"Non-exhaustive patterns in function" error when appending value before function call

Question

I'm not sure what I'm not handling. Suppose I have a function, that converts an integer to a string. Call it converter.

现在，要将位置整数转换为字符串，我只需调用converter。要将负整数转换为字符串，请附加-到converter调用中。

这是我的代码：

converter :: Integer -> String
converter x
    | x == 0 = "0"
    | x == 1 = "1"
    | x == 2 = "2"
    | x == 3 = "3"
    | x == 4 = "4"
    | x == 5 = "5"
    | x == 6 = "6"
    | x == 7 = "7"
    | x == 8 = "8"
    | x == 9 = "9"
    | x > 9 = z
    where
    (a, b) = divMod x 10
    z = (converter a) ++ (converter b)

negOrPosConverter :: NegOrPosInteger -> String
negOrPosConverter (ActualInt x)
    | x >= 0 = converter x
    | x < 0 = "-" ++ (converter x)

当我运行代码并尝试出现negOrPosConverter (ActualInt (-200))此错误时：

"-*** Exception: theConverter.hs:(19,1)-(27,32): Non-exhaustive patterns in function converter

知道为什么吗？

Answer 1

问题是converter仅为非负数定义。"-"当它为负数时，您可以在前面加上a ，但是却忘记了将传递给它的实际数字取反。尝试以下方法：

negOrPosConverter :: NegOrPosInteger -> String
negOrPosConverter (ActualInt x)
    | x >= 0 = converter x
    | x < 0 = '-' : converter (-x)

请注意，converter (-x)而不是converter x。

另外，如果这不仅是为了练习，请注意showPrelude中已经存在该函数，可以将数字（以及许多其他东西）转换为字符串。