My thought is that you could simply drop the IO from your type definition, type PDState = StateT PD You will need to change all of your type signature from "PDState <type>" to "PDState m <type>" to make them all polymorphic over the choice of monad. Then all you should need to do is to generalize the loop function to accept a line-fetching monad from the user: loop :: m a -> PDState m a loop getLine = forever $ do cmd <- lift getLine runCmd cmd Note how liftIO was changed to lift, which works for any monad and comes built-in with the StateT monad. Hope this helps! Cheers, Greg On Oct 5, 2009, at 4:56 PM, Floptical Logic wrote:
The code below is a little interactive program that uses some state. It uses StateT with IO to keep state. My question is: what is the best way to generalize this program to work with any IO-like monad/medium? For example, I would like the program to function as it does now using stdin but I would also like it to function over IRC using the Net monad from http://haskell.org/haskellwiki/Roll_your_own_IRC_bot. Thanks for any suggestions.
-- begin code -- import Control.Monad import Control.Monad.State import Data.List
data PD = PD { pdCount :: Int , pdList :: [String] } deriving (Show)
type PDState = StateT PD IO
main = runStateT loop (PD { pdCount = 0, pdList = [] })
loop :: PDState a loop = forever $ do cmd <- liftIO getLine runCmd cmd
runCmd :: String -> PDState () runCmd "Inc" = increment runCmd "PrintCount" = liftIO . print =<< getCount runCmd "PrintList" = liftIO . print =<< getList runCmd str | "Add " `isPrefixOf` str = addToList $ drop 4 str runCmd _ = return ()
getCount :: PDState Int getCount = pdCount `liftM` get
getList :: PDState [String] getList = pdList `liftM` get
increment :: PDState () increment = modify $ \st -> st { pdCount = pdCount st + 1 }
addToList :: String -> PDState () addToList str = modify $ \st -> st { pdList = pdList st ++ [str]} -- end code -- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe