freersimple1.2.1.2: A friendly effect system for Haskell.
An implementation of an effect system for Haskell (a fork of freereffects ), which is based on the work of Oleg Kiselyov et al.:
The key features are:
 An efficient effect system for Haskell  as a library!
 Reimplementations of several common Haskell monad transformers as effects.
 Core components for defining your own Effects.

Control

Monad

Control.Monad.Freer
Freer  an extensible effects library
 Control.Monad.Freer.Coroutine Composable coroutine effects layer.
 Control.Monad.Freer.Error An Error effect and handler.
 Control.Monad.Freer.Fresh Generation of fresh integers as an effect.
 Control.Monad.Freer.Internal Mechanisms to make effects work.
 Control.Monad.Freer.NonDet Non deterministic effects
 Control.Monad.Freer.Reader Reader effects, for encapsulating an environment.
 Control.Monad.Freer.State State effects, for statecarrying computations.
 Control.Monad.Freer.TH
 Control.Monad.Freer.Trace Composable Trace effects.
 Control.Monad.Freer.Writer Composable Writer effects.

Control.Monad.Freer
Freer  an extensible effects library

Monad

Data
 Data.FTCQueue Fast typealigned queue optimized to effectful functions.

Data.OpenUnion
Open unions (typeindexed coproducts) for extensible effects.
 Data.OpenUnion.Internal Open unions (typeindexed coproducts) for extensible effects.