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Monthly Downloads: 2
Programming language: Haskell
License: ISC License
Tags: Control     Monad    
Latest version: v0.1.0.0

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This Haskell package provides utilities for converting between computations parameterized via two different typeclasses, both of which can be used to abstract over monad transformer stacks with IO at the base. Unfortunately, both classes are frequently used in the Haskell ecosystem, since they have minor differences:

  • MonadIO comes from the base package (as of base version 4.9.0.0), and it provides a liftIO operation. It is an extremely simple typeclass, focusing exclusively on lifting IO actions through transformer stacks with IO at the base.

  • MonadBase comes from the transformers-base package, and it is a generalized version of MonadIO. It provides a more general liftBase function, which allows lifting to an arbitrary base monad.

    Generally, this additional power isn’t especially useful, but MonadBase appears most often through MonadBaseControl, a subclass from the monad-control package that enables lifting operations that accept an action in negative position. This class has no IO-specialized equivalent (not directly, at least), so it often appears in lifted “control” operations.

Due to these typeclasses being unrelated, it’s not entirely uncommon to end up with type signatures like (MonadIO m, MonadBaseControl IO m) => ..., which are a little silly, since MonadBaseControl IO really includes all the power of MonadIO.

For more information, see the documentation on Hackage.