Excerpts from Jacques Le Normand's message of Sun Dec 20 18:44:57 +0100 2009:
> Dear ocaml-list,
> the following code does not type check:
> 
> type foo = {bar : 'a. 'a -> 'a}
> let a : int -> int = fun x -> x
> let baz = {bar = Obj.magic a}
> 
> with the error
> 
> Error: This field value has type 'a -> 'a which is less general than
>          'b. 'b -> 'b
> 
> 
> my question is: how could I use obj.magic to get a polymorphic type?

Apart from the traditional
  "You should avoid Obj.magic as much as possible!"

I once faced this situation and the solution is to use modules.

module M : sig
  val f : 'a -> 'a
end = struct
  let f = Obj.magic
end ;;
let baz = {bar = M.f };;

Have fun!

-- 
Nicolas Pouillard
http://nicolaspouillard.fr