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).
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
