小编Jim*_* Hu的帖子

我们知道,fmap是fmap :: Functor f => (a -> b) -> f a -> f b和sum是sum :: (Num a, Foldable t) => t a -> a,但下面的代码让我困惑.

> :t (fmap sum Just)
(fmap sum Just) :: Num b => b -> b
> fmap sum Just 3
3

为什么？