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Flag to recompile modules which had warnings
by Kostiantyn Rybnikov 09 Sep '15

09 Sep '15

This is a call for comments before I'd create a ticket in corresponding project (I'm not sure if it should be ghc or stack/cabal). There's a common situation I get into: I compile with "stack build" (or cabal build), get a big list of warnings, fix few of them, then I re-compile project, but a lot of warnings are gone now. The only option I'm left with is to use "-fforce-recomp". Problems with -fforce-recomp are: - full-recompile is quite long for big projects - most people don't know about it, especially newbies I think some flag, which would be turned on by default, which would indicate to "recompile modules if they previously had warnings" would be awesome to have. 1. would you like to have that flag? 2. would you like it "on" by default? 3. should ticket go into ghc or stack/cabal-install project? Thank you.

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Can #Haskell be used to generate the #Java class libraries relatively error free? #Java8u60
by KC 09 Sep '15

09 Sep '15

Can #Haskell be used to generate the #Java class libraries relatively error free? #Java8u60 -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Functional counterpart of counting-sort?
by Nicola Gigante 09 Sep '15

09 Sep '15

Hi all, I’m sorry if this is a trivial question. Everybody knows the counting sort algorithm to sort an array of integer values, which runs in O(n) time, circumventing the O(n log n) lower bound by avoiding comparison operations. Of course, that only works for integers. However, it seems to me that it essentially relies on the availability of a O(1) indexing operation on the array, not on algebraic or logical properties of the set of the integers. Do any of you know if it is possible to sort a _list_ of integers in O(n) time in a (lazy) purely functional language? I’d say no, but given all the crazy laziness tricks out there, I can’t say for sure ;) Even if no: any theoretical considerations on why not? Greetings, Nicola

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Re: [Haskell-cafe] GHC Extension Proposal: ArgumentBlock
by James Burton 08 Sep '15

08 Sep '15

On 8 Sep 2015 3:12 pm, Carlo Nucera <meditans(a)gmail.com> wrote: > > -1 > > I wouldn't like to read code written with this extension. It feels to > me like an ad-hoc case, > hindering the general uniformity of the language. I agree, and think it could be confusing to switch between styles in different codebases or in a single codebase. I teach Haskell and encourage students to use brackets liberally if they're not sure when they're needed and experiment with removing redundant ones and replacing others with $ later on. I don't think this is one of the things they tend to get confused about. > It has been a nice > opportunity to read up > the haskell report, though :) > > 2015-09-08 15:35 GMT+02:00 Alexander Berntsen <alexander(a)plaimi.net>: > > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA512 > > > > ($) is just a function like any other*, so I don't think it's a great > > argument for or against a syntax extension. > > > > *Bar a few details like the special infix typing rule for > > accommodating impredicative polymorphism. > > - -- > > Alexander > > alexander(a)plaimi.net > > https://secure.plaimi.net/~alexander > > -----BEGIN PGP SIGNATURE----- > > Version: GnuPG v2 > > > > iQIcBAEBCgAGBQJV7uQDAAoJENQqWdRUGk8BA3wQAMdZ3xHQDYUy17HahMbMjxyT > > xkm2bZLDNMPwmyaciGHsC+BeRae9Oo1oVgjWDDzrL41zSf9e2GS5SmEQumJeAgLm > > TNkJgctJ44Ej+yPnJkik9OCK4YsFjZPFYdj8iI4T7uEBfC5vDQczODgKvcXo3WiY > > JmujgC+QFjU3PaDaz8HZ8UzDIRszBRPQyTTLzlYnsshQwws/eU7W1hBKBLNFjn90 > > t9n8xq49BfbNInHSzVy5CbUW5VyPCYikkCAz7zRBc25iWtaXODvYnDCyc4vwVg38 > > QsNfXvOsSrnE80WOQxXKbaHutMaf8QoiNQ5+M8gMNKWAHXZ0ROYgAE9lm2QfLMTE > > oidjVh9WrYmlXCmJl0pgFdN1Dc5J+KimoTK8ttvGcweRckUdgkBRQsmyqILtn+ql > > peNN9QU5W1V8rUfliTq9XCXDu1rAKuKtSPb3GQD9vXDQ22QSHQqjk8eLdd2bAK1n > > ji/r+Bjp9XNLvySaPi9neiEp3geFjZnN7b3pC2QtDsRJozv6prliNHOsGgLin5dN > > FmrwSXysha7hZtxy3ze4XnMa/91GZPB9Z91hZdALXyVSbDBrE4ukUABprI6tK8/6 > > O2pfmsdWzvGMNgXFibg1sgJk9e8aTvp6YPyi6Xyj/88hLD6YxB3/ekxxBy00EDW/ > > RFqKfswkYKypfKwx0FXq > > =8Rg1 > > -----END PGP SIGNATURE----- > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > ___________________________________________________________ > This email has been scanned by MessageLabs' Email Security > System on behalf of the University of Brighton. > For more information see http://www.brighton.ac.uk/is/spam/ > ___________________________________________________________ ___________________________________________________________ This email has been scanned by MessageLabs' Email Security System on behalf of the University of Brighton. For more information see http://www.brighton.ac.uk/is/spam/ ___________________________________________________________

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"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@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 06 Sep '15

06 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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