[Caml-list] still puzzled on generic types

[Walter Cazzola <cazzola@dico.unimi.it>]

Dear all,
waiting for some hints on my other issue for functors and generic I was
playing with an enumerate function (with tail recursion):

   let rec enumerate ?(l':((int * 'a) list)=[]) ?(n=0) l =
     match l with
          h::l1 -> enumerate (l'@[(n,h)]) (n+1) l1
     |  [] -> l'

this should just take a list and return a list of couple whose first
entry is the position in the list of the element; to be clearer:

   enumerate ['a'; 'b'; 'c'] -> [(0,'a'); (1,'b'); (2,'c')];

but this doesn't work as by type mismatch:

  Error: This expression has type (int * 'a) list
         but an expression was expected of type 'a list

Doesn't a matter if I've annotated the type (int * 'a) list
to the optional argument l' when I try to call the function with the new
value it says that the two things don't have the same type.

I'm quite puzzled by this behavior, any explanation/help is welcome as
usual.

Walter
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