September 2015 - Haskell-Cafe

"Assignment" in do blocks?
by Anupam Jain 08 Sep '15

08 Sep '15

Hi all, I'd like to propose an inbuilt "assignment" operator for use within do-blocks. Here's a common pattern - do foo <- someaction do something with foo -- Here we "update" the value of foo let foo' = bar foo -- The code from this point onwards shouldn't use foo -- but there is no way to enforce that What I'd like to do instead is something like - let foo = bar foo But to Haskell this is a recursive definition instead of an update to the previous value. I can do this instead - foo <- return $ bar foo which works fine but (I assume) GHC currently can't optimise it for arbitrary monads. Also it won't necessarily work if the monad laws don't hold. It would be nice to have an "assignment arrow", say (<-=), which lets GHC optimise this, so - foo <-= bar foo ... desugars to something like - foo' (bar foo) where foo' foo = ... -- Anupam

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Conduit and foldLines
by Kees Bleijenberg 07 Sep '15

07 Sep '15

My goal is to parse a big text-file line by line using the same parser for every line. I am new to Conduit and I've read this https://www.fpcomplete.com/user/chad/snippets/random-code-snippets/folding-l ines-in-conduit article. After adding import Control.Monad.Trans.Resource for runResourceT and adding rankNTypes, I got it working. Code in www.tbsoftware.nl/TestConduit/TestConduit7.hs I don't understand what is going on in myFunc. Does CL.fold fold over the already read lines? I wonder whether CL.fold loads the whole file in memory? if I change count <- CL.fold (\count t -> count + T.length t) 0 to count <- CL.fold (\count t -> T.length t) 0 nothing changes in the output. If count is always zero, why using a fold? How can I get the 'current line' in myFunc? Kees

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cabal or stack command to fetch and add dependency to .cabal file
by xu xiut 07 Sep '15

07 Sep '15

With node.js, you can run `npm install --save package(a)1.01`, and that version of the package will be retrieved and added to your list of dependencies. If you leave off the version number, it will retrieve the latest version from a default repository of packages. Are there any plans to add something this to either cabal or stack?

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ANNOUNCE: graphviz-2999.18.0.0
by Ivan Lazar Miljenovic 06 Sep '15

06 Sep '15

This is my "sorry it took so long" release: I'd like to apologise to all the people that have been nagging^W gently asking me when a GHC 7.10.*-compatible release would be available that it's taken me until now to do this, but I'm finally pleased to be able to say that you can now use my graphviz library to generate/parse Dot code and visualise graphs using the Graphviz suite of tools. As part of this version release, graphviz is now developed using git and can be found on GitHub: https://github.com/ivan-m/graphviz The major version bump is due to an extra attribute being made available, so in most (all?) cases you can safely version bump your existing dependencies. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Problem with lifted-async
by Lana Black 05 Sep '15

05 Sep '15

Hello, I'm writing a thread supervisor that allows implicitly passing some monadic context (e.g. ReaderT) using MonadBaseControl from monad-control. The problem is that I don't know how to tackle this error. GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( ex.hs, interpreted ) ex.hs:20:20: Couldn't match type ‘StM t a0’ with ‘StM t a’ NB: ‘StM’ is a type function, and may not be injective The type variable ‘a0’ is ambiguous Expected type: Async (StM t a0) Actual type: Async (StM t a) Relevant bindings include as :: Async (StM t a) (bound at ex.hs:19:30) t :: Task t a (bound at ex.hs:19:28) td :: TaskDescriptor t (bound at ex.hs:19:10) pollTask :: TaskDescriptor t -> TaskDescriptor t -> t (TaskDescriptor t) (bound at ex.hs:19:1) In the first argument of ‘poll’, namely ‘as’ In a stmt of a 'do' block: st <- poll as Failed, modules loaded: none. This minimal code snippet would be {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE FlexibleContexts #-} import Control.Concurrent.Async.Lifted (Async,async,asyncBound,poll,cancel) import Control.Monad.Trans.Control (MonadBaseControl,StM) data Task m a = Task { taskVal :: m a } data TaskDescriptor m = forall a. TaskDescriptor { task :: Task m a, asyncThread :: Async (StM m a) } runTask :: forall a m. (MonadBaseControl IO m) => Task m a -> m (TaskDescriptor m) runTask = undefined pollTask td(TaskDescriptor t as) = do st <- poll as case st of Nothing -> pure td Just r -> runTask t main = undefined StM is a type from MonadBaseControl typeclass, the definition is class MonadBase b m => MonadBaseControl b m | m -> b where type StM m a :: * liftBaseWith :: (RunInBase m b -> b a) -> m a restoreM :: StM m a -> m a type RunInBase m b = forall a. m a -> b (StM m a) I stripped the comments, the full version is here https://hackage.haskell.org/package/monad-control-1.0.0.4/docs/src/Control-…

