monad-classes: more flexible mtl

See this series of articles for the detailed description and motivation.

This is a more flexible version of mtl, the monad transformers library.

• You can have many layers of e.g. state transformers in your stack, and you don't have to explicitly lift your gets and puts, as soon as different state transformers carry different types of states.

  Example:

  a :: (MonadState Bool m, MonadState Int m) => m ()
  a = do
    put False -- set the boolean state
    modify (+ (1 :: Int)) -- modify the integer state
  

• mtl requires Θ(n²) instances (like MonadReader e (StateT s m)); monad-classes requires only Θ(n) of them (where n is the number of different transformer types).

  If you'd like to define your own monad-classes-style class, you have to write much less boilerplate code.