obj.magic for polymorphic record fields

obj.magic for polymorphic record fields

[Jacques Le Normand]

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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?


Jacques

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Re: [Caml-list] obj.magic for polymorphic record fields

[Nicolas Pouillard]

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