- Haskell-Cafe - Haskell.org

btc-e interfacing + bot
by Miro Karpis 26 Nov '13

26 Nov '13

Am thinking to inteface btc-e + make a bot. Anybody willing to join forces? m.

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Re: [Haskell-cafe] using external instance declarations
by Kata 26 Nov '13

26 Nov '13

The problem is that your definition of arbitrary for the Building instance isn’t indented under the ‘instance Arbitrary Building’, so it’s getting interpreted as two top-level statements: one that Building is an Arbitrary of Instance, and one that defines a value named ‘arbitrary’. Hence the error message. To fix this, just indent the definition of arbitrary like you did in the Glass definition. On Tuesday, November 26, 2013 at 11:06 AM, Kees Bleijenberg wrote: > I have a Building with a field glass: Glass. > I want to use quickcheck. Therefor I wrote a TestGlass.hs with the Arbitrary instance declaration for Glass. I also want to make Building an instance of Arbitrary. For the field glass in the Building I need the arbitrary :: Gen Glass function out of TestGlass.hs. > How do I export and use the instance declarations from TestGlass.hs? > > Glass.hs > module Glass( > Glass(..)) > where > data Glass = Glass Int > > TestGlass.hs > module TestGlass() > where > import Test.QuickCheck > import Test.QuickCheck.Gen > import Glass > > instance Arbitrary Glass where > arbitrary = do > n <- choose (1,8) :: Gen Int > return $ Glass n > > > Building.hs > module Building () > where > import Glass > import TestGlass > import Test.QuickCheck > import Test.QuickCheck.Gen > > data Building = Building { > glass :: Glass > } > instance Arbitrary Building where > arbitrary = do > g <- arbitrary :: (Gen Glass) > return Building { glass = g} > > > When I compile Building I get: > Building.hs:13:23: > Ambiguous occurrence `arbitrary' > It could refer to either `Building.arbitrary', > defined at Building.hs:12:1 > or `Test.QuickCheck.arbitrary', > imported from `Test.QuickCheck' at Building.hs:5:1-22 > (and originally defined in `Test.QuickCheck.Arbitrary') > > It looks like the compiler doesn’t look at the instance declarations in the TestGlass module. > What is wrong in my code? > I want to split the testcode as much as possible from the non-test code and I don’t want to merge the modules Building and Glass (they are both already big). > > Kees > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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using external instance declarations
by Kees Bleijenberg 26 Nov '13

26 Nov '13

I have a Building with a field glass: Glass. I want to use quickcheck. Therefor I wrote a TestGlass.hs with the Arbitrary instance declaration for Glass. I also want to make Building an instance of Arbitrary. For the field glass in the Building I need the arbitrary :: Gen Glass function out of TestGlass.hs. How do I export and use the instance declarations from TestGlass.hs? Glass.hs module Glass( Glass(..)) where data Glass = Glass Int TestGlass.hs module TestGlass() where import Test.QuickCheck import Test.QuickCheck.Gen import Glass instance Arbitrary Glass where arbitrary = do n <- choose (1,8) :: Gen Int return $ Glass n Building.hs module Building () where import Glass import TestGlass import Test.QuickCheck import Test.QuickCheck.Gen data Building = Building { glass :: Glass } instance Arbitrary Building where arbitrary = do g <- arbitrary :: (Gen Glass) return Building { glass = g} When I compile Building I get: Building.hs:13:23: Ambiguous occurrence `arbitrary' It could refer to either `Building.arbitrary', defined at Building.hs:12:1 or `Test.QuickCheck.arbitrary', imported from `Test.QuickCheck' at Building.hs:5:1-22 (and originally defined in `Test.QuickCheck.Arbitrary') It looks like the compiler doesn't look at the instance declarations in the TestGlass module. What is wrong in my code? I want to split the testcode as much as possible from the non-test code and I don't want to merge the modules Building and Glass (they are both already big). Kees

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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 24 Nov '13

24 Nov '13

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