遵循http://en.wikibooks.org/wiki/Haskell/Beginning中的示例
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?