- Haskell-Cafe - Haskell.org

ANN: tf-random 0.1 (splittable pseudorandom number generator)
by Michał Pałka 19 Sep '13

19 Sep '13

Dear all, I am happy to announce the release of tf-random 0.1. tf-random is a high-quality splittable pseudorandom number generator intended to address the random number quality problems of the standard Haskell StdGen generator. tf-random uses a cryptographic hash function under the hood to generate pseudorandom numbers, which is based on the ThreeFish block cipher. The design of the generator is explained in the paper 'Splittable Pseudorandom Number Generators Using Cryptographic Hashing' by Claessen, Pałka, which will be presented on the upcoming Haskell Symposium. The performance of tf-random was found to be about 8% slower than StdGen on typical QuickCheck properties, which make heavy use of splitting, but up to 2x faster in linear generation on a x86-64 processor. The package is available from hackage at: http://hackage.haskell.org/package/tf-random The code has not yet been thoroughly tested, so expect bugs. For example, it has never been tried on a big-endian architecture, and I would appreciate if somebody could check if the results returned by it are the same (with a given random seed) as on x86. In addition to the generator, the package also contains alternative versions of the RandomGen and Random classes. The most notable difference is that method 'next' from RandomGen returns a Word32 drawn uniformly from its the full range. The reason for this change is that I found it impossible to write efficient Random instances that are parametrized over the range of the generator (genRange). The instances currently defined in the random package make an (illegitimate) assumption that the range of StdGen is used, for that matter. I hope that this release will spark a discussion about the future of the standard PRNG and the RandomGen API in Haskell. Best regards, Michał PS. Please copy me when replying as I am not on the list.

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ANN Vigilance: Get notified when periodical tasks fail to run successfully
by Michael Xavier 19 Sep '13

19 Sep '13

Hey Cafe, Just wanted to announce a project I've been tinkering with for a while finally got to a state where I felt comfortable releasing it. Vigilance is a Dead Man's Switch system that notifies you when periodical tasks that fail to check in when you expected them to. An example of this could be registering the daily backups you do of your servers and have them send emails or send HTTP POST requests if backups ever fail to check in. Vigilance provides an executable for doing check-ins and inspecting your watches as well as a simple REST API if you need something embeddable for existing projects. HackageDB: http://hackage.haskell.org/package/vigilance Github: http://github.com/michaelxavier/vigilance Introductory blog post: http://michaelxavier.net/posts/2013-09-17-Announcing-Vigilance-An-Extensibl… -- Michael Xavier http://www.michaelxavier.net

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Re: [Haskell-cafe] OS X ghci problem
by Carter Schonwald 18 Sep '13

18 Sep '13

The ghci bug goes away when you use a ghc head snapshot from late summer? Great! On Wednesday, September 18, 2013, Jan-Philip Loos wrote: > Sadly I was not able to use your build directly, because ghc(i) > searched libs in a invalid path (it seems to be wired to one of your > directories). I tried to build this ghc version from source on my own, > without success beyond stage 1. This [1] is related to the build > failure. I tried new alex and happy versions, without any change. A > naive approach was to compile alex/happy with ghc_stage1 compiler, > without success (as I expected due missing TH). I tried several ghc > versions for bootstrapping and alex/happy rebuilding (i guess ghc > > 7.6 is necessary for a valid 7.7 alex/happy build with the new Bool > primops), without success too. > > Finally I ended in a fully self build ghc 7.7.20130730 ([2]) and with > this ghc I can confirm that this specific error with > Binding.GLFW.c'glfwInit is gone :) Thanks > > [1] http://www.mail-archive.com/ghc-devs@haskell.org/msg02357.html > [2] > http://darcs.haskell.org/ghcBuilder/uploads/tn23/ghc-7.7.20130730-src.tar.b… > > On Mon, Sep 16, 2013 at 2:52 PM, Christiaan Baaij > <christiaan.baaij(a)gmail.com <javascript:;>> wrote: > > Here's a binary dist of my build: > https://www.dropbox.com/s/d37rij0dnvjiqqy/ghc-7.7.20130915-x86_64-apple-dar… > > In case someone wants to confirm my findings. > > > > Cheers, > > > > Christiaan > > > > On Sep 16, 2013, at 2:24 PM, Christiaan Baaij < > christiaan.baaij(a)gmail.com <javascript:;>> wrote: > > > >> Hi, > >> > >> I saw the same issue/crash on my machine using ghc 7.6.3. > >> > >> I just build a "perf" build of GHC-head with > >> "85a9e2468dc74b9e5ccde0dd61be86219fd323a2" as the latest commit. > >> > >> Now running, I get: > >> 1) > cabal install bindings-glfw > >> 2) > ghci > >> 3) ghci> :m Bindings.GLFW > >> 4) ghci> Bindings.GLFW.c'glfwInit > >> 5) ghci> 1 > >> > >> And doing: > >> 1.) > cabal install GLFW-b > >> 2.) > ghci -package GLFW-b > >> 3.) ghci> import Graphics.UI.GLFW as GLFW > >> 4.) ghci> GLFW.init > >> 5.) ghci> True > >> > >> My platform: > >> - OSX 10.8.4 > >> - ghc(i) 7.7.20130915 > >> - cabal 1.18.0.1 (using 1.18.0 of the Cabal library) > >> - xcode cltools 4.6.2 > >> > >> So it seems that the new ghci linking infrastructure fixes things > >> > >> Cheers, > >> > >> Christiaan > >> > >> On Sep 14, 2013, at 9:00 PM, Jan-Philip Loos <maxdaten(a)gmail.com<javascript:;>> > wrote: > >> > >>>> What I do for GLFW is use a dylib, then you don't rely on GHCi's > static-ish linker. > >>>> The only wrinkle is figuring out where you want the dylib. > >>>> I think homebrew will put one in /usr/local/lib, which works out > nicely, but they don't have GLFW 3 yet. > >>>> Another option is to build the dylib yourself from the GLFW source > bundled with the GLFW-b package, then tell cabal where to find it. > >>> > >>> Hi, > >>> for me the problem relocates now to the "bindings-glfw" package, since > >>> the native bindings moved to this package and are wrapped up with > >>> "glfw-b". > >>> > >>> My way to the same exception already mentioned by Brian Lewis: > >>> 1) > cabal install bindings-glfw > >>> 2) > ghci > >>> 3) ghci> :m Bindings.GLFW > >>> 4) ghci> Bindings.GLFW.c'glfwInit > >>> 5) ghci terminates with exception: *** Terminating app due to uncaught > >>> exception 'NSInvalidArgumentException', reason: '-[NSAutoreleasePool > >>> init]: unrecognized selector sent to instance 0x7fc443c01b30' > >>> > >>> Anthony Cowley mentioned to use ghci with GLFW as a dylib, I have no > >>> clue how to do this. I built the according glfw version on my own as a > >>> dylib and loaded ghci with it explicitly, this didn't help. I guess > >>> the compiled bindings-glfw is already statically packed up. > >>> > >>> How can I get ghci to use the native glfw dylib in combination with > >>> bindings-glfw? If I have to compile bindings-glfw with different > >>> settings, which settings? I have some oversight over haskell but no > >>> really deep knowledge according to bindings and lib-loading of ghci, > >>> but I'm willing to learn it ;) > >>> > >>> My Platform: > >>> - OSX 10.8.5 > >>> - ghc(i) 7.6.3 > >>> - cabal 1.18.0.1 > >>> - xcode dev tools 4.6.3 > >>> > >>> Thanks and Greetings > >>> > >>> Jan > >>> _______________________________________________ > >>> Haskell-Cafe mailing list > >>> Haskell-Cafe(a)haskell.org <javascript:;> > >>> http://www.haskell.org/mailman/listinfo/haskell-cafe > >> > > >

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Multi-param typeclass vs locally constrained typeclass methods
by Jacques Carette 18 Sep '13

18 Sep '13

Could someone please explain what the difference (if any!), in semantics is between class Foo f => Bar f g where method1 :: f a -> g a and class Bar' g where method2 :: Foo f => f a -> g a ? Maybe the translation of the above to something lower level might help. [Note: "f a -> g a" is just an example, and is not meaningful to the question]. The best answer would contain a design guideline for typeclasses, which would spell out conditions in which to use each variant. Jacques

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future (editor) of HCAR?
by Janis Voigtlaender 18 Sep '13

18 Sep '13

Dear all, The Haskell Communities and Activities Report has been produced twice a year for more than ten years now: http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report I have been responsible for producing it the last few years, which was fun. I am now looking to pass that on, handing over the HCAR to someone else. Any takers? Editing HCAR is a few days work per year, but it is also a great way to be involved in the Haskell community - you get a lot of news first hand. There are various procedures in place that automate some of the work, and if someone is interested in taking over, I'm very willing to help along, for example producing the next edition of the report together for a smooth transition. Below I attach the previous call for contributions, which also answers some basic questions about the report. For further questions about the process etc., anybody interested simply get in touch with me. Also, such a transition is a good opportunity to rethink model and processes. Maybe you (anybody on the mailing list) have completely new ideas for how the HCAR should be organized? How it could function better? What other purposes it could serve than it has so far? ... (Or even that HCAR in its existing form is not needed anymore, but could be replaced by something great new?) Best, Janis. -------- Original-Nachricht -------- Betreff: Call for Contributions - Haskell Communities and Activities Report, May 2013 edition Datum: Mon, 08 Apr 2013 21:56:19 +0200 Von: Janis Voigtländer <jv(a)informatik.uni-bonn.de> An: Haskell <haskell(a)haskell.org> Dear all, I would like to collect contributions for the 24th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 1 May 2013 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2013/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2013/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should already have received your old entry as a template (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://people.cs.uu.nl/andres/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached the editor, and not ended up in a spam folder? A: Prior to publication of the final report, the editor will send a draft to all contributors, for possible corrections. So if you do not hear from the editor within two weeks after the deadline, it is safer to send another mail and check whether your first one was received.

