I ran into a simple falure with functional dependencies (in GHC 7.4.1):
> class Foo a ta | a -> ta
>
> foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool
> foo = (==)
I expected that the `a -> ta` functional dependency would suffice to prove
that `ta ~ tb`, but
Pixie/Bug1.hs:9:7:
Could not deduce (ta ~ tb)
from the context (Foo a ta, Foo a tb, Eq ta)
bound by the type signature for
foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool
at Pixie/Bug1.hs:9:1-10
`ta' is a rigid type variable bound by
the type signature for
foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool
at Pixie/Bug1.hs:9:1
`tb' is a rigid type variable bound by
the type signature for
foo :: (Foo a ta, Foo a tb, Eq ta) => ta -> tb -> Bool
at Pixie/Bug1.hs:9:1
Expected type: ta -> tb -> Bool
Actual type: ta -> ta -> Bool
In the expression: (==)
In an equation for `foo': foo = (==)
Failed, modules loaded: none.
Any insights about what's going wrong here?
-- Conal
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