filter -package:base -package:containers -package:unordered-containers package:base-compat

filter p xs removes any elements from xs that do not satisfy p.

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]

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

This generalizes the list-based filter function.

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