[mercury-users] Type class usage question

Ralph Becket rafe at cs.mu.OZ.AU
Mon Apr 5 17:18:25 AEST 2004

peter at hawkins.emu.id.au, Monday,  5 April 2004:
> Hi...
> On Mon, Apr 05, 2004 at 02:11:53PM +1000, Ralph Becket wrote:
> > Peter Hawkins, Monday,  5 April 2004:
> > The problem with `instance foo(set(int))' is that somewhere else someone
> > might declare `instance foo(set(T))' making it impossible for the
> > compiler to resolve which foo method calls should be made for set(int).
> Arguably that should only be an error if someone actually makes such a
> declaration...

If I recall correctly, it's because we support dynamic loading of
modules.  Your approach would make it possible to load a module at
runtime that would make ambiguous the semantics for a previously
unambiguous program.

> What's the recommended workaround? Wrap it in a discriminated union
> type?

'Fraid so!  It's usually not a big deal.

-- Ralph
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