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cabal: internal error when reading package index
by Henk-Jan van Tuyl 18 Sep '13

18 Sep '13

L.S., I was trying to install a package from a local drive and got the following message: >cabal install Resolving dependencies... cabal: internal error when reading package index: could not read tar file entryThe package index or index cache is probably corrupt. Running cabal update might fix it. Then I tried cabal update: >cabal update Downloading the latest package list from hackage.haskell.org Skipping download: Local and remote files match. Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install Trying the command cabal install cabal-install gave the same internal error message as for the first command. What can I do about this? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Telling Cassava to ignore lines
by Andrew Cowie 18 Sep '13

18 Sep '13

I'm happily using Cassava to parse CSV, only to discover that non-conforming lines in the input data are causing the parser to error out. let e = decodeByName y' :: Either String (Header, Vector Person) chugs along fine until line 461 of the input when "parse error (endOfInput) at ..." Ironically when my Person (ha) data type was all fields of :: Text it just worked, but now that I've specified one or two of the fields as Int or Float or whatever, it's mis-parsing. Is there a way to tell it to just ignore lines that don't parse, rather than it killing the whole run? Cassava understands skipping the *header* line (and indeed using it to do the -by-name field mapping). Otherwise the only thing I can see is going back to all the fields being :: Text, and then running over that as an intermediate structure and validating whether or not things parse to i.e. float. AfC Sydney

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Opportunity with Machine Learning Startup in Northern California
by Charles Weitzer 17 Sep '13

17 Sep '13

Hello, My name is Charles Weitzer. I have two friends who have started a quantitative hedge fund. One has his PhD in CS from Stanford, while the other has his PhD in Statistics from Berkeley. Company has around 12 people currently. They have made some unpublished discoveries in the field of Machine Learning and in other areas as well. They are looking for an extremely talented software engineer. This position could be a simple software engineer, but it could also be where you are "The Man", basically the head of systems development. These guys are going to be the next big thing in terms of quantitative strategies and systems. They are rebuilding their current system and also designing a new system. You could work on either or both. The title and level of seniority is very flexible. As well, given their returns, this is a chance to become extremely wealthy. They are currently managing a very healthy amount of capital. FYI, I am retained by them. They are located in Northern California. Their blurb below is really is just a starting point in terms of possible initial responsibility and seniority. They are flexible for the right person. Talk soon, Charles Weitzer CEO\Senior Recruiter Charles Weitzer and Associates, Inc. Global Financial Recruiting Services Charles(a)CharlesWeitzer.com Voice: USA (510) 558-9182 Fast-growing systematic-trading firm seeks an exceptional software developer. You will architect and implement a diverse set of core infrastructures, including new production systems, scientific-computing environments, and modern data stores. We seek candidates with a proven track record of writing correct, well-designed software, solving hard problems and delivering complex projects on time. You should preferably have experience with high assurance, distributed, fault-tolerant systems. Experience with functional programming as well as soft real-time, low-latency, cache-friendly systems is a bonus. We are getting big fast. Willingness to take initiative, and a gritty determination to productize, are essential. Join a team that includes faculty at premier universities and PhD's from top-tier schools, led by the founder and CEO of a successful Internet infrastructure startup. You will have a high impact, and you can expect frequent interaction with the researchers, officers, and founders. Compensation and benefits are highly competitive.

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Proposal: Pragma EXPORT
by Wvv 17 Sep '13

17 Sep '13

I suggest to add instead of (or with) export section Pragma EXPORT: We have 3 values: public, abstract and private. Data(with newtypes and types,..) could be public, like `Data(...)` or abstract `Data`. Other cases abstract = public. {-# EXPORT <smth> #-} pragma is valid till next {-# EXPORT <smth> #-}. We also can add local pragma: {-# EXPORT <smth> from #-} ... {-# EXPORT <smth> untill #-} Outside of block is rule of previous {-# EXPORT <smth> #-}. Finally we also have rule for 1: {-# EXPORT <smth> one #-} Example module C where {-# EXPORT public #-} data A1... data A2... {-# EXPORT abstract from #-} newtype A3... data A4... {-# EXPORT abstract until #-} type A5... {-# EXPORT private one #-} data A6... foo = ... {-# EXPORT private one #-} bar = ... baz = ... lorem = ... {-# EXPORT private #-} insput ... dolor = .. sit = ... {-# EXPORT public one #-} amen = ... consectetur = ... adipisicing = ... elit = ... sed = ... eiusmod = ... tempor = ... incididunt = ... {-# EXPORT public from #-} ut = ... labore = ... et = ... {-# EXPORT public until #-} dolore = ... magna = ... aliqua = ... is the same as module C ( A1(..) , A2(..) , A3 , A4 , A5(..) , foo , baz , lorem , amen , ut , labore , et ) where data A1... data A2... newtype A3... data A4... type A5... data A6... foo = ... bar = ... baz = ... lorem = ... insput ... dolor = .. sit = ... amen = ... consectetur = ... adipisicing = ... elit = ... sed = ... eiusmod = ... tempor = ... incididunt = ... ut = ... labore = ... et = ... dolore = ... magna = ... aliqua = ... We also could have complex pragma, like {-# EXPORT inherit one foo #-} bar=... Backward compatibility: module A where ... ~ module A where {-# EXPORT public #-} ... module B ( .... ) where ... ~ module B ( .... ) where {-# EXPORT private #-} ... -- View this message in context: http://haskell.1045720.n5.nabble.com/Proposal-Pragma-EXPORT-tp5736547.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Readable GHC 7.6.3 docs (Bootstrapped)
by Obscaenvs 17 Sep '13

17 Sep '13

At [1] you can find links to the GHC documentation that I use myself, since the official version is a bit too TimesNewRoman-y for my ..."developed" taste. It available in a downloadable Bzipped TAR aswell as being browsable online. [1] http://bugthunk.net/ /fredrik

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Re: [Haskell-cafe] reifying typeclasses
by oleg＠okmij.org 17 Sep '13

17 Sep '13

Evan Laforge wrote: > I have a typeclass which is instantiated across a closed set of 3 > types. It has an ad-hoc set of methods, and I'm not too happy with > them because being a typeclass forces them to all be defined in one > place, breaking modularity. A sum type, of course, wouldn't have that > problem. But in other places I want the type-safety that separate > types provide, and packing everything into a sum type would destroy > that. So, expression problem-like, I guess. > > It seems to me like I should be able to replace a typeclass with > arbitrary methods with just two, to reify the type and back. This > seems to work when the typeclass dispatches on an argument, but not on > a return value. E.g.: If the universe (the set of types of interest to instantiate the type class to) is closed, GADTs spring to mind immediately. See, for example, the enclosed code. It is totally unproblematic (one should remember to always write type signatures when programming with GADTs. Weird error messages otherwise ensue.) One of the most notable differences between GADT and type-class--based programming is that GADTs are closed and type classes are open (that is, new instances can be added at will). In fact, a less popular technique of implementing type classes (which has been used in some Haskell systems -- but not GHC)) is intensional type analysis, or typecase. It is quite similar to the GADT solution. The main drawback of the intensional type analysis as shown in the enclosed code is that it breaks parametricity. The constraint Eq a does not let one find out what the type 'a' is and so what other operations it may support. (Eq a) says that the type a supports (==), and does not say any more than that. OTH, Representable a tells quite a lot about type a, essentially, everything. > types. It has an ad-hoc set of methods, and I'm not too happy with > them because being a typeclass forces them to all be defined in one > place, breaking modularity. A sum type, of course, wouldn't have that Why not to introduce several type classes, even a type class for each method if necessary. Grouping methods under one type class is appropriate when such a grouping makes sense. Otherwise, Haskell won't lose in expressiveness if a type class could have only one method. {-# LANGUAGE GADTs #-} module G where data TRep a where TInt :: TRep Int TChar :: TRep Char class Representable a where repr :: TRep a instance Representable Int where repr = TInt instance Representable Char where repr = TChar argument :: Representable a => a -> Int argument x = go repr x where -- For GADTs, signatures are important! go :: TRep a -> a -> Int go TInt x = x go TChar x = fromEnum x -- just the `mirror inverse' result :: Representable a => Int -> a result x = go repr x where -- For GADTs, signatures are important! go :: TRep a -> Int -> a go TInt x = x go TChar x = toEnum x t1 = argument 'a' t2 = show (result 98 :: Char)

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Monomorphic containers, Functor/Foldable/Traversable WAS: mapM_ for bytestring
by Michael Snoyman 17 Sep '13

17 Sep '13

That's really funny timing. I started work on a very similar project just this week: https://github.com/snoyberg/mono-traversable It's not refined yet, which is why I haven't discussed it too publicly, but it's probably at the point where some review would make sense. There's been a bit of a discussion on a separate Github issue[1] about it. A few caveats: - The names are completely up for debate, many of them could be improved. - The laws aren't documented yet, but they mirror the laws for the polymorphic classes these classes are based on. - The Data.MonoTraversable module is the main module to look at. The other two are far more nascent (though I'd definitely appreciate feedback people have on them). I think this and mono-foldable have a lot of overlap, I'd be interested to hear what you think in particular John. Michael [1] https://github.com/snoyberg/classy-prelude/issues/18 On Wed, Sep 11, 2013 at 11:05 PM, John Lato <jwlato(a)gmail.com> wrote: > I agree with everything Edward has said already. I went through a similar > chain of reasoning a few years ago when I started using ListLike, which > provides a FoldableLL class (although it uses fundeps as ListLike predates > type families). ByteString can't be a Foldable instance, nor do I think > most people would want it to be. > > Even though I would also like to see mapM_ in bytestring, it's probably > faster to have a library with a separate monomorphic Foldable class. So I > just wrote one: > > https://github.com/JohnLato/mono-foldable > http://hackage.haskell.org/package/mono-foldable > > Petr Pudlak has done some work in this area. A big problem is that > foldM/mapM_ are typically implemented in terms of Foldable.foldr (or > FoldableLL), but this isn't always optimal for performance. They really > need to be part of the type class so that different container types can > have specialized implementations. I did that in mono-foldable, using > Artyom's map implementation (Artyom, please let me know if you object to > this!) > > pull requests, forks, etc all welcome. > > John L. > > > On Wed, Sep 11, 2013 at 1:29 PM, Edward Kmett <ekmett(a)gmail.com> wrote: > >> mapM_ is actually implemented in terms of Foldable, not Traversable, and >> its implementation in terms of folding a ByteString is actually rather slow >> in my experience doing so inside lens and isn't much faster than the naive >> version that was suggested at the start of this discussion. >> >> But as we're not monomorphizing Foldable/Traversable, this isn't a think >> that is able to happen anyways. >> >> -Edward >> >> >> On Wed, Sep 11, 2013 at 2:25 PM, Henning Thielemann < >> lemming(a)henning-thielemann.de> wrote: >> >>> >>> On Wed, 11 Sep 2013, Duncan Coutts wrote: >>> >>> For mapM etc, personally I think a better solution would be if >>>> ByteString and Text and other specialised containers could be an >>>> instance of Foldable/Traversable. Those classes define mapM etc but >>>> currently they only work for containers that are polymorphic in their >>>> elements, so all specialised containers are excluded. I'm sure there >>>> must be a solution to that (I'd guess with type families) and that would >>>> be much nicer than adding mapM etc to bytestring itself. We would then >>>> just provide efficient instances for Foldable/Traversable. >>>> >>> >>> I'd prefer to keep bytestring simple with respect to the number of type >>> extensions. Since you must implement ByteString.mapM anyway, you can plug >>> this into an instance definition of Traversable ByteString. >>> >> >> >> _______________________________________________ >> Libraries mailing list >> Libraries(a)haskell.org >> http://www.haskell.org/mailman/listinfo/libraries >> >> > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries > >

