December 2009 - Haskell-Cafe

Reading from a process
by Mitar 16 Dec '09

16 Dec '09

Hi! I would like to make an one-directional inter-process communication in a way that one process is printing out read-compatible Haskell data types and Haskell program is reading them from a pipe. I would like to make a function which would return Nothing if there is no data type in a pipe or Just last data type there was in a pipe (discarding all before). So it should behave in a non-blocking way. In an attempt I made this: getLastMyData :: MyState (Maybe MyData) getLastMyData = do p <- gets process case p of Nothing -> fail "No process" Just (MyProcess processOut processPid processBuffer) -> do processRunning <- io $ getProcessExitCode processPid case processRunning of Just _ -> processExited _ -> do ret <- io $ tryJust (guard . isEOFError) $ slurpInput processOut processBuffer case ret of Left _ -> processExited -- EOF Right currentBuffer -> do let (datalist, currentBuffer') = readData currentBuffer modify (\s -> s { process = Just (MyProcess processOut processPid currentBuffer') }) if null datalist then return Nothing else return $ Just $ head datalist -- MyData is stored in the reverse order so head is the last MyData from the process slurpInput :: Handle -> String -> IO String slurpInput h buffer = do ready <- hReady h if not ready then return buffer else do char <- hGetChar h slurpInput h (buffer ++ [char]) readData :: String -> ([MyData], String) readData buffer = readData' [] buffer where readData' datalist [] = (datalist, []) readData' datalist buf = case reads buf of [(x, rest)] -> readData' ((head x):datalist) rest -- x is a list and currently it has only one element [] -> if length buf > 5 * maxMyDataDescLength then error "Invalid data from process" else (datalist, buf) -- we probably do not have enough data to read MyData properly _ -> error "Ambiguous parse from process" (I have cleaned a code somewhat, I hope I have not introduced any errors. MyData is encapsulated in a list when printed from a process, this is why is there "head".) The problem is that I do not like this approach. And it does not look nice. For example I am reading byte per byte and appending it to the end. Then the problem is that if process is sending garbage faster then Haskell can consume it Haskell stays in slurpInput function and never gets to readData where it would found out that there is garbage coming in. I could use hGetBufNonBlocking? But it would still not solve garbage problem. So is there some better way to do it? Mitar

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Boxed Mutable Arrays
by Brad Larsen 16 Dec '09

16 Dec '09

Is anyone working on fixing ticket #650 <http://hackage.haskell.org/trac/ghc/ticket/650>? In short, STArray and the garbage collector don't play well together, resulting in array updates being non-constant time operations. This bug makes it very difficult/impossible to write efficient array algorithms that depend upon mutation in Haskell. On another note, does this (or perhaps better phrased, will this) bug also affect Data Parallel Haskell? I would really like to see highly efficient, mutable, boxed arrays in Haskell! Unfortunately, I don't have the know-how to fix Ticket 650. Sincerely, Brad

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Broken GHC doc links
by Edward Z. Yang 16 Dec '09

16 Dec '09

Hello, I think the recent GHC 6.12 release broke all of the old links to Haddock documentation; as of writing Hoogle still points to the wrong Haddock docs and stuff like: http://www.haskell.org/ghc/staging/docs/latest/html/libraries/mtl/Control-M… is now 404ing despite being the number one Google result. As the W3C would say, "Cool URLs don't Change". Can we at least setup redirects to the new pages? Thanks, Edward

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Optimizing Parsec 3 -- was: Wiki software?
by Paulo Tanimoto 16 Dec '09

