Control.Effects is a Haskell library for programming with effects, like in the the Eff language by Andrej Bauer and Matija Pretnar. Effects can be used instead of monad transformers, they are designed to be easier to use and to define.
cabal install effects
Here's an example how to use the state effect from Control.Effects.State.
example :: (Int, Int)
example = run $ do
with (ref 5) $ \x -> do
with (ref 10) $ \y -> do
x =: (+) <$> get x <*> get y
y =: (+) <$> get x <*> get y
(,) <$> get x <*> get y
Every instance of an effect is given a name (x and y in this example), which makes is possible to easily mix several instances of the same effect.
For more examples see examples.hs. Another good example is the implementation of a parser by Jesse McDonald.