Re: [Caml-list] explicit polymorphic type annotation

[octachron <octa@polychoron.fr>]

Hi,

In order to pass a polymorphic function to your rank-2 function f, you 
need to encapsulate it
eitheir in a record:

type poly = { f: 'a. 'a -> 'a }
let func = {f = (fun x -> x) }
let f func = func.f 1, func.f 2.

or in an object

let func = object method f:'a.'a->'a = fun x -> x end
let f (func:<f:'a. 'a -> 'a>) = func#f 1, func#f 2.;;

There is a section on polymorphic method in the manual here:
http://caml.inria.fr/pub/docs/manual-ocaml/objectexamples.html#sec34
and explicit polymorphic type for record field are briefly mentioned at 
the end of
http://caml.inria.fr/pub/docs/manual-ocaml/coreexamples.html#sec12.

Regards,
octachron


Le 03/31/16 14:06, William a écrit :
> Hi,
>
> I don't know how to handle this, and can not find how to do it in the 
> manual. I would like "func" to be polymorphic :
>
> let f func =
>   Printf.printf "%i\n%!" (func (1,2));
>   Printf.printf "%f\n%!" (func (1.,2.));;
> f fst;;
> f snd;;
>
> =>
> Characters 85-87:
> Printf.printf "%f\n%!" (func (1.,2.));;
>                               ^^
> Error: This expression has type float but an expression was expected 
> of type int
>
>
> let f (func:('a. 'a * 'a -> 'a)) =
>   Printf.printf "%i\n%!" (func (1,2));
>   Printf.printf "%f\n%!" (func (1.,2.));;
> f fst;;
> f snd;;
>
> =>
> Characters 15-16:
> let f (func:('a. 'a * 'a -> 'a)) =
>                ^
> Error: Syntax error: type expected.
>
>
>
> Could someone show me how to handle this ?
>
> Also, I did not find any relevant chapter in ocaml.org or ocaml 
> reference manual to explain how to do. Any pointers ?
>
> Best regards
>