# Monad Transformers

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** [http://hackage.haskell.org/package/transformers transformers]: Provide only concrete transformers like <hask>StateT</hask>. The monad <hask>State s a</hask> is only a type synonym for <hask>StateT s Identity a</hask>. Thus both <hask>State</hask> and <hask>StateT</hask> can be accessed by the same methods like <hask>put</hask> and <hask>get</hask>. However, this only works, if <hask>StateT</hask> is the top-most transformer in a monad transformer stack. This package is Haskell 98 and thus can be also used with [[JHC]]. | ** [http://hackage.haskell.org/package/transformers transformers]: Provide only concrete transformers like <hask>StateT</hask>. The monad <hask>State s a</hask> is only a type synonym for <hask>StateT s Identity a</hask>. Thus both <hask>State</hask> and <hask>StateT</hask> can be accessed by the same methods like <hask>put</hask> and <hask>get</hask>. However, this only works, if <hask>StateT</hask> is the top-most transformer in a monad transformer stack. This package is Haskell 98 and thus can be also used with [[JHC]]. | ||

** [http://hackage.haskell.org/package/monads-fd monads-fd]: Provides the same type classes with functional dependencies like MTL. They allow using <hask>State</hask> methods also for <hask>StateT</hask> transformers within a transformer stack. | ** [http://hackage.haskell.org/package/monads-fd monads-fd]: Provides the same type classes with functional dependencies like MTL. They allow using <hask>State</hask> methods also for <hask>StateT</hask> transformers within a transformer stack. | ||

− | ** [http://hackage.haskell.org/package/monads-tf monads-tf]: Provides a different abstraction using [[type families]]. Unfortunately the names of <code>monads-fd</code> and <code>monads-tf</code> clash, | + | ** [http://hackage.haskell.org/package/monads-tf monads-tf]: Provides a different abstraction using [[type families]]. Unfortunately the names of <code>monads-fd</code> and <code>monads-tf</code> clash, thus you can currently not import both packages in one package. |

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

− | + | ||

== How can I use MTL and transformers together? == | == 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 <code>-hide-package</code> option of GHC. [[Cabal]] uses the <code>-hide-all-packages</code> option and then explicitly makes every package visible with <code>-package</code>. | ||

== Shall I use MTL or transformers? == | == Shall I use MTL or transformers? == | ||

+ | |||

+ | Transformers is Haskell 98 and thus more portable. | ||

== How to move from MTL to transformers? == | == How to move from MTL to transformers? == | ||

− | + | Many package using <code>MTL</code> can be ported to <code>transformers</code> with only slight modifications. | |

Modules require the <code>Trans</code> infix, | Modules require the <code>Trans</code> infix, | ||

e.g. <hask>import Control.Monad.State ...</hask> must be replaced by <hask>import Control.Monad.Trans.State ...</hask>. | e.g. <hask>import Control.Monad.State ...</hask> must be replaced by <hask>import Control.Monad.Trans.State ...</hask>. | ||

Since <hask>State</hask> is only a type synonym, there is no longer a constructor named <hask>State</hask>. | Since <hask>State</hask> is only a type synonym, there is no longer a constructor named <hask>State</hask>. | ||

− | For constructing you must use the function <hask>state</hask> and instead of | + | For constructing you must use the function <hask>state</hask> and instead of matching patterns you must call <hask>runState</hask>. |

== See also == | == See also == |

## Revision as of 23:14, 5 March 2010

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 and abstractions using multi-parameter type classes with functional dependencies likeStateT. Monads likeMonadStateand their transformer counterparts likeStateare distinct types and can be accessed uniformly only through a type class abstraction likeStateT. Because of the functional dependencies, MTL can currently (2010-03) only used in Hugs and GHC. MTL was the first implementation.MonadState
- The newer implementation is derived from the former one and is split into the following components:
- transformers: Provide only concrete transformers like . The monadStateTis only a type synonym forState s a. Thus bothStateT s Identity aandStatecan be accessed by the same methods likeStateTandput. However, this only works, ifgetis the top-most transformer in a monad transformer stack. This package is Haskell 98 and thus can be also used with JHC.StateT
- monads-fd: Provides the same type classes with functional dependencies like MTL. They allow using methods also forStatetransformers within a transformer stack.StateT
- 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.

- transformers: Provide only concrete transformers like

## Contents |

## 1 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`

.

## 2 Shall I use MTL or transformers?

Transformers is Haskell 98 and thus more portable.

## 3 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,

import Control.Monad.State ...

import Control.Monad.Trans.State ...

State

State

state

runState

## 4 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