unsafeInterleaveIO allows embedding side effects into a pure computation. This means you can potentially observe if some pure value has been evaluated or not; the result of your code could change depending how lazy/strict it is, which is very hard to predict!
For example: > -- given > f :: Integer -> Integer > > main = do > r <- newIORef 0 > v <- unsafeInterleaveIO $ do > writeIORef r 1 > return 1 > x <- case f v of > 0 -> return 0 > n -> return (n - 1) > y <- readIORef r > print y > > -- a couple of examples: > f x = 0 -- program prints "0" > -- f x = x -- program prints "1" "f" is pure. But if f is nonstrict, this program prints 0, and if it's strict, it prints 1. The strictness of a pure function can change the observable behavior of your program! 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. Also, the compiller might notice that x is never used, and that "f" is total. So it could just optimize out the evaluation of "f v" completely, at which point the program always prints 0 again; changing optimization settings modifies the result of the program. This is why unsafeInterleaveIO is unsafe. -- ryan On Sun, Mar 15, 2009 at 11:31 AM, Yusaku Hashimoto <nonow...@gmail.com> wrote: > Hello, > > I was studying about what unsafeInterleaveIO is.I understood > unsafeInterleaveIO takes an IO action, and delays it. But I couldn't > find any reason why unsafeInterleaveIO is unsafe. > > I have already read an example in > http://www.haskell.org/pipermail/haskell-cafe/2009-March/057101.html > says lazy IO may break purity, but I think real matter in this example > are wrong use of seq. did I misread? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
