无法摆脱这种类型的错误

Question

我正在尝试解决 this problem。

这是我的代码：

import Data.List (nub)

main = interact $ unwords . map show . solve . map words . lines

solve :: [[String]] -> [Int]
solve (_:r:_:a) = map (rank . read) a
    where records = nub $ map read r :: [Int]
          rank n = (length $ takeWhile (> n) records) + 1

编译器抛出此错误：

    * Couldn't match type `[Char]' with `Char'
      Expected type: [String]
        Actual type: [[String]]
    * In the second argument of `map', namely `a'
      In the expression: map (rank . read) a
      In an equation for `solve':
          solve (_ : r : _ : a)
            = map (rank . read) a
            where
                records = nub $ map read r :: [Int]
                rank n = (length $ takeWhile (> n) records) + 1
  |
6 | solve (_:r:_:a) = map (rank . read) a
  |

我不明白这是什么问题。当我在 GHCi 中逐行拼凑时，它起作用了：

GHCi> import Data.List (nub)
GHCi> records = nub $ map read ["100", "100", "50", "40", "40", "20", "10"] :: [Int]
GHCi> rank n = (length $ takeWhile (> n) records) + 1
GHCi> a = ["5", "25", "50", "120"]
GHCi> map (rank . read) a
[6,4,2,1]

Answer 1

你进行了错误的模式匹配。由于 solve 必须接受大小正好为 4 的列表，模式匹配必须是这样的：

solve [_,r,_,a] = ...

可以脱糖为：

solve (_:r:_:a:[]) = ...

或进一步脱糖：

solve (_:(r:(_:(a:[])))) = ...

记住，: 左边是一个元素，右边是一个列表！

无法摆脱这种类型的错误

Can't get rid of this type error

haskell

ghci