Sam Tobin-Hochstadt <samth@...> writes:
> Can you say more about the problem here? The type of `assoc` mentions
> pairs, but you should be able to use it with lists, since they're made
> of pairs.
Yeah, that's what I had expected as well. This code produces type errors (in
5.3.3, too):
;; Definitions
(define: mystringlistlist : (Listof (Listof String)) (list (list "a"
"b") (list "c" "d")))
(define: mystringpairlist : (Listof (Pairof String String)) (list (cons
"a" "b") (cons "c" "d")))
; Works!
(assoc "a" mystringpairlist)
=> '("a" . "b")
; Type error
(assoc "a" mystringlistlist)
Type Checker: Polymorphic function assoc could not be applied to arguments:
Argument 1:
Expected: a
Given: String
Argument 2:
Expected: (Listof (Pairof a b))
Given: (Listof (Listof String))
in: (assoc "a" mystringlistlist)
I thought that Racket maybe didn't accept "a" as a String, so I defined it
as one and called assoc with that, but the error is exactly the same.
> Here's a version that type checks: https://gist.github.com/samth/5847063
>
> The key is giving a type annotation to `fname` using the #{} syntax.
> Typed Racket can figure out how to typecheck the expansion only if you
> give it that hint. And the #{} syntax is for adding such hints with
> macros like `command-line` that didn't anticipate Typed Racket.
Ah, that works, thank you :)
Did I not see that in the documentation or is this feature "experimental"?
Best Regards,
Tim
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