View source for New monads/MonadBase

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From New monads

= 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.

<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>