On Fri, Sep 24, 1999 at 10:15:52AM +0200, Peter Schrammel wrote:
> I wrote a Program:
> type 'i tree =
>     Empty
>   | Item of 'i
>   | Section of 'i * 'i
> 
> class ['i] ctree =
>   object (self : 'a)
>     val mutable content : 'i tree = Empty
> 
>     method get = content
>         
> end
> 
> class debug =
>   object
>     method debug = ()
>   end
> 
> class dtree =
>   object
>     inherit [#debug] ctree
>   end
> 
> But the compiler gives me the error:
> 
> Some type variables are unbound in this type:
>   class dtree :
>     object val mutable content : (#debug as 'a) tree method get : 'a
> tree end
> The method get has type #debug tree where .. is unbound
> make: *** [test.cmo] Error 2   
> 
> How can make trees of a certain (sub)class ? (I hope there's a simple
> solution)

Depending on your needs, you can either write
    class dtree =
      object
        inherit [debug] ctree
    end
or
    class ['a] dtree =
      object
        constraint 'a = #debug
        inherit ['a] ctree
    end
In the first case, the type of the contained object is fixed. In the
second case it is only required to be an instance of type
   #debug = < debug : unit; .. >
So, if you write a subclass debug2 of class debug, an object of type
"debug2 dtree" will contain objects of type "debug2".

Regards,

-- Jérôme