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" Does anyone know if the solutions are posted anywhere? Patrick On Fri, Jul 3, 2009 at 12:08 PM, Daniel Fischer wrote:

Am Freitag 03 Juli 2009 17:58:22 schrieb Patrick LeBoutillier:

...

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?

Exactly.

You want

banana :: (a -> (t -> b)) -> (t -> a) -> (t -> b) banana fun af = \tval -> ???

There's not much choice what you can do with those types and data.

...

Can anyone offer any insight?

Patrick

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-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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

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

...

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? Patrick

...

Does anyone know if the solutions are posted anywhere?

They're (under different names) in the standard libraries :)

I found it: forM

...

Patrick

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-- ===================== Patrick LeBoutillier Rosemère, Québec, Canada