December 2009 - Haskell-Cafe

Dead links for Web frameworks.
by Jason Dusek 20 Dec '09

20 Dec '09

I recently removed dead links on the web frameworks page, for "EdComb" and "Generative Implementation Strategies for Data-Centric Web Applications". http://www.haskell.org/haskellwiki/?title=Applications_and_libraries/Web_pr… I'm curious what happened to these two projects and if any docs remain for them. -- Jason Dusek

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Search a directory for files
by Eitan Goldshtrom 20 Dec '09

20 Dec '09

Hi, I'm trying to make a program to make it easy to rename files in bulk. What I'm wondering is how to get a list of all files in a particular directory. I found System.Posix.Files and I'm planning on using the rename function in it for actually renaming, but I can't find an easy way to get all filenames in a particular directory. Does anyone know how to do it? -Eitan

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sizeOf on a type
by Gregory Crosswhite 19 Dec '09

19 Dec '09

Hey everyone, I would like to write a routine like nextPtr :: Storable a => Ptr a -> Ptr a nextPtr = (`plusPtr` sizeOf (undefined :: a)) which increments a pointer by the correct amount, but GHC complains that the type variable "a" is ambiguous. I can see what's going on: it can't tell that the "a" I am writing there is the same "a" that's in the type specification, but is there any way that I can make it identify "a" with the "a" in the specification for nextPtr? Cheers, Greg

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How to use Control.Failure?
by ntupel 19 Dec '09

19 Dec '09

I have looked at the recently released Control.Failure library but I admit, I couldn't understand it completely. So given the example below, how would Control.Failure help me here? Thanks, nt -- Theirs (other library code stubs) data TheirError = TheirErrorCase deriving Show data TheirData = TheirData deriving Show theirFunc :: [String] -> Either TheirError TheirData theirFunc = undefined -- Mine (my own code stubs) data MyError = MyErrorCase deriving Show data MyData = MyData deriving Show myFuncA :: TheirData -> Either MyError MyData myFuncA = undefined -- Ugly. How to apply Control.Failure here? myFuncB :: IO (Either MyError MyData) myFuncB = do let x = theirFunc [] case x of Right x' -> return $ myFuncA x' Left _ -> return . Left $ MyErrorCase

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Haskell Weekly News: Issue 144 - December 19, 2009
by jfredett＠gmail.com 19 Dec '09

