January 2014 - Haskell-Cafe

Any projected data for new Haskell Platform (2013.4? 2014.2?)
by Stuart Mills 21 Jan '14

21 Jan '14

Thanks

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Associated patterns
by Alejandro Serrano Mena 21 Jan '14

21 Jan '14

Dear haskell-cafe, I've read in Reddit that pattern synonyms have been merged in GHC HEAD [ http://www.reddit.com/r/haskell/comments/1vpaey/pattern_synonyms_merged_int… ]. I would like to know whether associated patterns, that is, patterns which come under the umbrella of a type class, have also been implemented. I think that associated patterns would fill the gap in difference of features between type classes and common data types. For plain data types, you can declare both functions and patterns (either via constructors or now via pattern synonyms). However, you can only declare functions (either term-level or type-level) in type classes. This means that the pattern match mechanism, very useful to get clear code, is not useful if you want to use type classes. Alejandro.

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MissingH fails to compile
by Peter Colberg 21 Jan '14

21 Jan '14

Dear Haskell developers, I am trying to build a current version of git-annex, and stumble over a compilation failure of one of its dependencies, MissingH. I am using GHC 7.4.2 and Cabal 1.18.0.2, both bootstrapped on RHEL 5.x/6.x using a helper makefile [1] that has worked fine so far. The command to install git-annex is cabal install git-annex-5.20140108 --flags=-assistant --flags=-dbus --flags=-webapp --flags=-webdav --flags=-xmpp The build log for MissingH-1.2.0.2 is attached. I would be glad for any pointers on how to resolve this issue. Thanks, Peter [1] http://git.colberg.org/packages/plain/packages.mk

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ghc-mod list: cannot satisfy -package cairo
by Brian Victor 21 Jan '14

21 Jan '14

I'm receiving an error from ghc-mod in my cabal sandbox: %./.cabal-sandbox/bin/ghc-mod list Dummy:0:0:Error:<command line>: cannot satisfy -package cairo (use -v for more information) The obvious potential problem is that cairo isn't installed. However, %cabal repl Preprocessing executable 'Notation' for Notation-0.1.0.0... GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help ... Prelude Main> import Graphics.Rendering.Cairo Prelude Graphics.Rendering.Cairo Main> My cabal file includes: build-depends: base >=4.6 && <4.7, mtl >=2.1 && <2.2, containers >=0.4 && <0.5, cairo >= 0.12 && <= 0.13 I'm on OSX 10.9.1. I had to point cairo to gcc-4.9 instead of clang to make it compile. Where else can I look for the problem? -- Brian

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[Help] Trying to rewrite the NetBSD kernel using Haskell language.
by Kiwamu Okabe 21 Jan '14

21 Jan '14

Hi Haskell hackers, We start trying to rewrite NetBSD kernel using Haskell language. https://github.com/metasepi/netbsd-arafura-s1 But we find some problems. Could you have any ideas for them? ## Status * Trying rewrite AC''97 sound driver using Haskell * Before rewriting. (C language) https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… * After rewriting. (Haskell language) https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… * Have rewrited 2 functions, auich_open() and auich_close() * Not yet touch Haskell heap ## Problems Some time, C programmer writes following code. ~~~ static int auich_open(void *addr, int flags) { struct auich_softc *sc; sc = (struct auich_softc *)addr; mutex_spin_exit(&sc->sc_intr_lock); sc->codec_if->vtbl->lock(sc->codec_if); ~~~ Note the line "sc->codec_if->vtbl->lock(sc->codec_if);". It traces pointer tree regionally. Haskell's Strorable class is not good for the use case, because it copies the entire pointer tree! We avoid the problem using following messy code. https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… ~~~ auichOpen :: Ptr AuichSoftc -> Int -> IO Int auichOpen sc flags = do mutexp <- p_AuichSoftc_sc_intr_lock sc codeif <- peek =<< p_AuichSoftc_codec_if sc lock <- peek =<< p_Ac97CodecIfVtbl_lock =<< peek =<< p_Ac97CodecIf_vtbl codeif mutexSpinExit mutexp call_Ac97CodecIfVtbl_lock lock codeif -- snip -- p_AuichSoftc_sc_intr_lock :: Ptr AuichSoftc -> IO (Ptr KmutexT) p_AuichSoftc_sc_intr_lock p = return $ plusPtr p offsetOf_AuichSoftc_sc_intr_lock p_AuichSoftc_codec_if :: Ptr AuichSoftc -> IO (Ptr (Ptr Ac97CodecIf)) p_AuichSoftc_codec_if p = return $ plusPtr p offsetOf_AuichSoftc_codec_if p_Ac97CodecIf_vtbl :: Ptr Ac97CodecIf -> IO (Ptr (Ptr Ac97CodecIfVtbl)) p_Ac97CodecIf_vtbl p = return $ plusPtr p offsetOf_Ac97CodecIf_vtbl p_Ac97CodecIfVtbl_lock :: Ptr Ac97CodecIfVtbl -> IO (Ptr (FunPtr Ac97CodecIfVtbl_lock)) p_Ac97CodecIfVtbl_lock p = return $ plusPtr p offsetOf_Ac97CodecIfVtbl_lock ~~~ Haskell world has the answer for it already? Best regards, -- Kiwamu Okabe

