无法将预期类型“Bool”与实际类型“a -> Bool”匹配

Question

I want to write a function that returns the longest prefix of a list, where applying a function to every item in that prefix produces a strictly ascending list.

For example:

longestAscendingPrefix (`mod` 5) [1..10] == [1,2,3,4]

longestAscendingPrefix odd [1,4,2,6,8,9,3,2,1] == [1]

longestAscendingPrefix :: Ord b => (a -> b) -> [a] -> [a]
longestAscendingPrefix _ [] = []
longestAscendingPrefix f (x:xs) = takeWhile (\y z -> f y <= f z) (x:xs)

This code snippet produces the error message in the title. It seems the problem lies within that lambda function.

Answer 1

takeWhile has type takeWhile:: (a -> Bool) -> [a] -> [a] . The first parameter is thus a function that maps an element of the list to a Bool . Your lambda expression has type Ord b => a -> a -> Bool , which does not make much sense.

You can work with explicit recursion with:

longestAscendingPrefix :: Ord b => (a -> b) -> [a] -> [a]
longestAscendingPrefix f = go
    where go [] = []
          go [x] = …
          go (x1:x2:xs) = …

where you need to fill in the … parts the last one makes a recursive call to go .