G'day all. On Sun, Jun 30, 2002 at 01:51:56PM +0100, Peter G. Hancock wrote:
Why not have a monad m a = Int -> (a,Int) which is a state monad plus the operation bump : Int -> m Int
bump k n = (n,n+k)
Oh, ye of insufficient genericity. We can do better than that... import MonadTrans class (Monad m, Enum i) => MonadCounter i m | m -> i where bump :: Int -> m i newtype CounterT i m a = CounterT { runCounterT :: i -> m (a,i) } instance (Monad m, Enum i) => Monad (CounterT i m) where return a = CounterT $ \x -> return (a, x) m >>= k = CounterT $ \x -> do (a, x') <- runCounterT m x runCounterT (k a) x' fail str = CounterT $ \_ -> fail str instance (Monad m, Enum i) => MonadCounter i (CounterT i m) where bump k = CounterT $ \x -> let (next:_) = drop k [x..] in return (x, next) instance (Enum i) => MonadTrans (CounterT i) where lift m = CounterT $ \x -> do a <- m return (a, x) evalCounterT :: (Monad m, Enum i) => CounterT i m a -> i -> m a evalCounterT m x = do (a, _) <- runCounterT m x return a -- Example code follows main :: IO () main = evalCounterT count 0 count :: CounterT Int IO () count = do x1 <- bump 1 x2 <- bump 5 x3 <- bump 0 x4 <- bump 1 lift (putStrLn $ show [x1,x2,x3,x4]) I'd better get back to work now. Cheers, Andrew Bromage