December 2012 - Haskell-Cafe

forkProcess, forkIO, and multithreaded runtime
by Michael Snoyman 21 Jan '13

21 Jan '13

Hi all, I think I have a misunderstanding of how forkProcess should be working. Ultimately this relates to some bugs in the development version of keter, but I've found some behavior in a simple test program which I wouldn't have expected either, which may or may not be related. With the program at the end of this email, I would expect that, once per second, I would get a message printed from each forkIO'd green thread, the forked process, and the master process. And if I spawn 8 or less child threads that's precisely what happens. However, as soon as I up that number to 9, the child process is never run. The process is, however, created, as can be confirmed by looking at the process table. This only occurs when using the multithreaded runtime. In other words, compiling with "ghc --make test.hs" seems to always produce the expected output, whereas "ghc --make -threaded test.hs" causes the behavior described above. Having looked through the code for the process package a bit, my initial guess is that this is being caused by a signal being sent to the child process, but I'm not familiar enough with the inner workings to confirm or disprove this guess. If anyone has any ideas on this, I'd appreciate it. Michael import System.Posix.Process (forkProcess, getProcessID) import Control.Concurrent (forkIO, threadDelay) import System.IO (hFlush, stdout) import System.Posix.Signals (signalProcess, sigKILL) import Control.Exception (finally) main :: IO () main = do mapM_ spawnChild [1..9] child <- forkProcess $ do putStrLn "starting child" hFlush stdout loop "child" 0 print ("child pid", child) hFlush stdout -- I've commented out the "finally" so that the zombie process stays alive, -- to prove that it was actually created. loop "parent" 0 -- `finally` signalProcess sigKILL child spawnChild :: Int -> IO () spawnChild i = do _ <- forkIO $ loop ("spawnChild " ++ show i) 0 return () loop :: String -> Int -> IO () loop msg i = do pid <- getProcessID print (pid, msg, i) hFlush stdout threadDelay 1000000 loop msg (i + 1)

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Stackage mailing list
by Joachim Breitner 20 Jan '13

20 Jan '13

Dear Michael, I’m wondering if I missed something, but is there a mailing list for stackage? Or has one of the standard lists (-cafe, libraries) been designated for questions about stackage? What I want to know is if you plan to provide a website for stackage soon where the list of included packages and their dependencies, including the tested and approved version numbers can be seen. I’d like to take that and integrate it on http://people.debian.org/~nomeata/platform.html and http://people.debian.org/~nomeata/hackagevsdebian.html Also, once you do that, it should be possible to add Stackage to the list of Distributions listed on hackage, so that on hackage one already sees the package’s stackage status and whether it lags behind there. The integrations is rather simple, just provide a file with that format: http://people.debian.org/~nomeata/cabalDebianMap.txt and bug the hackage maintainers to add it. Greetings, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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Makefile for a Haskell Project
by xuan bach 16 Jan '13

16 Jan '13

Hi everyone, I'm a newbie in Haskell. I'm wondering that if there is any tool support creating Makefile for Haskell project like Ocamlbuild for Ocaml project? Thank you, Regards. -- *Le Dinh Xuan Bach* *Tel: 01234711869 or +65 86967149 * *Email: pig28789(a)gmail.com School of Information and Communication, * *Hanoi University of Science and Technology --------------------------------------------------------------------- レ。ディン。スアン。バイック電話番号：01234711869 or +65 86967149 メール： pig28789(a)gmail.com *

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Compliments on the "IO inside" tutorial
by Andriy Drozdyuk 05 Jan '13

05 Jan '13

I just wanted to express my gratitude to whoever wrote the "IO inside" tutorial: http://www.haskell.org/haskellwiki/IO_inside It is one of the best pieces of writing in Haskell I've ever come across: not only is it clear and very intuitive, but it also manages to strike a perfect balance between "programmers" and "beginner haskell programmers". It certainly ties together a lot of other stuff one reads in books and category theory - and actually makes sense! My sincere gratitude, and keep up great job. I can only hope to write something as clear and useful as this. My only complaint is that it is really wasting away inside the Haskell Wiki - which gives it a rather bad "reputation", since a lot of wiki pages are of much lower quality. This should be nicely formatted in a PDF file or a even part of a book! Make it pretty and get people to notice it!!! Now i must sleep, -Andriy Drozdyuk

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Object Oriented programming for Functional Programmers
by Daniel Díaz Casanueva 05 Jan '13

05 Jan '13

Hello, Haskell Cafe folks. My programming life (which has started about 3-4 years ago) has always been in the functional paradigm. Eventually, I had to program in Pascal and Prolog for my University (where I learned Haskell). I also did some PHP, SQL and HTML while building some web sites, languages that I taught to myself. I have never had any contact with JavaScript though. But all these languages were in my life as secondary languages, being Haskell my predominant preference. Haskell was the first programming language I learned, and subsequent languages never seemed so natural and worthwhile to me. In fact, every time I had to use another language, I created a combinator library in Haskell to write it (this was the reason that brought me to start with the HaTeX library). Of course, this practice wasn't always the best approach. But, why I am writing this to you, haskellers? Well, my curiosity is bringing me to learn a new general purpose programming language. Haskellers are frequently comparing Object-Oriented languages with Haskell itself, but I have never programmed in any OO-language! (perhaps this is an uncommon case) I thought it could be good to me (as a programmer) to learn C/C++. Many interesting courses (most of them) use these languages and I feel like limited for being a Haskell programmer. It looks like I have to learn imperative programming (with side effects all over around) in some point of my programming life. So my questions for you all are: * Is it really worthwhile for me to learn OO-programming? * If so, where should I start? There are plenty of "functional programming for OO programmers" but I have never seen "OO programming for functional programmers". * Is it true that learning other programming languages leads to a better use of your favorite programming language? * Will I learn new programming strategies that I can use back in the Haskell world? Thanks in advance for your kind responses, Daniel Díaz.

