**takeWhile** -bytestring +containers

*O(i)* applied to a predicate p and a sequence xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p.

*O(i)* applied to a predicate p and a sequence xs, returns the longest suffix (possibly empty) of xs of elements that satisfy p.
takeWhileR p xs is equivalent to reverse (takeWhileL p (reverse xs)).