- http://book.realworldhaskell.org/read/monads.html
- http://book.realworldhaskell.org/read/programming-with-monads.html
- http://book.realworldhaskell.org/read/monad-transformers.html
- http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html
- http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.html
- http://stackoverflow.com/questions/13352205/what-are-free-monads
- http://www.haskellforall.com/2012/07/purify-code-using-free-monads.html
- http://hackage.haskell.org/package/comonad-transformers-1.7/docs/Control-Monad-Trans-Free.html