Functor => Pointed => Applicative => Monad

[wren ng thornton]

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