Copyright | (c) Andy Gill 2001, (c) Oregon Graduate Institute of Science and Technology, 2001 |
---|---|

License | BSD-style (see the file LICENSE) |

Maintainer | libraries@haskell.org |

Stability | experimental |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell98 |

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.