Graded monads are a generalisation of monads where, from a functional programming perspective, a monadic type constructor is annotated (indexed) by another type representing effect information that explains the effects of a computation, i.e., values of type M r a are effectul computations of values of type a with effects r. They provide a way to give finer-grained information about impurity and capture a greater class of effectul computations that normal monads.

This library is based on the paper "Embedding Effect Systems in Haskell" (Orchard, Petricek, Haskell 2014). For more information, see talk slides and papers here:

   http://www.cs.kent.ac.uk/~dao7/publ/haskell14-effects.pdf
   http://www.cs.kent.ac.uk/~dao7/talks/haskell14-orchard-petricek-effects.pdf

This is also available on hackage (http://hackage.haskell.org/package/effect-monad) so you can get this by doing:

   cabal install effect-monad