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

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

[Christoph Höger]

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

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

[Jeremie Dimino]

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On Thu, Aug 27, 2015 at 10:59 AM, Christoph Höger <
christoph.hoeger@tu-berlin.de> wrote:

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


​In a type annotation, the scope of variables is only the type annotation.
So in your example the ['a] in the annotation of [a_to_opt] and the ['a] in
the annotation of [None] are distinct. To refer to the same type, you need
to use a local abstract type:

    let rec a_to_opt : type a. (a -> a option) = fun x -> (None : a
​ option​
)​​

-- 
Jeremie

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