:: (a -> Bool) -> [a] -> [a] -package:extra -package:prelude-compat -package:Cabal -package:utility-ht

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]

takeWhile, applied to a predicate p and a list xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p.

>>> takeWhile (< 3) [1,2,3,4,1,2,3,4]
[1,2]

>>> takeWhile (< 9) [1,2,3]
[1,2,3]

>>> takeWhile (< 0) [1,2,3]
[]

dropWhile p xs returns the suffix remaining after takeWhile p xs.

>>> dropWhile (< 3) [1,2,3,4,5,1,2,3]
[3,4,5,1,2,3]

>>> dropWhile (< 9) [1,2,3]
[]

>>> dropWhile (< 0) [1,2,3]
[1,2,3]

The dropWhileEnd function drops the largest suffix of a list in which the given predicate holds for all elements. For example:

>>> dropWhileEnd isSpace "foo\n"
"foo"

>>> dropWhileEnd isSpace "foo bar"
"foo bar"

dropWhileEnd isSpace ("foo\n" ++ undefined) == "foo" ++ undefined

Like filter, only it reverses the sense of the test

takeWhile, applied to a predicate p and a list xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p:

takeWhile (< 3) [1,2,3,4,1,2,3,4] == [1,2]
takeWhile (< 9) [1,2,3] == [1,2,3]
takeWhile (< 0) [1,2,3] == []

dropWhile p xs returns the suffix remaining after takeWhile p xs:

dropWhile (< 3) [1,2,3,4,5,1,2,3] == [3,4,5,1,2,3]
dropWhile (< 9) [1,2,3] == []
dropWhile (< 0) [1,2,3] == [1,2,3]

O(n). 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]

dropWhileEndLE p is equivalent to reverse . dropWhile p . reverse, but quite a bit faster. The difference between "Data.List.dropWhileEnd" and this version is that the one in Data.List is strict in elements, but spine-lazy, while this one is spine-strict but lazy in elements. That's what LE stands for - "lazy in elements". Example:

>>> safeTail $ Data.List.dropWhileEnd (<3) [undefined, 5, 4, 3, 2, 1]
*** Exception: Prelude.undefined
...

>>> safeTail $ dropWhileEndLE (<3) [undefined, 5, 4, 3, 2, 1]
[5,4,3]

>>> take 3 $ Data.List.dropWhileEnd (<3) [5, 4, 3, 2, 1, undefined]
[5,4,3]

>>> take 3 $ dropWhileEndLE (<3) [5, 4, 3, 2, 1, undefined]
*** Exception: Prelude.undefined
...

takeWhileEndLE p is equivalent to reverse . takeWhile p . reverse, but is usually faster (as well as being easier to read).

Return the longest suffix of elements that satisfy a given predicate.