Costin Stefan wrote (on 07-08-01 12:41 +0300):
> I have the following problem : want to define lists but using only
> functions :

Sorry, I was not reading very carefully and now recognize that my post was a
complete non-answer.

> So why the type of the result of hd may be the same of the type of the
> result of tl !
> 
> Why the type of the result of a cons isn't a polymorphic function ?

Type variables of the form '_a are not generalized; they can be instantiated
only once. This is a conservative criterion to ML-like languages use to handle
the interaction between polymorphism and imperative side-effects.

You can read about it in the FAQ:

  http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#variables_de_types_faibles

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