给出一个Maybe Int,我试着将它mappend到自己。
$let x = Just 55 :: Maybe Int
$mappend x x
<interactive>:126:1:
No instance for (Monoid Int) arising from a use of `mappend'
In the expression: mappend x x
In an equation for `it': it = mappend x x
看着,我看到:
么半群a =>么半群(可能是a)
由于不实现Monoid类型类