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From: Goswin von Brederlow <goswin-v-b@web.de>
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Subject: Re: [Caml-list] implicit subtyping fails with recursive classes

On Sat, Jul 20, 2013 at 08:06:43PM +0100, Leo White wrote:
> 
> I am not sure, but I think the problem is that you are using #foo before
> it is fully defined, so it is becoming unified with the universal 'a and
> then when #foo is generalised the universal is escaping its scope.
> 
> One way to avoid the problem is to expand out the type of #foo:
> 
>   # class type t = object method push : 'b. (< foo : t -> unit; ..> as 'b) -> unit end;;
>   class type t = object method push : < foo : t -> unit; .. > -> unit end
>   # class type foo = object method foo : t -> unit end;;
>   class type foo = object method foo : t -> unit end
> 
> if foo has a lot of methods, and you want to avoid typing them out

Yeah, the real code has rather a few more methods (9 occurances of t
in foo) so expanding out expands a lot. And since I'm still developing
the classes still change and then the change has to be copied to each
expanded type again. That is realy painfull.

> multiple times, you could try:
> 
>   # class type ['a] foo' = object method foo : 'a -> unit end;;
>   class type ['a] foo' = object method foo : 'a -> unit end
>   # class type t = object method push : 'b. (t #foo' as 'b) -> unit end;;
>   class type t = object method push : t #foo' -> unit end
>   # class type foo = [t] foo';;
>   class type foo = [t] foo'
> 
> Regards,
> 
> Leo

I came up with that idea independently, too. But I parameterized t
instead of foo. This causes and Assert_failure in the toplevel:

http://caml.inria.fr/mantis/view.php?id=6083

But the way you suggest with ['a] foo', t and foo works. Luckily so far I
only need subtyping in t.

The other idea to circumvent his I had was using recursive modules but
I haven't tried that.

MfG
	Goswin