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Proposal: New syntax for Haskell
by Niklas Hambüchen 17 Sep '13

17 Sep '13

Impressed by the productivity of my Ruby-writing friends, I have recently come across Cucumber: http://cukes.info It is a great tool for specifying tests and programs in natural language, and especially easy to learn for beginners. I propose that we add a Cucumber syntax for Haskell, with the extension ".chs", next to .hs and .lhs. Code written in cucumber syntax is concise and easy to read: You can find some example code in https://gist.github.com/nh2/6505995. Quoting from that: Feature: The Data.List module In order to be able to use lists As a programmer I want a module that defines list functions Scenario: Defining the function foldl Given I want do define foldl Which has the type (in brackets) a to b to a (end of brackets), to a, to list of b, to a And my arguments are called f, acc, and l When l is empty Then the result better be acc Otherwise l is x cons xs Then the result should be foldl f (in brackets) f acc x (end of brackets) xs PS: People even already started a testing framework for Haskell in it: https://github.com/sol/cucumber-haskell#cucumber-for-haskell

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An APL library for Haskell
by Simon Peyton-Jones 17 Sep '13

17 Sep '13

Friends Many of you will know the array language APL<http://en.wikipedia.org/wiki/APL_%28programming_language%29>. It focuses on arrays and in particular has a rich, carefully-thought-out array algebra. An obvious idea is: what would a Haskell library that embodies APL's array algebra look like? In conversation with John Scholes and some of his colleagues in the APL community a group of us developed some ideas for a possible API, which you can find on the Haskell wiki here: http://www.haskell.org/haskellwiki/APL However, we have all gone our separate ways, and I think it's entirely possible that that the idea will go no further. So this message is to ask: * Is anyone interested in an APL-style array library in Haskell? * If so, would you like to lead the process of developing the API? I think there are quite a few people who would be willing to contribute, including some core gurus from the APL community: John Scholes, Arthur Whitney, and Roger Hui. Simon

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Re: [Haskell-cafe] OS X ghci problem
by Jan-Philip Loos 16 Sep '13

16 Sep '13

> What I do for GLFW is use a dylib, then you don't rely on GHCi's static-ish linker. > The only wrinkle is figuring out where you want the dylib. > I think homebrew will put one in /usr/local/lib, which works out nicely, but they don't have GLFW 3 yet. > Another option is to build the dylib yourself from the GLFW source bundled with the GLFW-b package, then tell cabal where to find it. Hi, for me the problem relocates now to the "bindings-glfw" package, since the native bindings moved to this package and are wrapped up with "glfw-b". My way to the same exception already mentioned by Brian Lewis: 1) > cabal install bindings-glfw 2) > ghci 3) ghci> :m Bindings.GLFW 4) ghci> Bindings.GLFW.c'glfwInit 5) ghci terminates with exception: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSAutoreleasePool init]: unrecognized selector sent to instance 0x7fc443c01b30' Anthony Cowley mentioned to use ghci with GLFW as a dylib, I have no clue how to do this. I built the according glfw version on my own as a dylib and loaded ghci with it explicitly, this didn't help. I guess the compiled bindings-glfw is already statically packed up. How can I get ghci to use the native glfw dylib in combination with bindings-glfw? If I have to compile bindings-glfw with different settings, which settings? I have some oversight over haskell but no really deep knowledge according to bindings and lib-loading of ghci, but I'm willing to learn it ;) My Platform: - OSX 10.8.5 - ghc(i) 7.6.3 - cabal 1.18.0.1 - xcode dev tools 4.6.3 Thanks and Greetings Jan

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ANN: Cabal v1.18.0 released
by Johan Tibell 16 Sep '13

16 Sep '13

Hi all, On behalf of the cabal maintainers and contributors I'm proud to announce the Cabal (and cabal-install) 1.18.0 release. To install run cabal update && cabal install Cabal-1.18.0 cabal-install-1.18.0 With 854 commits since the last release there are two many improvements and bug fixes to list them here, but two highlights are: * Hermetic builds using sandboxes. This should reduce the number of "dependency hell" and broken package DB problems. * GHCi support. It's now much easier to use ghci when developing your packages, especially if those packages require preprocessors (e.g. hsc2hs). Here's how working on a package might look like using the new features: # Only once: cabal sandbox init cabal install --only-dependencies --enable-tests # Configure, build, and run tests: cabal test # now implies configure and build # Play around with the code in GHCi: cabal repl Mikhail wrote a bit more about the user visible changes on his blog: http://coldwa.st/e/blog/2013-08-21-Cabal-1-18.html For a complete list of changes run git log cabal-install-v1.16.0.2..cabal-install-v1.18.0 in the cabal repo or look at the GitHub compare page: https://github.com/haskell/cabal/compare/cabal-install-v1.16.0.2...cabal-in… (only shows the last 250 commits). 57 people contributed to this release! 503 Mikhail Glushenkov 99 Johan Tibell 41 Duncan Coutts 39 Ian Lynagh 19 Brent Yorgey 19 Thomas Tuegel 18 Ben Millwood 16 Eyal Lotem 10 Thomas Dziedzic 7 Andres Loeh 6 John Wiegley 6 Benno Fünfstück 5 Gregory Collins 4 Herbert Valerio Riedel 4 Simon Hengel 3 Joachim Breitner 3 Luke Iannini 3 Bryan Richter 3 Richard Eisenberg 3 Tuncer Ayaz 3 Jens Petersen 2 Arun Tejasvi Chaganty 2 Bryan O'Sullivan 2 Eric Kow 2 Jookia 2 Paolo G. Giarrusso 2 Paolo Capriotti 1 Sönke Hahn 1 Yitzchak Gale 1 Albert Krewinkel 1 stepcut 1 Alexander Kjeldaas 1 Austin Seipp 1 Bardur Arantsson 1 Ben Doyle 1 Ben Gamari 1 Bram 1 Carter Tazio Schonwald 1 Clint Adams 1 Daniel Wagner 1 David Lazar 1 Erik Hesselink 1 Eugene Sukhodolin 1 Gabor Greif 1 Jack Henahan 1 Jason Dagit 1 Ken Bateman 1 Mark Lentczner 1 Masahiro Yamauchi 1 Merijn Verstraaten 1 Michael Thompson 1 Niklas Hambüchen 1 Oleksandr Manzyuk 1 Patrick Premont 1 Roman Cheplyaka 1 Sergei Trofimovich 1 Stephen Blackheath -- Johan, on behalf of the cabal maintainers and contributors.

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Recent problems with -cafe -- Fixed (I hope)
by Gershom Bazerman 16 Sep '13

16 Sep '13

There were problems with the -cafe mailinglist today. Best I can tell, we had an unplanned system reboot last night. In the course of it going down and back up, the configuration for -cafe got corrupted and the auto-fixed configuration had roughly 3/4 of the membership deleted. I've gone in and replaced it with a recent backup config, which should have fixed the problem. If this email goes through, then we're in good shape -- sorry about the mess. --Gershom

1 0

reifying typeclasses (resend)
by Evan Laforge 15 Sep '13

15 Sep '13

[ This is the second time I sent this, the first time it said it was awaiting moderation because I'm not subscribed to haskell-cafe, which is weird because I thought I was. Did a bunch of people get unsubscribed? ] I'm sure this is old-hat to typeclass wizards, but I've managed to get pretty far without understanding them too well, so here's a basic question. I haven't seen it phrased this way before though: I have a typeclass which is instantiated across a closed set of 3 types. It has an ad-hoc set of methods, and I'm not too happy with them because being a typeclass forces them to all be defined in one place, breaking modularity. A sum type, of course, wouldn't have that problem. But in other places I want the type-safety that separate types provide, and packing everything into a sum type would destroy that. So, expression problem-like, I guess. It seems to me like I should be able to replace a typeclass with arbitrary methods with just two, to reify the type and back. This seems to work when the typeclass dispatches on an argument, but not on a return value. E.g.: {-# LANGUAGE ScopedTypeVariables #-} class Taggable a where toTagged :: a -> Tagged toTaggedType :: a -> TaggedType fromTagged :: Tagged -> Maybe a m_argument :: a -> Int m_result :: Int -> a data Tagged = TInt Int | TChar Char deriving (Show) data TaggedType = TypeInt | TypeChar deriving (Show) instance Taggable Int where toTagged = TInt toTaggedType _ = TypeInt fromTagged (TInt x) = Just x fromTagged _ = Nothing m_argument = id m_result = id instance Taggable Char where toTagged = TChar toTaggedType _ = TypeChar fromTagged (TChar x) = Just x fromTagged _ = Nothing m_argument = fromEnum m_result = toEnum argument :: (Taggable a) => a -> Int argument a = case toTagged a of TInt x -> x TChar c -> fromEnum c result :: forall a. (Taggable a) => Int -> a result val = case toTaggedType (undefined :: a) of TypeInt -> val TypeChar -> toEnum val Say m_argument and m_result are the ad-hoc methods I'd like to get out of the typeclass. I can do that well enough for 'argument', but 'result' runs into trouble. One is the ugly undefined trick with toTaggedType, but the bigger one is that ghc says 'Could not deduce (a ~ Int) from the context (Taggable a)'. I wasn't really expecting it to work, because it would entail a case with multiple types. As far as I know, the only way for that to happen is with GADTs. But I don't see how they could help me here. So, perhaps my intuition was wrong. toTagged and fromTagged methods give you the power to go between value and type level, but apparently that's not enough power to express what typeclasses give you. Also it seems like there's a fundamental difference between dispatching on argument vs dispatching on result. Is there a way to more formally understand the extents of what typeclasses provide, and what a toTagged fromTagged scheme gives me, so I can have a better intuition for how to go between value and type levels? Also, the toTaggedType thing is pretty ugly. Not just passing it undefined, but how it has to repeat the types. I don't really see a way to get around that though.

