Transformers provides functor and monad transformers, inspired by the paper "Functional Programming with Overloading and Higher-Order Polymorphism", by Mark P Jones, in Advanced School of Functional Programming, 1995 http://web.cecs.pdx.edu/~mpj/pubs/springschool.html.

This package contains:

• the monad transformer class (in Control.Monad.Trans.Class)

• concrete functor and monad transformers, each with associated operations and functions to lift operations associated with other transformers.

This package can be used on its own in portable Haskell code, in which case operations need to be manually lifted through transformer stacks (see Control.Monad.Trans.Class for some examples). Alternatively, it can be used with the non-portable monad classes in the mtl or monads-tf packages, which automatically lift operations introduced by monad transformers through other transformers.