为什么Int不实现'Monoid'？

Question

鉴于Maybe Int,我试着mappend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Maybe,我看到:

Monoid a => Monoid(也许是a)

由于Int没有实现Monoid类型类,这就解释了为什么我不能使用mappend它Maybe Int.

但是,我记得LYAH我可以使用Sum:

ghci> let x = Sum 55
ghci> mappend x x
Sum {getSum = 110}

但是,为什么不是IntMonoid？

Answer 1

Int不是Monoid因为有不止一个明显的Monoid实现Int.

instance Monoid Int where
    mempty  = 0
    mappend = (+)

instance Monoid Int where
    mempty  = 1
    mappend = (*)

该newtype小号Sum和Product定义在Data.Monoid让你轻松选择哪些Monoid情况下用数字来使用.