[Caml-list] why is this definition less general?

["Christoph Höger" <christoph.hoeger@tu-berlin.de>]

Dear all,

I found that I can evade the "first occurence unifies" behavior of
recursive polymorphic functions by using explicit polymorphic types:

utop # let rec a_to_opt : 'a . ('a -> 'a option) = fun x -> None and
int_to_opt (i:int) = a_to_opt i and str_to_opt (s:string)= a_to_opt s ;;

val a_to_opt : 'a -> 'a option = <fun>


               val int_to_opt : int -> int option = <fun>


                              val str_to_opt : bytes -> bytes option = <fun>

When I try to annotate the type in the body of a_to_opt, however, I get
a "less general" error:

utop # let rec a_to_opt : 'a . ('a -> 'a option) = fun x -> (None : 'a
option) and int_to_opt (i:int) = a_to_opt i and str_to_opt (s:string)=
a_to_opt s ;;
Error: This definition has type 'a -> 'a option which is less general
than

                          'a0. 'a0 -> 'a0 option

Obviously, 'a is bound by its first occurrence in the type annotation,
here. Can someone explain this behavior?

-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de