Monad Transformers

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Revision as of 22:58, 5 March 2010 by Lemming (talk | contribs) (explanation of transformers set of packages)

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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?

Shall I use MTL or transformers?

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