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
  |

我不明白问题是什么。当我在GHCi中将它们逐行拼凑在一起时，它会起作用：

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

您进行了错误的模式匹配。由于solve必须接受大小为4的列表，因此模式匹配必须像这样：

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

哪些可以放心使用：

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

甚至进一步贬低：

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

请记住，:在左边带有一个元素，在右边带有一个列表！

无法摆脱这种类型的错误

1 个答案: