# The monad-wrap package

This package allows you to invoke a function on one monadic
type passing it an argument of a different monadic type. The
canonical example is using a function such as ` finally` or

`to ensure cleanup is run regardless of any exceptions thrown by a computation such as`

`bracket``x ::`. If

`ReaderT`MyConfig`IO`b`x`uses the

`ReaderT`function

`ask`, it cannot be re-written to run in the

`IO`monad, and hence cannot be executed with a construction like

`. Instead, you must use the`

`lift`(x ``finally`` cleanup)`wrap`method, provided by module

`Control.Monad.Wrap`in this package.

This package contains several other modules:
`Control.Monad.WrapIO` wraps an IO action through multiple
monad transformers. `Control.Monad.WrapBase` generalizes the
concept to other base monads besides IO.
`Control.Monad.MultiWrap` implements `mwrap`, a method that
behaves like `wrap` but allows wrapping through multiple
nested layers of monad transformer. The module
`Control.Monad.MultiLift` provides `mlift`, a version of
`lift` that similarly lifts through multiple nested monad
transformers.

Since this library was first released, an alternate approach
was introduced by the `monad-control` package.
`monad-control` is now the standard. However, this package,
`monad-wrap`, stands as an example of accomplishing similar
goals with fewer language extensions and less complexity. In
particular, `monad-wrap` does not use `RankNTypes` or
`TypeFamilies`, both required by `monad-control`. Moreover,
`monad-wrap` is much smaller--no `MonadWrap` method requires
more than one line of code. Both `monad-wrap` and
`monad-control` require `UndecidableInstances`, but
`Control.Monad.Wrap` itself does not require that extension,
only the other modules.

## Properties

Versions | 0.0, 0.1 |
---|---|

Dependencies | base (<6), transformers [details] |

License | BSD3 |

Author | David Mazieres |

Maintainer | http://www.scs.stanford.edu/~dm/addr/ |

Stability | experimental |

Category | Control |

Uploaded | Sat Apr 27 07:52:54 UTC 2013 by DavidMazieres |

Distributions | NixOS:0.1 |

Downloads | 449 total (2 in the last 30 days) |

Votes | |

Status | Docs uploaded by user Build status unknown [no reports yet] |

## Downloads

- monad-wrap-0.1.tar.gz [browse] (Cabal source package)
- Package description (included in the package)