Re: [Haskell-cafe] Beginner: IORef constructor?

1 Dec 2006


      Donald:
...
Now, if you wanted to pass that ref to other functions, you'd have to
thread it explicitly -- unless you store it in a state monad :)
i.e. do ref <- theGlobalVariable
            ...
            .. f ref
            ...
f r = do
            ...
            .. g r
            ...
I kind of jumped ahead that step, and went straight to the implicitly
threaded version.
-- Don
Tested my code again and it doesn't work as expected. I don't
understand what "threading" means, but is that the reason I can't have
this:

----------------------------------------------------------
module StateTest where

import Data.IORef

theGlobalVariable = newIORef []

modify1 = do ref <- theGlobalVariable
             original <- readIORef ref
             print original
             writeIORef ref $ original ++ [1]
             new <- readIORef ref
             print new

modify2 = do ref <- theGlobalVariable
             original <- readIORef ref
             print original
             writeIORef ref $ original ++ [2]
             new <- readIORef ref
             print new

doIt = do modify1
          modify2
----------------------------------------------------------

TJ