Re: [Haskell-beginners] Help with "20 intermediate haskell exercises"

On Mon, Jul 06, 2009 at 03:55:34PM -0400, Patrick LeBoutillier wrote:

Hi,

On Sun, Jul 5, 2009 at 3:44 PM, Daniel Fischer wrote:

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Am Sonntag 05 Juli 2009 21:05:20 schrieb Patrick LeBoutillier:

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Hi,

Thanks for the help. I figured it out after that. I'm having a hard time with the other exercises though, I'm currently stuck at 14:

class Misty m where   banana :: (a -> m b) -> m a -> m b   unicorn :: a -> m a

-- Exercise 14 -- Relative Difficulty: 6 moppy :: (Misty m) => [a] -> (a -> m b) -> m [b] moppy = error "todo"

moppy [] mop = ? moppy (a:as) mop = (mop a) ?? (moppy as mop)

use (among other things) banana and unicorn to replace the question marks

I came up with this:moppy [] mop = unicorn [] moppy (a:as) mop = banana (\b -> banana (\bs -> unicorn (b:bs)) (moppy as mop)) (mop a)

moppy [] mop = unicorn [] moppy (a:as) mop = banana (\b -> banana (\bs -> unicorn (b:bs)) (moppy as mop)) (mop a)

How do I make the second one nicer/shorter?

Great! You can't make it much shorter just using banana and unicorn. But you could make it a little nicer like so: ananab = flip banana moppy (a:as) mop = mop a `ananab` (\b -> moppy as mop `ananab` (\bs -> unicorn (b:bs)) That makes it a bit more obvious where the 'b' and 'bs' are coming from. You can also do something like this: liftBanana2 f mx my = mx `ananab` (\x -> my `ananab` (\y -> unicorn (f x y))) moppy (a:as) mop = liftBanana2 (:) (mop a) (moppy as mop) I'll let you figure out what 'ananab' and 'liftBanana2' are called in the standard libraries. =) -Brent