On Sun, 2009-03-15 at 18:11 -0700, Ryan Ingram wrote: > On Sun, Mar 15, 2009 at 1:56 PM, Jonathan Cast > <jonathancc...@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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
