--- | The monoid class.
--- A minimal complete definition must supply 'mempty' and 'mappend',
--- and these should satisfy the monoid laws.
+-- | The class of monoids (types with an associative binary operation that
+-- has an identity). Instances should satisfy the following laws:
+--
+-- * @mappend mempty x = x@
+--
+-- * @mappend x mempty = x@
+--
+-- * @mappend x (mappend y z) = mappend (mappend x y) z@
+--
+-- * @mconcat = 'foldr' mappend mempty@
+--
+-- The method names refer to the monoid of lists under concatenation,
+-- but there are many other instances.
+--
+-- Minimal complete definition: 'mempty' and 'mappend'.
+--
+-- Some types can be viewed as a monoid in more than one way,
+-- e.g. both addition and multiplication on numbers.
+-- In such cases we often define @newtype@s and make those instances
+-- of 'Monoid', e.g. 'Sum' and 'Product'.