haskell - 为什么会出现这种类型错误？

Question

solveRPN :: String -> Double
solveRPN rpnString = head foldl . foldingFunction [] . words
    where foldingFunction (x:y:ys) "*" = (x * y):ys
          foldingFunction (x:y:ys) "+" = (x + y):ys
          foldingFunction (x:y:ys) "-" = (x - y):ys
          foldingFunction xs numberString = read numberString:xs

上面的代码产生下面的错误。我尝试将 String 更改为 [Char] 并将双引号替换为单引号，但我一直收到此错误:

_01.hs:874:50: error:
    • Couldn't match type ‘Char’ with ‘[Char]’
      Expected type: String
        Actual type: Char
    • In the first argument of ‘read’, namely ‘numberString’
      In the first argument of ‘(:)’, namely ‘read numberString’
      In the expression: read numberString : xs
    | 874 |           foldingFunction xs numberString = read numberString:xs
    |                                                  ^^^^^^^^^^^^ Failed, 1 module loaded.

Answer 1

Learn you a Haskell 书中的原始解决方案如下所示

solveRPN :: (Num a, Read a) => String -> a  
solveRPN = head . foldl foldingFunction [] . words  
    where   foldingFunction (x:y:ys) "*" = (x * y):ys  
            foldingFunction (x:y:ys) "+" = (x + y):ys  
            foldingFunction (x:y:ys) "-" = (y - x):ys  
            foldingFunction xs numberString = read numberString:xs

当我将它与您的代码段进行比较时，有两件事导致了错误:

1) 您在合成链中放错了合成运算符。而不是这个

head foldl . foldingFunction [] . words

你需要这个

head . foldl foldingFunction [] . words

调用words后，用foldingFunction折叠后，您可以将其理解为取head，其中每个“after”都是组合运算符。

2) 你使用了一个组合，但你仍然留下参数 rpnString。如果你想使用一个组合，你必须省略链中的参数。所以你可以省略参数:

solveRPN :: String -> Double
solveRPN = head . foldl foldingFunction [] . words
    where   foldingFunction (x:y:ys) "*" = (x * y):ys
            foldingFunction (x:y:ys) "+" = (x + y):ys
            foldingFunction (x:y:ys) "-" = (y - x):ys
            foldingFunction xs numberString = read numberString:xs

或者你可以省略合成链:

solveRPN :: String -> Double
solveRPN rpnString = head (foldl foldingFunction [] (words rpnString))
    where   foldingFunction (x:y:ys) "*" = (x * y):ys
            foldingFunction (x:y:ys) "+" = (x + y):ys
            foldingFunction (x:y:ys) "-" = (y - x):ys
            foldingFunction xs numberString = read numberString:xs