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Re: [Haskell-cafe] Help with Binary Serialization
by Lennart Kolmodin 21 Jan '14

21 Jan '14

Hi, the format is quite simple, and it's the same in both binary and cereal. For data types with only one constructor, only the values of the constructor are encoded - not the constructor itself. data Foo = Foo Int Int deriving (Generic) So Foo is encoded as if it was only the Ints without the Foo constructor. encode (Foo 1 2) = 0x 00000001 00000002 If there are multiple constructors a tag is encoded representing the constructor, and then the values of that constructor. The tag will use as many bytes as it needs to be to fit the tag. 2-255 constructors will use 1 byte, and so on. data Fruit = Apple Int | Orange Foo encode (Apple 3) = 0x 00 00000003 First byte is zero to represent the Apple constructor, then the Int. encode (Orange (Foo 4 5)) = 0x 01 00000004 00000005 First byte is 0x01 to represent the Orange constructor, then the Foo value which is just to subsequent Ints. The format will not change without warning, and has not changed since it was implemented. That said, the generic format is mostly meant for being written and read by the haskell app that defines the data types, as it's easy to break compatibility even within the same app. JSON is probably not a bad choice, try it out http://hackage.haskell.org/package/aeson-0.6.1.0/docs/Data-Aeson.html Hope it helps! Lennart On Monday, 20 January 2014 21:28:33 UTC+4, Joey Eremondi wrote: > > Makes sense. I've had trouble finding documentation on the format used by > aeson, any links to that? > On 2014-01-19 10:46 AM, "John Lato" <jwl...(a)gmail.com <javascript:>> > wrote: > >> I think this approach will likely lead to problems later on. First, >> undocumented formats are liable to change without warning. Secondly, it's >> conceivable that the format could change due to changes in ghc's generics >> support, or internal (and hence unstable) data structures of some component >> type. >> >> Would it be possible to just define your own format instead, or use >> something like JSON that's already well-defined? >> >> On Jan 18, 2014 12:17 PM, "Joey Eremondi" <jmit...(a)gmail.com<javascript:>> >> wrote: >> > >> > I was wondering if somebody could talk me through the default derived >> format for binary serialization used, either by binary or by cereal. >> > >> > I'm trying to share data between Haskell and another function language >> (Elm) which also supports algebraic data types, so the conversion of data >> should be pretty trivial. I'd like to be able to just derive encode and >> decode in Haskell using either binary/cereal, and then write a parser for >> the same format in Elm. The trick is, I don't know what that format is. >> > >> > Is there any documentation on it, or if not, is anybody familiar enough >> with it that they could explain the format to me? >> > >> > Thanks! >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskel...(a)haskell.org <javascript:> >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >

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Hacking GHC's Stack for Fun and Profit (featuring The Glorious Haskell Debugger v0.0.1 Pre-alpha)
by Tim C. Schroeder 20 Jan '14

20 Jan '14

Hello, I have developed various profiling and debugging tools in the past, but haven't done any work in that direction for GHC compiled Haskell programs. I thought a good start might be to write a document detailing how the call stack works in GHC, and also write a basic debugger stack trace tool. Here: https://github.com/blitzcode/ghc-stack I hope this is helpful to people trying to pin down crashes, and people interested in working on debugging / profiling tools. I'd also really appreciate if somebody with more GHC / RTS knowledge than me would have a look and see if I got things right. Cheers, Tim

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Re: [Haskell-cafe] different behaviours with or without putStrLn
by Corentin Dupont 20 Jan '14

