Haskell where 类型声明

Question

我是 Haskell 新手，在类型系统方面遇到问题。我有以下功能:

threshold price qty categorySize
    | total < categorySize = "Total: " ++ total ++ " is low"
    | total < categorySize*2 = "Total: " ++ total ++ " is medium"
    | otherwise = "Total: " ++ total ++ " is high"
    where total =  price * qty

Haskell 回应:

No instance for (Num [Char])
      arising from a use of `*'
    Possible fix: add an instance declaration for (Num [Char])
    In the expression: price * qty
    In an equation for `total': total = price * qty
    In an equation for `threshold':
     ... repeats function definition

我认为问题是我需要以某种方式告诉 Haskell 总计的类型，并且可能将其与类型类 Show 相关联，但我不知道如何实现这一点。感谢您的帮助。

Answer 1

问题是您将 total 定义为乘法的结果，这迫使它成为 Num a => a，然后您将其用作++ 带有字符串，强制其为 [Char]。

您需要将total转换为String:

threshold price qty categorySize
    | total < categorySize   = "Total: " ++ totalStr ++ " is low"
    | total < categorySize*2 = "Total: " ++ totalStr ++ " is medium"
    | otherwise              = "Total: " ++ totalStr ++ " is high"
    where total    = price * qty
          totalStr = show total

现在，它将运行，但代码看起来有点重复。我建议这样:

threshold price qty categorySize = "Total: " ++ show total ++ " is " ++ desc
    where total = price * qty
          desc | total < categorySize   = "low"
               | total < categorySize*2 = "medium"
               | otherwise              = "high"