list - 当函数在 haskell 中不正确时，如何只对列表进行切片？

Question

我想在函数不正确时对我的列表进行切片，但我不知道在其他情况下我必须返回什么。你有什么想法吗？

Example : 
sliceBy odd [1..5] == [[1],[2],[3],[4],[5]]
sliceBy odd [1,3,2,4,5,7,4,6] == [[1,3],[2,4],[5,7],[4,6]]
sliceBy even [1,3,2,4,5,7,4,6] == [[],[1,3],[2,4],[5,7],[4,6]]

sliceBy :: (a -> Bool) -> [a] -> [[a]]
sliceBy _ []  = []
sliceBy _ [x] = [[x]]
sliceBy f (x:xs)
  | f x   = [x] : sliceBy f xs
  | otherwise = ?? 

Answer 1

您可以使用 span :: (a -> Bool) -> [a] -> ([a], [a])和 break :: (a -> Bool) -> [a] -> ([a], [a])获取列表不满足/不满足给定谓词的最长前缀。因此，您可以使用它来创建两个相互调用的函数 sliceBy 和 sliceByNot:

sliceBy :: (a -> Bool) -> [a] -> [[a]]
sliceBy _ [] = []
sliceBy f xs = … : …
    where (xp, xnp) = <strong>span</strong> f xs

sliceByNot :: (a -> Bool) -> [a] -> [[a]]
sliceByNot _ [] = []
sliceByNot f xs = … : …
    where (xnp, xp) = <strong>break</strong> f xs

需要填写…部分的地方。因此这两个函数应该相互调用。