haskell - 涉及预期类型的​​错误

Question

以下代码段(摘自 Graham Hutton 的“Programming in Haskell”(第 9 章))

:{
Prelude| data Op = Add | Sub | Mul | Div
Prelude| instance Show Op where
Prelude|    show Add = "+"
Prelude|    show Sub = "-"
Prelude|    show Mul = "*"
Prelude|    show Div = "/"
Prelude| valid :: Op -> Int -> Int -> Bool
Prelude| valid Add _ _ = True
Prelude| valid Sub x y = x > y
Prelude| valid Mul _ _ = True
Prelude| valid Div x y = x `mod` y == 0
Prelude| apply :: Op -> Int -> Int -> Int
Prelude| apply Add x y = x + y
Prelude| apply Sub x y = x - y
Prelude| apply Mul x y = x * y
Prelude| apply Div x y = x `div` y
Prelude| data Expr = Val Int | App Op Expr Expr
Prelude| instance Show Expr where
Prelude|       show (Val n) = n
Prelude|       show (App o l r) = brak l ++ show o ++ brak r 
Prelude|                          where
Prelude|                             brak (Val n ) = show n
Prelude|                             brak e = "(" ++ show e ++ ")"
Prelude| :}

产生这些错误:

<interactive>:455:22: error:
• Couldn't match type ‘Int’ with ‘[Char]’
  Expected type: String
    Actual type: Int
• In the expression: n
  In an equation for ‘Prelude.show’: Prelude.show (Val n) = n
  In the instance declaration for ‘Show Expr’

<interactive>:456:26: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘brak l’
  In the expression: brak l ++ show o ++ brak r
  In an equation for ‘Prelude.show’:
      Prelude.show (App o l r)
        = brak l ++ show o ++ brak r
        where
            brak (Val n) = show n
            brak e = "(" ++ show e ++ ")"

<interactive>:456:36: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘show o’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’
  In the expression: brak l ++ show o ++ brak r

<interactive>:456:41: error:
• Couldn't match expected type ‘Ghci41.Expr’ with actual type ‘Op’
• In the first argument of ‘show’, namely ‘o’
  In the first argument of ‘(++)’, namely ‘show o’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’

<interactive>:456:46: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the second argument of ‘(++)’, namely ‘brak r’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’
  In the expression: brak l ++ show o ++ brak r

<interactive>:458:50: error:
• Couldn't match expected type ‘Ghci41.Expr’ with actual type ‘Int’
• In the first argument of ‘show’, namely ‘n’
  In the expression: show n
  In an equation for ‘brak’: brak (Val n) = show n

<interactive>:459:38: error:
• Couldn't match expected type ‘Int’ with actual type ‘[Char]’
• In the expression: "(" ++ show e ++ ")"
  In an equation for ‘brak’: brak e = "(" ++ show e ++ ")"
  In an equation for ‘Prelude.show’:
      Prelude.show (App o l r)
        = brak l ++ show o ++ brak r
        where
            brak (Val n) = show n
            brak e = "(" ++ show e ++ ")"

<interactive>:459:45: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘show e’
  In the second argument of ‘(++)’, namely ‘show e ++ ")"’
  In the expression: "(" ++ show e ++ ")"

<interactive>:459:50: error:
• Couldn't match expected type ‘Ghci41.Expr’
              with actual type ‘Expr’
  NB: ‘Expr’ is defined at <interactive>:453:1-38
      ‘Ghci41.Expr’ is defined at <interactive>:246:1-35
• In the first argument of ‘show’, namely ‘e’
  In the first argument of ‘(++)’, namely ‘show e’
  In the second argument of ‘(++)’, namely ‘show e ++ ")"’

我不明白出了什么问题。有人可以帮忙吗？为了处理第一个错误，我是否需要将 n 的类型声明为字符串？其他错误报告指出了什么？

如果我看到类似这些特定错误报告，我应该采用什么策略来避免它们？

---

编辑:

我已经尝试了下面答案中的代码，但收到以下错误:

