Re: [Caml-list] Recursive Parametric class type Typing

[Guillaume Yziquel <guillaume.yziquel@citycable.ch>]

Le Saturday 19 Mar 2011 à 20:20:36 (+0100), Raphael Proust a écrit :
> Le samedi 19 mars 2011 18:48:03, Guillaume Yziquel a écrit :
> > Le Saturday 19 Mar 2011 à 16:48:40 (+0100), Raphael Proust a écrit :
> > > Hi list,
> > > 
> > > Trying to bind a javascript library for js_of_ocaml, I encountered the
> > > following pattern (here drastically simplified):
> > > 
> > > class type ['t] c =
> > >   object
> > >     method plus: 't -> unit
> > >     method minus: unit -> 't
> > >     method container: unit -> container
> > >   end
> > > and container =
> > >   object
> > >     method int: unit -> int c
> > >     method string: unit -> string c
> > >   end
> > > 
> > > The following error is raised at compile time:
> > > Error: This type string should be an instance of type int
> > > 
> > > In the use case:
> > > - instead of [int] and [string] there are six different [class type]s;
> > > - [classe type]s have more methods; and
> > > - there are additional recursive [class type]s
> > > 
> > > Why is there such a limitation on the types? Is there a work around that
> > > wouldn't induce (too  much) code duplication?
> > 
> > The reason why there is this behaviour is unification within the 'and'
> > declaration. method int says that 't in ['t] c is of type int,
> > therefore it should also be an int c in method string instead of a
> > string c.
> 
> It makes sens…
> 
> > 
> > Workaround: Use recursive modules. They are a solution for keeping
> > recursion while breaking type unification. Something in the taste of:
> > 
> > yziquel@seldon:~$ ocaml
> >         Objective Caml version 3.12.0
> > 
> > # module rec Q : sig
> >     class type ['w] e = object method r : 'w end
> >   end = struct
> >     class type t = object
> >       method z : int Q.e
> >       method u : string Q.e
> >     end
> >     class type ['w] e = object method r : 'w end
> >   end;;
> > module rec Q : sig class type ['a] e = object method r : 'a end end
> > 
> > Not sure if this fits your bill when it comes to avoiding code
> > duplication.
> 
> Because I want both [t] and [e] available (in fact there are 4 recursive classes
> I want available), I made several recursive modules. Beside having each type
> written twice, it still had a typing error:
> 
> Error: In the definition of Paper.paper, type
>        Svg.circle_attr Elem.element
>        should be
>        'a Elem.element

Noticed it too. Do not know where that comes from.

> I found a (half satisfying) workaround using the fact that methods that makes
> parametrization necessary and methods that makes recursivity necessary do not
> intersect. So I ended up with a parametric class that is not part of the
> recursion. The "polymorphic recursion" of sort is broken but I need a different
> class for each type I want to instantiate the type with.

A perhaps more natural way would be to use object types. Something like

type t = private < z : int e; u : string e >

is easier to declare, and easier to declare recursively. Drawbacks: you
still have issues when you want to declare the meat of your classes; and
doesn't work well with ocamldoc.

-- 
     Guillaume Yziquel