16 Dec '09

Hello! On Mon, Nov 23, 2009 at 12:29 PM, Antoine Latter <aslatter(a)gmail.com> wrote: > On Sun, Nov 22, 2009 at 11:56 AM, Antoine Latter <aslatter(a)gmail.com> wrote: >> >> Running 'pandoc --strict' over the Markdown readme.text takes: >> >> ~0.09s with pandoc built against parsec-2 >> ~0.19s with pandoc built against parsec-3 >> >> on my machine. >> >> I have a branch of parsec-3 which seems to brings us back to parsec-2 >> numbers, but also fails the rst-reader test-case in the pandoc testing >> suite: >> >> http://community.haskell.org/~aslatter/code/parsec/cps > > In reply to my own post, the branch of parsec posted now passes all of > the pandoc test cases. > > If there are any other consumers of the parsec library that have tests > I can run let me know. > > The 'many' combinator is one of those things that can look right, be > wrong, yet work for almost everything. > I finally had some time to test it. After running it multiple times (of course, it would be nice to use criterion here), I'm getting numbers in this neighborhood: $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.0.20091121 $ time pandoc --strict README > /dev/null parsec 2: --- real 0m0.140s user 0m0.130s sys 0m0.010s parsec 3 - antoine's: --- real 0m0.151s user 0m0.150s sys 0m0.010s parsec 3 - hackage: --- real 0m0.243s user 0m0.240s sys 0m0.010s Nice work, Antoine! Paulo

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Design question
by Jeff Wheeler 16 Dec '09

16 Dec '09

On Wed, Dec 16, 2009 at 10:40 AM, haskell(a)kudling.de <haskell(a)kudling.de> wrote: > 1) Is the way i define and use "SubSet", the only/valid way to define > subsets? > > 2) What's the best way to make "doSomethingElse" polymorphic? I'm not very familiar with them, so I'm not sure if it's totally applicable, but generalized algebraic datatypes (GADTs) might be what you're looking for. -- Jeff Wheeler (Sorry, sent this to the author originally.)

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changelogs for packages on Hackage
by Valery V. Vorotyntsev 16 Dec '09

16 Dec '09

It would be nice if Hackage displayed ``recent changes'' of a package. [severity: wishlist] You see, I am subscribed to the ``hackage - recent additions'' feed [http://hackage.haskell.org/packages/archive/recent.rss] and receive entries that look like this: Cabal 1.8.0.2 Added by DuncanCoutts, Wed Dec 16 04:19:24 UTC 2009. A framework for packaging Haskell software This is sweet. But how can I tell what's new in Cabal since 1.6.0.3? (`Cabal' is just an example here.) * * * -- these are snowflakes Dear Santa, This year, I have been a very good boy (in his thirties). I have not pillaged, and I have often helped my children with their homework. And I always say `thank you', which makes me polite and so I deserve lots of presents this year! Please bring this stuff for me and the people in Haskell community. -- vvv

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Difference in Runtime but no explanation
by Johann Höchtl 16 Dec '09

16 Dec '09

Hello, I'm still to Haskell, and after I read through http://users.aber.ac.uk/afc/stricthaskell.html#seq I thought, that these tow fragments after term rewriting are really the same: myLength :: [a] -> Integer myLength xs = len xs 0 where len [] l = l len (x:xs) l = l `seq` len xs (l+1) main = print $ myLength [1..10000000] -- vs. myLength :: [a] -> Integer myLength xs = len xs 0 where len [] l = l len (x:xs) l = len xs $! (l+1) main = print $ myLength [1..10000000] main = print $ myLength [1..10000000] But the first expression evaluates more then twice as fast as the second one. Tested on GHC 6.10.4 and Windows XP, dual core (for what it's worth) It's on http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=5321#a5321 btw. I can't see the difference, especially as $! is expressed in terms of seq

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[ANN] text 0.7, fast Unicode text
by Bryan O'Sullivan 16 Dec '09

16 Dec '09

The new 0.7 release of the text package<http://hackage.haskell.org/package/text>adds support for Unicode I/O, using either the new locale-aware Handle code in 6.12 or a fallback on older releases. Details: http://www.serpentine.com/blog/2009/12/15/data-text-0-7-gains-io-support/

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consulting and contracting
by Tim Newsham 16 Dec '09

16 Dec '09

I was wondering how many haskell consultants and contractors (ie. freelance programmers) there are and how much demand there is for their work. Also what kind of work do most haskell consultants and contractors get? Is it primarily focused around developing and maintaining haskell compilers and tools? Mostly related to academia? Spread out around several areas? Tim Newsham | www.thenewsh.com/~newsham | thenewsh.blogspot.com

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ANN: atom-0.1.4
by Tom Hawkins 16 Dec '09

16 Dec '09

A bug fix affecting floating point variable initialization. http://hackage.haskell.org/package/atom

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