November 2013 - Haskell-Cafe

Witnessing lifted types?
by Andras Slemmer 25 Nov '13

25 Nov '13

Hi, not sure whether this has been addressed before. Basically the goal is to have a standardised way of witnessing lifted types (=simulating pattern matching on type indices). A particular problem this would be useful for: Given a standard natural-indexed vector (Vec n) provide an Applicative instance for it with n left polymorphic. The issue is that we don't have a way of pattern matching on n, therefore we cannot implement pure/<*>. The way I addressed situations like this before was to have two Applicative instances, one for Vec Zero and one for Vec (Succ n), however I think there is a way to decouple this way of "pattern matching on types" by introducing a GADT: data WNat n where WZero :: WNat Zero WSucc :: WNat n -> WNat (Succ n) and a class class WNatClass n where witness :: WNat n instance WNatClass Zero where witness = WZero instance (WNatClass n) => WNatClass (Succ n) where witness = WSucc witness Now whenever we need to pattern match on a natural index we can just put a WNatClass restriction on it and pattern match on the automatically constructed witness. For the Vec example: instance (WNatClass n) => Applicative (Vec n) where pure = pure' witness where pure' :: WNat n -> a -> Vec n a pure' WZero _ = VecNil pure' (WSucc w) a = a ::: pure' w a (<*>) = app' witness where app' :: WNat n -> Vec n (a -> b) -> Vec n a -> Vec n b app' WZero _ _ = VecNil app' (WSucc w) (f ::: fs) (a ::: as) = f a ::: app' w fs as We can also generalise this concept to any index type with PolyKinds, although I'm not 100% sure why it works in general: class Construct (a :: k) where data Wit a :: * class (Construct a) => Witness a where witness :: Wit a The idea is that each instance of Construct will leave the type 'a' polymorphic and will only bind the kind 'k' and provide an indexed witness: instance Construct (n :: Nat) where data Wit n where WZero :: Wit Zero WSucc :: Wit n -> Wit (Succ n) and the Witness instances will be the same as with WNatClass. We need two classes because one will define the witness type, the other will do the "pattern matching". The part I don't understand is that together with the (n :: Nat) instance we can also have instance Construct (a :: SomeOtherType) where ... without triggering any 'overlapping' diagnostic, even though both instances are "fully polymorphic". I am assuming this is because internally the compiler adds a kind parameter to the class that disambiguates the instances? If so then I think this is a nice uniform way of handling lifted type witnesses. (The instances can be generated automatically too). What do you think? Is there a better way of doing this? Is the decoupling even worth the trouble? Is the performance hit big compared to the explicit instance splitting?

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IDE
by Corentin Dupont 25 Nov '13

25 Nov '13

Hi cafe, I would like to know which IDE are you using? I use Leksah, I like the GUI design and constant compilation process. However, the development seems to be slowing down: last version date from early 2012. The installation process is very painful. There is FPComplete IDE coming up, but it's commercial. I'm experimenting with it, anyway: how to compile using cabal files? It doesn't seem to recognize them. Will there be an off-line version? I'm interested to know if you are using vim to develop: which extension are you using? I'm interested in continuous compilation, project files browsing, code completion, search-in-files. Best, Corentin

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New Functional Programming Job Opportunities
by Functional Jobs 25 Nov '13

25 Nov '13

Here are some functional programming job opportunities that were posted recently: Software Developer - Functional Programming at Genetec http://functionaljobs.com/jobs/8660-software-developer-functional-programmi… Cheers, Sean Murphy FunctionalJobs.com

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[ANNOUNCE]hPDB - Is it the fastest parallel PDB parser? Part of structural bioinformatics library collection...
by Michal J Gajda 24 Nov '13

24 Nov '13

Dear Haskellers, I would like to present a benchmark of Protein Databank parsers that indicates that one written in Haskell seems to outpace all others when using 4 or more of parallel cores: hPDB - Haskell library for processing atomic biomolecular structures in Protein Data Bank format -- Michal Jan Gajda BMC Research Notes.2013, 6:483. DOI: 10.1186/1756-0500-6-483 URL: http://www.biomedcentral.com/1756-0500/6/483 Please let me know if you know of any other parsers that could be added to this benchmark. Along with hTalos, and parseSTAR parser libraries for nuclear magnetic resonance data it adds to growing collection of bioinformatic libraries written in Haskell. Together with CloudHaskell and modern 48-core machines, they allow to process multigigabyte bioinformatic databases in a matter of few minutes (slightly over 8 minutes in case of over 10GB of PDB.) If interested, please see: http://biohaskell.org/. -- Best regards Michał J. Gajda

