On 12/14/2010 09:30 PM, Alexander Bernauer wrote:
The code
---8<---
let foo ():string =
let f: ('a -> string) -> 'a -> string = fun g v -> g v
in let h = string_of_int
in let i = string_of_float
in let x = f h 23
in let y = f i 23.0
in x ^ y
--->8---
leads to the following error message
---8<---
File "test.ml", line 6, characters 14-15:
Error: This expression has type float -> string
but an expression was expected of type int -> string
--->8---
The scope for type variables is implicitly defined as the smallest
surrounding structure item. In your case, the scope for 'a is the "let
foo..." declaration, not the local declaration "let f".
There are two solution to do what you want in OCaml 3.12.
* Use the syntax introduced for polymorphic recursion:
let f: 'a. ('a -> string) -> 'a -> string = fun g v -> g v
* Create a locally abstract type:
let f (type a) (g : a -> string) (v : a) : string = g v
-- Alain
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