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Re: [Haskell-cafe] reifying typeclasses
by oleg＠okmij.org 15 Sep '13

15 Sep '13

[I too had the problem sending this e-mail to Haskell list. I got a reply saying the message awaits moderator approval] Evan Laforge wrote: > I have a typeclass which is instantiated across a closed set of 3 > types. It has an ad-hoc set of methods, and I'm not too happy with > them because being a typeclass forces them to all be defined in one > place, breaking modularity. A sum type, of course, wouldn't have that > problem. But in other places I want the type-safety that separate > types provide, and packing everything into a sum type would destroy > that. So, expression problem-like, I guess. > > It seems to me like I should be able to replace a typeclass with > arbitrary methods with just two, to reify the type and back. This > seems to work when the typeclass dispatches on an argument, but not on > a return value. E.g.: If the universe (the set of types of interest to instantiate the type class to) is closed, GADTs spring to mind immediately. See, for example, the enclosed code. It is totally unproblematic (one should remember to always write type signatures when programming with GADTs. Weird error messages otherwise ensue.) One of the most notable differences between GADT and type-class--based programming is that GADTs are closed and type classes are open (that is, new instances can be added at will). In fact, a less popular technique of implementing type classes (which has been used in some Haskell systems -- but not GHC)) is intensional type analysis, or typecase. It is quite similar to the GADT solution. The main drawback of the intensional type analysis as shown in the enclosed code is that it breaks parametricity. The constraint Eq a does not let one find out what the type 'a' is and so what other operations it may support. (Eq a) says that the type a supports (==), and does not say any more than that. OTH, Representable a tells quite a lot about type a, essentially, everything. > types. It has an ad-hoc set of methods, and I'm not too happy with > them because being a typeclass forces them to all be defined in one > place, breaking modularity. A sum type, of course, wouldn't have that Why not to introduce several type classes, even a type class for each method if necessary. Grouping methods under one type class is appropriate when such a grouping makes sense. Otherwise, Haskell won't lose in expressiveness if a type class could have only one method. {-# LANGUAGE GADTs #-} module G where data TRep a where TInt :: TRep Int TChar :: TRep Char class Representable a where repr :: TRep a instance Representable Int where repr = TInt instance Representable Char where repr = TChar argument :: Representable a => a -> Int argument x = go repr x where -- For GADTs, signatures are important! go :: TRep a -> a -> Int go TInt x = x go TChar x = fromEnum x -- just the `mirror inverse' result :: Representable a => Int -> a result x = go repr x where -- For GADTs, signatures are important! go :: TRep a -> Int -> a go TInt x = x go TChar x = toEnum x t1 = argument 'a' t2 = show (result 98 :: Char)

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LATA 2014: 3rd call for papers
by GRLMC 15 Sep '13

15 Sep '13

*To be removed from our mailing list, please respond to this message with UNSUBSCRIBE in the subject line* ************************************************************************* 8th INTERNATIONAL CONFERENCE ON LANGUAGE AND AUTOMATA THEORY AND APPLICATIONS LATA 2014 Madrid, Spain March 10-14, 2014 Organized by: Research Group on Implementation of Language-Driven Software and Applications (ILSA) Complutense University of Madrid Research Group on Mathematical Linguistics (GRLMC) Rovira i Virgili University http://grammars.grlmc.com/lata2014/ ********************************************************************* AIMS: LATA is a yearly conference on theoretical computer science and its applications. Following the tradition of the diverse PhD training events in the field developed at Rovira i Virgili University in Tarragona since 2002, LATA 2014 will reserve significant room for young scholars at the beginning of their career. It will aim at attracting contributions from both classical theory fields and application areas (bioinformatics, language technology, artificial intelligence, etc.). VENUE: LATA 2014 will take place in Madrid, the capital of Spain. The venue will be the School of Informatics of Complutense University. SCOPE: Topics of either theoretical or applied interest include, but are not limited to: algebraic language theory algorithms for semi-structured data mining algorithms on automata and words automata and logic automata for system analysis and programme verification automata, concurrency and Petri nets automatic structures cellular automata codes combinatorics on words compilers computability computational complexity data and image compression decidability issues on words and languages descriptional complexity DNA and other models of bio-inspired computing digital libraries and document engineering foundations of finite state technology foundations of XML fuzzy and rough languages grammars (Chomsky hierarchy, contextual, unification, categorial, etc.) grammatical inference and algorithmic learning graphs and graph transformation language varieties and semigroups language-based cryptography language-theoretic foundations of artificial intelligence and artificial life natural language and speech automatic processing parallel and regulated rewriting parsing patterns power series quantum, chemical and optical computing semantics string and combinatorial issues in computational biology and bioinformatics string processing algorithms symbolic dynamics symbolic neural networks term rewriting transducers trees, tree languages and tree automata weighted automata STRUCTURE: LATA 2014 will consist of: invited talks invited tutorials peer-reviewed contributions INVITED SPEAKERS: Javier Esparza (Munich Tech, DE), On Trees and Fixed Point Equations (tutorial) Leslie A. Goldberg (Oxford, UK), The Complexity of Approximate Counting Oscar H. Ibarra (Santa Barbara, US), tba Sanjeev Khanna (Philadelphia, US), tba Helmut Seidl (Munich Tech, DE), tba PROGRAMME COMMITTEE: Dana Angluin (Yale, US) Eugene Asarin (Paris Diderot, FR) Jos Baeten (Amsterdam, NL) Christel Baier (Dresden, DE) Jan Bergstra (Amsterdam, NL) Jin-Yi Cai (Madison, US) Marek Chrobak (Riverside, US) Andrea Corradini (Pisa, IT) Mariangiola Dezani (Turin, IT) Ding-Zhu Du (Dallas, US) Michael R. Fellows (Darwin, AU) Jörg Flum (Freiburg, DE) Nissim Francez (Technion, IL) Jürgen Giesl (Aachen, DE) Annegret Habel (Oldenburg, DE) Kazuo Iwama (Kyoto, JP) Sampath Kannan (Philadelphia, US) Ming-Yang Kao (Northwestern, US) Deepak Kapur (Albuquerque, US) Joost-Pieter Katoen (Aachen, DE) S. Rao Kosaraju (Johns Hopkins, US) Evangelos Kranakis (Carleton, CA) Gad M. Landau (Haifa, IL) Andrzej Lingas (Lund, SE) Jack Lutz (Iowa State, US) Ian Mackie (École Polytechnique, FR) Carlos Martín-Vide (Tarragona, ES, chair) Giancarlo Mauri (Milan, IT) Faron G. Moller (Swansea, UK) Paliath Narendran (Albany, US) Enno Ohlebusch (Ulm, DE) Helmut Prodinger (Stellenbosch, ZA) Jean-François Raskin (Brussels, BE) Wolfgang Reisig (Humboldt Berlin, DE) Marco Roveri (Bruno Kessler, Trento, IT) Michaël Rusinowitch (LORIA, Nancy, FR) Yasubumi Sakakibara (Keio, JP) Davide Sangiorgi (Bologna, IT) Colin Stirling (Edinburgh, UK) Jianwen Su (Santa Barbara, US) Jean-Pierre Talpin (IRISA, Rennes, FR) Andrzej Tarlecki (Warsaw, PL) Rick Thomas (Leicester, UK) Sophie Tison (Lille, FR) Rob van Glabbeek (NICTA, Sydney, AU) Helmut Veith (Vienna Tech, AT) ORGANIZING COMMITTEE: Adrian Horia Dediu (Tarragona) Ana Fernández-Pampillón (Madrid) Carlos Martín-Vide (Tarragona, co-chair) Antonio Sarasa (Madrid) José-Luis Sierra (Madrid, co-chair) Bianca Truthe (Magdeburg) Florentina Lilica Voicu (Tarragona) SUBMISSIONS: Authors are invited to submit non-anonymized papers in English presenting original and unpublished research. Papers should not exceed 12 single-spaced pages (including eventual appendices) and should be formatted according to the standard format for Springer Verlag's LNCS series (see http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0) Submissions have to be uploaded to: https://www.easychair.org/conferences/?conf=lata2014 PUBLICATIONS: A volume of proceedings published by Springer in the LNCS series will be available by the time of the conference. A special issue of a major journal will be later published containing peer-reviewed extended versions of some of the papers contributed to the conference. Submissions to it will be by invitation. REGISTRATION: The period for registration is open from July 15, 2013 to March 10, 2014. The registration form can be found at: http://grammars.grlmc.com/lata2014/Registration.php DEADLINES: Paper submission: October 14, 2013 (23:59 CET) Notification of paper acceptance or rejection: November 25, 2013 Final version of the paper for the LNCS proceedings: December 2, 2013 Early registration: December 9, 2013 Late registration: February 24, 2014 Starting of the conference: March 10, 2014 End of the conference: March 14, 2014 Submission to the post-conference journal special issue: June 14, 2014 QUESTIONS AND FURTHER INFORMATION: florentinalilica.voicu(a)urv.cat POSTAL ADDRESS: LATA 2014 Research Group on Mathematical Linguistics (GRLMC) Rovira i Virgili University Av. Catalunya, 35 43002 Tarragona, Spain Phone: +34-977-559543 Fax: +34-977-558386 ACKNOWLEDGEMENTS: Departament dEconomia i Coneixement, Generalitat de Catalunya Universidad Complutense de Madrid Universitat Rovira i Virgili

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Quick Angel User's Survey
by Michael Xavier 15 Sep '13

15 Sep '13

Hey Cafe, I am the maintainer of Angel, the process monitoring daemon. Angel's job is to start a configured set of processes and restart them when they go away. I was responding to a ticket and realized that the correct functionality is not obvious in one case, so I figured I'd ask the stakeholders: people who use Angel. From what I know, most people who use Angel are Haskellers so this seemed like the place. When Angel is terminated, it tries to cleanly shut down any processes it is monitoring. It also shuts down processes that it spawned when they are removed from the config and the config is reloaded via the HUP signal. It uses terminateProcess from System.Process which sends a SIGTERM to the program on *nix systems. The trouble is that SIGTERM can be intercepted and a process can still fail to shut down. Currently Angel issues the SIGTERM and hopes for the best. It also cleans pidfiles if there were any, which may send a misleading message. There are a couple of routes I could take: 1. Leave it how it is. Leave it to the user to make sure stubborn processes go away. I don't like this solution so much as it makes Angel harder to reason about from a user's perspective. 2. Send a TERM signal then wait for a certain number of seconds, then send an uninterruptable signal like SIGKILL. There are some caveats with #2. I think I'd prefer the timeout to be configurable per-process. I think I'd also prefer that if no timeout is specified, we assume the user does not want us to use a SIGKILL. SIGKILL can be very dangerous for some processes like databases. I want explicit user permission to do something like this. If Angel generated a pidfile for the process, if it should only be cleaned if Angel can confirm the process is dead. Otherwise they should be left so the user can handle it. So the real question: is the extra burden of an optional configuration flag per process worth this feature? Are my assumptions about path #2 reasonable. Thanks for your feedback! -- Michael Xavier http://www.michaelxavier.net

