Scott, Here's the most straightforward way to do it: -- process :: Integer -> Integer -> StateT Int IO Integer process x y = do s <- get put $ s + 1 return $ 2 * x * y doit :: StateT Int IO () doit = do p <- process 42 43 liftIO $ printf "f x y = %d\n" p main :: IO () main = do n <- execStateT doit 0 putStrLn $ "done "++show n++" times" -- One thing you'll note is that the type of 'doit' has changed. There's no way to pass state "through" a function without it being reflected in the type, and in many ways, that's the point of Haskell - to make potentially dangerous things explicit. An alternative is to use an IORef, but that makes your code completely imperative style, which is not very Haskellish. One thing you'll notice is that process is now in IO, which is not desirable, since it's pure. On occasions I've written this helper function: -- | Adapt a StateT to a pure state monad. purely :: Monad m => State s a -> StateT s m a purely code = do s <- get let (ret, s') = runState code s put s' return ret With this you could re-write it as... -- process :: Integer -> Integer -> State Int Integer process x y = do s <- get put $ s + 1 return $ 2 * x * y doit :: StateT Int IO () doit = do p <- purely $ process 42 43 liftIO $ printf "f x y = %d\n" p -- Monad transformer stacks aren't perfect, but they're good if used appropriately. If you use them a lot, then it can lead to a necessity to unstack and re-stack them like I did here. I think monads work best if you initially think of your code in plain Haskell terms, and introduce them later as a convenience. As I'm sure you know, the "Haskell way" is to make code as pure as possible, using IO types only where necessary. Steve Scott Thoman wrote:
Since I'm very new to Haskell I have what is probably a simple question yet I'm having trouble finding a clear example of how it works. The basic question is: how do I pass state through existing code without the intermediate code knowing about it. If I have, for example, several layers of function calls and the innermost function needs to access some state that is only "seeded" by the outermost function, how do I do that without the functions in between knowing about the additional state being threaded through them?
I have a simple example (that may *not* be good idiomatic Haskell):
-- process :: Integer -> Integer -> Integer process x y = 2 * x * y
doit :: IO () doit = do printf "f x y = %d\n" $ process 42 43
main :: IO () main = do doit putStrLn "done" --
(I'm not totally sure about the type of "doit" but the code compiles and runs as expected)
What I want to do is add some state handing to "process" to have it, say, count the number of times it's been called (putting threading/thread-local concerns aside for the moment). I'm trying to understand how to add state to "process" along the lines of:
-- process :: Integer -> Integer -> State Integer Integer process x y = do s <- get put $ s + 1 return $ 2 * x * y --
but I want to only seed the state from "main" without "doit" having to change -- I can call "process" from "doit" like "(execState (process 42 43) 0)" but I want the initial state to be determined at the top level, from main.
I have a feeling there's some kind of "ah ha" moment that I'm just not seeing yet. Any help or pointers to where I can look for myself would be greatly appreciated.
Thanks in advance,
-thor _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners