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From: =?iso-8859-1?Q?Laurent_Ch=E9no?= <laurent.cheno@noos.fr>
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Subject: [Caml-list] False polymorphic
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(please excuse my poor english)

David said well :

> # let id x =3D x;;
> val id : 'a -> 'a =3D <fun>
>
> This expression is fully polymorphic.
>
> # let id2 =3D id id;;
> val id2 : '_a -> '_a =3D <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 =3D x ;;
> id : 'a -> 'a =3D <fun>
> #let id3 x =3D (id id) x ;;
> id3 : 'a -> 'a =3D <fun>
> #id3 ;;
> - : 'a -> 'a =3D <fun>
> #id3 3 ;;
> - : int =3D 3
> #id3 "abc" ;;
> - : string =3D "abc"
> #
>

You can retain that this '_a problem can be avoided if you explicitly=20
give all expected args.

Best regards,

Laurent
(who never learned english)


--
Laurent Ch=E9no
Prof. de math=E9matiques en PSI*
et de l'option informatique en MP/MP*
Lyc=E9e Louis-le-Grand, Paris

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