overlapping instances and constraints

[Taral]

On 2/28/06, Claus Reinke <claus.reinke at talk21.com> wrote:
> I'm not sure what you're talking about? both hugs and ghc clearly
> specify that the more specific instance declaration is chosen when
> overlapping instance declarations are permitted.

I have trouble with this "more specific" term. It's not well-defined,
i.e. not a total order on possible instance declarations.

The point I was trying to make was that in the case where a
precondition (in this case, Fail a) does not apply, what stops the
resolution algorithm falling back on the other instance declaration?

--
Taral <taralx at gmail.com>
"Computer science is no more about computers than astronomy is about
telescopes."
    -- Edsger Dijkstra