Re: [Caml-list] typing mutually recursive classes

[Gabriel Scherer <gabriel.scherer@gmail.com>]

I don't really understand what you are trying to achieve with this
#foo types. What would even be the type of "set" in your example? You
cannot hold a mutable reference to a polymorphic type (#element list),
so I'm not sure what you would have but (element list). If you're
going to coerce your elements into the common (element) supertype
anyway, why insist on having flexible bounds? You could just use
(registry) and (element), coerce when needed (foo :> element), and get
rid of those pesky typing issues.

On Wed, Oct 24, 2012 at 8:03 PM, Christopher Zimmermann
<madroach@gmerlin.de> wrote:
> Hi,
>
> I have a problem with typing a system of mutually recursive classes.
>
> This piece of code fails to compile:
>
> class a =
>   object end
> and b =
>   object
>     method foo: a -> int =
>       fun s -> 3
>   end;;
>
> Error: The universal type variable 'a cannot be generalized:
>        it escapes its scope.
>
>
> But this compiles fine:
>
> class a =
>   object end
> class b =
>   object
>     method foo: 'a. (#a as 'a) -> int =
>       fun s -> 3
>   end;;
>
>
> What I actually want to do is this:
>
> class element id (registry :#registry) =
>   object
>     method registry = registry
>   end
>
> and registry =
>   object
>     val set = []
>     method register :'a. (#element as 'a) -> unit =
>       fun s ->
>         set <- s :: set
>   end
>
>
> Any ideas how to do this without parametrizing the classes?
>
> Christopher
>
>
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