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Do you want to maintain bindings-DSL?
by Maurício Antunes 13 Sep '13

13 Sep '13

Hi, bindings-DSL[1] is a stable and reliable macro package for FFI. It just got a new tutorial and its repository is now in git. If no one else wants to maintain it, I'll still be looking at issue reports and fixing possible problems, but I've not been actively using Haskell for some time, and won't add anything new or make plans for future changes. The original idea behind bindings-DSL was to build a wide set of community maintained raw bindings to interesting C libraries, on which Haskell programmers could safely depend when creating their own higher level packages. Although that idea never really took off, bindings-DSL itself got some popularity, probably because it makes instancing Storable easy. If you want to take maintenance, you have some options. You could just kill the bindings-* idea and add some flexibility to new macros, which I always avoided to enforce consistency. Or you could keep the original goal, and maybe add a mailing list and a web page where users could request new libraries and discuss solutions to trick issues. A possible plan for version 2 is to be independent of hsc2hs, and use a new syntax instead of C macros. Thanks. Best, Maurício [1]: http://bitbucket.org/mauricio/bindings-dsl

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Package compatibility (SemVer, PVP, ECT)
by Thiago Negri 13 Sep '13

13 Sep '13

I've just read Semantic Versioning (SemVer) [1], Package Versioning Policy (PVP) [2] and Eternal Compatibility in Theory (ECT) [3]. Is PVP the one that every package on Hackage should use? SemVer seems to make more sense to me. Also, ECT looks very promising, but it is dated back to 2005 and I didn't see any package using it. Why? If Cabal could map the package version number to the versioned module name, there will be no need to "uglify" the client code. I mean, when I import module A and define in dependencies "a == 1.8.3", Cabal would map that to module A_7 and bind that to the "A" I've imported. We would be locked in with Cabal, but that seems to be a good tradeoff. [1] http://semver.org [2] http://www.haskell.org/haskellwiki/Pvp [3] http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue2/EternalCompatibi…

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Bytestring map/zipWith rationale
by Scott Lawrence 13 Sep '13

13 Sep '13

Hello all, Something's always bothered me about map and zipWith for ByteString. Why is it map :: (Word8 -> Word8) -> ByteString -> ByteString but zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a] ? Obviously they can be transformed into each other with pack/unpack, and as I understand it, the compiler performs sufficient optimizations so that there's no performance hit to doing things like (pack $ zipWith xor a b), but it still seems inconsistent. Is there a deep reason for this? -- Scott Lawrence Linux baidar 3.10.9-1-ARCH #1 SMP PREEMPT Wed Aug 21 13:49:35 CEST 2013 x86_64 GNU/Linux

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dns library version 1.0.0
by Kazu Yamamoto 13 Sep '13

13 Sep '13

Hi all, I have released dns library version 1.0.0. This version provides new APIs. Thus, version is now 1.0.0. The design and implementation was done by Michael Orlitzky based on his experience. Enjoy! --Kazu

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GLFW not working in Cabal 1.18
by Thiago Negri 13 Sep '13

13 Sep '13

The package GLFW is not building in Cabal 1.18. Setup.hs [1] depends on `rawSystemStdInOut` [2] that changed signature between 1.16 and 1.18. Is this considered a public API of Cabal? Cabal 1.16 rawSystemStdInOut :: Verbosity -> FilePath -> [String] -> Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode) Cabal 1.18 rawSystemStdInOut :: Verbosity -> FilePath -> [String] -> Maybe FilePath -- new arg -> Maybe [(String, String)] -- new arg -> Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode) Compilation output: [1 of 1] Compiling Main ( /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs, /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/dist/setup/Main.o ) /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs:167:33: Couldn't match expected type `IO (t0, t1, ExitCode)' with actual type `Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode)' In the return type of a call of `rawSystemStdInOut' Probable cause: `rawSystemStdInOut' is applied to too few arguments In a stmt of a 'do' block: (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, "-o", objPath] ++ flags) Nothing False In the expression: do { hClose outHandle; hPutStr inHandle contents; hClose inHandle; (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, ....] ++ flags) Nothing False; .... } /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs:167:113: Couldn't match expected type `Maybe [(String, String)]' with actual type `Bool' In the fifth argument of `rawSystemStdInOut', namely `False' In a stmt of a 'do' block: (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, "-o", objPath] ++ flags) Nothing False In the expression: do { hClose outHandle; hPutStr inHandle contents; hClose inHandle; (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, ....] ++ flags) Nothing False; .... } Failed to install GLFW-0.5.1.0 [1] http://code.haskell.org/GLFW/Setup.hs [2] https://github.com/haskell/cabal/blob/d16c307c33fb7af19d8f17a2ad8be4902a3af…

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Hackage 2 now available for beta testing
by Duncan Coutts 13 Sep '13

13 Sep '13

Well-Typed and the Industrial Haskell Group (IHG) are very pleased to announce that Hackage 2 is now available for public beta testing. The plan is to do the final switchover in late September, to coincide with ICFP. http://beta.hackage.haskell.org/ Read on for details of how to help with the public beta, an overview of the new features and what the IHG has been doing to help. Support from the Industrial Haskell Group ========================================= The IHG is a consortium of companies that rely on Haskell. The IHG members have funded the effort to get Hackage 2 up to feature parity and get it ready for the switchover. The IHG funded this effort because while the volunteer effort got us the "first 90%" of the way there (including adding a number of new features) there was still the "last 90%" to do to get it production ready. The IHG members decided to fund Hackage 2 not just because they are good citizens, but out of enlightened self-interest. Hackage has over 5000 packages written by over 1000 people -- including the world's best Haskell developers. This is a massive resource. The IHG members recognise that improvements to the tools and infrastructure that the community uses helps the community to produce more and better code. This is a benefit to everyone in the community -- including the commercial users. The IHG is keen to increase its membership so that more resources can be dedicated to improving the Haskell development platform. If your organisation relies on Haskell in some way then you may want to consider joining. See the IHG website for more details or contact info(a)industry.haskell.org. [IHG website]: http://industry.haskell.org/ Despite the help of the IHG in getting to this point, Hackage is a community project, and its success depends on the community maintaining and further improving the new server. The code is now on github so it is easier to contribute, and now that the server is live there is more immediate gratification for volunteers contributing fixes and new features. Public beta =========== We would like to encourage you to take part in the public beta testing. We need help both from package authors as well as other users of the site. Please report any problems you find using the issue tracker on the hackage-server github site. [issue tracker]: https://github.com/haskell/hackage-server/issues [github site]: https://github.com/haskell/hackage-server We are mirroring packages from the old server (every 30min) so it is suitable to use as your main hackage server with some caveats: we are allowing package authors to upload (as well as doing the mirroring) so you may find a slightly different set of packages on this server. If you are a package author then you are welcome to poke about and upload packages. We have imported user accounts from the old server (except for a small number of early adopters of the original server who will need to contact an administrator). Note that before we do the final switchover we will *delete everything* from the beta instance and do a fresh import from the old hackage server. Configuring cabal-install ------------------------- Edit your ~/.cabal/config file. Comment-out the existing "remote-repo" line near the top of the file and add in a new one like this: --remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive remote-repo: beta.hackage.haskell.org:http://beta.hackage.haskell.org/ New features ============ Though our main priority has been feature parity so that we can switch over, volunteers have contributed several new features, including better package search, a new site theme, improved security, the ability to fix package dependencies after a release, changelogs, and a REST-style interface. See the beta site for more details on these new features, plus details of other features that are partially implemented or are in need of improvement. [new features]: http://beta.hackage.haskell.org/new-features Contributing to the development =============================== The code is on github and we welcome pull requests. There are open tickets describing existing bugs and features that we want or that are in need of improvement. Help on any of these would be greatly appreciated. There is some developer and user documentation on the github wiki, including a quick guide to getting your own server instance up and running. You can ask questions on the cabal-devel mailing list or on IRC in the #hackage channel on freenode. [code]: https://github.com/haskell/hackage-server [github wiki]: https://github.com/haskell/hackage-server/wiki [cabal-devel]: http://www.haskell.org/mailman/listinfo/cabal-devel -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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ANNOUNCE: HDBI-1.1.1 and friends
by Alexey Uimanov 12 Sep '13

12 Sep '13

Hello, haskellers! I am glad to announce new tested version of HDBI-1.1.1 (Haskell Database Independent interface). Now it becomes much more databae-independent than before because of SQlite3 driver in addition to PostrgreSQL driver. What changed from version 1.0.0: 1. removed `affectedRows` from `Statement`: sqlite does not support this feature directly. You can still use this feature of PostgreSQL with function `pgAffectedRows` 2. fixed default implementation of `executeMany`: perform reset after execute, statement will be in New state after `executeMany`. 3. "stm" is now a dependency 4. DriverUtils: ChildList implemented with STM, `closeAllChildren` now wait for all child statement finalizers and reduced the probability of blocking. 5. Tests improved. 6. Common tests moved to separate package http://hackage.haskell.org/package/hdbi-tests HDBI available on Hackage http://hackage.haskell.org/package/hdbi http://hackage.haskell.org/package/hdbi-postgresql http://hackage.haskell.org/package/hdbi-sqlite Here is github: https://github.com/s9gf4ult/hdbi https://github.com/s9gf4ult/hdbi-postgresql https://github.com/s9gf4ult/hdbi-sqlite https://github.com/s9gf4ult/hdbi-tests https://github.com/s9gf4ult/hdbi-svc Look at the issues list https://github.com/s9gf4ult/hdbi/issues?state=open Your issues and comments are welcome. Pull requests and contribution are welcome as well. Have a nice day.

