Thanks for the demo. I don't actually understand what's going on yet,
but your code doesn't really use a global variable, does it? From
what I can understand, the main function is passing the State to the
other functions.
I think I was careless about mixing "IO functions" and normal
functions. Now that I think about it, my "global variable" really
should only be available to IO functions, so the following should be
just fine:
----------------------------------------------------------
module Global where
import Data.IORef
theGlobalVariable = newIORef []
testIt = do ref <- theGlobalVariable
original <- readIORef ref
print original
writeIORef ref [1,2,3]
new <- readIORef ref
print new
----------------------------------------------------------
I've got a lot to learn about Haskell...
On 12/1/06, Donald Bruce Stewart
tjay.dreaming:
Thanks. I've been reading the docs and examples on State (in Control.Monad.State), but I can't understand it at all. ticks and plusOnes... All they seem to do is return their argument plus 1...
Here's a little demo. (I agree, the State docs could have nicer demos)
Play around with the code, read the haddocks, and it should make sense eventually :)_
-- Don
import Control.Monad.State
-- -- the type for a 'global' 'variable' -- data T = T { ref :: Int }
-- Run code with a single global 'ref', initialised to 0 main = evalStateT g $ T { ref = 0 }
-- set it to 10 g = do printio "g" putRef 10 printio "modified state" f
-- read that ref, print it f = do r <- getRef printio r return ()
getRef = gets ref
putRef x = modify $ \_ -> T { ref = x }
printio :: Show a => a -> StateT T IO () printio = liftIO . print