# Monad Transformers

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## Revision as of 13:34, 15 December 2010

There are currently several packages that implement similar interfaces to monad transformers (besides an additional package with a similar goal but different API named MonadLib):

- transformers: provides the classes andMonadTrans, as well as concrete monad transformers such asMonadIO. 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
- mtl (Monad Transformer Library) comes in two versions:
- version 1 was the first implementation, containing the classes andMonadTrans, concrete monad transformers such asMonadIOand multi-parameter type classes with functional dependencies such asStateT. Monads likeMonadStateand their transformer counterparts likeStateare distinct types and can be accessed uniformly only through a type class abstraction likeStateT. This version is now obsolete.MonadState
- version 2 re-exports the classes and monad transformers of the transformers package, and adds multi-parameter type classes with functional dependencies such as .MonadState

- Version 2 of the MTL has some small incompatibilities relative to version 1. See "Upgrading from MTL 1 to MTL 2" for instructions on how to make code written for version 1 work with version 2.

- version 1 was the first implementation, containing the classes

- Because of the functional dependencies, MTL can currently (2010-03) only used in Hugs and GHC. MTL was the first implementation.

- monads-fd: this was the prototype of the new mtl implementation. It is now obsolete, and simply re-exports mtl.
- monads-tf: Provides a different abstraction using type families. Unfortunately the module names of
`mtl`

and`monads-tf`

clash, so you can currently not import both packages in one package.

## Contents |

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

MTL and transformers use different module names, but share common classes, type constructors and functions, so they are fully compatible.

## 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
- 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