monad-wrap

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 bracket to ensure cleanup is run regardless of any exceptions thrown by a computation such as x :: ReaderT MyConfig IO b. If 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 lift (x `finally` cleanup). Instead, you must use the 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.