- Haskell-Cafe - Haskell.org

Help me understand general recursion from cata- and anamorphism
by Takayuki Muranushi 23 Jun '13

23 Jun '13

In an attempt to understand why cata- and anamorphisms are considered so important, I found multiple implications that you can write any recursive functions in terms of nonrecursive functions and ana, cata (am I right here?) so I'm trying to practice the rewrite by a few functions. I'm following a recipe found here: http://lambda-the-ultimate.org/node/4290 ~~~ Given a function that recurses on itself, do a partial CPS transform so that it only ever recurses on itself with tail calls. Then, convert the recursive calls to codata returns, so that the function either returns TheAnswer or StillWorking with enough parameters to describe the recursive call / continuation state. This codata can be built with an unfold and can be collapsed back down to the final answer with a fold. ~~~ https://github.com/nushio3/practice/blob/master/lens/banana/CollatzTest.hs https://github.com/nushio3/practice/blob/master/lens/banana/FibTest.hs I find it difficult to understand the terminology, and the above attempts are only halfway done. I guess ( TheAnswer or StillWorking ) structure is the one found in iteratee/enumeratee. But I don't know how to "build a codata with unfold." I'd appreciate any advice. Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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data constructor names
by Brian Lewis 23 Jun '13

23 Jun '13

Say you write data Callback = Error ... | ... because one of the kinds of callbacks you need to model is an error callback. Then, later, you write data Error = ... to model some error that can happen. They're both good names, but there's a conflict. So I started thinking I should prefix my constructor names like data Callback = CallbackError ... | ... It will work, but it's not the nicest looking. I discovered I can write data Callback = Callback'Error ... | ... Where can I find the syntax reference to see what's allowed? Will people kill me if I start doing this? I could import qualified, but in my case, there would be too many modules. Thanks for any advice.

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<<loop>> error message
by Omari Norman 22 Jun '13

22 Jun '13

I compiled some code with GHC 7.6.3 that produces a simple error at runtime myProgramName: <<loop>> At which point the program exits with code 1. Is there documentation for this error anywhere? Does it mean I have some infinite loop in my code somewhere? If so, does GHC catch all infinite loops? I have never gotten this error before. Thanks.

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The promising world of Monadic formlets
by Alberto G. Corona 22 Jun '13

22 Jun '13

Hi, This is just to let you know the promising results of some experimentation: Formlets are about applicative instances, but what about monadic instances? What a Monad instance of formlets means? I recently experimented with this and the results are very interesting and powerful- It mixes the best of web forms, with the flexibility of console applications. ???!!!!!! Althoug this example is for the formlets of the MFlow <https://github.com/agocorona/MFlow>framework , it can be ported to other formlet implementations. Although the MFLow formlets include web formatting that is not supported in other formlets implementations. Static HTML templating don´t work well with monadic formlets, so it is important to include the formatting as a part of the computation: import MFlow.Wai.Blaze.Html.All dynamicForm= wform $ do (n,s) <- (,) <- p << "Who are you?" ++> getString Nothing <! hint "name" <++ br <*> getString Nothing <! hint "surname" <++ br <** submitButton "ok" <++ br flag <- b << "do you " ++> getRadio[radiob "work?",radiob "study?"] <++ br r<-case flag of "work" -> pageFlow "l" $ Left <- b << "do you enjoy your work? " ++> getBool True "yes" "no" <** submitButton "ok" <++ br "study"-> pageFlow "r" $ Right <- b << "do you study in " ++> getRadio[radiob "University" ,radiob "High School"] p << ("You are "++n++" "++s) ++> case r of Left fl -> p << ("You work and it is " ++ show fl ++ " that you enjoy your work") ++> noWidget Right stu -> p << ("You study at the " ++ stu) ++> noWidget hint s= [("placeholder",s)] onClickSubmit= [("onclick","this.form.submit()")] radiob s n= text s ++> setRadio s n <! onClickSubmit Here wform, getBool, getString , getRadio etc are formlet elements The first sentence is an applicative composition that generate a 2 tuple, to show that applicative and monadic can be mixed. the operations ++> add html to the formlet. the operatior <! add attributes to the formlet element.. noWidget is a dumb formlet that does not validate. The second monadic statement is an election between two options. The beauty of the monadic instance is that the rest of the form can vary depending on the previous answers. Since the formlets validate the input, unless the election is made, the radio will not validate, so the monadic execution will be aborted beyond any unanswered question, so nothing will appear after the question. The rest of the form will appear when the user choose one of the two options. once one or the other option is chosen, then another binary question is presented. (either he likes his work or where he study). When the questions are finised, the results are presented. I hope that you get the idea. The benefit is not only the familiar coding and presentation of a sequential console application: Since the form encloses all the fields, At any time the user can change previous inputs and the form will reflect these changes. For example if the user change from work to study (second statements) the "where do you study will appear and the work related questions and answers will disappear. That is wonderfully useful for heavily interactive applications. There is a problem however and it is the issue of the formlet identifiers. Unlike in an applicative presentation, now the number and type of the formlets will vary, so the response to a previous form create a new kind of form, and the post response can be misinterpreted. To avoid that , the pageFlow call creates fixed field labels for each branch of execution. I will release a version of MFlow that support this kind of monadic composition of fomlets, but In essence it is nothing but to add Monad instance to formlets. A single server procedure, that executes the formlet code can support all the interaction so any framework can do it. The usability of that is huge:It is possible to interact in a web page in a console style with questions and answers with the versatitly of a dynamic foms: Any modification in the form change the subsequent flow of interaction. Another application of this monadic style is to ease multistep processes such are registration, check-out and payment ad so on. Even a entire interactive dynamic application can be coded in a single page. And no javascript is needed!. To run this formlet in MFlow: main=do addMessageFlows [("" , transient $ runFlow $ ask dynamicForm )] wait $ run port waiMessageFlow This video show how the presentation of this example vary with the user input: http://youtu.be/DryBQc9agFg I hope that you find the idea interesting. If you want to experiment with this in MFlow, I have to say that the implementation of this feature is in an early stage. The code is in the head branch https://github.com/agocorona/MFlow/tree/head Alberto.

