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 can I use MTL and transformers together?

Q: When I use ghc or ghci it complains about the same module names in mtl and transformers or monads-fd. How can I resolve these name clashes? A: You can use the -hide-package option of GHC. Cabal uses the -hide-all-packages option and then explicitly makes every package visible with -package.

Shall I use MTL or transformers?

Transformers is Haskell 98 and thus more portable.

How to move from MTL to transformers?

Many package using MTL can be ported to transformers with only slight modifications. 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 matching patterns you must call runState.

See also

• Monad Transformers Explained
• Monad Transformers Step by Step (PDF)
• All About Monads
• 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