filter -package:base -package:containers -package:unordered-containers -package:hedgehog package:protolude

filter :: (a -> Bool) -> [a] -> [a]
protolude Protolude
filter, applied to a predicate and a list, returns the list of those elements that satisfy the predicate; i.e., 
filter p xs = [ x | x <- xs, p x]

Examples

>>> filter odd [1, 2, 3]
[1,3]

>>> filter (\l -> length l > 3) ["Hello", ", ", "World", "!"]
["Hello","World"]

>>> filter (/= 3) [1, 2, 3, 4, 3, 2, 1]
[1,2,4,2,1]
filterM :: Applicative m => (a -> m Bool) -> [a] -> m [a]
protolude Protolude.Monad
This generalizes the list-based filter function.
mfilter :: MonadPlus m => (a -> Bool) -> m a -> m a
protolude Protolude.Monad
Direct MonadPlus equivalent of filter.

Examples

The filter function is just mfilter specialized to the list monad: 
filter = ( mfilter :: (a -> Bool) -> [a] -> [a] )
An example using mfilter with the Maybe monad: 
>>> mfilter odd (Just 1)
Just 1

>>> mfilter odd (Just 2)
Nothing