On Wed, 2008-11-12 at 23:02 +0000, David MacIver wrote: > On Wed, Nov 12, 2008 at 10:46 PM, Don Stewart <[EMAIL PROTECTED]> wrote: > > david.maciver: > >> On Wed, Nov 12, 2008 at 8:35 PM, Lennart Augustsson > >> <[EMAIL PROTECTED]> wrote: > >> > Actually, unsafeInterleaveIO is perfectly fine from a RT point of view. > >> > >> Really? It seems easy to create things with it which when passed to > >> ostensibly pure functions yield different results depending on their > >> evaluation order: > >> > >> module Main where > >> > >> import System.IO.Unsafe > >> import Data.IORef > >> > >> main = do w1 <- weirdTuple > >> print w1 > >> w2 <- weirdTuple > >> print $ swap w2 > >> > >> swap (x, y) = (y, x) > >> > >> weirdTuple :: IO (Int, Int) > >> weirdTuple = do it <- newIORef 1 > >> x <- unsafeInterleaveIO $ readIORef it > >> y <- unsafeInterleaveIO $ do writeIORef it 2 >> return 1 > >> return (x, y) > >> > >> [EMAIL PROTECTED]:~$ ./Unsafe > >> (1,1) > >> (1,2) > >> > >> So show isn't acting in a referentially transparent way: If the second > >> part of the tuple were evaluated before the first part it would give a > >> different answer (as swapping demonstrates). > > > > Mmmm? No. Where's the pure function that's now producing different > > results? I only see IO actions at play, which are operating on the > > state of the world. > > I suppose so. The point is that you have a pure function (show) and > the results of evaluating it totally depend on its evaluation order.
Sure. But only because the argument to it depends on its evaluation order, as well. jcc _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
