相关疑难解决方法(0)

以http://en.wikibooks.org/wiki/Haskell/Beginning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？

以下所有表达式都得到评估而不会发生意外:

(+2) 1 -- 3
(*2) 1 -- 2
((-)2) 1 -- 1
(2-) 1 -- 1   
(/2) 1 -- 0.5
(2/) 1 -- 2.0

但不是这个:

(-2) 1 -- the inferred type is ambiguous

GHC抛出一些关于推断类型不明确的错误.为什么？