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From: Ruslan Ledesma Garza <ruslanledesmagarza@gmail.com>
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Date: Sun, 14 Aug 2011 15:46:23 +0200
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To: OCaml List <caml-list@inria.fr>
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Subject: [Caml-list] Type generalization problem

Dear list,

Consider the following OCaml session.

        Objective Caml version 3.12.0

# let f : 'v . 'v -> 'v = fun x -> x;;
val f : 'a -> 'a = <fun>
# let g : 'v . 'v -> 'v = (fun x -> x) (fun x -> x);;
Error: This definition has type 'a -> 'a which is less general than
         'b. 'b -> 'b
# 

Why doesn't OCaml generalize the type 'a -> 'a? According to the typing rules in "Principal type-schemes for functional programs" ( http://portal.acm.org/citation.cfm?id=582176 ), the type 'a -> 'a can be generalized.

Something similar happens in the following OCaml session.

        Objective Caml version 3.12.0

# let f : 'v . 'v -> 'v = fun x -> x;;
val f : 'a -> 'a = <fun>
# let h = f f;;
val h : '_a -> '_a = <fun>
# h 1;;
- : int = 1
# h h;;
Error: This expression has type int -> int
       but an expression was expected of type int
# 

I want the type of h to be \forall 'a . 'a -> 'a.         :'(

Best regards,
Ruslán.