #f
; in: method m
; (I/c (m (-> I? 1)))
; [...removed the rest of the error...]
; 1
(module gen-contracts racket/base
(require racket/contract racket/exn racket/generic)
(define-generics I [m I] #:fallbacks [(define (m _) 1)])
(struct noop () #:methods gen:I [])
(struct impl () #:methods gen:I [(define (m _) 1)])
(define (try ctor)
(displayln
(with-handlers ([exn:fail:contract? exn->string])
(m (invariant-assertion (I/c [m (-> I? 1)])
(ctor))))))
(try noop)
(try impl))
I expected the output to be just two "1" lines, because I assumed the contract combinator from `define-generics` would consider fallback implementations. I do think it makes sense for `I/c` to fail to apply a contract to a method in `noop` because `noop` doesn't implement `m`. But I think the combinator reuses some logic from whatever is making `#:defined-predicate` work, since it represents missing methods as `#f`, and doesn't look at the fallbacks.
But... I wanted a contract that's okay with a
missing method if there's a fallback. The only contracts
I can write with the `I/c`
combinator seem require me to either A) write repetitive method
definitions that just parrot the fallback, or B) write its method
contracts like `(I/c [m (or/c #f (-> I?
1))])`, which feels icky.
Apologies if I'm totally off the mark or just missed something in
the manual, but is there a way to define a contract for a generics
implementation that won't complain if it sees a fallback for a
missing method?
-- ~slg
Sage Gerard
For the time being I've decided to use this macro and attach
contracts to the fallbacks a different way. Please let me know if
there's a better solution.
(define-syntax-rule (I/weak/c [method-id
method-contract] ...)
(I/c [method-id (or/c #f method-contract)] ...)
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-- ~slg