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New monads

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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.
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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. 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.

Retrieved from "http://www.haskell.org/haskellwiki/New_monads"