Christophe TROESTLER wrote:
> On Sat, 09 Jul 2005, "Matthew O'Connor" <angagon@earthlink.net> wrote:
> 
>>Here is a version of what I would like to be able to do, but can't or
>>don't know how.  I would like to use polymorphic variants to enable
>>easier extensibility of the hierarchy.
> 
> 
> To understand what does not work, try
> 
> class entity  =
> object
>   method is_a v = (v = `Entity)
> end
> 
> You get an error message of which the important part is "The method
> is_a has type ([> `Entity ] as 'a) -> bool where 'a is unbound".  It
> says that the method [is_a] is of type 'a -> bool with a constraint on
> 'a.  Since you do not want a method of type (forall 'a) 'a -> bool
> (written "'a. ([> ] as 'a) -> bool" in your error message), you
> parametrize the class by that type 'a (the rest is in interactive
> mode):

Ahh, thank you for the explanation.

> # class ['a] entity =
> object
>   method is_a (v:'a) = (v = `Entity)
> end;;
>   class ['a] entity :
>   object constraint 'a = [> `Entity ] method is_a : 'a -> bool end

Yes, this is exactly what I was looking for.  Again, thanks to all who
answered.


Matt