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

[Raphael Proust <raphlalou@gmail.com>]

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

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.

-- 
______________
Raphaël Proust