Re: [Caml-list] explicit polymorphic type annotation

[Goswin von Brederlow <goswin-v-b@web.de>]

On Thu, Mar 31, 2016 at 02:26:37PM +0200, octachron wrote:
> 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
> >

Or in a GADT.

MfG
	Goswin