[Haskell-cafe] What unsafeInterleaveIO is unsafe

[Daniel Fischer]

Am Sonntag, 15. März 2009 21:25 schrieb Jonathan Cast:
> On Sun, 2009-03-15 at 13:02 -0700, Ryan Ingram wrote:
>
> > Furthermore, due to the monad laws, if f is total, then reordering the
> > (x <- ...) and (y <- ...) parts of the program should have no effect.
> > But if you switch them, the program will *always* print 0.
>
> I'm confused.  I though if f was strict, then my program *always*
> printed 1?

But not if you switch the (x <- ...) and (y <- ...) parts:

main = do
    r <- newIORef 0
    v <- unsafeInterleaveIO $ do
        writeIORef r 1
        return 1
    y <- readIORef r
    x <- case f v of
            0 -> return 0
            n -> return (n - 1)
    print y

Now the IORef is read before the case has a chance to trigger the writing.