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To: nadji@noos.fr
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] polymorphic method question
In-Reply-To: <3D640AF1.C24688A@noos.fr>
References: <3D640AF1.C24688A@noos.fr>
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Message-Id: <20020822075744N.garrigue@kurims.kyoto-u.ac.jp>
Date: Thu, 22 Aug 2002 07:57:44 +0900
From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
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From: nadji@noos.fr
> I am puzzled by this behaviour, can a guru explain me ?
> I have two classes
[...]
> I want to do a class gentoto :
> class gentoto = object
>   method gen : 'a . 'a -> 'a basic =
>     fun x -> (new toto x :> 'a basic )
> end;;
> But the compiler complains that :
>       Characters 64-98:
>       fun x -> (new toto x :> 'a basic )
>       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> This method has type 'a -> 'a basic which is less general than
>   'b. 'b -> 'b basic
> 
> ... but if I do :
> let newtotoasbasic x = (new toto x :> 'a basic );;
[...]
> The compiler agrees with me ...
> What is wrong with the first declaration ?

There is a problem with the scoping of named type variables in the
language, which makes them incompatible with polymorphic methods.
You can solve this by using anonymous variables:
# class gentoto = object
    method gen : 'a . 'a -> 'a basic =
      fun x -> (new toto x :> _ basic)
  end;;
class gentoto : object method gen : 'a -> 'a basic end

Jacques Garrigue
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