For example, how can I write a version of map
that will work with polymorphic functions in Typed Racket? I use a simple id
function defined as:
(: id : (All (A) A -> A))
(define (id x) x)
When I try to map it over a list i get an error:
> (map id '(1 2 3))
Type Checker: Polymorphic function `map' could not be applied to arguments:
Types: (-> a b ... b c) (Listof a) (Listof b) ... b -> (Listof c)
(-> a c) (Pairof a (Listof a)) -> (Pairof c (Listof c))
Arguments: (All (A) (-> A A)) (List One Positive-Byte Positive-Byte)
Expected result: AnyValues
in: (map id (quote (1 2 3)))
You have to manually instantiate the polymorphism in this case:
-> (map (inst identity Integer) '(1 2 3))
- : (Listof Integer) [more precisely: (Pairof Integer (Listof Integer))]
'(1 2 3)
The reason is explained in the Typed Racket Guide here:
Typed Racket’s local type inference algorithm is currently not able to infer types for polymorphic functions that are used on higher-order arguments that are themselves polymorphic.
(see docs for more explanation and examples)
来源:https://stackoverflow.com/questions/26212222/how-do-i-write-higher-order-functions-that-take-polymorphic-functions-as-argumen