Re: [Haskell-cafe] Delaling with State StateT and IO in the same function

I don't know if this will help or not, but there's a basic StateT example on the haskell wiki that you could look at, to see how to deal with State in general. The code is at http://www.haskell.org/haskellwiki/Simple_StateT_use and is thanks to Don Stewart. Maybe I'll just paste the code with a few more comments (with the warning that I'm a newbie as well): import Control.Monad.State main :: IO () main = runStateT code [1..] >> return () -- Here, the state is just a simple stack of integers. runStateT is the equivalent -- in the StateT monad of runState in the State monad code :: StateT [Integer] IO () code = do x <- pop -- pop an item out of the stack io $ print x -- now in the INNER monad, perform an action return () -- -- pop the next unique off the stack -- pop :: StateT [Integer] IO Integer -- This type signature is correct, but it's the reason you have to be -- careful with StateT. pop really has nothing to do with IO, but it has -- been 'tainted' by IO because it's being done together with it pop = do (x:xs) <- get -- get the list that's currently in the stack put xs -- put back all but the first return x -- return the first io :: IO a -> StateT [Integer] IO a -- transform an action from being in the inner monad (in this case IO), to -- being in the outer monad. since IO is so familiar, it's been written already -- and it's called liftIO io = liftIO Gurus, please check my comments to be sure I haven't said something stupid! Hope this helps. Andrew On 2/26/07, Alfonso Acosta wrote:

On 2/27/07, Kirsten Chevalier wrote:

...

So what if you changed your netlist function so that the type sig would be:

netlist :: DT.Traversable f => (State s (S HDPrimSignal) -> State s v ) -> -- new (State s (Type,v) -> S v -> State s ()) -> -- define State s (f HDPrimSignal) -> -- the graph IO (State s ())

...

Or why not:

netlist :: DT.Traversable f => (State s (S HDPrimSignal) -> State s v ) -> -- new (State s (Type,v) -> S v -> State s ()) -> -- define State s (f HDPrimSignal) -> -- the graph IO s

Uhm, this looks better, I'll try with this one and see what I get, I anyway suspect I'll have a hard time because of the nested monads _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe