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 run XXX to unwrap the transformer, exposing a computation of the inner monad.