On Friday 13 March 2009, Cristiano Paris wrote: > 2009/3/13 Marcin Kosiba <marcin.kos...@gmail.com>: > > Hi, > > I've already checked those out. I tried using your yield > > implementation and while it works, I couldn't get it to work with the > > state monad. > > So while: > > data RecPair a b = Nil | RP (b, a -> RecPair a b) > > yield x = Cont $ \k -> RP (x, k) > > > > got me half-way to my goal, I couldn't figure out how to make > > something like: > > > > yield' = do > > state <- get > > state' <- yield state > > put state' > > Basically, the yield is built upon the Cont monad which has a > transformer counter part, ContT. You could try and re-implement the > yield under ContT instead of just Cont then you can stack ContT on top > of State (or StateT if you need more monads) and have a state (i.e. > get/put) and the yield.
Great! That helped a lot. I'm attaching a ConT yield implementation and
another one which also handles a return statement with a different type. Hope
someone finds them useful.
Thanks!
Marcin Kosiba
{-# OPTIONS -XNoMonomorphismRestriction #-}
module Main where
import Control.Monad
import Control.Monad.Cont
import Control.Monad.State
import Control.Monad.Identity
data (Monad m) => RecPair m a b = Nil | RP (b, a -> m (RecPair m a b))
yield :: (Monad m) => r -> ContT (RecPair m a r) m a
yield x = ContT $ \k -> return $ RP(x, k)
f'cps = return 2
test = do
x <- f'cps
yield x
yield (x + 1)
return ()
testSt :: (MonadState s m, Num s) => ContT (RecPair m a s) m ()
testSt = do
y <- f'cps
v <- get
put (y + 1)
yield v
v' <- get
yield v'
return ()
getRP :: RecPair Identity a a -> [a]
getRP Nil = []
getRP (RP (b, f)) = b : (getRP $ runIdentity $ f b)
runYield :: ContT (RecPair m a1 b) Identity a -> RecPair m a1 b
runYield f = runIdentity $ runContT f (\_ -> return Nil)
--result is [2,3]
runTest = getRP $ runYield test
getRPSt :: (RecPair (State t) a a, t) -> [a]
getRPSt (Nil, _) = []
getRPSt (RP (b, f), s) = b : (getRPSt $ runState (f b) s)
runYieldSt :: (Num s) => s -> (RecPair (State s) a s, s)
runYieldSt iState = runState (runContT testSt (\_ -> return Nil)) iState
--result is [iState, 3]
runTestSt iState = getRPSt $ runYieldSt iState{-# OPTIONS -XNoMonomorphismRestriction #-}
module Main where
import Control.Monad
import Control.Monad.Cont
import Control.Monad.State
import Control.Monad.Identity
data (Monad m) => RecPair m a b r = Nil r | RP (b, a -> m (RecPair m a b r))
yield :: (Monad m) => r -> ContT (RecPair m a r v) m a
yield x = ContT $ \k -> return $ RP(x, k)
f'cps = return 2
test = do
x <- f'cps
yield x
yield (x + 1)
return (-1)
testSt = do
y <- f'cps
v <- get
put (y + v)
yield v
testSt
getRP :: RecPair Identity a a a -> [a]
getRP (Nil x) = [x]
getRP (RP (b, f)) = b : (getRP $ runIdentity $ f b)
runYield :: ContT (RecPair m a1 b a) Identity a -> RecPair m a1 b a
runYield f = runIdentity $ runContT f (\x -> return $ Nil x)
--result is [2,3, -1]
runTest = getRP $ runYield test
getRPSt :: (RecPair (State t) v v v, t) -> [v]
getRPSt (Nil x, _) = [x]
getRPSt (RP (b, f), s) = b : (getRPSt $ runState (f b) s)
runYieldSt :: ContT (RecPair m a1 b a) (State s) a
-> s
-> (RecPair m a1 b a, s)
runYieldSt f iState = runState (runContT f (\x -> return $ Nil x)) iState
--result is [iState, iState + 2..]
runTestSt iState = getRPSt $ runYieldSt testSt iState
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