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

On Wed, 2008-11-12 at 23:02 +0000, David MacIver wrote:

On Wed, Nov 12, 2008 at 10:46 PM, Don Stewart wrote:

...

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.

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