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Monad Transformers

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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.
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 version 2.
  • 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. But this also means it lacks the monad classes, so you'll have to lift operations to the composite monad yourself (examples).

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,

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
.

4 See also