[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions

[Achim Schneider]

Henning Thielemann <lemming at henning-thielemann.de> wrote:

> 
> On Fri, 2 Jan 2009, Achim Schneider wrote:
> 
> > Henning Thielemann <schlepptop at henning-thielemann.de> wrote:
> >
> >> If it is generally possible to use unsafeInterleaveIO such that it
> >> executes actions in the right order, wouldn't this allow the
> >> definition of a general lazy IO monad?
> >>
> > The question is what "right order" means.
> >
> > Let B1..Bn be some arbitrary IO-Actions.
> > Let A1..An be some arbitrary IO Actions passed to unsafeInterleaveIO
> >
> > You're guaranteed that
> > a) Bk+1 is executed after Bk
> > b) Ak+1 is executed after Ak
> >
> > , all by virtue of the IO Monad.
> 
> If all Ak's are defered using individual unsafeInterleaveIO's then it
> is not guaranteed that A[k+1] is executed after A[k]. That's my
> problem.
> 
> Check:
> Prelude> fmap snd $ Monad.liftM2 (,) (unsafeInterleaveIO getLine)
> Prelude> (unsafeInterleaveIO getLine)
> 
> If unsafely interleaved actions would be executed in order, then this 
> would first ask you for the first pair member, then for the second
> one, then echo the second one. Actually it asks only for the second
> one and prints it.

module Main where
import System.IO.Unsafe

chooseAct :: String -> IO (IO ())
chooseAct s = do
    putStrLn $ s ++ "?"
    l <- getLine
    if (l == s)
        then return $ putStrLn $ "w00t! a " ++ s
        else return $ putStrLn "bah"

getActs :: IO [IO ()]
getActs = mapM chooseAct ["foo", "bar", "baz"]

main0 = unsafeInterleaveIO getActs >>= unsafeInterleaveIO . sequence_
main1 = unsafeInterleaveIO getActs >>= sequence_

main = main0 >> main1

There you've got the ordering.

It's quite easy to write a haskell program that reduces itself to 
main = return (), though.

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