Haskell Language Monoid An instance of Monoid for lists
Example
instance Monoid [a] where
mempty = []
mappend = (++)
Checking the Monoid laws for this instance:
mempty `mappend` x = x <-> [] ++ xs = xs -- prepending an empty list is a no-op
x `mappend` mempty = x <-> xs ++ [] = xs -- appending an empty list is a no-op
x `mappend` (y `mappend` z) = (x `mappend` y) `mappend` z
<->
xs ++ (ys ++ zs) = (xs ++ ys) ++ zs -- appending lists is associative
