Re: [Caml-list] Polymorphic values in local modules

Re: [Caml-list] Polymorphic values in local modules

[Tiphaine Turpin]

Hans Ole Rafaelsen a écrit :

> Hi,
>
> I'm trying to construct a module that have a function taking
> polymorphic arguments. I want to use this module as a local module in
> different places, where the function implementation is different. The
> module has the signature:
>
> module type Foo_sig =
> sig
>  val foo : 'a -> 'a
> end
>
> let test f =
>  let module Foo : Foo_sig =
>      struct
>    let foo = f
>      end
>  in
>    ()
The problem here is that the test function itself is not typeable
(because the 'a parameter would have to be quantified under an arrow,
which the type system does not directly allows.
It is possible however, provided you wrap the argument inside a
polymorphic record field (or object) as follows:

type t = {f : 'a. 'a -> 'a}

let test f =
 let module Foo : Foo_sig =
     struct
   let foo = f.f
     end
 in
   ()

Tiphaine

Re: [Caml-list] Polymorphic values in local modules

[Alain Frisch]

On 03/02/2010 11:05, Tiphaine Turpin wrote:
> It is possible however, provided you wrap the argument inside a
> polymorphic record field (or object) as follows:
>
> type t = {f : 'a. 'a ->  'a}
>
> let test f =
>   let module Foo : Foo_sig =
>       struct
>     let foo = f.f
>       end
>   in
>     ()

Another option (that will be available in OCaml 3.12) is to build the 
module as a first-class value outside the function "test".

=================================
module type Foo_sig = sig
   val f: 'a -> 'a
end

let test foo =
   let module Foo = (val foo : Foo_sig) in
   (* use the polymorphic function here... *)
   ()

let () =
   let foo_impl = (module struct let f a = a end : Foo_sig) in
   test foo_impl
=================================

The type for test is:

val test : (module Foo_sig) -> unit



Alain