   <interactive>:520:20: error:
• Couldn't match type ‘Int’ with ‘[Char]’
  Expected type: String
    Actual type: Int
• In the expression: show n
  In an equation for ‘Prelude.show’: Prelude.show (Val n) = show n
  In the instance declaration for ‘Show Expr’

<interactive>:520:25: error:
• Couldn't match expected type ‘Ghci41.Expr’ with actual type ‘Int’
• In the first argument of ‘show’, namely ‘n’
  In the expression: show n
  In an equation for ‘Prelude.show’: Prelude.show (Val n) = show n

<interactive>:521:24: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘brak l’
  In the expression: brak l ++ show o ++ brak r
  In an equation for ‘Prelude.show’:
      Prelude.show (App o l r)
        = brak l ++ show o ++ brak r
        where
            brak (Val n) = show n
            brak e = "(" ++ show e ++ ")"

<interactive>:521:34: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘show o’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’
  In the expression: brak l ++ show o ++ brak r

<interactive>:521:39: error:
• Couldn't match expected type ‘Ghci41.Expr’ with actual type ‘Op’
• In the first argument of ‘show’, namely ‘o’
  In the first argument of ‘(++)’, namely ‘show o’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’

<interactive>:521:44: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the second argument of ‘(++)’, namely ‘brak r’
  In the second argument of ‘(++)’, namely ‘show o ++ brak r’
  In the expression: brak l ++ show o ++ brak r

<interactive>:522:34: error:
• Couldn't match expected type ‘Ghci41.Expr’ with actual type ‘Int’
• In the first argument of ‘show’, namely ‘n’
  In the expression: show n
  In an equation for ‘brak’: brak (Val n) = show n

<interactive>:523:23: error:
• Couldn't match expected type ‘Int’ with actual type ‘[Char]’
• In the expression: "(" ++ show e ++ ")"
  In an equation for ‘brak’: brak e = "(" ++ show e ++ ")"
  In an equation for ‘Prelude.show’:
      Prelude.show (App o l r)
        = brak l ++ show o ++ brak r
        where
            brak (Val n) = show n
            brak e = "(" ++ show e ++ ")"

<interactive>:523:30: error:
• Couldn't match expected type ‘[Char]’ with actual type ‘Int’
• In the first argument of ‘(++)’, namely ‘show e’
  In the second argument of ‘(++)’, namely ‘show e ++ ")"’
  In the expression: "(" ++ show e ++ ")"

<interactive>:523:35: error:
• Couldn't match expected type ‘Ghci41.Expr’
              with actual type ‘Expr’
  NB: ‘Expr’ is defined at <interactive>:518:1-38
      ‘Ghci41.Expr’ is defined at <interactive>:246:1-35
• In the first argument of ‘show’, namely ‘e’
  In the first argument of ‘(++)’, namely ‘show e’
  In the second argument of ‘(++)’, namely ‘show e ++ ")"’

Answer 1

我在这里发现的唯一错误是 n 是一个 Int，因此您需要使用 show 将其转换为 字符串:

instance Show Expr where
    show (Val n) = <b>show</b> n
    show (App o l r) = brak l ++ show o ++ brak r 
        where brak (Val n) = show n
              brak e = "(" ++ show e ++ ")"

此外，您没有显示 Op 的类型，但这应该是 Show 实例的类型。同样，因此您应该定义一个实例 Show Op。

例如:

Prelude> data Op = Add | Sub | Mul | Div
Prelude> :{
Prelude| instance Show Op where
Prelude|     show Add = "+"
Prelude|     show Sub = "-"
Prelude|     show Mul = "*"
Prelude|     show Div = "/"
Prelude| :}
Prelude> data Expr = Val Int | App Op Expr Expr
Prelude> :{
Prelude| instance Show Expr where
Prelude|     show (Val n) = show n
Prelude|     show (App o l r) = brak l ++ show o ++ brak r 
Prelude|         where brak (Val n) = show n
Prelude|               brak e = "(" ++ show e ++ ")"
Prelude| :}

然后我们可以这样查询:

Prelude> show (App Mul (Val 3) (App Add (Val 3) (Val 2)))
"3*(3+2)"