list - 从列表中获取偶数位置的元素

Question

问题:使用折叠，从列表中取出偶数位置的元素:

GHCi> evenOnly [1..10]
 [2,4,6,8,10]
GHCi> evenOnly ['a'..'z']
 "bdfhjlnprtvxz"

evenOnly :: [a] -> [a]
evenOnly = undefined

我首先决定获取一个交替的 0-es 和 1-s 的列表:[0,1,0,1..]

Prelude> let g = iterate (\x -> (x + 1) `mod` 2) 0
Prelude> take 10 $ g
[0,1,0,1,0,1,0,1,0,1]

然后将其与原始列表zip，得到一个对列表:[(x1, 0), (x2,1), (x3,0) .. (xn, ?)]:

Prelude> zip g [1,2,3,4,5]
[(0,1),(1,2),(0,3),(1,4),(0,5)]

之后，foldr具有过滤功能的对列表和一个空列表作为初始化值。

所以我认为这会起作用:

evenOnly :: [a] -> [a]
evenOnly xs = let g = iterate (\x -> (x + 1) `mod` 2) 0
  in
  foldr (\ (x, n) s -> if n == 1 then x : s else s) [] . (zip g xs)

但它给出了一个我不明白的错误:

foldr.hs:44:59: error:
    • Couldn't match expected type ‘a0 -> t0 (a1, Integer)’
                  with actual type ‘[(Integer, a)]’
    • Possible cause: ‘zip’ is applied to too many arguments
      In the second argument of ‘(.)’, namely ‘(zip g xs)’
      In the expression:
        foldr (\ (x, n) s -> if n == 1 then x : s else s) [] . (zip g xs)
      In the expression:
        let g = iterate (\ x -> (x + 1) `mod` 2) 0
        in
          foldr (\ (x, n) s -> if n == 1 then x : s else s) [] . (zip g xs)
    • Relevant bindings include
        xs :: [a] (bound at foldr.hs:42:10)
        evenOnly :: [a] -> [a] (bound at foldr.hs:42:1)

我认为我的想法是正确的，我只是在语法上做错了。

Answer 1

(.) 是函数组合，但 zip g xs 是列表而不是函数。您可以直接将结果列表作为参数应用到 foldr。请注意，您的参数 g 和 xs 顺序错误:

evenOnly :: [a] -> [a]
evenOnly xs = let g = iterate (\x -> (x + 1) `mod` 2) 0
  in
  foldr (\ (x, n) s -> if n == 1 then x : s else s) [] (zip xs g)