[Caml-list] False polymorphic

["Laurent Chéno" <laurent.cheno@noos.fr>]

(please excuse my poor english)

David said well :

> # let id x = x;;
> val id : 'a -> 'a = <fun>
>
> This expression is fully polymorphic.
>
> # let id2 = id id;;
> val id2 : '_a -> '_a = <fun>
>
> This one is not (notice the underscore (_) before the type variable
> a). It is a monomorphic type that is right know unknown but will be
> determined at first instantiation.

and you should read indeed

> For further information on this subject, have a look at the FAQ:
> http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#variables_de_types_faibles
>


I just add this point :

> #let id x = x ;;
> id : 'a -> 'a = <fun>
> #let id3 x = (id id) x ;;
> id3 : 'a -> 'a = <fun>
> #id3 ;;
> - : 'a -> 'a = <fun>
> #id3 3 ;;
> - : int = 3
> #id3 "abc" ;;
> - : string = "abc"
> #
>

You can retain that this '_a problem can be avoided if you explicitly 
give all expected args.

Best regards,

Laurent
(who never learned english)


--
Laurent Chéno
Prof. de mathématiques en PSI*
et de l'option informatique en MP/MP*
Lycée Louis-le-Grand, Paris

http://pauillac.inria.fr/~cheno/
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