- Haskell-Cafe - Haskell.org

Suggestiong for inter-thread communication
by Erik de Castro Lopo 28 Jan '13

28 Jan '13

Hi all, I am in the process of writing a Haskell program consisting of two threads, one for performing a calculation and one for an Ncurses UI (haskell-ncurses). The calculation thread needs to feed a stream of numbers back to the UI thread (about 1 value per second) and the UI needs to take input from the user and will pass parameter changes to the calculation thread using an IORef and atomicModifyIORef. However, I'm not sure how to hande the UI thread. The UI thread would normally wait for Ncurses input using getEvent, but I need advice on how to the the data from the calculation thread. Any advice or things to try? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

8 8

Munich Haskell Meeting
by Heinrich Hördegen 28 Jan '13

28 Jan '13

Dear Haskellers, our monthly get-togethers in Munich this year start on Thursday, 31ts of January at 19h30. Our meeting venue will be Cafe Puck once again. If you want to join, please go to http://www.haskell-munich.de/dates and click the button. Of course, you can also invite all those interested in functional programming. Have a successful week, Heinrich

1 0

GHC.Generics and newtypes
by Roman Cheplyaka 27 Jan '13

27 Jan '13

Hi, Is it possible to generate different instances for newtypes and datatypes using GHC.Generics? Roman

2 4

Where did the Cabal manual go?
by Richard Cobbe 27 Jan '13

27 Jan '13

I'm not able to access the cabal manual today: links to my local copy and links to the copy at haskell.org result in a 404. I'm running the Haskell Platform, 2012.4.0.0, 64-bit, OS X 10.8.2. On my first attempt this morning, I loaded file:///Library/Haskell/doc/start.html in my browser and clicked on the "Cabal" link. The browser can't find file:///Library/Haskell/doc/ghc-doc/Cabal/index.html, and indeed the directory /Library/Haskell/doc/ghc-doc/Cabal doesn't exist. Ok, I think, so my local copy's gotten corrupted. One "sudo /Library/Haskell/bin/uninstall-hs --remove thru 7.4.2" and reinstall later, I try again; no change. I also can't find any docs by doing find /Library/Haskell -iname \*cabal\* find /Library/Frameworks/GHC.framework -iname \*cabal\* either. I also get a 404 when I click on the "Cabal" link from <http://lambda.haskell.org/platform/doc/current/start.html>. I can probably recover a copy from system backups, but what's going on? Richard

2 3

How to store Fixed data type in the database with persistent ?
by s9gf4ult＠gmail.com 27 Jan '13

27 Jan '13

All modern databases has field type NUMERIC(x, y) with arbitrary precision. I need to store financial data with absolute accuracy, and I decided to use Fixed. How can I store Fixed data type as NUMERIC ? I decided to use Snoyman's persistent, bit persistent can not use it from the box and there is a problem with custom field declaration. Here is the instance of PersistField for Fixed I wrote instance (HasResolution a) => PersistField (Fixed a) where toPersistValue a = PersistText $ T.pack $ show a -- fromPersistValue (PersistDouble d) = Right $ fromRational $ toRational d fromPersistValue (PersistText d) = case reads dpt of [(a, "")] -> Right a _ -> Left $ T.pack $ "Could not read value " ++ dpt ++ " as fixed value" where dpt = T.unpack d fromPersistValue a = Left $ T.append "Unexpected data value can not be converted to Fixed: " $ T.pack $ show a sqlType a = SqlOther $ T.pack $ "NUMERIC(" ++ (show l) ++ "," ++ (show p) ++ ")" where p = round $ (log $ fromIntegral $ resolution a) / (log 10) l = p + 15 -- FIXME: this is maybe not very good isNullable _ = False I did not found any proper PersistValue to convert into Fixed from. As well as converting Fixed to PersistValue is just a converting to string. Anyway the saving works properly, but thre reading does not - it just reads Doubles with rounding error. If you uncomment the commented string in instance you will see, that accuracy is not absolute. Here is test project to demonstrate the problem. https://github.com/s9gf4ult/xres If you launch main you will see that precission is not very good because of converting database value to Double and then converting to Fixed. How can i solve this with persistent or what other framework works well with NUMERIC database field type ?

