Subtyping for Mercury?

[Ralph Becket]

Hi,

has anybody considered augmenting the Mercury type scheme with a
subtyping relation a la Modula 3?  This would be very handy for
implementing OO support for Mercury.

Let's say I have a class foo with attributes foo_a_1 to foo_a_3 and
methods foo_m_1 to foo_m_4 (sorry about the boring names).

Now I want a subclass bar which inherits all of the above, but also
adds attributes bar_a_1 to bar_a_2 and methods bar_m_1 to bar_m3 and
overrides foo_m_1 with bar_m_4.

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.

Maybe I'm missing something, but it's not obvious to me how I could
implement such a scheme in Mercury without having to also add my own
runtime type checking (which would (a) be a shame and (b) not be in the
Mercury style).  If Mercury had subtyping and ADTs (i.e. classes) there
would, of course, be no problem!

Any suggestions?

Ralph