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Date: Fri, 19 Dec 2014 14:30:39 +0100
From: Maxence Guesdon <Maxence.Guesdon@inria.fr>
To: "caml-list@inria.fr" <caml-list@inria.fr>
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Subject: [Caml-list] Function not polymorphic enough

Hello,

With the following code:

let (fn, extend) =
  let module M = struct
    type t = { mutable f : 'a. ('a -> unit) -> 'a list -> unit }
    let v = 
       { f = fun (type a) -> fun (poly_a : a -> unit) -> fun _ ->
         assert false 
       }
  end in
  let fn = fun f x -> M.v.M.f x in
  let extend g = M.v.M.f <- g M.v.M.f in
  (fn, extend)

I get the following message about " g M.v.M.f":
Error: This field value has type ('b -> unit) -> 'b list -> unit
       which is less general than 'a. ('a -> unit) -> 'a list -> unit

Does anyone know how to make "g M.v.M.f" general enough ?

Regards,

Maxence