Objects and variants

Objects and variants

[Samuel Mimram]

Hi,

When I try to compile the following code:

class a =
object
  method f (x:[>]) = ()
end

I get the following error:

Error: Some type variables are unbound in this type:
         class a : object method f : [>  ] -> unit end
       The method f has type ([>  ] as 'a) -> unit where 'a is unbound

Is the binding of [>] to 'a really necessary here? In particular it is
not mentioned in the inferred type...

Of course I can parametrize the class over 'a as

class ['a] a =
object
  method f (x:[>] as 'a) = ()
end

which now has type

class ['a] a : object constraint 'a = [>  ] method f : 'a -> unit end

But this seems rather cumbersome. Is there a way to handle this without
mentioning 'a?

Thanks!

Cheers,

Samuel.

Re: [Caml-list] Objects and variants

[Peng Zang]

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You're looking for a polymorphic method:

class a = object
  method f : 'a. ([>] as 'a) -> unit = fun _ -> ()
end

Peng

On Friday 06 March 2009 10:57:43 am Samuel Mimram wrote:
> Hi,
>
> When I try to compile the following code:
>
> class a =
> object
>   method f (x:[>]) = ()
> end
>
> I get the following error:
>
> Error: Some type variables are unbound in this type:
>          class a : object method f : [>  ] -> unit end
>        The method f has type ([>  ] as 'a) -> unit where 'a is unbound
>
> Is the binding of [>] to 'a really necessary here? In particular it is
> not mentioned in the inferred type...
>
> Of course I can parametrize the class over 'a as
>
> class ['a] a =
> object
>   method f (x:[>] as 'a) = ()
> end
>
> which now has type
>
> class ['a] a : object constraint 'a = [>  ] method f : 'a -> unit end
>
> But this seems rather cumbersome. Is there a way to handle this without
> mentioning 'a?
>
> Thanks!
>
> Cheers,
>
> Samuel.
>
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