Monad Transformers

There are currently two sets of packages that implement similar interfaces to monad transformers, besides a third package with a similar goal but different API named MonadLib:

• MTL - Monad Transformers Library: provides concrete monad transformers like StateT and abstractions using multi-parameter type classes with functional dependencies like MonadState. Monads like State and their transformer counterparts like StateT are distinct types and can be accessed uniformly only through a type class abstraction like MonadState. Because of the functional dependencies, MTL can currently (2010-03) only used in Hugs and GHC. MTL was the first implementation.
• The newer implementation is derived from the former one and is split into the following components:
  • transformers: Provide only concrete transformers like StateT. The monad State s a is only a type synonym for StateT s Identity a. Thus both State and StateT can be accessed by the same methods like put and get. However, this only works, if StateT is the top-most transformer in a monad transformer stack. This package is Haskell 98 and thus can be also used with JHC.
  • monads-fd: Provides the same type classes with functional dependencies like MTL. They allow using State methods also for StateT transformers within a transformer stack.
  • monads-tf: Provides a different abstraction using type families. Unfortunately the names of monads-fd and monads-tf clash,

thus you can currently not import both packages in one package.

How to move from MTL to transformers?

Any package using MTL can be ported to transformers and monads-fd with only slight variations. Modules require the Trans infix, e.g. import Control.Monad.State ... must be replaced by import Control.Monad.Trans.State .... Since State is only a type synonym, there is no longer a constructor named State. For constructing you must use the function state and instead of pattern matching you must call runState.

See also

• Monad Transformers Explained
• http://www.haskell.org/pipermail/libraries/2009-March/011415.html
• http://www.haskell.org/pipermail/libraries/2009-December/012914.html
• http://www.haskell.org/pipermail/haskell-cafe/2010-January/071842.html
• http://www.mail-archive.com/debian-haskell@lists.debian.org/msg01241.html