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how to debug stack overflow?
by Johannes Waldmann 22 Jun '13

22 Jun '13

What is the recommended method to find the exact reason for a stack overflow (when running a Haskell program compiled with ghc)? When I compile with -prof -auto-all, and run with +RTS -xc, I see a very short call stack, which can't be right. But that's probably because I am calling some library method that was compiled without -auto-all so it's invisible in the trace? I can avoid the overflow by +RTS -K1G or something but I want to investigate because it might indicate some inefficiency in the code (evaluations happening at the wrong time). - J.W.

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writing a function to make a correspondance between type-level integers and value-level integers
by oleg＠okmij.org 22 Jun '13

22 Jun '13

I'm late to this discussion and must have missed the motivation for it. Specifically, how is your goal, vector/tensor operations that are statically assured well-dimensioned differs from general well-dimensioned linear algebra, for which several solutions have been already offered. For example, the Vectro library has been described back in 2006: http://ofb.net/~frederik/vectro/draft-r2.pdf http://ofb.net/~frederik/vectro/ The paper also talks about reifying type-level integers to value-level integers, and reflecting them back. Recent GHC extensions (like DataKinds) make the code much prettier but don't fundamentally change the game.

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Nightly builds of GHC HEAD for OS X 10.8
by John Wiegley 22 Jun '13

22 Jun '13

Since mid-January, I’ve been running nightly builds of GHC on my Mac Pro for 10.8.x, 64-bit. I’ve decided to make these results publically downloadable here: http://ghc.newartisans.com. The installer tarballs are in dist, while the fulltest and nofib logs are in logs. According to Jenkins this build takes 8h15m minutes, so I figured this might save others some CPU heat. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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How to make this data type work?
by Magicloud Magiclouds 21 Jun '13

21 Jun '13

data ActionData = AD { oldData :: (FromJSON j, ToJSON j) => j , newData :: (FromJSON j, ToJSON j) => j} instance ToJSON ActionData where toJSON (AD o n) = object [ "oldData" .= o , "newData" .= n ] instance FromJSON ActionData where parseJSON (Object v) = AD <$> v .: "oldData" <*> v .: "newData" parseJSON _ = mzero I got when compile: No instance for (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) arising from a use of `.:' Possible fix: add an instance declaration for (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) In the second argument of `(<$>)', namely `v .: "oldData"' In the first argument of `(<*>)', namely `AD <$> v .: "oldData"' In the expression: AD <$> v .: "oldData" <*> v .: "newData" -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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TH clause Pat selection
by Brian Lewis 21 Jun '13

