[Haskell-cafe] What unsafeInterleaveIO is unsafe

[Jonathan Cast]

On Sun, 2009-03-15 at 18:11 -0700, Ryan Ingram wrote:
> On Sun, Mar 15, 2009 at 1:56 PM, Jonathan Cast
> <jonathanccast at fastmail.fm> wrote:
> >> 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?
> 
> You don't really have any guarantee; the compiler is free to assume
> that v is a pure integer and that f is a pure function from integers
> to integers.  Therefore, it can assume that the only observable affect
> of calling f v is non-termination.  Note that unsafeInterleaveIO
> *breaks* this assumption;

[Ignored; begging the question]

jcc