# Monad Transformers

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
`MonadTrans`

and`MonadIO`

, as well as concrete monad transformers such as`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. - mtl (Monad Transformer Library) comes in two versions:
- version 1 was the first implementation, containing the classes
`MonadTrans`

and`MonadIO`

, concrete monad transformers such as`StateT`

and multi-parameter type classes with functional dependencies such as`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`

. This version is now obsolete. - 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

## 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.

## 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