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To: caml-list@inria.fr
Subject: Re: [Caml-list] Polymorphic methods
References: <3D6658D0.F18A453D@inrialpes.fr>
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From: Remi VANICAT <vanicat@labri.u-bordeaux.fr>
Date: Fri, 23 Aug 2002 20:21:54 +0200
In-Reply-To: <3D6658D0.F18A453D@inrialpes.fr> (Frederic Tronel's message of
 "Fri, 23 Aug 2002 17:46:24 +0200")
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Frederic Tronel <Frederic.Tronel@inrialpes.fr> writes:

> Hello list,
>
>
> I've just compiled the new 3.05 version (I will change for 3.06
> tomorrow),
> and start playing a little bit with polymorphic methods. 
> But this small piece of code does not compile:
>
> class node =
> object 
> val mutable t = None
> method set_t: 'a. 'a -> unit = fun nt -> t <- Some nt
> end
>
> File "essai.ml", line 4, characters 31-53:
> This method has type 'a -> unit which is less general than 'b. 'b ->
> unit

you should look to this bug :
http://caml.inria.fr/bin/caml-bugs/fixed?id=1274;page=44;user=guest
(bug which had be fixed). 

To be more precise, the type of t can't be polymorphic, so the type of
set_t can't too.

look to this example :

class node =
object 
  val mutable t = None
  method set_t: 'a. 'a -> unit = fun nt -> t <- Some nt
  method get_t: 'a. unit -> 'a option = fun () -> t
end

the one could do this :

let t : int =
  let a = new node
  a #set_t 1.23
  match a #get_t () with Some x -> x

which is obviously wrong.

you probably want :

class ['a] node =
object 
  val mutable t = None
  method set_t: 'a -> unit = fun nt -> t <- Some nt
  method get_t: unit -> 'a option = fun () -> t
end


-- 
Rémi Vanicat
vanicat@labri.u-bordeaux.fr
http://dept-info.labri.u-bordeaux.fr/~vanicat
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