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. But you're still in the IO monad at this point so I suppose it technically "doesn't count" because it's only observable as the result of IO. It's pretty suspect behaviour, but not actually a failure of referential transparency. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
