这在 Haskell 中有什么作用？

Question

f = filter . flip notElem

我想知道这条线是如何工作的，这是一个应该接受两个字符串并返回一个字符串的函数。

Answer 1

flip :: (b -> a -> c) -> a -> b -> c flips the first two parameters of a function. So since notElem :: Eq a => a -> [a] -> Bool takes a value and a list to check if the value is not in the list, flip notElem :: Eq a => [a] -> a -> Bool takes a list and a value to check if it is not in the list.

We thus will first partially apply the first parameter in f to flip notElem , and then filter a list. Your expression is thus a short form of:

f :: Eq a => [a] -> [a] -> [a]
f xs ys = filter (\y -> notElem y xs) ys

It will thus return all elements in ys (the second parameter) that are not in xs (the first parameter). For example:

Prelude> f [1,3,0,2] [1,4,2,5]
[4,5]

Both 4 and 5 are elements of the second list, but not of the first one.