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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 == |