[Haskell-cafe] What unsafeInterleaveIO is unsafe

[Daniel Fischer]

Am Sonntag, 15. März 2009 21:56 schrieb 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?
>

None?

Wasn't that Ryan's point, that without the unsafeInterleaveIO, that reordering 
wouldn't matter, but with it, it does?

> jcc