On Fri, Jul 03, 2009 at 11:58:22AM -0400, Patrick LeBoutillier wrote:
Hi,
I'm running through these Haskell exercises:
http://dibblego.wordpress.com/2008/09/18/20-intermediate-haskell-exercises/
and I'm stuck at number 9:
class Misty m where banana :: (a -> m b) -> m a -> m b unicorn :: a -> m a
-- Exercise 9 -- Relative Difficulty: 6 instance Misty ((->) t) where banana = ??? unicorn x = (\t -> x)
I can picture it when m is "[]" or "Maybe", but I can't wrap my head around the banane implementation for "((->) t)". I can see that this somewhat looks like a Monad, with unicorn = return and banana = (flip >>=) or something. Perhaps some kind of reader Monad?
Precisely. ((->) t) is the reader monad. Just follow the types! banana :: (a -> m b) -> m a -> m b substituting ((->) t) for m (remembering that (->) associates to the right): banana :: (a -> t -> b) -> (t -> a) -> t -> b banana f g t = ? Can you figure out how to apply f :: (a -> t -> b) and g :: (t -> a) to a 't' in order to get a 'b'? -Brent