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Bullet bindings with Bullet 2.83
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I'm trying to get the bullet binding working with Bullet 2.83.5. I installed libbullet-dev 2.83.5 from the Debian testing archive (I'm running Debian Stretch) and installed the Haskell binding (with cabal install --enable-documentation). Compiling the example results in a bunch of undefined reference errors like this: /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/bullet-0.2.4/libHSbullet-0.2.4.a(Bullet.o):(.data.rel.ro._ZTV20btTetrahedronShapeEx[_ZTV20btTetrahedronShapeEx]+0x60): undefined reference to `btPolyhedralConvexShape::initializePolyhedralFeatures()' I guess this is because, the Debian package doesn't come with a static bullet libary, so I tried to compile it with dynamic libraries (with the -dynamic flag), but it just fails with other undefined references. The installation instructions for the Bullet binding say, it works with Bullet 2.73, so I'm afraid this is some version incompatibility. Does anyone know, how to get the binding working with Bullet 2.83 or what other causes the missing symbols can have? Regards Sven

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Anyone familiar with Hpysics?
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I recently found the physic library/engine Hpysics. It looks really nice and would like to use it for a project, because it's more natural to use in Haskell, than Bullet is. The problem with the library, seems to be, that it doesn't seem, like it retrieved a lot of maintainer attention lately, e.g. after downloading it, I had to fix some type errors in the library (some incompatibility to GL floats), to get it working. The author field in the .cabal file lacks an email, so I can't contact him and ask if he still cares about the package. Given that there doesn't seem to be an active maintainer and I need the package, I would be willing to take over its maintenance. The documentation is not really verbose about most stuff, so there are some things, where I don't know how they are supposed to work. Is there maybe someone, who was involved in writing the library or maintaining it, or someone, who just used it and can help me, with getting it in shape again, or maybe someone, who still feels responsible for it and just didn't update it, because there was no interest? PS: The package can be found on http://code.haskell.org/hpysics/, but not on hackage (another reason, why I believe it's not actively mantained anymore). Regards Sven

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Physics engine for Haskell
by Sven Bartscher 05 Sep '15

05 Sep '15

Greetings, I'm looking for a game physics engine for Haskell. As Bullet is widely used in this field, I went to use the Haskell Bullet binding, but had to find, that it is very inconvenient to use, because of things like making optional parameters necessary in the binding. Is there any higher level binding to Bullet or some other Physics library that is more convenient to use on Haskell? Regards Sven

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GHC 7.8 undefined symbol
by Sven Bartscher 04 Sep '15

04 Sep '15

Greetings, I recently installed a library (namely Hpysics, not on hackage) with shared libraries (they were on by default). When trying to do some stuff with the library (program attached), I get a bunch of undefined symbol errors like this: $ ghc --make Main.hs Linking Main ... /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0x46e): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_constructBoundingSpherezugo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0x4f9): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_zdwlgo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0xaca6): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_constructBoundingSpherezugo_info' /home/sven/.cabal/lib/i386-linux-ghc-7.8.4/Hpysics-0.0/libHSHpysics-0.0.a(Body.o):(.text+0xad31): undefined reference to `Hpysicszm0zi0_PhysicsziHpysicsziBoundingSphere_zdwlgo_info Does anyone know, what this is about? PS: Hpysics can be found at http://code.haskell.org/hpysics/ Regards Sven

