Haskell在哪里进行类型声明

Question

我是Haskell的新手,并且在类型系统方面遇到了麻烦.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总类型,并且可能将它与类型Show相关联,但我不知道如何实现它.谢谢你的帮助.

Answer 1

问题是你定义total为乘法的结果,它强制它成为a Num a => a然后你用它作为++字符串的参数,强制它[Char].

您需要转换total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,这将运行,但代码看起来有点重复.我会建议这样的事情:

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