| > class C a b | a -> b where
| > foo :: a -> b
| > foo = error "Yo dawg."
| >
| > instance C a b where
|
| The instance 'C a b' blatantly violates functional dependency and
| should not have been accepted. The fact that it was is a known bug in
| GHC. The bug keeps getting mentioned on Haskell mailing lists
| about every year. Alas, it is still not fixed. Here is one of the
| earlier messages about it:
|
| http://www.haskell.org/pipermail/haskell-cafe/2007-March/023916.html
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.
It's just the same for
instance C a b
It is rejected, with the same message, unless you add -XUndecidableInstances.
Do you think the two are different? Do you argue for unconditional rejection
of everything not satisfying the Coverage Condition, regardless of flags?
Simon
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