21 Jun '13

I want to use TH to generate functions like foo :: c -> h foo ... = ... foo ... = ... ... from lists of pairs :: [(c, h)] For example, $(genFoo ''Int ''Bool [(0,False), (1,True)]) would generate foo 0 = False foo 1 = True The problem is, I don't know how to generate the function's clauses. "foo 0 = ..." seems to be a LitP pattern. But "foo True = ..." seems to be a ConP pattern. The appropriate pattern depends on type c. Here's code with more explanation and examples: http://hpaste.org/90163

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Ann: Cascading 0.1.0, DSL for Cascading Style Sheets
by Ertugrul Söylemez 20 Jun '13

20 Jun '13

Hi there, I've just released the first version of [cascading], library for writing/generating Cascading Style Sheets (CSS) in Haskell. The API design is inspired by many other projects, including blaze-html, HSP and Yesod's Lucius. [cascading]: <http://hackage.haskell.org/package/cascading> Features: * Style sheets are monoids and can be MonadWritered. * All non-appendix properties of CSS level 2.1 are implemented and exposed through a type-safe API. It's easy enough to set properties for which no type-safe API exists. See the example below. * Easy to integrate into all monadic web frameworks, easy to interleave stylesheets with framework operations. * Builtin support for web-routes. * Color space awareness through the colour library. * Fully encoding-aware with one caveat: If you bypass PropValue and use ByteStrings as property values directly, then UTF-8 is assumed. Example: stylesheet :: (MonadWriter CSS m) => m () stylesheet = onAll $ do select ["p"] $ do margin . Edges $ [zeroLen] padding . Edges $ [_Em # 1, _Ex # 2] select ["em"] $ do fontWeight BolderWeight select ["ul", "ol"] . below ["li"] $ do borderColor (LeftEdge green) borderWidth (LeftEdge ThickWidth) vendors $ "borderRadius" $= ("3mm" :: PropValue) This results in the following stylesheet: p { margin: 0; padding: 1em 2ex } em { font-weight: bolder } ul li, ol li { border-left-color: #008000; border-left-width: thick; border-radius: 3mm; -moz-border-radius: 3mm; -ms-border-radius: 3mm; -o-border-radius: 3mm; -webkit-border-radius: 3mm } Please visit the [homepage]. Feedback is always welcome. If you find bugs, please don't hesitate to [report] them. [homepage]: <http://hub.darcs.net/ertes/cascading> [report]: <http://hub.darcs.net/ertes/cascading/issues> Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

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Why isn't ArrowChoice a parent of of ArrowApply?
by Petr Pudlák 20 Jun '13

20 Jun '13

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Re: [Haskell-cafe] Remove Enum from Float and Double
by Doug McIlroy 20 Jun '13

20 Jun '13

> it's the entire Enum class which doesn't have a consistent or sensible semantics Yes, for a language with generally sound mathematical roots, Enum is an embarrassment. My pet peeve is that Rationals are enumerable, but not with Enum. And the differing lengths of [1,4..6] and [1.0,4.0..6.0] will shock even the non-mathematically inclined. Much of the craziness can be hidden if in place of [a..b] for nonintegral numeric types, one uses (map fromInteger [a..b]). Perhaps that should happen automatically. As for the horror mentioned at the end of the first paragraph, the language definition's oafish attempt to "get right" things like [0.0,0.1..1.0] needs major revision. Doug McIlroy

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GADTs and pattern matching
by Francesco Mazzoli 19 Jun '13

19 Jun '13

Hi list, I have stumbled upon a strange annoyance: {-# LANGUAGE GADTs #-} data Foo v where Foo :: Foo (Maybe v) -- This doesn't work foo1 :: a -> Foo a -> Int foo1 Nothing Foo = undefined foo1 (Just x) Foo = undefined -- This does foo2 :: a -> Foo a -> Int foo2 x Foo = foo2' x foo2' :: Maybe a -> Int foo2' Nothing = undefined foo2' (Just x) = undefined The first definition fails with the error Couldn't match expected type `a' with actual type `Maybe t0' `a' is a rigid type variable bound by the type signature for foo1 :: a -> Foo a -> Int at /tmp/foo_flymake.hs:8:9 In the pattern: Nothing In an equation for `foo1': foo1 Nothing Foo = undefined Now, GHC can clearly derive and use the type equalities correctly, given that the second definition works, but why can’t I pattern match directly? Thanks, Francesco

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ANNOUNCE: Ajhc 0.8.0.6 Release
by Kiwamu Okabe 19 Jun '13

19 Jun '13

We are happy to announce Ajhc 0.8.0.6. Major change on the release is supporting THREAD. Yeah! You can get Ajhc using "cabal install ajhc" command. Ajhc's project web site is found at http://ajhc.metasepi.org/. You find the source code at https://github.com/ajhc/ajhc/tags. Welcome sending any bugs or your ideas to https://github.com/ajhc/ajhc/issues. ## Good news * You can use forkOS interface on Ajhc. * Your program runs on pthread with compiling -fpthread option. See below. $ cat Main.hs import Control.Monad import Control.Concurrent main :: IO () main = do l <- putStrLn "Type some string and enter." >> getLine forkOS $ (forever $ putChar '*') forkOS $ (forever $ putStr l) forever $ putChar '.' $ ajhc -fpthread Main.hs $ ./hs.out ## Bad news * Ajhc guards critical section only for the runtime. * Example: StablePtr isn't guarded with lock. * Ajhc's thread can't share any objects without Ptr type. * And you may find funny pthread bugs. Please send us the report! ## Other changes * No more depend on DrIFT. * Add _JHC_JGC_LIMITED_NUM_GC_STACK flag to set number of limited gc_stack entries. * Add _JHC_JGC_FIXED_MEGABLOCK flag to set number of limited megablock entries. * Add compile flags -fcustomthread -fpthread -fnothread. * You can implement your own thread API for Ajhc, if you choose -fcustomthread. Enjoy! ;) - - - Metasepi team

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ANN: Nomyx 0.2 beta, the game where you can change the rules
by Corentin Dupont 19 Jun '13

19 Jun '13

Hello everybody! Here it comes, the second beta release [1] of Nomyx, the only game where You can change the rules!! This is an implementation of a Nomic [2] game in Haskell (I believe the first complete implementation of a Nomic game on a computer). In a Nomyxgame you can change the rules of the game itself while playing it. The players can submit new rules or modify existing ones, thus completely changing the behaviour of the game through time. The rules are managed and interpreted by the computer. They must be written in the Nomyx language, which is a subset of Haskell. At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Here is a video introduction and first tutorial of the game: http://vimeo.com/58265498 The game is running here: www.nomyx.net:8000/Nomyx I have set up a forum where players can learn about Nomyx and discuss the rules they intend to propose: www.nomyx.net/forum The example file gives a lot of examples of rules that you can submit: www.nomyx.net:8000/src/Language/Nomyx/Examples.hs Changes from V0.1: - new login system: you can now login with your Google, Yahoo, Live Journal, Myspace, OpenId or Facebook accounts (thanks to happstack-authenticate)! - new DSL for voting (see below) - styling: rule code colorized, better settings and help - use cookies to store the user ID (as suggested on this mailing list) - new error system to handle exceptions in rules (with ErrorT) - use lenses I set up a little DSL to create elections (elect one of the players for a special role) or referendums (a yes/no question). You create in one line within Nomyx an vote with unanimity or majority, a quorum and different ballot systems. See here: http://www.nomyx.net/forum/viewtopic.php?f=3&t=1518 Let's test it! If you are interested, please go to this forum thread and we'll set up a small team to start a match! http://www.nomyx.net/forum/viewtopic.php?f=4&t=1517 The demo game is running here: www.nomyx.net:8000/Nomyx then select game "demo2". As the first player of the game, I changed the initial unanimity vote to a simple majority, with a minimum of 2 players voting. Having your new rules accepted will be easy! Let's see who will win :) Cheers, Corentin [1] http://hackage.haskell.org/package/Nomyx [2] www.nomic.net

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hand over maintenance of a package
by Corentin Dupont 19 Jun '13

19 Jun '13

Hi Cafe, How to hand over the maintenance of a hackage package? Thanks Corentin

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ML 2013: last call for presentations
by Daan Leijen 19 Jun '13