19 Dec '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20091219 Issue 144 - December 19, 2009 --------------------------------------------------------------------------- Welcome to issue 144 of HWN, a newsletter covering developments in the [1]Haskell community. The big news this week is the release of the Glorious Glasgow Haskell compiler version 6.12.1, lots of exciting new improvements, congratulations from the whole community go out to the devs, good work GHC Team! Otherwise, this week has been quiet, with some interesting discussions about associated types and the new GHC, and some excellent new packages announced. Finally, due to the incoming holidays, there won't be an HWN for the next two weeks, I'll see you Haskellers in 2010, until then, your Haskell Weekly News! Announcements GHC version 6.12.1. Ian Lynagh [2]announced the (Interactive) Glasgow Haskell Compiler, version 6.12.1. Including improved support for parallel execution, Dynamic linking support on Linux, and many other improvments. atom-0.1.4. Tom Hawkins [3]announced a bug fix release of the atom package, affecting floating point variable initialization. text 0.7, fast Unicode text. Bryan O'Sullivan [4]announced a new release of the text package adds support for Unicode I/O explicit-iomodes-0.1. Bas van Dijk [5]announced explicit-iomodes version 0.1, a wrapper around System.IO that provides a Handle type which is parametrized with the IOMode it is in. Hemkay, the 100% Haskell MOD player. Patai Gergely [6]announced a Hemkay, an old-school MOD player in pure Haskell. GT-VMT 2010: deadlines extended. Emilio Tuosto [7]announced a deadline extension to the GT-VMT call for papers. Discussion Restrictions on associated types for classes. Stephen Lavelle [8]asked if there was a way to wrap a associated type in a class constraint. Yay 6.12.1! Evan Laforge [9]talked about some of the changes in the new version of GHC. Blog noise [10]Haskell news from the [11]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Michael Snoyman: [12]data-object family. * Bryan O'Sullivan: [13]Making GHC's I/O manager more scalable. * Conor McBride: [14]Epilogue for Epigram: 2+2=4. * Sebastian Fischer: [15]Thesis submitted. * Bryan O'Sullivan: [16]Data.Text 0.7 gains I/O support. * Ketil Malde: [17]Searching for poly(A) tails. * Mark Jason Dominus: [18]Monads are like burritos. * Sean Leather: [19]"Upwards and downwards accumulations on trees" translated into Haskell. Quotes of the Week * yrlnry: <byorgey> yrlnry: just some lady who had a lot of money and decided she wanted poor grad students to have some of it after she died... pretty nice of her, actually <yrlnry> senile dementia, more likely. * Philonous: A 'newbie', in Haskell, is someone who hasn't yet implemented a compiler. They've only written a monad tutorial. * kmc: Right, for example Either (pun not intended) * kmc: One thing I've learned from open-source software is that if you invent enough wheels, one of them will turn out to be round and capable of rolling. * PhilipWadler: To see that "in" and "out" are inverses, stare at the following diagram * kmc: Data.Map is like a C++ STL map<k,v>, without the suck * edwardkmett: Most monad tutorials are written by people who barely understand monads, if at all, but unfortunately nothing can stop someone from writing a monad tutorial. We've tried, there was blood everywhere. * Cale: A small category is a monad in the 2-category of spans of sets. About the Haskell Weekly News New editions are posted to [20]the Haskell mailing list as well as to [21]the Haskell Sequence and [22]Planet Haskell. [23]RSS is also available, and headlines appear on [24]haskell.org. To help create new editions of this newsletter, please see the information on [25]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [26]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17678 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/68009 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/67946 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/67931 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/67923 7. http://article.gmane.org/gmane.comp.lang.haskell.general/17676 8. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/68084 9. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/67950 10. http://planet.haskell.org/ 11. http://haskell.org/haskellwiki/Blog_articles 12. http://snoyberg.wordpress.com/2009/12/17/data-object-family/ 13. http://www.serpentine.com/blog/2009/12/17/making-ghcs-io-manager-more-scala… 14. http://www.e-pig.org/epilogue/?p=266 15. http://www-ps.informatik.uni-kiel.de/~sebf/haskell/thesis.html 16. http://www.serpentine.com/blog/2009/12/15/data-text-0-7-gains-io-support/ 17. http://blog.malde.org/index.php/2009/12/14/searching-for-polya-tails/ 18. http://blog.plover.com/prog/burritos.html 19. http://feedproxy.google.com/~r/splonderzoek/~3/4E8TZPDZ-aM/upwards-and-down… 20. http://www.haskell.org/mailman/listinfo/haskell 21. http://sequence.complete.org/ 22. http://planet.haskell.org/ 23. http://sequence.complete.org/node/feed 24. http://haskell.org/ 25. http://haskell.org/haskellwiki/HWN 26. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Re: Pattern matching, and bugs
by John Lato 19 Dec '09

19 Dec '09

> From: Felipe Lessa <felipe.lessa(a)gmail.com> > > On Fri, Dec 18, 2009 at 02:13:02PM +0000, John Lato wrote: >> So now the program needs a result of some type (String in your >> example) and gets an undefined, and then immediately crashes >> with an "Exception - undefined" error. > > I think this is the second time this is said in this thread, but > please don't as it may confuse the OP. "undefined"s, including > those generated by missing cases, do *not* crash the program. > They also do not *immediately* close the program. They are > propagated as exceptions are, however most of the time we write > programs without explicitly catching them. Of course you're correct. I was careful to note that undefined's generate exceptions, but left out the rest on the assumption that the OP would be familiar with exception handling and propagation. But it is best to be perfectly explicit. > > So the proper wording would be that the "undefined"s propagate > through the functions and whenever they reach the top function > (i.e. main), they are caught by the runtime system, which prints > the String associated with the "undefined" and exits the proccess > with an error code. > > I should note also that *pure* code *can't* catch those > "undefined"s. If they could, then Nasty Things Would Happen(TM) > and we don't want that. However IO code may catch them without > any problem using Control.Exception's functions. I agree this is an important point, particularly as it's different from declarative/OOP exception mechanisms. John

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Re: [Haskell-cafe] Double free-ing (was: Reading from a process)
by Mitar 18 Dec '09

