January 2013 - Haskell-Cafe

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

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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 ?

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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.

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SDL and ALUT on Windows: "stdin Bad file descriptor"
by Thiago Negri 26 Jan '13

26 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.

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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

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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

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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

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Re: [Haskell-cafe] ANN: monad-bool 0.1
by wren ng thornton 26 Jan '13

26 Jan '13

On 1/23/13 3:00 AM, Alexander Kjeldaas wrote: > If we step back, I think the lesson here is that Haskell libraries exist, > but the concepts are far enough from what you expect to exist given > background knowledge from another programming language. > > So what is actually needed is not monad-bool, but the equivalent > documentation that makes a programmer which is not an expert in Haskell > connect the dots like you did today. Definitely. We could even set up NotOnHackage: a "package" repository just like Hackage, except the packages are just documentation on why there is no such package. Implementation-wise it's just a wiki; but the idea is about how to organize the wiki. NotOnHackage should be organized and searchable similar to Hackage itself, so that people can look there when nothing looks promising on Hackage. -- Live well, ~wren

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How to escape from typecheck error: Duplicate instance declarations ?
by s9gf4ult＠gmail.com 26 Jan '13

26 Jan '13

Hello, haskellers. I am trying to write some generic subtyping issue. Here upcast is always safe operation because of subtype is always behaves like the parrent type. downcast is not the safe becase of not every parrent type value can be converted to children type. Rangeable here is the typeclass of values in some range, so downcasting to Rang1 or Range2 or any other type, having instance for Rangeable can be done by checking if value is in proper range. The same for MultipleTo, downcasting can be done with checking if value is multiple to some value. {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FunctionalDependencies, FlexibleContexts, UndecidableInstances, OverlappingInstances, IncoherentInstances #-} class SubtypeOf a b | a -> b where upcast :: a -> b downcastSafe :: b -> Maybe a downcast :: b -> a downcast b = case downcastSafe b of Nothing -> error $ "can not downcast the value" Just a -> a class (Ord a) => Rangable t a | t -> a where lowLim :: t -> a highLim :: t -> a class Packable t a | t -> a where pack :: a -> t unpack :: t -> a class MultipleTo t a | t -> a where multiple :: t -> a instance (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b >= (lowLim $ pb) && b <= (highLim $ pb) = Just $ pb | otherwise = Nothing where pb = pack b instance (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a where upcast = unpack downcastSafe b | b `mod` (multiple pb) == 0 = Just pb | otherwise = Nothing where pb = pack b newtype Range1 a = Range1 {unRange1 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range1 a) a where lowLim _ = 0 highLim _ = 10 instance (Num a, Ord a) => Packable (Range1 a) a where pack = Range1 unpack = unRange1 newtype Range2 a = Range2 {unRange2 :: a} deriving Show instance (Num a, Ord a) => Rangable (Range2 a) a where lowLim _ = -10 highLim _ = 200 instance (Num a, Ord a) => Packable (Range2 a) a where pack = Range2 unpack = unRange2 but there is compilation error: Duplicate instance declarations: instance [incoherent] (Num a, Ord a, Rangable range a, Packable range a) => SubtypeOf range a -- Defined at ...:22:10 instance [incoherent] (Integral a, Packable range a, MultipleTo range a) => SubtypeOf range a -- Defined at ...:29:10 Failed, modules loaded: none. If I remove one of instances of SubtypeOf the program is compiling. How to write this instances properly, or to write proper type casting ? Thanks PS. My english is not very good, but I hope this is understandable.

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why no replace function in our regular expression libs ?
by Simon Michael 25 Jan '13

25 Jan '13

People have put a lot of work into regular expression libraries on haskell. Yet it seems very few of them provide a replace/substitute function - just regex-compat and regepr as far as I know. Why is that ? #haskell says: <sclv> iirc its because that's a really mutatey operation in the underlying c libs <sclv> should be simple enough to write a general purpose wrapper layer that uses captures to create the effect Secondly, as of today what do y'all do when you need that functionality ? -Simon

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