You can use 'mapM' similar to 'map', to get the resultant list in the
monad, and then liftM the function 'or' into it. This way you don't
need to recurse explicitly, like in or' and and'.
many :: Monad m => (a -> m Bool) -> [a] -> m Bool
many p list = or `liftM` mapM p list
(Type of mapM is: Monad m => (a -> m b) -> [a] -> m [b])
On Thu, Dec 29, 2011 at 7:45 AM, Manfred Lotz
Hi there, might be trivial but anyway.
I have a usage of 'any' and 'all' but I only have a predicate p :: a -> IO Bool.
I wrote my own functons for this:
mor :: Monad m => [m Bool] -> m Bool mor = liftM or . sequence
mand :: Monad m => [m Bool] -> m Bool mand = liftM and . sequence
or' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] or' _ [] = [] or' p (x:xs) = p x : or' p xs
and' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] and' _ [] = [] and' p (x:xs) = p x : and' p xs
myany :: Monad m => (a -> m Bool) -> [a] -> m Bool myany p = mor . or' p
myall :: Monad m => (a -> m Bool) -> [a] -> m Bool myall p = mand . and' p
which seems to do what I want.
Question: Is there any libray function I could use to do this?
-- Thanks, Manfred
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