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Monad Transformers

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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
    StateT
    and abstractions using multi-parameter type classes with functional dependencies like
    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
    . Because of the functional dependencies, MTL can currently (2010-03) only used in Hugs and GHC. MTL was the first implementation.
  • The newer implementation is derived from the former one and is split into the following components:
    • transformers: Provide only concrete transformers like
      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.
    • monads-fd: Provides the same type classes with functional dependencies like MTL. They allow using
      State
      methods also for
      StateT
      transformers within a transformer stack.
    • 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.


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,

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 pattern matching you must call
runState
.

4 See also