June 2009 - Haskell-Cafe

Compiling to C
by Greg Santucci 19 Jun '09

19 Jun '09

Hi all, I'd like to call Haskell functions from C and receive its output as a string. Is compiling a Haskell program to C using ghc -C HaskellSource.hs the preferred method of doing this? Regards, Greg

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About the Monad Transformers
by .shawn 19 Jun '09

19 Jun '09

On page 141 of "Yet another Haskell Tutorial" (9.7 Monad Transformers) mapTreeM action (Leaf a) = do lift (putStrLn ("Leaf" ++ show a)) b <- action a return (Leaf b) mapTreeM :: (MonadTrans t, Monad (t IO), Show a) => (a -> t IO a1) -> Tree a -> t IO (Tree a1) Why does the type signature of mapTreeM look like this? And what does it mean by "The lift tell us that we're going to be executing a command in an enclosed monad. In this case the enclosed monad is IO"? Why does the author put lift here? How does the lift work? I have no idea about the explanation in the book...is there anyone can give me some hints about this? Thank you in advance! _________________________________________________________________ Messenger10年嘉年华，礼品大奖等你拿！ http://10.msn.com.cn

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Fwd: [Haskell-cafe] Re: curious about sum
by Alberto G. Corona 18 Jun '09

18 Jun '09

However it does not work as I expected. I ´m interested in memory management. I though that ghci> let l= [1..1000000] ghci> foldl' (+) 0 l would produce a stack overflow, since the list can not be freed, because l points to the beginning of the list however it succeed My conclussion is that, in the case of sum, with the lazy evaluation, isn´t the list what is the cause of the stack overflow, but the lazy structure 0 + 1 +2 + 3 +4.... created by foldl before the final evaluation I tried to test how I can force a stack overflow, in the strict case. Since foldl' is strict, the only way is to create a long long initial list. ghci> let l= [1..100000000] -- 100 times larger, two 0's added ghci> foldl' (+) 0 l as in the previous case, l will not garbage collect, and foldl' must build the real list [1,2,3,4..] . at the end perhaps a stack overflow willl be produced This is not the case in my windows system.ghc 6.10.1. instead of that, ghci grow to gigabyte size and more. I stopped the process or my machine would be irresponsive. My question is: Why the process does not grow also in the lazy case and instead produces a stack overflow inmediately? Yes I know that all of this is bizantine since strictness analysis would solve this when compiled, but I think that there are somethig that i don´t understand. It´s a bug perhaps??? 2009/6/18 Edward Kmett <ekmett(a)gmail.com> > On Thu, Jun 18, 2009 at 9:14 AM, Gleb Alexeyev <gleb.alexeev(a)gmail.com>wrote: > >> Thomas Davie wrote: >> >>> No, I think it's extremely useful. It highlights that numbers can both >>> be lazy and strict, and that the so called "useless" lazy sum, is in fact, >>> useful. >>> >>> But lazy sum should have beed defined in terms of foldr, not foldl. And >> foldl is not strict enough for strict sum. Therefore the current choice in >> the worst of both worlds. >> > > > I definitely agree with that sentiment. > > -Edward Kmett > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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how to #include files within parsec ... without unsafePerformIO?
by Leonard Siebeneicher 18 Jun '09

18 Jun '09

Dear reader, I wonder whether there is a 'general' working solution to include files within a parsec parser. Without the need of unsafePerformIO. Appending an example program, using unsafePerformIO. Thanx for reading. Greetings, Leonard Siebeneicher --- Begin: experiment.hs --- import Text.ParserCombinators.Parsec import System.IO.Unsafe my_str :: Parser String my_str = many1 anyToken wrap_input :: Parser String -> Parser String wrap_input p = do i <- getInput setInput readI a <- my_str setInput i b <- my_str return $ a ++ " //\n\n " ++ b where {- Aaaah ... any solution without unsafePerformIO? -} readI = unsafePerformIO (readFile "experiment.hs") main = case parse (wrap_input my_str) "" "eintest" of Left err -> putStrLn "Error raised" Right ostr -> putStrLn ostr --- End: experiment.hs --- ___ Thinking about a special type like data MyInclude = PlainText String | IncludeFile String the parser could generate [MyInclude] data, but it does not work generally.