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Munich Haskell Meeting
by Heinrich Hördegen 24 Nov '13

24 Nov '13

Dear all, on Wed, 27th of November, once again, our monthly Haskell meeting will take place in Munich at 19h30 at Cafe Puck. If you plan to join, please go to: http://www.haskell-munich.de/dates and hit the button. Have a nice week! Heinrich

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Stacked return
by Dmitry Bogatov 24 Nov '13

24 Nov '13

Hi, list! I want to write function, that will stack `return` as much times, as necessery. In code, I want <$> magicLift 2 :: IO Int <$> magicLift 2 :: Maybe [Int] both be valid. My best approach is following (not work) {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} class Monad m => MonadS m where liftS :: a -> m a instance (Monad m) => MonadS m where liftS = return but <$> :t liftS 2 liftS 2 :: (Monad m, Num a) => m a What would you suggest? -- Best regards, Dmitry Bogatov <KAction(a)gnu.org>, Free Software supporter and netiquette guardian. git clone git://kaction.name/rc-files.git --depth 1 GPG: 54B7F00D Html mail and proprietary format attachments are forwarded to /dev/null.

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by Ed Soares 23 Nov '13

23 Nov '13

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Hackage home page content should feature cabal install instructions
by John D. Ramsdell 23 Nov '13

23 Nov '13

As I read the content at hackage.haskell.org, I thought about what a new user interesting in using the site would like to know. The most important question is how to use the programs and libraries provided by the site. Thus, the content that should be featured is how to install packages using cabal install assuming Haskell platform has been installed. Exploring the site might be the next topic, followed by the support section and the other topics. In short, most users want to know how to use the packages provided by the site, and not so much about the server that powers the site. John

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Is there any "purely algebraic" Linear Algebra library?
by Hiromi ISHII 23 Nov '13

23 Nov '13

Hi, I'm recently implementing computational algebra library (available on http://hackage.haskell.org/package/computational-algebra ), build on top of ekmett's `algebra` library. My `computational-algebra` library currently provides very basic functionalities which is rather slow. There are many techniques to speeding up calculation utilizing linear algebra. So I want to implement those efficient algorithms. But, I couldn't find any *purely algebraic* linear-algebra library. As for the most famous library `hmatrix`, this library requires "Field" to be convertible between Double or Real numbers, and due to such constraints, it cannot treat genral coefficient fields, such as Rational (which is not closed under sin or exponentials) or finite fields. So I have to give up using this library. I noticed there are some additional candidates such as: * `linear` by Edward Kmett * `vector-space` by Conal Elliot But neither of them provides necessary functionalities which is available in `hmatrix`, such as determinant for arbitrary dimensional matrices and rank function , etc. Is there any other library providing the functionalities like above which can be used with general coefficient fields? Or is there any good document on the efficient implementation for linear algebra on Haskell? -- Hiromi ISHII konn.jinro(a)gmail.com

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Dynamic package information with Setup.hs
by Thiago Padilha 21 Nov '13

21 Nov '13

I'm writing a cabal package that provides bindings to a C library. I've written a shell script that will download and build this C library to a subdirectory of the user's HOME(~/.cabal-tmp/{lib,include}). Currently I have the following Setup.hs that automatically runs the script before installing: import Distribution.Simple import Distribution.Simple.Setup import Distribution.PackageDescription import System.Process main :: IO () main = defaultMainWithHooks simpleUserHooks { preConf = buildDeps } buildDeps :: Args -> ConfigFlags -> IO HookedBuildInfo buildDeps _ _ = do rawSystem "sh" ["./scripts/build-deps.sh"] return emptyHookedBuildInfo I've tried adding the following lines: extra-lib-dirs: ${HOME}/.cabal-tmp/lib include-dirs: ${HOME}/.cabal-tmp/include But it complains about syntax errors and I found no other way to expand the user's HOME directory in the .cabal file. If I use relative paths in the 'extra-lib-dirs' an error is reported(cabal 1.16). Full paths are also needed for linking against this package as ghc needs to know the linker flags for the native library. Obviously I cant know the full path the library will be installed in advance, as I dont know the HOME directory of the user installing the package until the Setup.hs script is running. My question: How can I use the Setup.hs hooks to update those two fields dynamically?

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