20 Jan '14

One question: Since it works with putStrLn, can I simulate the behaviour of putStrLn without actually... printing anything? As a workaround... On Mon, Jan 13, 2014 at 12:40 AM, Corentin Dupont <corentin.dupont(a)gmail.com > wrote: > I doesn't seems to be easy, either with deepseq-th or deepseq-generic... > With deepseq-generic, the derivation of Generic doesn't work because I'm > using a GATD. > It says *"NewVar must be a vanilla data constructor"*. > > For deepseq-th, I have this error: > > *deriveNFData: 'forall' not supported in constructor declaration* > I have some complex data constructor... > > > So it looks like both deepseq-th and deepseq-generic are out of the game > :(( > > > > > On Mon, Jan 13, 2014 at 12:08 AM, Erik de Castro Lopo < > mle+hs(a)mega-nerd.com> wrote: > >> James ‘Twey’ Kay wrote: >> >> > You can (probably) automatically derive it via Template Haskell using >> > the deepseq-th package: >> > >> > {-# LANGUAGE TemplateHaskell #-} >> > import Control.DeepSeq.TH >> > >> > data MyState = ... >> > >> > $(deriveNFData ''MyState) >> >> In most cases where you think >> >> You should probably try the deepseq-generics package before you try >> deepseq-th. >> >> The generics version was suggested to me by the author of the TH version, >> Herbert Valerio Riedel. It seems to be just as fast as the TH version and >> doesn't require TH. >> >> Cheers, >> Erik >> -- >> ---------------------------------------------------------------------- >> Erik de Castro Lopo >> http://www.mega-nerd.com/ >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Combining databases from packages installed with cabal install
by Marco Túlio Pimenta Gontijo 20 Jan '14

20 Jan '14

Hi. I'm configuring haddock via cabal install (see [0]) to build the hoogle database. The database is being installed in ~/.cabal/share/doc/$package-$version/html/$package.txt, but is not being combined with the default database. That is, if right after the installation I try to search with the hoogle command for some a function, it will not work. I wrote the following script, which I called cabal-install: #!/bin/sh set -e set -x cabal \ install \ --enable-documentation \ --enable-library-profiling \ --haddock-hyperlink-source \ --haddock-hoogle \ --haddock-html \ "$@" cd ~/.cabal/share/hoogle-4.2.13/databases/ for file in ~/.cabal/share/doc/*/html/*.txt do hoo=`echo $file | sed 's/.txt$/.hoo/;s#.*/##'` if [ ! -f $hoo ] then hoogle convert $file $hoo || true hoogle combine default.hoo $hoo -o /tmp/cabal-install-$$.hoo mv /tmp/cabal-install-$$.hoo default.hoo fi done Basically, it searches for .txt hoogle databases installed that were not combined yet with the default database, and combines them. I think it would be good if this was the default behaviour of cabal install when called with --haddock-hoogle. Is this a bug? Greetings. 0: http://hackage.haskell.org/trac/hackage/ticket/517 -- marcot http://marcot.eti.br/

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ANN: unsafely, Flexible access control for unsafe operations and instances
by Hiromi ISHII 19 Jan '14

19 Jan '14

Yesterday, I uploaded the library `unsafely` to Hackage: http://hackage.haskell.org/package/unsafely This package provides you the functionality for access control for unsafe operations and instances. This purpose is somewhat similar to GHC's `NullaryTypeClasses`[^1] extension, but permits more flexible access control. With this package, you can tag functions and type-class instances as *unsafe* in type constraint. This library is useful when: * You want to restrict the access to *unsafe* operations by type constraint * You have to provide some *unsafe* type-instances for practical reasons. For example, when writing computer algebra system with type-classes, `Double` type doesn't even form a semi ring, but we need the instance `Semiring Double` if we want to combine the symbolic computations and the numerical methods. A simple example: ```haskell {-# LANGUAGE FlexibleContexts, FlexibleInstances, RankNTypes #-} {-# LANGUAGE UndecidableInstances #-} module Main where import Data.Constraint.Unsafely import Data.IORef import Data.Proxy import System.IO.Unsafe saferUnsafePerformIO :: Unsafely IO => IO a -> a saferUnsafePerformIO = unsafePerformIO global :: Unsafely IO => IORef Int global = saferUnsafePerformIO $ newIORef 0 unsafelyIO :: (Unsafely IO => a) -> a unsafelyIO = unsafely (Proxy :: Proxy IO) main :: IO () main = do unsafelyIO $ readIORef global -- | uncommenting following line causes type-error! -- readIORef global return () ``` For more detail, please read Haddock[^2]. [^1]: https://ghc.haskell.org/trac/ghc/ticket/7642 [^2]: http://hackage.haskell.org/package/unsafely-0.1.0.0.1/docs/Data-Constraint-… -- Hiromi ISHII konn.jinro(a)gmail.com

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