2007/6/22, Hugo Pacheco <[EMAIL PROTECTED]>:
class Functor f => C f a b | f a -> b where
ftest :: f a -> b
I want to write some function
test :: (C f a b) => (a -> b)
test = ftest . undefined
I'm not sure whether this is what you want, but the "obvious" way to
make this type-check would seem to be to add a functional dependency
and a type signature for 'undefined,' like this:
class Functor f => C f a b | f a -> b, a b -> f where
ftest :: f a -> b
test :: (C f a b) => (a -> b)
test = ftest . (undefined :: a -> f a)
- Benja
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