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Haskell Weekly News: Issue 279
by Daniel Santa Cruz 12 Sep '13

12 Sep '13

Welcome to issue 279 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 25 to September 7, 2013. Quotes of the Week * dmwit: I typically figure a parse error on line 500 means I have 500 lines to inspect. * bos: bah, all you category wonks spoil the atmosphere. we were simple peasants, and proud, once. * merijn: edwardk's delegation has reached the point where he can have the language implementor explain his libraries... * ReinH: haasn asks an offhand question. edwardk wrote a thesis on it. * JaffaCake: children are automatically killed if their parents die for any reason * geekosaur: if the platform is batteries included, lens is a full fledged transformer robot included :) * FreeFull: Mathematical laws are so much nicer than legal laws * twey: to edwardk, v. to be working on a larger problem and absent-mindedly spin out a package to satisfy dependencies, accidentally solving the problem more elegantly than existing solutions (but perhaps incomprehensibly to anyone who can't dualize in their head) * elliott: lens is the DLC expansion pack for Haskell * monochrom: new catchy thing to say: an IORef Int contains an Int in the same sense as an IO Int contains an Int Top Reddit Stories * Announcing GHC iOS Domain: self.haskell, Score: 259, Comments: 52 On Reddit: [1] http://goo.gl/28XFCH Original: [2] http://goo.gl/28XFCH * The Haskell Cast: Episode 1 - Edward Kmett on Lenses Domain: haskellcast.com, Score: 101, Comments: 35 On Reddit: [3] http://goo.gl/umnk1Y Original: [4] http://goo.gl/0vKtZ5 * The Haxl Project at Facebook (slides from my talk at ZuriHac) Domain: github.com, Score: 97, Comments: 76 On Reddit: [5] http://goo.gl/1nWvfs Original: [6] http://goo.gl/pWk1Ll * Haskell on the Daily WTF Domain: thedailywtf.com, Score: 96, Comments: 14 On Reddit: [7] http://goo.gl/kJs9FG Original: [8] http://goo.gl/qdM3sz * Cabal 1.18.0 released Domain: groups.google.com, Score: 92, Comments: 30 On Reddit: [9] http://goo.gl/QbNCmx Original: [10] http://goo.gl/OtYTZG * I've got functional curse. I lost my ability to write in C++. I have to ship C++. What do I do? Domain: self.haskell, Score: 69, Comments: 92 On Reddit: [11] http://goo.gl/oQBzBo Original: [12] http://goo.gl/oQBzBo * `pipes-4.0.0` release candidate Domain: self.haskell, Score: 58, Comments: 42 On Reddit: [13] http://goo.gl/NQDJyT Original: [14] http://goo.gl/NQDJyT * Lenses from Scratch Domain: fpcomplete.com, Score: 54, Comments: 29 On Reddit: [15] http://goo.gl/jYq0ob Original: [16] http://goo.gl/6aeyUP * Controlling Chromium in Haskell Domain: jaspervdj.be, Score: 51, Comments: 3 On Reddit: [17] http://goo.gl/W7VVPh Original: [18] http://goo.gl/WD32IC * Interactive Programming: Hot-swapping in Elm Domain: elm-lang.org, Score: 50, Comments: 6 On Reddit: [19] http://goo.gl/D7LYnM Original: [20] http://goo.gl/A9TwJ5 * Lens/Aeson Traversal/Prism Domain: fpcomplete.com, Score: 48, Comments: 10 On Reddit: [21] http://goo.gl/9o9r3R Original: [22] http://goo.gl/IuFJiC * Mandelbrot Domain: fpcomplete.com, Score: 42, Comments: 6 On Reddit: [23] http://goo.gl/pY3MHE Original: [24] http://goo.gl/gySnCb * ANNOUNCE: posix-paths, for faster file system operations Domain: permalink.gmane.org, Score: 41, Comments: 29 On Reddit: [25] http://goo.gl/vI3UCF Original: [26] http://goo.gl/PlQsAO * Control Text LCD with Haskell code. The code runs on mbed. The RAM size is 32kB. Using Ajhc Haskell compiler. Domain: youtube.com, Score: 41, Comments: 8 On Reddit: [27] http://goo.gl/jchNCd Original: [28] http://goo.gl/CqjxBp * DeepSeq Once Domain: fpcomplete.com, Score: 37, Comments: 57 On Reddit: [29] http://goo.gl/WBohP4 Original: [30] http://goo.gl/zRupez * DSLs and Towers of Abstraction with Gershom Bazerman at NY Haskell Domain: vimeo.com, Score: 36, Comments: 6 On Reddit: [31] http://goo.gl/737bji Original: [32] http://goo.gl/rzHY63 * Port Catch to GHC Domain: self.haskell, Score: 36, Comments: 10 On Reddit: [33] http://goo.gl/6mKcFZ Original: [34] http://goo.gl/6mKcFZ * Reflecting values to types and back Domain: fpcomplete.com, Score: 33, Comments: 11 On Reddit: [35] http://goo.gl/id2AiE Original: [36] http://goo.gl/LJHQfW * Haskell Game Programming Book? Domain: self.haskell, Score: 33, Comments: 25 On Reddit: [37] http://goo.gl/qJDSWv Original: [38] http://goo.gl/qJDSWv * ANN: Cabal v1.18.0 released Domain: haskell.org, Score: 33, Comments: 0 On Reddit: [39] http://goo.gl/w08cdI Original: [40] http://goo.gl/aYSkJf * How could little me make Haskell better? Domain: self.haskell, Score: 31, Comments: 31 On Reddit: [41] http://goo.gl/qtnee1 Original: [42] http://goo.gl/qtnee1 Top StackOverflow Questions * Examples of a monad whose Applicative part can be better optimized than the Monad part votes: 19, answers: 3 Read on SO: [43] http://goo.gl/06Awvz * Haskell: unnecessary reevaluations of constant expressions votes: 17, answers: 2 Read on SO: [44] http://goo.gl/82hb99 * equal sign with haskell literals votes: 16, answers: 3 Read on SO: [45] http://goo.gl/F08H9Z * Haskell standalone desktop application authentication (using google / facebook / twitter / etc accounts) votes: 15, answers: 1 Read on SO: [46] http://goo.gl/JQaIfD * Continue debugging in GHC after interrupt votes: 12, answers: 0 Read on SO: [47] http://goo.gl/ZpQms7 * How to automatically generate (and see the code for) Read and Show in Haskell votes: 12, answers: 1 Read on SO: [48] http://goo.gl/dET9H6 * Killing a Haskell binary votes: 11, answers: 2 Read on SO: [49] http://goo.gl/FDnsvz * Haskell: What does ((->) t) mean? votes: 10, answers: 3 Read on SO: [50] http://goo.gl/HAV96m * What does the term “reason about” mean in computer science? votes: 10, answers: 5 Read on SO: [51] http://goo.gl/CV6EE5 * Idiomatic bidirectional Pipes with downstream state without loss votes: 9, answers: 3 Read on SO: [52] http://goo.gl/rjX2uK Until next time, [53]+Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/1lboh4/announcing_ghc_ios/ 2. http://www.reddit.com/r/haskell/comments/1lboh4/announcing_ghc_ios/ 3. http://www.haskellcast.com/episode/001-edward-kmett-on-lenses/ 4. http://www.reddit.com/r/haskell/comments/1l45c3/the_haskell_cast_episode_1_… 5. https://github.com/meiersi/HaskellerZ/blob/master/meetups/20130829-FPAftern… 6. http://www.reddit.com/r/haskell/comments/1le4y5/the_haxl_project_at_faceboo… 7. http://thedailywtf.com/Articles/OMGWTF2-The-Honorable-Mentions.aspx 8. http://www.reddit.com/r/haskell/comments/1locjo/haskell_on_the_daily_wtf/ 9. https://groups.google.com/forum/#!topic/haskell-cafe/SFoNwaq8wdc 10. http://www.reddit.com/r/haskell/comments/1lqo13/cabal_1180_released/ 11. http://www.reddit.com/r/haskell/comments/1lrrhk/ive_got_functional_curse_i_… 12. http://www.reddit.com/r/haskell/comments/1lrrhk/ive_got_functional_curse_i_… 13. http://www.reddit.com/r/haskell/comments/1lh5u7/pipes400_release_candidate/ 14. http://www.reddit.com/r/haskell/comments/1lh5u7/pipes400_release_candidate/ 15. https://www.fpcomplete.com/user/tel/lenses-from-scratch 16. http://www.reddit.com/r/haskell/comments/1l8ld7/lenses_from_scratch/ 17. http://jaspervdj.be/posts/2013-09-01-controlling-chromium-in-haskell.html 18. http://www.reddit.com/r/haskell/comments/1liux8/controlling_chromium_in_has… 19. http://elm-lang.org/blog/Interactive-Programming.elm 20. http://www.reddit.com/r/haskell/comments/1ln75a/interactive_programming_hot… 21. https://www.fpcomplete.com/user/tel/lens-aeson-traversals-prisms 22. http://www.reddit.com/r/haskell/comments/1l64oe/lensaeson_traversalprism/ 23. https://www.fpcomplete.com/user/edwardk/snippets/mandelbrot 24. http://www.reddit.com/r/haskell/comments/1lkkb0/mandelbrot/ 25. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19946 26. http://www.reddit.com/r/haskell/comments/1liahv/announce_posixpaths_for_fas… 27. http://www.youtube.com/watch?v=fOsjEfNmTkY 28. http://www.reddit.com/r/haskell/comments/1lkboo/control_text_lcd_with_haske… 29. https://www.fpcomplete.com/user/edwardk/snippets/once 30. http://www.reddit.com/r/haskell/comments/1lo15w/deepseq_once/ 31. https://vimeo.com/72870861 32. http://www.reddit.com/r/haskell/comments/1l4ph3/dsls_and_towers_of_abstract… 33. http://www.reddit.com/r/haskell/comments/1lj39q/port_catch_to_ghc/ 34. http://www.reddit.com/r/haskell/comments/1lj39q/port_catch_to_ghc/ 35. https://www.fpcomplete.com/user/thoughtpolice/using-reflection 36. http://www.reddit.com/r/haskell/comments/1l9qzw/reflecting_values_to_types_… 37. http://www.reddit.com/r/haskell/comments/1lgano/haskell_game_programming_bo… 38. http://www.reddit.com/r/haskell/comments/1lgano/haskell_game_programming_bo… 39. http://www.haskell.org/pipermail/haskell-cafe/2013-September/108722.html 40. http://www.reddit.com/r/haskell/comments/1lqo00/ann_cabal_v1180_released/ 41. http://www.reddit.com/r/haskell/comments/1lf6z5/how_could_little_me_make_ha… 42. http://www.reddit.com/r/haskell/comments/1lf6z5/how_could_little_me_make_ha… 43. http://stackoverflow.com/questions/18590639/examples-of-a-monad-whose-appli… 44. http://stackoverflow.com/questions/18655561/haskell-unnecessary-reevaluatio… 45. http://stackoverflow.com/questions/18598365/equal-sign-with-haskell-literals 46. http://stackoverflow.com/questions/18472448/haskell-standalone-desktop-appl… 47. http://stackoverflow.com/questions/18504432/continue-debugging-in-ghc-after… 48. http://stackoverflow.com/questions/18538055/how-to-automatically-generate-a… 49. http://stackoverflow.com/questions/18430748/killing-a-haskell-binary 50. http://stackoverflow.com/questions/18561859/haskell-what-does-t-mean 51. http://stackoverflow.com/questions/18666821/what-does-the-term-reason-about… 52. http://stackoverflow.com/questions/18471706/idiomatic-bidirectional-pipes-w… 53. https://plus.google.com/105107667630152149014/about

