comonads, io

[Andrew J Bromage]

G'day all.

On Wed, Jan 01, 2003 at 01:15:09PM +0100, Nicolas.Oury wrote:

> From this, I think the safety become intuitive.

Maybe.

Using the OI module from Richard Kieburtz' paper, I can write
this:

	-- Bootstrap into the OI comonad
	main :: IO ()
	main = return $! comain stdOI

	-- The following are the OI functions which we use.
	-- stdGetChar :: OI () -> Char
	-- stdPutStrLn :: OI String -> ()

	comain :: OI a -> ()
	comain w
	    = coeval (w .>> show (a,b) =>> stdPutStrLn)
	    where
		a = coeval (w .>> () =>> stdGetChar)
		b = coeval (w .>> () =>> stdGetChar)

Even though a and b are identical, they return different values.  I see
two possibilities: Either my intuition is way off, or the OI comonad
breaks referential transparency.

It's possible that it's a fault in the implementation.  It was clearly
intended as an example only, after all.

Cheers,
Andrew Bromage