Re: [Haskell-cafe] What unsafeInterleaveIO is unsafe

15 Mar 2009


      On Sun, 2009-03-15 at 18:11 -0700, Ryan Ingram wrote:
...
On Sun, Mar 15, 2009 at 1:56 PM, Jonathan Cast
 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