- Haskell-Cafe - Haskell.org

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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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 22 Nov '13

22 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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ANN: New Haskell.org committee members
by Jason Dagit 21 Nov '13

21 Nov '13

The Haskell.org committee has selected new members following the October self-nomination period. The new members are: * Adam Foltzer * Nicolas Wu * Andres Loeh Thank you to everyone who submitted a self-nomation. We had many very strong candidates and it was not easy for us to narrow the selection down to just three people. I would like to add, that if you've self-nominated in the past but not be picked, please self-nominate again in the future. Cheers, Jason

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Product Profunctor and Contravariant
by Tom Ellis 20 Nov '13

20 Nov '13

Does anyone recognise these typeclasses: import Data.Profunctor (Profunctor) import Data.Functor.Contravariant (Contravariant) class Profunctor p => ProductProfunctor p where empty :: p () () (***!) :: p a b -> p a' b' -> p (a, a') (b, b') class Contravariant f => ProductContravariant f where point :: f () (***<) :: f a -> f b -> f (a, b) They are both a bit like Applicative, and ProductProfunctor is basically Arrow without the Category part. I'm finding ProductProfunctor useful for marshalling data from a database into Haskell, and both of them come up a lot inside my database library. Has anyone ever seen these before? Has Edward Kmett written a library for these already? Thanks, Tom

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QuickCheck-like tests with GHC.Generic deriving
by Alexander Solla 20 Nov '13

20 Nov '13

I am looking for a QuickCheck-like test framework which exposes data generators (like QuickCheck's Arbitrary class) with Generic defaults. I have seen that SmallCheck's Serial class has Generic defaults, but it isn't particularly good for my purpose (I'd like to use the generator to populate deeply nested data types, and SmallCheck's generator requires computing a few million values before I get any interesting ones) Something "random" would be more useful for my purpose. I know that the plan depends on my vaguely abusing the test-case generator, but I plan on doing some testing anyway, so I figured I should reuse the instance. That said, I'd be willing to use any random value type class with Generic-default instances. Does anybody have any suggestions?

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Re: [Haskell-cafe] QuickCheck-like tests with GHC.Generic, deriving
by Janis Voigtlaender 20 Nov '13

20 Nov '13

Alexander Solla writes: > I am looking for a QuickCheck-like test framework which exposes data > generators (like QuickCheck's Arbitrary class) with Generic defaults. > > I have seen that SmallCheck's Serial class has Generic defaults, but it > isn't particularly good for my purpose (I'd like to use the generator to > populate deeply nested data types, and SmallCheck's generator requires > computing a few million values before I get any interesting ones) > Something "random" would be more useful for my purpose. > > I know that the plan depends on my vaguely abusing the test-case generator, > but I plan on doing some testing anyway, so I figured I should reuse the > instance. > > That said, I'd be willing to use any random value type class with > Generic-default instances. > > Does anybody have any suggestions? I think what you want is: http://hackage.haskell.org/package/testing-feat See also the paper: http://dx.doi.org/10.1145/2430532.2364515 Specifically, the technique solves the "populate deeply nested data types" issue. You can efficiently jump to the so-and-so-many-millionth value in the enumeration without having to create all smaller values. Best, Janis.

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Need help with very simple C++ FFI call
by Thiago Padilha 19 Nov '13

19 Nov '13

Hi I'm trying write a ffi binding to the v8 javascript engine (C++ library). Theres one already at https://github.com/sol/v8 but I want to do it again as an exercise to improve by haskell skills. I chose this library be because it will force me to deal with impure functions using monads. Since its not a trivial library, it will also improve my knowledge of writing FFI bindings and cabal packages in case I want to wrap more C++ libraries in the future. My first step is to port the 'hello world' example in v8 embedders guide(https://developers.google.com/v8/get_started) as a haskell binding. So I have downloaded/compiled v8 and created the following directory structure: src/V8.hs cbits/haskell-v8.cpp # v8 libraries deps/lib/libv8_base.a deps/lib/libv8_snapshot.a # v8 headers deps/include/ Here's the contents of the haskell-v8.cpp file: http://pastebin.com/RfYCCxFQ (basically a copy of the v8 embedders example function, except that it returns a char pointer). and here's the haskell module: http://pastebin.com/fnXVbEeU I compiled a shared library exposing the c "hello" function with the following command: $ g++ \ -shared -o libhaskellv8.so \ -I./deps/include \ -L./deps/lib \ ./cbits/haskell-v8.cpp \ -lv8_base \ -lv8_snapshot \ -lpthread \ -lrt And loaded into ghci with the following: $ ghci -L. -lhaskellv8 src/V8.hs The problem is that I get a segmentation fault when I call the 'hello' function from ghci. To ensure the library was compiled correctly, I successfully called it from python: $ python -c \ 'from ctypes import CDLL, c_char_p; lv8=CDLL("libhaskellv8.so"); lv8.hello.restype = c_char_p; print lv8.hello()' Hello, World! I have selectively uncommented lines from the c++ file, and the segmentation faults starts happening when I uncomment line 14 (HandleScope handle_scope(isolate)). What am I missing here?

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