19 Jun '13

There are still a few days left to write a short talk proposal for the ML workshop 2013! Since there are no official proceedings, this is an ideal venue if you are working on a full submission for some other conference but want to talk about the work early on and get useful community feedback. Hope to see you there, -- Daan ACM SIGPLAN Workshop on ML Sunday, September 22, 2013, Boston MA (co-located with ICFP) http://research.microsoft.com/en-us/um/people/daan/mlworkshop2013 ======================================================================= The ML family of programming languages includes dialects known as Standard ML, OCaml, and F#. These languages have inspired a large amount of computer-science research, both practical and theoretical. This workshop aims to provide a forum where users, developers and researchers of ML languages and related technology can interact and discuss ongoing research, open problems and innovative applications. The ML workshop has adopted an informal model since 2010. It is a workshop with presentations selected from submitted abstracts. There are no published proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format encourages the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. SCOPE ----- We seek research presentations on topics related to ML, including but not limited to * Applications: case studies, experience reports, pearls, etc. * Extensions: higher forms of polymorphism, generic programming, objects, concurrency, distribution and mobility, semi-structured data handling, etc. * Type systems: inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * Implementation: compilers, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * Environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * Semantics: operational, denotational, program equivalence, parametricity, mechanization, etc. Three kinds of submissions will be accepted: Research Presentations, Experience Reports and Demos. * Research Presentations: Research presentations should describe new ideas, experimental results, significant advances in ML-related projects, or informed positions regarding proposals for next-generation ML-style languages. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. These presentations should be structured in a way which can be, at least in part, of interest to (advanced) users. * Experience Reports: Users are invited to submit Experience Reports about their use of ML languages. These presentations do not need to contain original research but they should tell an interesting story to researchers or other advanced users, such as an innovative or unexpected use of advanced features or a description of the challenges they are facing or attempting to solve. * Demos: Live demonstrations or short tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. (Please note that you will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. SUBMISSION INSTRUCTIONS ----------------------- Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. Submissions longer than half a page should include a one-paragraph synopsis suitable for inclusion in the workshop program. Submissions must be uploaded to the following website before the submission deadline (2013-06-21): https://www.easychair.org/conferences/?conf=ml2013 For any question concerning the scope of the workshop or the submission process, please contact the program chair (daan at microsoft.com) IMPORTANT DATES --------------- * Friday, June 21 : Submission * Monday, July 22 : Notification * Sunday, September 22: Workshop PROGRAM COMMITTEE ----------------- Daan Leijen (chair) (Microsoft Research, US) Jesse A. Tov (Harvard University, US) Derek Dreyer (MPI-SWS, Germany) Atsushi Ohori (Univ. of Tohoku, Japan) Lars Bergstrom (Univ. of Chicago, US) Jean Yang (MIT CSAIL, US) Gavin Bierman (Microsoft Research, Cambridge, UK) Tomas Petricek (Univ. of Cambridge, UK) Yukiyoshi Kameyama (Univ. of Tsukuba, Japan) Peter Thiemann (Univ. of Freiburg, Germany) STEERING COMMITTEE ------------------ Matthew Fluet (Chair, Rochester Institute of Technology) Alain Frisch (LexiFi) Jacques Garrigue (Nagoya University) Yaron Minsky (Jane Street) Greg Morrisett (Harvard University) Chung-chieh Shan (Indiana University) Anil Madhavapeddy (Cambridge University) Daan Leijen (Microsoft Research)

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[ANNOUNCE] Juicy.Pixels 3.1
by Vincent Berthoux 18 Jun '13

18 Jun '13

Hello, I'm pleased to announce the release of Juicy.Pixels 3.1 AKA. the "Squeezed juice" release Hackage : http://hackage.haskell.org/package/JuicyPixels GitHub : https://github.com/Twinside/Juicy.Pixels Change list : * Addition of Tiff reading : - 2, 4, 8, 16 bit depth reading (planar and contiguous for each) - CMYK, YCbCr, RGB, Paletted, Greyscale - Uncompressed, PackBits, LZW * Adding basic handling of 16bits pixel types * Some new tiny helper functions (nothing too fancy) * Performances : now between 2x and 3x of the performance of ImageMagick! (some benchmarks at https://soundofapixel.wordpress.com/2013/06/17/juicy-pixels-3-1/ ) Big thanks to Alp Mestanogullari and Jason Dagit who helped making this release possible. Vincent Berthoux

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Strange cabal failure
by Jacques Carette 18 Jun '13

18 Jun '13

In trying to install the lens package, it eventually tries to install transformers-compat-0.1.1.1 which in turn depends on transformers-0.3.0.0 -- however that asksk for transformers-0.3.0.0-3006d6ea13a2c10770bffd4de7a96dc9 which 1) is weird, and 2) doesn't exist!' What gives? Jacques

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Re: [Haskell-cafe] Promoting Haskell via Youtube movies
by Walter Askew 18 Jun '13