4 7

Use forM_ with Maybe, it's Foldable!
by Felipe Almeida Lessa 27 Jan '13

27 Jan '13

Hey, Just wanted to share this little insight I had that I don't think is a common idiom. Often times I need to define: whenJust :: Monad m => Maybe a -> (a -> m b) -> m () whenJust (Just x) f = f x whenJust Nothing _ = return () It always bugged me that you can't find this function anywhere. Actually, you can find it on at least a couple of packages [1], but you can't find it on a standard place. Or can you? A few days ago I decided to hoogle the type of whenJust [2] and what I discovered is that import Data.Foldable (forM_) whenJust = forM_ For example, instead of: mfoo <- doSomething case mfoo of Just foo -> ... Nothing -> return () You may just write: forM_ mfoo $ \foo -> ... There you go. I hope this is useful to someone =). Cheers, [1] http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=whenJust [2] http://www.haskell.org/hoogle/?hoogle=Maybe%20a%20-%3E%20(a%20-%3E%20m%20b)… -- Felipe.

2 2

SDL and ALUT on Windows: "stdin Bad file descriptor"
by Thiago Negri 27 Jan '13

27 Jan '13

I'm trying SDL on Windows, and things are getting really weird. I can compile the code (links on the end). When I run it, if I try using `stdin` the program crashes with this message: <stdin>: hGetLine: invalid argument (Bad file descriptor) Is it something to do with SDL itself? What am I doing wrong? :( Code: http://hpaste.org/81321 Cabal file: http://hpaste.org/81322 Thanks.

3 3

cabal install choosing an older version
by Ozgur Akgun 26 Jan '13

26 Jan '13

Hi, I noticed a weird behaviour with cabal-install. When I run `cabal install hspec --dry-run -v` cabal-install correctly picks hspec-1.4.3 (the latest version). However, when I run `cabal install ansi-terminal hspec --dry-run -v`cabal-install tries to install hspec-0.3.0 for no apparent reason. This is with a clean user package db. Following is some info about my system. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.1 $ cabal --version cabal-install version 1.16.0.2 using version 1.16.0.3 of the Cabal library $ ghc-pkg list --user $ cabal install hspec --dry-run -v Reading available packages... Choosing modular solver. Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.1 (new package) ansi-terminal-0.5.5.1 (new package) hspec-expectations-0.3.0.3 (new package) random-1.0.1.1 (new package) QuickCheck-2.5.1.1 (new package) setenv-0.1.0 (new package) silently-1.2.4.1 (new package) transformers-0.3.0.0 (new package) hspec-1.4.3 (new package) $ cabal install ansi-terminal hspec --dry-run -v Reading available packages... Choosing modular solver. Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.1 (new package) ansi-terminal-0.6 (new package) extensible-exceptions-0.1.1.4 (new package) random-1.0.1.1 (new package) QuickCheck-2.5 (new package) hspec-0.3.0 (new package) -- Ozgur Akgun

3 4

Writing config files for configurator library
by Daniel Austin 26 Jan '13

26 Jan '13

Hello Haskellers, What is the typical way to write out configuration files for use by the configurator library? From asking around, seems like the library does not (obviously) support writing out config files to disc from scratch. What I want to do is write out a default config file to disc, or write out the configuration settings that a user selects. I want to avoid having the user edit the config files by hand in a text editor, as well as avoid rolling my own function to write it out if possible. Both of those solutions leave a sour taste in my mouth. Has there been any work toward this end? Thanks much, Dan

1 0

aeson-0.6.1.0 deriveJSON error
by jims＠stuttard.org 26 Jan '13

26 Jan '13

ghc doesn't seem to be unifying deriveJSON (String->String) parameter with id :: a -> a. examples/TemplateHaskell.hs:22:14: Couldn't match expected type `Data.Aeson.TH.Options' with actual type `a0 -> a0' In the first argument of `deriveJSON', namely `id' In the expression: deriveJSON id ''Coord ghc --make: /usr/hs/ghc/7.4.1/bin/ghc failure (return code=1) ghc-7.4.1 x86_64 Centos-6 [jim@localhost aeson]$ ghc-pkg list --package-conf=../cabal-dev/packages-7.4.1.conf ../cabal-dev/packages-7.4.1.conf HTTP-4000.2.6 aeson-0.6.1.0 attoparsec-0.10.3.0 attoparsec-conduit-0.5.0.3 base-unicode-symbols-0.2.2.4 blaze-builder-0.3.1.0 blaze-builder-conduit-0.5.0.3 case-insensitive-0.4.0.4 conduit-0.5.6 dlist-0.5 hashable-1.2.0.5 http-types-0.7.3.0.1 lifted-base-0.2.0.2 monad-control-0.3.1.4 nats-0.1 network-2.4.1.0 network-conduit-0.6.2.2 parsec-3.1.3 primitive-0.5.0.1 resourcet-0.4.4 semigroups-0.9 simple-sendfile-0.2.10 transformers-base-0.4.1 unix-compat-0.4.1.0 unordered-containers-0.2.3.0 vault-0.2.0.4 vector-0.10.0.1 void-0.5.11 wai-1.3.0.1 warp-1.3.7.1

2 4