# Monad Transformers

### From HaskellWiki

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(initial version with links to many discussions of MTL and Transformers) |
(explanation of transformers set of packages) |
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besides a third package with a similar goal but different API named [[MonadLib]]: |
besides a third package with a similar goal but different API named [[MonadLib]]: |
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− | * [http://hackage.haskell.org/package/mtl MTL] - Monad Transformers Library: provides concrete monad transformers like <hask>StateT</hask> and abstractions using multi-parameter classes with functional dependencies like <hask>MonadState</hask> |
+ | * [http://hackage.haskell.org/package/mtl MTL] - Monad Transformers Library: provides concrete monad transformers like <hask>StateT</hask> and abstractions using [[multi-parameter type class]]es with [[functional dependencies]] like <hask>MonadState</hask>. Monads like <hask>State</hask> and their transformer counterparts like <hask>StateT</hask> are distinct types and can be accessed uniformly only through a type class abstraction like <hask>MonadState</hask>. Because of the functional dependencies, MTL can currently (2010-03) only used in [[Hugs]] and [[GHC]]. MTL was the first implementation. |

− | * [http://hackage.haskell.org/package/transformers transformers] |
+ | * The newer implementation is derived from the former one and is split into the following components: |

+ | ** [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]]. |
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+ | ** [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. |
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+ | ** [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, |
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+ | thus you can currently not import both packages in one package. |
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+ | |||

+ | |||

+ | == How can I use MTL and transformers together? == |
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+ | |||

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

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

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

## Revision as of 22:58, 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,

- transformers: Provide only concrete transformers like

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

## Contents |

## 1 How can I use MTL and transformers together?

## 2 Shall I use MTL or transformers?

## 3 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,

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