On 12/4/10 6:32 AM, Sebastian Fischer wrote:

Hi Wren,

On Sat, 2010-12-04 at 01:25 -0500, wren ng thornton wrote:

...

data SliceUnder a b = SliceUnder (b->a)

instance Functor (SliceUnder a) where fmap f (SliceUnder g) = SliceUnder (g . f)

Couldn't match type `a1' with `b'

Ha, righto! That one's contravariant, whoops. class ContraFunctor f where contraMap :: (a -> b) -> (f b -> f a) instance ContraFunctor (SliceUnder a) where contraMap f (SliceUnder g) = SliceUnder (g . f) That'll teach me to rattle things off without typechecking them. -- Live well, ~wren