[mercury-users] Subtyping for Mercury?

[Tyson Richard DOWD]

> 
> > Anything which accepts a foo as a class should also accept a bar (but
> > not vice versa).  Moreover, there should be a way of narrowing the type
> > of a bar object in order to access the hidden foo methods.
> 
> This is achieved by using a `context' in a predicate --- ie. by placing 
> constraints upon type variables eg:
> 
>     % T must be in the class `foo'. `foo' methods can be used upon things of
>     % type T.
> :- pred baz(T::in, int::out) is det.
> 
>     % T must be in the class `bar'. `bar' methods can be used upon things of
>     % type T, as can `foo' (because of the class hierarchy).
> :- pred kabam(T::in, int::out) is semidet.
> 

Did I miss something? Where is the context? (I realize this is just
syntax, but the syntax appears to be missing).

-- 
       Tyson Dowd           #
                            #             Sign on refrigerator:
     trd at cs.mu.oz.au        #           Refrigerate after opening. 
http://www.cs.mu.oz.au/~trd #                  - C. J. Owen.