18 Dec '09

Hi! On Thu, Dec 17, 2009 at 5:28 AM, Jason Dusek <jason.dusek(a)gmail.com> wrote: > It seems like the message delimiter to me because you keep > buffering till you receive it. Hm, true. I have changed code to this: getLastMyData :: MySate (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) $ slurpMyData processOut processBuffer case ret of Left _ -> processExited -- EOF Right (datalist, currentBuffer) -> do 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 slurpMyData :: Handle -> DataBuffer -> IO ([MyData], DataBuffer) slurpMyData = slurpMyData' [] where slurpMyData' datalist h buffer@(DataBuffer capacity array bytecount) = do ready <- hReady h if not ready then return (datalist, buffer) else do let array' = advancePtr array bytecount count <- hGetBufNonBlocking h array' (capacity - bytecount) let bytecount' = bytecount + count chars <- peekArray bytecount' array let (d, rest) = readData . dataToString $ chars rest' = stringToData rest if null d then if length rest' == capacity then error "Invalid data from the process" else return (datalist, buffer) else do assert (length rest' <= capacity) $ pokeArray array rest' let buffer' = DataBuffer capacity array (length rest') slurpMyData' (d ++ datalist) h buffer' readData :: String -> ([MyData], String) readData = readData' [] where readData' datalist "" = (datalist, "") readData' datalist string = case reads string of [(x, rest)] -> readData' ((head x):datalist) rest -- x is a list and currently it has only one element [] -> (datalist, string) -- we probably do not have enough data to read MyData properly _ -> error "Ambiguous parse from the process" And now it works. I just do not like using malloc and free. And talking about free, I have now a problem of double freeing this buffer. I am getting it after I send ctrl-c to the main process (with underlying process I am communicating with). I am freeing it in processExited: processExited :: MyState a processExited = do terminateDataProcess fail "Process exited" terminateDataProcess :: MyState () terminateDataProcess = do p <- gets process case p of Just (MyProcess _ processPid (DataBuffer _ array _)) -> do modify (\s -> s { process = Nothing }) io $ free array io $ terminateProcess processPid _ -> return () So I run processExited if there is EOF from underlying process. But I also run terminateDataProcess in a bracket I am calling getLastMyData. Code is like this: bracket initDataProcess terminateDataProcess (... read getLastMyData repeatedly and process it ...) Why I am getting this double free-ing errors? Should I introduce some locks on terminateDataProcess? I am using Linux 2.6.30 amd64 and 6.10.4. Mitar

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getPtrSize works, but is annoying
by Gregory Crosswhite 18 Dec '09

18 Dec '09

Ugh, I figured out how to write code to do what I wanted, but it seems like an ugly solution: getPtrSize :: Ptr a -> Int getPtrSize = getFrom dummy where getFrom :: a -> Ptr a -> Int getFrom dummy _ = sizeOf dummy Any thoughts on a less ugly solution, or is this really the best that I can do? Cheers, Greg

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Cabal
by Philippos Apolinarius 18 Dec '09

18 Dec '09

I made small improvements in the Small Japi Binding, and asked how to make it available. I received a few private messages advising me to build and package the library using a tool called cabal. Since I have used installation tools for PLT, R and LaTeX libraries, I thought cabal was something similar. However, I noticed that there are a lot of complaints against cabal. Therefore, I decided to install cabal, and try it. It seems that it is easier to install packages by hand than using cabal. Here is a typical cabal session: D:\ghc\ghcapi>cabal update Downloading the latest package list from hackage.haskell.org D:\ghc\ghcapi>cabal install mkcabal Resolving dependencies... Downloading pcre-light-0.3.1... Configuring pcre-light-0.3.1... Downloading readline-1.0.1.0... Configuring readline-1.0.1.0... cabal: Error: some packages failed to install: mkcabal-0.4.2 depends on readline-1.0.1.0 which failed to install. pcre-light-0.3.1 failed during the configure step. The exception was: exit: ExitFailure 1 readline-1.0.1.0 failed during the configure step. The exception was: exit: ExitFailure 1 As one can see, it is much more difficult to use than R installation tools, or MikTeX installation tools. If cabal is so difficult, why use it? I am sure that I have done something very stupid mistake, but believe me, a lot of people is trying to use cabal and failing. I am not sure that it is a good idea to distribute libraries using cabal. However, this is irrelevant, because I am sure that creating a package with cabal is well beyond my powers. It is much easier to write a complete JAPI library, than packaging it in cabal. __________________________________________________________________ Looking for the perfect gift? Give the gift of Flickr! http://www.flickr.com/gift/

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Allowing hyphens in identifiers
by Deniz Dogan 18 Dec '09

18 Dec '09

Has there been any serious suggestion or attempt to change the syntax of Haskell to allow hyphens in identifiers, much like in Lisp languages? E.g. hello-world would be a valid function name. -- Deniz Dogan

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