Monad Transformers

From HaskellWiki

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? 4 See also

1 How can I use MTL and transformers together?

Q: When I use ghc or ghci it complains about the same module names in mtl and transformers or monads-fd. How can I resolve these name clashes? A: You can use the -hide-package option of GHC. Cabal uses the -hide-all-packages option and then explicitly makes every package visible with -package.

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,

e.g. must be replaced by . Since is only a type synonym, there is no longer a constructor named . For constructing you must use the function and instead of matching patterns you must call .

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

Categories: Packages | Monad | FAQ