On Wed, Jun 15, 2011 at 3:25 AM, Simon Peyton-Jones
<[email protected]> wrote:
> Wait. What about
> instance C [a] [b]
> ? Should that be accepted? The Coverage Condition says "no", and indeed it
> is rejected. But if you add -XUndecidableInstances it is accepted.
This 'clearly' violates the functional dependency as well.
However, I must admit, it surprises me that GHC or Hugs ever detected
this, and I imagine there's no general way to detect 'acceptable'
instances.
> Do you think the two are different? Do you argue for unconditional rejection
> of everything not satisfying the Coverage Condition, regardless of flags?
One obvious difference from the instances that appear (depending on
how smart you're pretending to be as a compiler) bad but are
nevertheless okay is that these have no contexts. If you can detect
that, then:
instance C a b
instance C [a] [b]
clearly have multiple independent instantiations on both sides, and so
the relation is clearly non-functional. A simple heuristic might be to
reject those, but allow:
instance (..., D .. b .., ...) => C a b
trusting that the context determines b in the right way. Is this
possibly what GHC used to do? Of course, that allows 'Show b => C a b'
so it's pretty weak.
A slightly more intelligent heuristic might be to see if the fundeps
in the context determine b, but that sounds like it might be leaving
the realm of what's checkable.
-- Dan
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