I would like to slice my list when the function is not true, but I do not have an idea what I have to give back in the otherwise case. Do you have any idea ?
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 = ??
You can make use of span :: (a -> Bool) -> [a] -> ([a], [a]) and break :: (a -> Bool) -> [a] -> ([a], [a]) to get the longest prefixes where the list does/does not satisfy a given predicate. You thus can use this to make two functions sliceBy and sliceByNot that call each other:
sliceBy :: (a -> Bool) -> [a] -> [[a]]
sliceBy _ [] = []
sliceBy f xs = … : …
where (xp, xnp) = span f xs
sliceByNot :: (a -> Bool) -> [a] -> [[a]]
sliceByNot _ [] = []
sliceByNot f xs = … : …
where (xnp, xp) = break f xsWhere you need to fill in the … parts. The two functions thus should call each other.