Haskell有一个takeUntil函数吗？

Question

Currently I am using

takeWhile (\x -> x /= 1 && x /= 89) l

to get the elements from a list up to either a 1 or 89. However, the result doesn't include these sentinel values. Does Haskell have a standard function that provides this variation on takeWhile that includes the sentinel in the result? My searches with Hoogle have been unfruitful so far.

Answer 1

Since you were asking about standard functions, no . But also there isn't a package containing a takeWhileInclusive , but that's really simple:

takeWhileInclusive :: (a -> Bool) -> [a] -> [a]
takeWhileInclusive _ [] = []
takeWhileInclusive p (x:xs) = x : if p x then takeWhileInclusive p xs
                                         else []

The only thing you need to do is to take the value regardless whether the predicate returns True and only use the predicate as a continuation factor:

*Main> takeWhileInclusive  (\x -> x /= 20) [10..]
[10,11,12,13,14,15,16,17,18,19,20]

Answer 2

Is span what you want?

matching, rest = span (\x -> x /= 1 && x /= 89) l

then look at the head of rest .