[Caml-list] Re: typing mutually recursive classes

[Christopher Zimmermann <madroach@gmerlin.de>]

On Wed, 24 Oct 2012 20:40:27 +0200
Gabriel Scherer <gabriel.scherer@gmail.com> wrote:

> I don't really understand what you are trying to achieve with this
> #foo types.

It's the simplest statement possible to demonstrate the typing error I 
ran into. Even simpler:

class type a = object end
and b =
  object
    method foo: 'a. (#a as 'a) -> unit
  end

  fails, but

class type a = object end
class type b =
  object
    method foo: 'a. (#a as 'a) -> unit
  end


works fine. Why?

> 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).
That's correct. It should read
  val mutable set = []
  method register :'a. (#element as 'a) -> unit =
    fun s ->
      set <- (s : #element :> element) :: set

> 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.

That's my current workaround for this issue. But I would prefer a 
solution where the coercion happens in the registry.

> 
> 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