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{------------------------------------------------------------------------------
Control.Monad.Operational
Example:
List Monad Transformer
------------------------------------------------------------------------------}
{-# LANGUAGE GADTs, Rank2Types, FlexibleInstances #-}
module ListT where
import Control.Monad
import Control.Monad.Operational
import Control.Monad.Trans
{------------------------------------------------------------------------------
A direct implementation
type ListT m a = m [a]
would violate the monad laws, but we don't have that problem.
------------------------------------------------------------------------------}
data MPlus m a where
MZero :: MPlus m a
MPlus :: ListT m a -> ListT m a -> MPlus m a
type ListT m a = ProgramT (MPlus m) m a
-- *sigh* I want to use type synonyms for type constructors, too;
-- GHC doesn't accept MonadMPlus (ListT m)
instance Monad m => MonadPlus (ProgramT (MPlus m) m) where
mzero = singleton MZero
mplus m n = singleton (MPlus m n)
runListT :: Monad m => ListT m a -> m [a]
runListT = eval <=< viewT
where
eval :: Monad m => ProgramViewT (MPlus m) m a -> m [a]
eval (Return x) = return [x]
eval (MZero :>>= k) = return []
eval (MPlus m n :>>= k) =
liftM2 (++) (runListT (m >>= k)) (runListT (n >>= k))
testListT :: IO [()]
testListT = runListT $ do
n <- choice [1..5]
lift . print $ "You've chosen the number: " ++ show n
where
choice = foldr1 mplus . map return
-- testing the monad laws, from the Haskellwiki
-- http://www.haskell.org/haskellwiki/ListT_done_right#Order_of_printing
a,b,c :: ListT IO ()
[a,b,c] = map (lift . putChar) ['a','b','c']
-- t1 and t2 have to print the same sequence of letters
t1 = runListT $ ((a `mplus` a) >> b) >> c
t2 = runListT $ (a `mplus` a) >> (b >> c)
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