class Monoid m where
mempty :: m
mappend :: m -> m -> m
mconcat :: [m] -> m
mconcat = foldr mapend mempty
其中m是一个具体类型（:k m = *）。
注：
<u></u>foldr定义为：
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr step zero (x:xs) = step x (foldr step zero xs)
foldr _ zero [] = zero
Monoid Law
（1）mempty `mapend` x = x
（2）x `mappend` mempty = x
（3）(x `mappend` y) `mappend` z = x `mappend` (y `mappend` z)
Examples
（1）[a]是Monoid类型类的实例
instance Monoid [a] where
mempty = []
mappend = (++)
ghci> [1, 2, 3] `mappend` [4, 5, 6]
[1, 2, 3, 4, 5, 6]
ghci> (“one ” `mappend` “two ”) `mappend` “three”
“one two three”
其中，:t “one” = [Char]
（2）Any是Monoid类型类的实例
newtype Any = Any { getAny :: Bool }
deriving (Eq, Ord, Read, Show, Bounded)
instance Monoid Any where
mempty = Any False
Any x `mappend` Any y = Any (x || y)
注：
（1）:k Any = *
（2）“instance Monoid Any where”中的Any是类型，“mempty = Any False”和“Any x `mappend` Any y = Any (x || y)”中的Any是值构造器
ghci> getAny $ Any True `mappend` Any False
True
其中getAny是Any类型的字段读取器。
（3）Maybe a是Monoid类型类的实例
instance Monoid a => Monoid (Maybe a) where
mempty = Nothing
Nothing `mappend` m = m
m `mappend` Nothing = m
Just m1 `mappend` Just m2 = Just (m1 `mappend` m2)
ghci> Nothing `mappend` Just “andy”
Just “andy”
