Re: [Haskell] Class type constraining?

2007/6/22, Hugo Pacheco <hpacheco@gmail.com>:

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