[Caml-list] Polymorphic method problem

[Caml-list] Polymorphic method problem

[Christian Szegedy]

Can anyone tell me, why does the following piece of
code fail to compile, and how can I achieve
equivalent effect in the most convenient way?

class a () =
   object(self)
      method f : 'a. ((< f : unit; .. > as 'a) -> unit) = fun other -> 
other#f 
      method g = (self :> a)
   end

Thanks in advance!


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Re: [Caml-list] Polymorphic method problem

[Alex Baretta]

Christian Szegedy wrote:
> Can anyone tell me, why does the following piece of
> code fail to compile, and how can I achieve
> equivalent effect in the most convenient way?
> 
> class a () =
>   object(self)
>      method f : 'a. ((< f : unit; .. > as 'a) -> unit) = fun other -> 
> other#f      method g = (self :> a)
>   end
> 
> Thanks in advance!
> 

Your type annotations are not correct. Omit the ones which are not 
strictly needed.

class a =
object (self)
   method f : 'a.((<f:unit; ..> as 'a) -> unit) > as 'a) -> unit) =
     fun x -> x # f
   method g = self
end

Alex

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Re: [Caml-list] Polymorphic method problem

[Christian Szegedy]

Dear Alex!

I tried to run your code, but it has obvious syntax errors.

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Re: [Caml-list] Polymorphic method problem

[William Lovas]

On Wed, Jul 07, 2004 at 12:00:04AM +0200, Christian Szegedy wrote:
> Dear Alex!
> 
> I tried to run your code, but it has obvious syntax errors.

I suspect a copy/paste error.  Here's the corrected code:

    class a =
    object (self)
      method f : 'a.((<f:unit; ..> as 'a) -> unit) =
          fun x -> x # f
      method g = self
    end

Hope this helps.

cheers,
William

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Re: [Caml-list] Polymorphic method problem

[Olivier Andrieu]

 Christian Szegedy [Tue, 06 Jul 2004]:
 > Can anyone tell me, why does the following piece of
 > code fail to compile, and how can I achieve
 > equivalent effect in the most convenient way?
 >
 > class a () =
 >    object(self)
 >       method f : 'a. ((< f : unit; .. > as 'a) -> unit) = fun other -> 
 > other#f 
 >       method g = (self :> a)
 >    end

Defining a class type beforehand helps:

,----
| # class type a_t = object method f : < f : unit; ..> -> unit method g : a_t end ;;
| class type a_t =
|   object method f : < f : unit; .. > -> unit method g : a_t end
| 
| # class a () : a_t = object (self) 
|     method f : 'a. ((< f : unit; .. > as 'a) -> unit) = fun other ->
|   other#f 
|     method g = (self :> a_t) 
|   end ;;
| class a : unit -> a_t
`----

-- 
   Olivier

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Re: [Caml-list] Polymorphic method problem

[Alex Baretta]

William Lovas wrote:
> On Wed, Jul 07, 2004 at 12:00:04AM +0200, Christian Szegedy wrote:
> 
>>Dear Alex!
>>
>>I tried to run your code, but it has obvious syntax errors.

Indeed! DdR's ledit is a very nice tool to quickly type code into the 
interpreter, but it has the obvious disadvantage of not allowing easy 
copying out of the line buffer.

Sorry for the mistake, and thanks to William for the correction.

> 
> I suspect a copy/paste error.  Here's the corrected code:
> 
>     class a =
>     object (self)
>       method f : 'a.((<f:unit; ..> as 'a) -> unit) =
>           fun x -> x # f
>       method g = self
>     end
> 


Alex

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Re: [Caml-list] Polymorphic method problem

[Jacques GARRIGUE]

From: szegedy@t-online.de (Christian Szegedy)

> Can anyone tell me, why does the following piece of
> code fail to compile, and how can I achieve
> equivalent effect in the most convenient way?
> 
> class a () =
>    object(self)
>       method f : 'a. ((< f : unit; .. > as 'a) -> unit) = fun other -> 
> other#f 
>       method g = (self :> a)
>    end

There was a bug in the subtyping algorithm.
Hopefully, it's now fixed in CVS.

A workaround at the source level is to define your type in advance:

class type ta = object
  method f : <f : unit; ..> -> unit
  method g : ta
end
class a () =
  object(self : #ta)
    method f other = other#f
    method g = (self :> ta)   
  end

Jacques Garrigue

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Re: [Caml-list] Polymorphic method problem

[Christian Szegedy]

> There was a bug in the subtyping algorithm.
> Hopefully, it's now fixed in CVS.

Thanks for your fast reaction!


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