# New monads

### From HaskellWiki

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== MonadBase == |
== MonadBase == |
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− | It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc. |
+ | It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc. View [[MonadBase]]. |

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− | <haskell> |
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− | -- By Chris Kuklewicz |
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− | module MonadBase(MonadBase,liftBase,MonadIO',liftIO') where |
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− | import Data.Monoid(Monoid) |
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− | import Control.Monad.Trans(lift,MonadTrans) |
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− | -- All the base monads with GHC |
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− | import Control.Monad.ST.Strict as S(ST) |
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− | import Control.Monad.ST.Lazy as L(ST) |
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− | import Control.Concurrent.STM(STM) |
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− | import Control.Monad.Identity(Identity) |
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− | import Text.ParserCombinators.Parsec(GenParser) |
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− | -- And all the MonadIO instances: |
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− | import Control.Monad.List(ListT) |
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− | import Control.Monad.Cont(ContT) |
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− | import Control.Monad.Error(ErrorT,Error) |
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− | import Control.Monad.Reader(ReaderT) |
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− | import Control.Monad.State(StateT) |
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− | import Control.Monad.Writer(WriterT) |
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− | import Control.Monad.RWS(RWST) |
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− | class (Monad m,Monad b) => MonadBase b m where |
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− | liftBase :: b a -> m a |
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− | -- One can recover MonadIO and liftIO from MonadBase |
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− | class (MonadBase IO m) => MonadIO' m where |
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− | liftIO' :: IO a -> m a |
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− | liftIO' = liftBase |
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− | |||

− | -- Base monads |
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− | instance MonadBase IO IO where liftBase = id |
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− | instance MonadBase STM STM where liftBase = id |
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− | instance MonadBase (GenParser tok st) (GenParser tok st) where liftBase = id |
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− | instance MonadBase (S.ST s) (S.ST s) where liftBase = id |
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− | instance MonadBase (L.ST s) (L.ST s) where liftBase = id |
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− | instance MonadBase Maybe Maybe where liftBase = id |
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− | instance MonadBase [] [] where liftBase = id |
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− | instance (Error e) => MonadBase (Either e) (Either e) where liftBase = id |
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− | instance MonadBase ((->) a) ((->) a) where liftBase = id |
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− | instance MonadBase Identity Identity where liftBase = id |
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− | -- Trans monads |
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− | instance MonadBase b m => MonadBase b (ListT m) where |
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− | liftBase = lift . liftBase |
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− | instance MonadBase b m => MonadBase b (ContT r m) where |
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− | liftBase = lift . liftBase |
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− | instance (Error e, MonadBase b m) => MonadBase b (ErrorT e m) where |
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− | liftBase = lift . liftBase |
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− | instance MonadBase b m => MonadBase b (ReaderT r m) where |
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− | liftBase = lift . liftBase |
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− | instance MonadBase b m => MonadBase b (StateT s m) where |
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− | liftBase = lift . liftBase |
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− | instance (Monoid w, MonadBase b m) => MonadBase b (WriterT w m) where |
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− | liftBase = lift . liftBase |
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− | instance (Monoid w, MonadBase b m) => MonadBase b (RWST r w s m) where |
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− | liftBase = lift . liftBase |
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− | </haskell> |
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− | |||

− | And an artificial example: |
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− | <haskell> |
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− | module Main where |
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− | import MonadBase |
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− | import Control.Monad.Reader |
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− | import Control.Monad.Writer |
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− | type Foo a = (WriterT [Int] (ReaderT String [])) a |
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− | foo :: Foo String |
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− | foo = do |
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− | x <- liftBase [1,2,3] |
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− | s <- ask |
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− | tell [succ x] |
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− | return (s ++ show x) |
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− | test = runReaderT (runWriterT foo) "hello" |
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− | </haskell> |
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− | <pre> |
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− | *Main> test |
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− | [("hello1",[2]),("hello2",[3]),("hello3",[4])] |
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− | </pre> |
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− | |||

== MonadLib == |
== MonadLib == |

## Revision as of 17:10, 24 August 2006

## Contents |

## 1 MonadBase

It seems that the liftIO function from MonadIO can be generalized to access whatever the base of a transformer stack happens to be. So there is no need for a liftSTM, liftST, etc. View MonadBase.

## 2 MonadLib

This is by Iavor S. Diatchki and can be found at http://www.cse.ogi.edu/~diatchki/monadLib/

It is a new version of the mtl package with transformers: ReaderT WriterT StateT ExceptT SearchT ContT

It also defines BaseM which is like MonadBase above.