[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
Henning Thielemann schlepp...@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. However, usage of unsafeInterleaveIO means that you aren't guaranteed that A1..An happens _exactly_ between any pair of Bk and Bk+1: The total ordering of actions is circumvented. Semantically, this is equivalent to sparking a thread on a uniprocessor, and equivalent to sparking a thread on a multiprocessor for all but the rarest combinations of IO actions. There are no lazy monads. Monads imply explicit sequencing... writing kiss girl == speakTo girl , denoting either (fist kiss the girl, then pass the result of that to speakTo girl) or (first speak to the girl, then pass the result of that to kiss girl) would, OTOH, certainly makes sense and certainly (well, depending on girl and setting) is undeterministic in both execution order and result. The question thus becomes: Does arbitrary ordering of actions result in the same (in the id sense) result, and do I care more about the aggregate result than its ordering, or do I only care about ordering of actions for a certain definition of 'same result'? Implementation of such a beast is left to the semantically inclined reader ;) In the meantime, I'm happy to just carelessly use unsafe*IO. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On Fri, 2 Jan 2009, Achim Schneider wrote: Henning Thielemann schlepp...@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) (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. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On Thu, Jan 1, 2009 at 7:39 PM, Achim Schneider bars...@web.de wrote: There are no lazy monads. Monads imply explicit sequencing... Huh? How are you defining lazy monad? -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On Fri, 2 Jan 2009, Achim Schneider wrote: There are no lazy monads. Monads imply explicit sequencing... writing I think this is an extremely bad thing to say and is a source of misunderstanding about monads and evaluation. Most monads _are_ lazy, and it is important to understand that when trying to understand the run-time properties of your monadic code. Monads sequence effects, but evaluation is an almost orthogonal issue. Here is a recent thread where I talk about laziness: http://www.reddit.com/r/haskell/comments/7itbi/mapm_mapm_and_monadic_statements/c06s6pm (for the short short story, simply try out take 10 $ execWriter (sequence_ (repeat (tell x))) ) Furthermore, the code in my article on recursive do from The.Monad.Reader issue #6 http://www.haskell.org/sitewiki/images/1/14/TMR-Issue6.pdf requires the monads to be lazy in order to tie the knot. -- Russell O'Connor http://r6.ca/ ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.'' ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
Henning Thielemann lemm...@henning-thielemann.de wrote: On Fri, 2 Jan 2009, Achim Schneider wrote: Henning Thielemann schlepp...@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. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On 2009 Jan 1, at 20:08, David Menendez wrote: On Thu, Jan 1, 2009 at 7:39 PM, Achim Schneider bars...@web.de wrote: There are no lazy monads. Monads imply explicit sequencing... Huh? How are you defining lazy monad? We've had this discussion before; somewhere in the archives is an example of a State monad doing things in data-driven order instead of the apparently explicit monadic sequencing. Monads don't insure sequencing unless designed to do so (as, for example, IO). -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
Achim Schneider bars...@web.de wrote: [...] actually, even better: main = do acts - unsafeInterleaveIO getActs putStrLn dumdidum sequence_ acts , which, at least on my machine, prints dumdidum before asking for foo. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On 2009 Jan 1, at 20:08, David Menendez wrote: On Thu, Jan 1, 2009 at 7:39 PM, Achim Schneider bars...@web.de wrote: There are no lazy monads. Monads imply explicit sequencing... Huh? How are you defining lazy monad? We've had this discussion before; somewhere in the archives is an example of a State monad doing things in data-driven order instead of the apparently explicit monadic sequencing. Monads don't insure sequencing unless designed to do so (as, for example, IO). The more I try to put stuff into words, the more I part from lambda calculus into the regions of syntactical pitfalls. Therefore, I'm just going to shut up. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
On Thu, Jan 1, 2009 at 8:29 PM, Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On 2009 Jan 1, at 20:08, David Menendez wrote: On Thu, Jan 1, 2009 at 7:39 PM, Achim Schneider bars...@web.de wrote: There are no lazy monads. Monads imply explicit sequencing... Huh? How are you defining lazy monad? We've had this discussion before; somewhere in the archives is an example of a State monad doing things in data-driven order instead of the apparently explicit monadic sequencing. Monads don't insure sequencing unless designed to do so (as, for example, IO). Certainly. I asked because Achim might have been making a point about about call-by-need versus call-by-value, or something. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: unsafeInterleaveIO respecting order of actions
David Menendez d...@zednenem.com wrote: On Thu, Jan 1, 2009 at 8:29 PM, Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On 2009 Jan 1, at 20:08, David Menendez wrote: On Thu, Jan 1, 2009 at 7:39 PM, Achim Schneider bars...@web.de wrote: There are no lazy monads. Monads imply explicit sequencing... Huh? How are you defining lazy monad? We've had this discussion before; somewhere in the archives is an example of a State monad doing things in data-driven order instead of the apparently explicit monadic sequencing. Monads don't insure sequencing unless designed to do so (as, for example, IO). Certainly. I asked because Achim might have been making a point about about call-by-need versus call-by-value, or something. Nah, I was speculating about (possibly better) ways to specify dependencies of side-effects. Ways, that is, that enable the computer to directly implement your perception of priorities like importance of ordering vs. importance of results. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
