Re: [Haskell-cafe] Proof that Haskell is RT

david.maciver:

On Wed, Nov 12, 2008 at 8:35 PM, Lennart Augustsson 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)

david@mel:~$ ./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.