haskell - "No instance for"错误

Question

以 http://en.wikibooks.org/wiki/Haskell/Beginning 中的示例为例

Prelude> let abs x = if x < 0 then -x else x
Prelude> abs 5
5
Prelude> abs -3

<interactive>:1:6:
    No instance for (Num (a0 -> a0))
      arising from the literal `3'
    Possible fix: add an instance declaration for (Num (a0 -> a0))
    In the second argument of `(-)', namely `3'
    In the expression: abs - 3
    In an equation for `it': it = abs - 3

怎么了？

Answer 1

Haskell 认为您正在尝试减去 3来自 abs ，并提示 abs不是一个数字。使用一元否定运算符时需要添加括号:

abs (-3)