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curious about sum
by Keith Sheppard 18 Jun '09

18 Jun '09

Is there any reason that sum isn't strict? I can't think of any case where that is a good thing. Prelude> sum [0 .. 1000000] *** Exception: stack overflow -Keith -- keithsheppard.name

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Unicode workaround for getDirectoryContents under Windows?
by Shu-yu Guo 18 Jun '09

18 Jun '09

Hello all, It seems like getDirectoryContents applies codepage conversion based on the default program locale under Windows. What this means is that if my default codepage is some kind of Latin, Asian glyphs get returned as '?' in the filename. By '?' I don't mean that the font is lacking the glyph and rendering it as '?', but I mean 'show (head (getDirectoryContents "C:\\Music"))' returns something that looks like like "?? ????". This is a problem as I can't get the filenames of my music directory, some of which are in Japanese and Chinese, some of which have accents. If I change the default codepage to Japanese, say, then I get the Japanese filenames in Shift-JIS and I lose all the accented letters. I have filed this as a bug already, but is there a workaround in the meantime (I don't know the Win32 API, but didn't see anything that looked like it would help under System.Win32 anyways) that lets me gets the list of files in a directory that's encoded in some kind of Unicode? Cheers, -- shu

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Profiling/cost centre question
by Sebastian Reese 18 Jun '09

18 Jun '09

Hi there, I mailed to this list in May (http://www.haskell.org/pipermail/haskell-cafe/2009-May/062126.html) with no answer at all. So I wrote a smaller program to demonstrate my problem/question. Maybe now someone can help me now. I wrote a small program that does nothing but listening on a TCP port. After connection is done it simply terminates. #> cat Test.hs module Main where import Network main :: IO () main = (do servSock <- {-# SCC "cc1" #-}listenOn . PortNumber $ 10000 (handle, host, port) <- {-# SCC "cc2" #-}accept servSock return () )`catch` (putStrLn . show) #> I compile, run (connection from elsewhere, so program terminates) and watch the .prof output #> ghc --make -threaded -O2 -prof -caf-all -auto-all Test.hs Linking Test ... #> ./Test +RTS -p #> cat Test.prof Thu Jun 18 12:24 2009 Time and Allocation Profiling Report (Final) Test +RTS -p -RTS total time = 0.32 secs (16 ticks @ 20 ms) total alloc = 32,384 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc MAIN MAIN 100.0 7.8 CAF GHC.Conc 0.0 4.0 CAF GHC.Handle 0.0 26.8 cc1 Main 0.0 3.8 cc2 Main 0.0 55.8 ... snip ... #> My actual question is, where does the 100% individual time from MAIN come from, how can I debug that in other programs and where does this MAIN cost centre come from? thanks for your help Sebastian ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.

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ANNOUNCE: hyena
by Johan Tibell 18 Jun '09

18 Jun '09

I am pleased to announce the first release of hyena, a library for building web servers, based on the work on iteratee style I/O by Oleg Kiselyov. The library allows you to create web servers that consume their input incrementally, without resorting to lazy I/O. This should lead to more predictable resource usage. This is an early alpha release so expect the API to change in the future. In particular, I'm working on converting the current definition of iteratees/enumerators used to the one in the iteratee package [1]. I decided to release this version, based on a simple left fold, due to requests by several people who already started using hyena. Get it: cabal install hyena And on Hackage: http://hackage.haskell.org//package/hyena 1. http://hackage.haskell.org/package/iteratee -- Johan

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Haskell on Android
by Vasili I. Galchin 18 Jun '09

18 Jun '09

Hello, Let me change the subject ... I think everybody understood my "thrust" but let me make more provocative. Don, please let me expose my ignorance for the greater good and time my personal scorn ;^) ... "EDSL" time => Embedded Domain-Specific Language?? If so, can you please be more specific! I don't mind to be a grunt for Haskell. Kind regards, Vasili

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Haskell-based iPhone development
by Conal Elliott 18 Jun '09

18 Jun '09

If you are working with Haskell and making iPhone apps, or if you intend to soon, please say so at http://haskell.org/haskellwiki/IPhone. By helping each other out, I hope we can work more productively and have more fun in the process. At this point, I'd like to find out what you'd like to share, what help you'd like from others, and generally what kind of apps you want to make. We'll see what evolves from there. - Conal

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