1 0

GHC API + Cabal API + Cabal version checks: is there a way out?
by JP Moresmau 11 Sep '13

11 Sep '13

Hello all, I have an issue that has been nagging me for a while, and I'd like to make sure I haven't missed a solution to it. I'm the maintainer for the buildwrapper package. This package has dependencies on the Cabal and GHC libraries, and also uses the cabal-install executable. For example it will run cabal configure by invoking the executable, then it will load the LocalBuildInfo via the Cabal API, and extract the relevant information to start a GHC session with the proper flags, etc. Now yesterday some great news were announced, a new version of Cabal! However, I can't use it. I can of course install the Cabal library and rebuild the cabal executable but - buildwrapper will only use the version of the Cabal library that was bundled with GHC, since the GHC library has a dependency on that version of Cabal - so the library access will be using Cabal 1.16, but the executable will be 1.18 - and the library checks when reading the setup-config file that the Cabal versions match. Hence, running configure with 1.18 create build information I can't read back in 1.16. So I'm stuck. People can easily install cabal 1.18 and build their projects with it, but I can't use it in conjunction with buildwrapper. Some solutions I've considered - do not use the Cabal API and write my own code to read the local build info, and to replace all the Cabal library code I use - do not use the cabal-install executable but do everything by the API Both solutions reek of madness, and involve rewriting code that others have already written, and a maintenance nightmare with each Cabal release. So, do I have to wait a new release of GHC/Haskell Platform or is there a better solution? I suspect ghc-mod and other similar packages have the same issues. Thanks for any help! -- JP Moresmau http://jpmoresmau.blogspot.com/

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GHC API question: Getting access to cpp processed source
by AlanKim Zimmerman 11 Sep '13

11 Sep '13

Hi Cafe I have just discovered that GHC.getTokenStream fails if it is used on a module with CPP directives in it. This is reported in http://ghc.haskell.org/trac/ghc/ticket/8265 Is there an easy way to get access to the pre-processed source, without having to explicitly write it to an output file in a temporary location? In other words an equivalent to getModuleSourceAndFlags that does the right thing. This currently prevents HaRe from processing files with preprocessor directives in them, I would like to come up with a workaround for current GHC versions, rather than having to wait for a future one. Regards Alan

3 4

Reasoning about performance
by Scott Pakin 11 Sep '13

11 Sep '13

I'm a Haskell beginner, and I'm baffled trying to reason about code performance, at least with GHC. For a program I'm writing I needed to find all pairs of elements of a list. That is, given the list "ABCD" I wanted to wind up with the list [('A','B'),('A','C'),('A','D'),('B','C'),('B','D'),('C','D')], where the order of elements in the resulting list is immaterial, but I don't want both ('A', 'B') and ('B', 'A'), for example. My first attempt looked like this: allPairs1 :: [a] -> [(a, a)] allPairs1 [] = [] allPairs1 (x:xs) = map (\a -> (x, a)) xs ++ allPairs1 xs Benchmarking with ghci's ":set +s" reveals the following performance (median of three runs shown): $ ghci GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :!ghc -c -O2 allpairs.hs Prelude> :load allpairs Ok, modules loaded: AllPairs. Prelude AllPairs> :m +Control.DeepSeq Prelude Control.DeepSeq AllPairs> :show modules AllPairs ( allpairs.hs, allpairs.o ) Prelude Control.DeepSeq AllPairs> :set +s Prelude Control.DeepSeq AllPairs> deepseq (allPairs1 [1..10000]) True True (4.85 secs, 4004173984 bytes) A colleague suggested getting rid of the list append as follows: allPairs2 :: [a] -> [(a, a)] allPairs2 [] = [] allPairs2 (x:xs) = allPairs2' x xs xs where allPairs2' x [] [] = [] allPairs2' x (y:ys) zs = (x,y) : allPairs2' x ys zs allPairs2' x [] (z:zs) = allPairs2' z zs zs Not surprisingly, that runs faster: Prelude Control.DeepSeq AllPairs> deepseq (allPairs2 [1..10000]) True True (2.14 secs, 4403686376 bytes) I then figured I could do even better by rewriting the code tail-recursively: allPairs3 :: [a] -> [(a, a)] allPairs3 [] = [] allPairs3 (x:xs) = allPairs3' x xs xs [] where allPairs3' :: a -> [a] -> [a] -> [(a, a)] -> [(a, a)] allPairs3' h (x:xs) ys result = allPairs3' h xs ys ((h, x):result) allPairs3' _ [] (y:ys) result = allPairs3' y ys ys result allPairs3' _ [] [] result = result This takes half the memory as the above (yay!) but runs substantially *slower* (and appears to be thrashing my computer's memory system): Prelude Control.DeepSeq AllPairs> deepseq (allPairs3 [1..10000]) True True (10.58 secs, 2403820152 bytes) What gives? Why does my tail-recursive implementation run even slower and presumably produce more garbage than my initial, naive implementation? Is there some optimization GHC is performing for allPairs1 and allPairs2 that it can't apply to allPairs3? Similarly, how should I, as a newcomer who wants to write efficient Haskell code, reason about whether a code change is likely to improve rather than degrade performance? Guidelines like "per-iteration appends = bad" and "tail recursion = good" are great, but the preceding data indicate that something subtler is taking precedence over those guidelines with respect to code performance. Thanks in advance, -- Scott

5 8

Mistakes in documentation for weak pointers?
by Heinrich Apfelmus 10 Sep '13

10 Sep '13

Dear café, I'm currently studying weak pointers in order to implement garbage collection for a small JavaScript FFI used by the threepenny-gui library [1]. While the paper [2] is fairly clear, it seems that the documentation in System.Mem.Weak [3] differs in certain aspects. Could someone help me and clarify this? I have two questions: 1. The sentence in the documentation "References from the finalizer to the key are treated in the same way as references from the value to the key: they do not keep the key alive." seems incorrect in the subordinate clause. Namely, the paper only states that the weak pointer object does not keep the key alive. However, I understood that it is perfectly acceptable for the value to keep the key alive, and that this relation is not changed by mkWeak . Is this still correct? (The memo table example in the paper never stores the value itself, only a weak pointer to it, so it doesn't matter whether the value is reachable from the key or not.) 2. The sentence in the documentation "A heap object is reachable if: ... It is a weak pointer object whose key is reachable." seems to differ from the paper, which states that while weak pointers are always retained if their keys are alive, they can very much be unreachable. But if I think about it, the formulation in the documentation seems to be equivalent to the paper (as far as the semantics are concerned, it does not matter whether unreachable weak pointers are tombstoned or not.). Is that correct? [1]: http://www.haskell.org/haskellwiki/Threepenny-gui [2]: http://community.haskell.org/~simonmar/papers/weak.pdf [3]: http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-Mem… Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

1 0

Transforming AST parametrized with type
by Sergey Mironov 10 Sep '13

10 Sep '13

Hi. Pat asked a question [1] about AST parametrized with types. People suggest to use Functor machinery if possible. Have anything changed since them? Do we have a way to safely transform the tree like data Expr a = Id { id :: Id a } | Op { op :: Char, expra :: (Expr a) , exprb :: (Expr a) } data Id a = Id { name :: a } from Expr String to, say, Expr Int where Int names may represent some keys in the hash map? Regards, Sergey [1] - http://stackoverflow.com/questions/5434889/is-it-possible-to-use-syb-to-tra… PS sclv suggested to use synthesize from SYB to solve the problem. Could anyone provide me with example of this method?

1 0

Looking for GUI examples
by Sergey Mironov 10 Sep '13

10 Sep '13

Hi, Cafe. I'd like to write simple GUI utility containing tray icon and the menu. Could you please suggest Haskell example to make my start easier? Regards, Sergey

3 3

Lenses: Should I declare Getters?
by Charlie Paul 10 Sep '13

10 Sep '13

I've been looking through Edward Kmett's lens library, and I'm a bit befuddled about Getters. In my own code, why would I want to have something be a Getter instead of a plain function? As far as I can see, a plain function is simpler to use, and can be converted to a Getter with "to" if we want to use it as a Fold. Is there a situation where writing a Getter is superior than writing a function, then lifting it as needed? Thank you, Charles Paul

3 2

Re: [Haskell-cafe] [Haskell] Hackage 2 now available for beta testing
by Ross Paterson 10 Sep '13

10 Sep '13

On Mon, Sep 09, 2013 at 07:23:59PM +0100, Duncan Coutts wrote: > Well-Typed and the Industrial Haskell Group (IHG) are very pleased to > announce that Hackage 2 is now available for public beta testing. The > plan is to do the final switchover in late September, to coincide > with ICFP. > > http://beta.hackage.haskell.org/ What's the story with haddock documentation? I see that some packages have docs imported from the old server, some have newly generated docs, and some have none, but no indication whether a bot has tried to build it or not. There's mention of maintainer uploads of docs as a fallback, but I couldn't find where one would do that. (It would also need to document the flags needed to get the links right.)

