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New monads

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== MonadBase ==
 
== 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.
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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]].
 
<haskell>
 
-- By Chris Kuklewicz
 
module MonadBase(MonadBase,liftBase,MonadIO',liftIO') where
 
 
import Data.Monoid(Monoid)
 
import Control.Monad.Trans(lift,MonadTrans)
 
-- All the base monads with GHC
 
import Control.Monad.ST.Strict as S(ST)
 
import Control.Monad.ST.Lazy as L(ST)
 
import Control.Concurrent.STM(STM)
 
import Control.Monad.Identity(Identity)
 
import Text.ParserCombinators.Parsec(GenParser)
 
-- And all the MonadIO instances:
 
import Control.Monad.List(ListT)
 
import Control.Monad.Cont(ContT)
 
import Control.Monad.Error(ErrorT,Error)
 
import Control.Monad.Reader(ReaderT)
 
import Control.Monad.State(StateT)
 
import Control.Monad.Writer(WriterT)
 
import Control.Monad.RWS(RWST)
 
 
class (Monad m,Monad b) => MonadBase b m where
 
liftBase :: b a -> m a
 
 
-- One can recover MonadIO and liftIO from MonadBase
 
class (MonadBase IO m) => MonadIO' m where
 
liftIO' :: IO a -> m a
 
liftIO' = liftBase
 
 
-- Base monads
 
instance MonadBase IO IO where liftBase = id
 
instance MonadBase STM STM where liftBase = id
 
instance MonadBase (GenParser tok st) (GenParser tok st) where liftBase = id
 
instance MonadBase (S.ST s) (S.ST s) where liftBase = id
 
instance MonadBase (L.ST s) (L.ST s) where liftBase = id
 
instance MonadBase Maybe Maybe where liftBase = id
 
instance MonadBase [] [] where liftBase = id
 
instance (Error e) => MonadBase (Either e) (Either e) where liftBase = id
 
instance MonadBase ((->) a) ((->) a) where liftBase = id
 
instance MonadBase Identity Identity where liftBase = id
 
 
-- Trans monads
 
instance MonadBase b m => MonadBase b (ListT m) where
 
liftBase = lift . liftBase
 
instance MonadBase b m => MonadBase b (ContT r m) where
 
liftBase = lift . liftBase
 
instance (Error e, MonadBase b m) => MonadBase b (ErrorT e m) where
 
liftBase = lift . liftBase
 
instance MonadBase b m => MonadBase b (ReaderT r m) where
 
liftBase = lift . liftBase
 
instance MonadBase b m => MonadBase b (StateT s m) where
 
liftBase = lift . liftBase
 
instance (Monoid w, MonadBase b m) => MonadBase b (WriterT w m) where
 
liftBase = lift . liftBase
 
instance (Monoid w, MonadBase b m) => MonadBase b (RWST r w s m) where
 
liftBase = lift . liftBase
 
</haskell>
 
 
And an artificial example:
 
 
<haskell>
 
module Main where
 
 
import MonadBase
 
import Control.Monad.Reader
 
import Control.Monad.Writer
 
type Foo a = (WriterT [Int] (ReaderT String [])) a
 
 
foo :: Foo String
 
foo = do
 
x <- liftBase [1,2,3]
 
s <- ask
 
tell [succ x]
 
return (s ++ show x)
 
 
test = runReaderT (runWriterT foo) "hello"
 
</haskell>
 
 
<pre>
 
*Main> test
 
[("hello1",[2]),("hello2",[3]),("hello3",[4])]
 
</pre>
 
 
   
 
== 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.