[Haskell-cafe] What unsafeInterleaveIO is unsafe

[Jonathan Cast]

On Sun, 2009-03-15 at 21:43 +0100, Daniel Fischer wrote:
> 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.

But if the compiler is free to do this itself, what guarantee do I have
that it won't?

jcc