2 1

resources on static analysis
by Maarten Faddegon 10 Sep '13

10 Sep '13

Dear list, I am interested in learning more about static analysis of Haskell code. Specifically of the relation between arguments of recursive and non-recursive calls. For example if we look at the ++ function from Prelude: (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys amongst others, we could infer the relations: ys_i+1 = ys_i (x:xs)_i+1 = xs_i Searching the web I found several tools (HLint, Haskabelle, Sourcegraph), but I am interested in the theory behind this. If you could recommend a paper or a book on this topic I would be grateful. Thanks, Maarten Faddegon

3 2

Please excuse brief service disruption
by John Wiegley 10 Sep '13

10 Sep '13

The e-mail services at haskell.org were switched from exim4 to postfix tonight, please excuse the service disruption. For about one hour e-mails were not being accepted to the mailing lists. All should be resolved now. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

1 0

This is a mail system test, please ignore
by John Wiegley 10 Sep '13

10 Sep '13

Testing the new e-mail services at haskell.org. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

1 0

Cabal --enable-tests
by satvik chauhan 10 Sep '13

10 Sep '13

Hi cafe, I wanted to ask this as I couldn't find this in cabal documentation. Is there any CCP macro set when a package is configured with --enable-testing? If not is there a way to do that? -Satvik

2 1

An Extra Empty Line
by Hong Yang 10 Sep '13

10 Sep '13

Hi Dear Haskellers, I have this small program to grep recursively in parallel. It works fine, but generates the last line empty. Is this empty line coming from mapConcurrently()? Thanks, Hong -- mygrepr.hs -- lrf.pl is an old Perl script to get all non-duplicate files recursively under the current directory, since some of directories have tons of links -- the program runs concurrently on either specified number of threads or 3/4 of the number of available cores module Main (main) where import Control.Concurrent.Async import Control.Monad import Data.List import Data.List.Split import GHC.Conc import System.Environment ( getArgs ) import System.Exit import System.Process import Text.Regex.Posix main :: IO () main = do hs_argv <- getArgs if null hs_argv || any (=~ "-h") hs_argv || any (=~ "--h") hs_argv then putStrLn "mygrepr [+RTS -N[x] -RTS] [OPTION]... PATTERN" else do numCores <- getNumProcessors numCapas <- getNumCapabilities let numT | numCapas > 1 = numCapas | otherwise = max numCapas (numCores `div` 4 * 3) _ <- setNumCapabilities numT let numThreads = fromIntegral numT :: Double findResult <- readProcess "lrf.pl" [] [] let files = lines findResult let num_of_files = fromIntegral $ length files :: Double let chunks = chunksOf (ceiling (num_of_files/numThreads)) files results <- mapConcurrently (grep hs_argv) chunks let (_, grepResult, _) = unzip3 results putStr $ unlines $ nub $ filter (\line -> not (line =~ "Binary file .* matches")) $ lines (concat grepResult) grep :: [String] -> [String] -> IO (ExitCode, String, String) grep hs_argv files = readProcessWithExitCode "/tool/pandora64/.package/grep-2.5.4/bin/grep" (hs_argv ++ files) []

1 0

FLTK GUI Binding in progress. Call for participation.
by aditya siram 09 Sep '13

09 Sep '13

Hi all, I'm working on a FLTK [1] GUI binding [2]. The attraction of FLTK is that it is portable across many platforms, meaning it is easy to: 1. provide a binary for your application that works without installing anything else. eg. no need to install X on Mac. Just double-click! 2. develop on other platforms because both the library and the bindings have few dependencies and compile from scratch without a fuss. 3. provide native solutions that don't require the browser. To my knowledge no current bindings have all those advantages. Most of the work of wrapping the C++ in C is done and a number of the examples that ship with FLTK have been ported to C and seem to be compatible. I have only just started the Haskell bindings. This is a great time to: 1. help with the low-level porting. Most of it is just busy work. 2. significantly influence what the Haskelly abstraction layer will look like. 3. tell me if I'm doing something wrong in the C/C++ code. I'm a newbie to both languages. I've learned what I needed to know to get this far. Thanks for your attention. -deech [1] http://www.fltk.org/index.php [2] http://github.com/deech/fltkhs

3 3

a little parsec enhancement
by Petr Pudlák 09 Sep '13

09 Sep '13

Hi, when thinking about this SO question <http://stackoverflow.com/q/18583416/1333025>, I couldn't find a combinator that allows a parser to /optionally/ fail without consuming input, or consume input and return its value. So I'm suggesting such a function: |-- | @emptyIf p@ parses @p@ and if its return value is @Nothing@, pretends -- that an error has occured with no input consumed. -- -- If @p@ fails and consumes some input, so does @emptyIf p@. Combine with -- 'try' if this is undesirable. emptyIf :: (Stream s m t) =>ParsecT s u m (Maybe a) ->ParsecT s u m a emptyIf p =ParsecT $ \s cok cerr eok eerr -> let cok' (Just x) s e = cok x s e cok'Nothing _ e = eerr e eok' (Just x) s e = eok x s e eok'Nothing _ e = eerr e in unParser p s cok' cerr eok' eerr| With this function, the answer to the SO question becomes really easy: |rcomb :: (Stream s m t) =>ParsecT s u m a ->ParsecT s u m b ->ParsecT s u m b rcomb p q = emptyIf $ runMaybeT (opt p *> opt q) where opt =MaybeT . optional-- optional from Control.Applicative!| Whenever |p| or |q| fails without consuming input, then |rcomb p q| fails without consuming input. Unfortunately |ParsecT| constructor isn't exported so I'm not able to implement it outside /parsec/. (Perhaps it would make sense to export |ParsecT| in some module such as |Text.Parsec.Internal|?) Therefore I'm suggesting to add such a function to /parsec/ (darcs patch included). Perhaps change the name, I couldn't think of anything better. Best regards, Petr

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New Functional Programming Job Opportunities
by Functional Jobs 09 Sep '13

09 Sep '13

Here are some functional programming job opportunities that were posted recently: Haskell Programmer at Zalora Southeast Asia http://functionaljobs.com/jobs/8645-haskell-programmer-at-zalora-southeast-… Cheers, Sean Murphy FunctionalJobs.com

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ANN: unix-bytestring 0.3.7
by wren ng thornton 09 Sep '13

09 Sep '13

-------------------------------------------- -- unix-bytestring 0.3.7 -------------------------------------------- The unix-bytestring package offers a full selection of Unix/Posix-specific functions for reading and writing ByteStrings to Fds. -------------------------------------------- -- Changes (since 0.3.4) -------------------------------------------- Most of the changes are for portability with newer versions of GHC, though the 0.3.7 release corrects a major bug on certain systems. I'd like to thank John Lato, Kazu Yamamoto, and Gergely Risko for their help with these patches. 0.3.5 (2011-06-29): - System.Posix.IO.ByteString: added fdSeek, tryFdSeek. - Foreign.C.Error.Safe: added. 0.3.5.3 (2012-02-21): - Foreign.C.Error.Safe: changed (Num a) to (Eq a, Num a) for GHC 7.4 0.3.5.4 (2012-08-23): - System.Posix.IO.ByteString: adjusted some imports for GHC 7.6 - Removed the custom Setup.hs, for compatibility with GHC 7.6 0.3.6 (2013-05-29): - System.Posix.Types.Iovec: adjusted unsafeForeignPtrToPtr for GHC 7.8 0.3.7 (2013-08-08): - System.Posix.IO.ByteString: corrected a bug in fdSeek and tryFdSeek where the return value was always 0 on largefile i686 systems due to the underlying C function being (undocumentedly) implemented as a macrocall. This had been fixed in unix-2.6.0.1, and I've just backported their fix, cf: * <https://github.com/ghc/packages-base/blob/master/System/Posix/Internals.hs#…> * <https://github.com/ghc/packages-unix/blob/master/System/Posix/IO/Common.hsc…> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unix-bytestring Darcs: http://community.haskell.org/~wren/unix-bytestring Haddock (Darcs version): http://community.haskell.org/~wren/unix-bytestring/dist/doc/html/unix-bytes… -- Live well, ~wren

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Impredicative types and Lens?
by Artyom Kazak 09 Sep '13

09 Sep '13

Here’s a small example, which, when compiled, gives an error. Why? {-# LANGUAGE FlexibleInstances, ImpredicativeTypes, TemplateHaskell #-} import Control.Lens class Item a where name :: a -> String instance Item (String, Int) where name = fst type ItemFilter = Item a => a -> Bool data ItemBox = ItemBox { _itemFilter :: ItemFilter } makeLenses ''ItemBox The error is Couldn't match type `a0 -> Bool' with `forall a. Item a => a -> Bool' Expected type: ItemFilter Actual type: a0 -> Bool In the expression: b_aaZE In the first argument of `iso', namely `\ (ItemBox b_aaZE) -> b_aaZE' In the expression: iso (\ (ItemBox b_aaZE) -> b_aaZE) ItemBox I’m using GHC 7.6.2, if it’s important.

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Any Haskell events in Madrid next week?
by Joachim Breitner 07 Sep '13

07 Sep '13

Hi, I’ll be visiting Madrid next week (research visit) and I’m wondering if there are any Haskell or FP Group meeting or other events that might be interesting? I could possibly contribute a talk. (Both preferably in English.) Wednesday or Thursday evening might would most convenient. Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata(a)debian.org

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Unary functions and infix notation
by Johannes Emerich 07 Sep '13

07 Sep '13

As is well known, any binary function f can be turned into an infix operator by surrounding it with backticks: f a b -- prefix application a `f` b -- infix application It is then possible to take left and right sections, i.e. partially applying f: (a `f`) -- equivalent to \b -> a `f` b (`f` b) -- equivalent to \a -> a `f` b This extends relatively naturally to functions of arity greater than two, where usage of a function in infix notation produces a binary operator that returns a function of arity n-2. Weirdly, however, infix notation can also be used for unary functions with polymorphic types, as the following ghci session shows: Prelude> :t (`id` 1) (`id` 1) :: Num a => (a -> t) -> t Prelude> (`id` 1) (\y -> show y ++ ".what") "1.what" Desugaring of an equivalent source file shows that id is applied to the anonymous function, which is then applied to 1. The following example of a function that is not polymorphic in its return type behaves closer to what I would have expected: It does not work. Prelude> let z = (\y -> True) :: a -> Bool Prelude> :t (`z` True) <interactive>:1:2: The operator `z' takes two arguments, but its type `a0 -> Bool' has only one In the expression: (`z` True) What is the purpose/reason for this behaviour? Thank you, --Johannes

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sequence causing stack overflow on pretty small lists
by Niklas Hambüchen 06 Sep '13

06 Sep '13

On #haskell we recently had a discussion about the following: import System.Random list <- replicateM 1000000 randomIO :: IO [Int] I would think that this gives us a list of a million random Ints. In fact, this is what happens in ghci. But with ghc we get: Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize -RTS' to increase it. This is because sequence is implemented as sequence (m:ms) = do x <- m xs <- sequence ms return (x:xs) and uses stack space when used on some [IO a]. >From a theoretical side, this is an implementation detail. From the software engineering side this disastrous because the code is * obviously correct by itself * the first thing people would come up with * not exaggerating: a million elements is not much * used a lot of places: mapM, replicateM are *everywhere* and yet it will kill our programs, crash our airplanes, and give no helpful information where the problem occurred. Effectively, sequence is a partial function. (Note: We are not trying to obtain a lazy list of random numbers, use any kind of streaming or the likes. We want the list in memory and use it.) We noticed that this problem did not happen if sequence were implemented with a difference list. What do you think about this? Should we "fix" functions like this, probably trading off a small performance hit, or accept that idiomatic Haskell code can crash at any time?

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