For clarity, one trick that uses "unsafePerformIO" which you may have
seen posted on this list earlier today is the following way of
creating a globally visible IORef:
import Data.IORef
import System.IO.Unsafe
*** counter = unsafePerformIO $ newIORef 0 ***
next = do
modifyIORef counter (+1)
readIORef counter
The key line has been starred; this created a counter IORef that
"next" could access without having to be explicitly given it on each
call.
Personally, though, I never use this approach because it creates a
race condition. Because Haskell is lazy, "counter" will not be
evaluated until the first time that it is accessed. If two threads
access it at the same time before it has been evaluated, then in GHC
there are conditions under which they could both evaluate it and
create a new IORef simultaneously, resulting in each thread
effectively ending up with its own counter variable when you
specifically wanted a globally shared counter variable.
This is why I personally use the ReaderT monad if I want to implicitly
share a global IORef, rather than using this trick.
Cheers,
Greg
On Oct 21, 2009, at 10:50 PM, Colin Paul Adams wrote:
zaxis> thank you! In fact i really donot understand
zaxis> "unsafePerformIO" very much !
Then all you have to understand is - never use it!
--
Colin Adams
Preston Lancashire
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