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Re: [Haskell-cafe] [Haskell-beginners] ghc and android
by Nathan Hüsken 03 Jan '13

03 Jan '13

After googling a little more, do I understand correctly that to compile haskell (with ghc) for Android, one would have to compile and run ghc itself on android (because it can not cross compile)? That seems almost impossible, I would need an android device with unusual capacity. I have heard that ghc for iphone is possible. Anyone knows how that is done? What is the difference for android? Regards, Nathan

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ANNOUNCE: graphviz-2999.15.0.0
by Ivan Lazar Miljenovic 02 Jan '13

02 Jan '13

After much prodding and poking by end-users for the past few months, I'm finally able to announce a GHC-7.6.1 compatible release of my Haskell bindings to the Graphviz graph visualisation toolkit: http://hackage.haskell.org/package/graphviz-2999.15.0.0 (It took me so long to release a version that allowed bytestring-0.10.* as I was waiting for upstream to get back to me about an inconsistency I found; in the end I decided to just try and cope with how it seems to work.) The list of changes since the last release are: * The repository is now hosted at http://hub.darcs.net/ivanm/graphviz, and using the bug-tracker there. * Updates to various `Attribute` definitions: - The list of available shapes has been expanded to take into account the new synthetic biology shapes. - The `Size` and `FontNames` attributes now have specified data types. - The optional start and end points for `Spline`s were previously the wrong way round; this has now been fixed. - Explicitly only print 2-dimensional `Point` values for `Rect` and `DPoint` (previously only 2-dimensional values where parsed, but it was possible to print a 3-dimensional value). - HTML-like labels previously disallowed empty textual label components when parsing; spotted by Daniel Hummel. * `GraphvizParams` now lets you specify whether a "cluster" is actually a cluster or a sub-graph. Requested by Gabor Greif. * Fixed an error where printing edges whose attributes contained a `ColorScheme` attribute, that attribute would stay in the state and the old color scheme would override the _parent's_ state. * Previously, some malformed attributes were accepted when being parsed as they silently became parsed as an `UnknownAttribute`. Now, attributes where the attribute name and the equal sign are successfully parsed **but** the value of the attribute is _not_ successfully parse will throw an unrecoverable error. **Note**: this does mean that some Dot graphs that are accepted by Graphviz (as they separate tokenizing from parsing; as such something like `"0.1"` is successfully accepted as an integer, specifically `0`) are no longer accepted when parsing them in. * Miscellaneous parsing improvements: - Whilst not specified anywhere, Graphviz seems to treat empty quotes as values for attributes expecting a number as their default; as such, this is now taken into account when parsing. - `DPoint` values can now parse an optional `+` prefix. - Whitespaces after commas in HSV colors are now accepted. * Error messages from parsing are improved to help you track down where the parsing error occurred (in that it's easier to find which attribute failed to parse, etc.). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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A big hurray for lambda-case (and all the other good stuff)
by Ben Franksen 02 Jan '13

02 Jan '13

Hi Everyone just wanted to drop by to say how much I like the new lambda case extension. I use it all the time and I just *love* how it relieves me from conjuring up dummy variables, which makes teh code not only esier to write but also to read. A big, huge thank you to the ghc developers. This has been sooooo long on my wish list. Also much appreciated and long awaited: tuple sections (though I use them not quite as often). Both should *definitely* go into Haskell'13. Of course, thank you also for all the other beautiful stuff in ghc-7.6.1, especially PolyKinds, DataKinds etc. GHC is just simply amazing. You guys RULE THE WORLD! Cheers -- Ben Franksen () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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Mancunian Haskellers out there?
by Alfredo Di Napoli 02 Jan '13

02 Jan '13

Morning Cafe, I've been living in Manchester for 1 month now and I'm wondering if some on you are from the Greater Manchester area, so that we could chat about out beloved language in front of a glass of ale / tea :P Happy new year to everyone! A.

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Backtracking in HXT
by Mateusz Kowalczyk 01 Jan '13

01 Jan '13

Hello, I'm not sure if this is the right place to ask this but here goes. I'm currently working with HXT and I quite like it. There's one issue I can't solve (at least now without some dirty, dirty hacking) and I'm sure it's fairly simple. Consider the following mark-up <a href="example.com/somelink.html">Hello One!</a> <a href="example.com/anotherlink.html">Hello Two!</a> Now, what I'm trying to achieve is to get the href based on the text. I can test for what the text is by traversing <a>s, then using the getChildren >>> getText arrow. What I can't figure out is how to check the text and then get the href as by the time I get to the text, I'll further down the tree! I imagine it would look something along the lines of (getChildren >>> getText <?*> "Hello Two") `guards` getAttrValue "href". <?*> is just a random operator I made up for illustration purposes that works as a predicate over arrows. Is this the right approach? Is there a built-in that already does what I want? It seems like a common task... Mateusz Kowalczyk

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