Classes for monad transformers.

A monad transformer makes new monad out of an existing monad, such that computations of the old monad may be embedded in the new one. To construct a monad with a desired set of features, one typically starts with a base monad, such as Identity, [] or IO, and applies a sequence of monad transformers.

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.

Each monad transformer also comes with an operation runXXX to unwrap the transformer, exposing a computation of the inner monad.