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 <[EMAIL PROTECTED]> wrote:
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
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