The identity functor and monad.

This trivial type constructor serves two purposes:

• It can be used with functions parameterized by a Functor or Monad.
• It can be used as a base monad to which a series of monad transformers may be applied to construct a composite monad. Most monad transformer modules include the special case of applying the transformer to Identity. For example, State s is an abbreviation for StateT s Identity.