Re: [Haskell-cafe] safe lazy IO or Iteratee?

Thanks for the detailed response below... I must be able to understand how the resources will be used in my system for mission-critical, long-running applications. Dave On Thu, Feb 4, 2010 at 12:51 PM, John Millikin wrote:

Both have advantages and disadvantages. The primary advantage of lazy IO over iteratees is that it's much, *much* easier to understand -- existing experience with monads can be used immediately. The downsides of lazy IO, of course, are well documented[1][2][3].

Some are fixed by the safe/strict IO packages. However, safe lazy IO is still "unpredictable" in that it's difficult to know how many resources will be used, the order of some operations (eg, releasing handles), or whether some particular expression will throw an exception.

Iteratees are useful because the author can make and verify guarantees about performance (eg, "this code will never read more than 4 KiB at once"). They also allow pure code to determine the behavior of IO, safely, which is useful when writing libraries which must deal with large amounts of data (eg, streaming a file over HTTP). The Hyena web server is written using iteratees, and from what I've heard the authors have been quite happy with their properties.

I've also found iteratees to perform somewhat better than lazy IO, though I don't know why.

Downside: iteratees are very hard to understand. I wrote a decently-sized article about them[4] trying to figure out how to make them useful, and some comments in one of Oleg's implementations[5] suggest that the "iteratee" package is subtly wrong. Oleg has written at least three versions (non-monadic, monadic, monadic CPS) and I've no idea why or whether their differences are important. Even dons says he didn't understand them until after writing his own iteratee-based IO layer[6].

tl;dr: Lots of smart people, with a history of being right about this sort of thing, say iteratees are better. Evidence suggests iteratee-based IO is faster and more predictable than lazy IO. Iteratees are really hard to understand.

[1] http://okmij.org/ftp/Haskell/Iteratee/Lazy-vs-correct.txt [2] http://www.kuliniewicz.org/blog/archives/2010/01/27/happstack-and-streaming-... [3] http://forums.xkcd.com/viewtopic.php?f=11&t=46780 [4] http://ianen.org/articles/understanding-iteratees/ [5] http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs , search for "Drawbacks of this encoding" [6] http://stackoverflow.com/questions/1319705/introduction-or-simple-examples-f...

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Hi everyone, This is not an attempt to start a flame war. I'm just trying to get a good feel for the advantages and disadvantages of the newer safe lazy io lib available on Hackage vs using Iteratee. It does appear to me that using something like Itereatee gives a bit of room to really tweak the low level enumerator/iteratee relationship, and, if you asked my old boss, I'm just a big fan of folds too, I use them a lot in my Erlang :-), so Iteratee really does appeal to me on many levels. Yet at the same time, I'm quite enamored with the beauty of "interact" and functions of that sort. I realize mixing the effects of the lazy IO and pure code may not be the clearest way to write code for everyone, but

On Thu, Feb 4, 2010 at 08:29, David Leimbach wrote: there

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is something about being able to get linewise data as interact (unlines . fmap someLineWiseFunction . lines) that is just kind of cool. Dave _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe