Dan,
class C a where
foo :: a -> String
instance C a where
foo _ = "universal"
instance C Int where
foo _ = "Int"
[...]
Now, IncoherentInstances is something most people would suggest you
don't use
(even people who are cool with OverlappingInstances). However, it
turns out
that ExistentialQuantification does the same thing, because we can
write:
data Ex = forall a. Ex a
baz :: a -> String
baz x = case Ex x of
Ex x' -> foo x'
and this is accepted, and always yields "universal", just like bar.
So, things
that you get out of an existential are allowed to make use of the
general
versions of overlapping instances if any fit.
I don't think it's the same thing. The whole point of the existential
is that at the creation site of any value of type Ex the type of the
value being packaged is hidden. At the use site, therefore, the only
suitable instance is the one for C a. In particular, there is no way
for the baz to tell whether an Int is wrapped in the existential.
However, if we pack a dictionary along, as in
data Ex = forall a. C a => Ex a
then, you'll find that baz will pick the dictionary supplied with the
existential package rather than the one from the general instance.
Cheers,
Stefan
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