span, applied to a predicate
p and a list
xs,
returns a tuple where first element is the longest prefix (possibly
empty) of
xs of elements that satisfy
p and second
element is the remainder of the list:
span p xs is equivalent to
(takeWhile p xs,
dropWhile p xs), even if
p is
_|_.
Laziness
>>> span undefined []
([],[])
>>> fst (span (const False) undefined)
*** Exception: Prelude.undefined
>>> fst (span (const False) (undefined : undefined))
[]
>>> take 1 (fst (span (const True) (1 : undefined)))
[1]
span produces the first component of the tuple lazily:
>>> take 10 (fst (span (const True) [1..]))
[1,2,3,4,5,6,7,8,9,10]
Examples
>>> span (< 3) [1,2,3,4,1,2,3,4]
([1,2],[3,4,1,2,3,4])
>>> span (< 9) [1,2,3]
([1,2,3],[])
>>> span (< 0) [1,2,3]
([],[1,2,3])