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ANN: stack-0.1.4.0 (and 0.1.4.1)
by Emanuel Borsboom 04 Sep '15

04 Sep '15

See README <https://github.com/commercialhaskell/stack#how-to-install> for installation and upgrade instructions. Note: while stack-0.1.4.1 is on Hackage, the only change is to a comment so that stack’s haddocks build. Therefore, no new binaries were generated. Functionally, v0.1.4.0 is identical to v0.1.4.1. Note: we have stopped uploading new versions to the Fedora 20 repository, since that version of Fedora has reached end of life <https://fedoraproject.org/wiki/End_of_life>. If you are using Fedora 20, use stack upgrade or download the general Linux binary <https://github.com/commercialhaskell/stack/wiki/Downloads#linux>. Major changes: - You now have more control over how GHC versions are matched, e.g. “use exactly this version,” “use the specified minor version, but allow patches,” or “use the given minor version or any later minor in the given major release.” The default has switched from allowing newer later minor versions to a specific minor version allowing patches. For more information, see #736 <https://github.com/commercialhaskell/stack/issues/736> and #784 <https://github.com/commercialhaskell/stack/pull/784>. - Support added for compiling with GHCJS - stack can now reuse prebuilt binaries between snapshots. That means that, if you build package foo in LTS-3.1, that binary version can be reused in LTS-3.2, assuming it uses the same dependencies and flags. #878 <https://github.com/commercialhaskell/stack/issues/878> Other enhancements: - Added the --docker-env argument, to set environment variables in Docker container. - Set locale environment variables to UTF-8 encoding for builds to avoid “commitBuffer: invalid argument” errors from GHC #793 <https://github.com/commercialhaskell/stack/issues/793> - Enable translitation for encoding on stdout and stderr #824 <https://github.com/commercialhaskell/stack/issues/824> - By default, stack upgrade automatically installs GHC as necessary #797 <https://github.com/commercialhaskell/stack/issues/797> - Added the ghc-options field to stack.yaml #796 <https://github.com/commercialhaskell/stack/issues/796> - Added the extra-path field to stack.yaml - Code page changes on Windows only apply to the build command (and its synonyms), and can be controlled via a command line flag (still defaults to on) #757 <https://github.com/commercialhaskell/stack/issues/757> - Implicitly add packages to extra-deps when a flag for them is set #807 <https://github.com/commercialhaskell/stack/issues/807> - Use a precompiled Setup.hs for simple build types #801 <https://github.com/commercialhaskell/stack/issues/801> - Set —enable-tests and —enable-benchmarks optimistically #805 <https://github.com/commercialhaskell/stack/issues/805> - --only-configure option added #820 <https://github.com/commercialhaskell/stack/issues/820> - Check for duplicate local package names - Stop nagging people that call stack test #845 <https://github.com/commercialhaskell/stack/issues/845> - --file-watch will ignore files that are in your VCS boring/ignore files #703 <https://github.com/commercialhaskell/stack/issues/703> - Add --numeric-version option Bug fixes: - stack init --solver fails if GHC_PACKAGE_PATH is present #860 <https://github.com/commercialhaskell/stack/issues/860> - stack solver and stack init --solver check for test suite and benchmark dependencies #862 <https://github.com/commercialhaskell/stack/issues/862> - More intelligent logic for setting UTF-8 locale environment variables #856 <https://github.com/commercialhaskell/stack/issues/856> - Create missing directories for stack sdist - Don’t ignore .cabal files with extra periods #895 <https://github.com/commercialhaskell/stack/issues/895> - Deprecate unused --optimizations flag - Truncated output on slow terminals #413 <https://github.com/commercialhaskell/stack/issues/413>

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