18 Jun '13

I think something like Erlang: The Movie is more Haskell's style: http://www.youtube.com/watch?v=uKfKtXYLG78

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[haskell.org Google Summer of Code] The Summer of Code is officially under way.
by Edward Kmett 18 Jun '13

18 Jun '13

The "getting to know your mentor" period has passed and the Summer of Code has officially begun. Keep in mind that mid-term evaluations will begin July 29th, so students will need to hit the ground running. I'm looking forward to seeing what we can accomplish together this year! -Edward

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opengl type confusion
by briand＠aracnet.com 17 Jun '13

17 Jun '13

This, wireframe :: Double -> Double -> Double -> IO () wireframe wx wy wz = do -- yz plane renderPrimitive LineLoop $ do vertex $ Vertex3 0.0 0.0 0.0 vertex $ Vertex3 0.0 wy 0.0 vertex $ Vertex3 0.0 wy wz vertex $ Vertex3 0.0 0.0 wz produces this: No instance for (VertexComponent Double) arising from a use of `vertex' Possible fix: add an instance declaration for (VertexComponent Double) In the expression: vertex In a stmt of a 'do' block: vertex $ Vertex3 0.0 wy 0.0 In the second argument of `($)', namely `do { vertex $ Vertex3 0.0 0.0 0.0; vertex $ Vertex3 0.0 wy 0.0; vertex $ Vertex3 0.0 wy wz; vertex $ Vertex3 0.0 0.0 wz }' and thusly this :-( Changing the declaration to GLdouble -> GLdouble -> GLdouble -> IO() and using (0.0::GLdouble) fixes it, and I'm not clear on why it's not automagic. There are many times I see the compiler doing type conversion an numerican arguments although sometimes the occasional fracSomethingIntegralorOther is required. I was hoping for some enlightenment. Thank you. Brian

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ANN: custom-hackage
by Niklas Hambüchen 17 Jun '13

17 Jun '13

https://github.com/nh2/custom-hackage An (almost trivial) script to generate 00-index.tar.gz which is necessary to run your own `remote-repo`. If you are a company that has to rely on that not everybody with a Hackage account can run arbitrary code on your computer at your next cabal install, and you want to make available only select versions of packages (as opposed to a full hackage mirror), this is for you. Just drop your tars into the directory structure, run the script and serve it over HTTP.

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ANNOUNCE: som-4.0 (for data analysis and visualisation)
by Amy de Buitléir 17 Jun '13

17 Jun '13

Do you have some data that you'd like to understand better? I'm happy to announce a new release of a package called som that may help: http://hackage.haskell.org/package/som https://github.com/mhwombat/som/wiki (wiki) A Kohonen Self-organising Map (SOM) maps input patterns onto a regular grid (usually two-dimensional) where each node in the grid is a model of the input data, and does so using a method which ensures that any topological relationships within the input data are also represented in the grid. This implementation supports the use of non-numeric patterns. In layman's terms, a SOM can be useful when you want to discover the underlying structure of some data. I have a brief tutorial in the wiki, which I hope to expand over the next few weeks. WHAT'S NEW - It is now easier to for non-math types to create a SOM (see defaultSOM) - Added another example to the wiki

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Call for Papers IFL 2013
by publicityifl＠gmail.com 16 Jun '13

16 Jun '13

Hello, Please, find below the second call for papers for IFL 2013. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL CALL FOR PAPERS 25th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2013 RADBOUD UNIVERSITY NIJMEGEN, THE NETHERLANDS ACM In-Cooperation / ACM SIGPLAN AUGUST 28 - 30 2013 "Landgoed Holthurnsche Hof" http://ifl2013.cs.ru.nl We are proud to announce that the 25th edition of the IFL series returns to its roots at the Radboud University Nijmegen in the Netherlands. The symposium is held from 28th to 30th of August 2013. Scope ----- The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2013 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Following the IFL tradition, IFL 2013 will use a post-symposium review process to produce the formal proceedings which will be published in the ACM Digital Library. All participants of IFL 2013 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Invited Speaker --------------- Lennart Augustsson, currently employed by the Standard Chartered Bank, well-known for his work on Haskell, parallel Haskell, Cayenne, and Bluespec, is the invited speaker of IFL 2013. He will be talking about practical applications of functional programming. Submission Details ------------------ Submission deadline draft papers: July 31 Notification of acceptance for presentation: August 2 Early registration deadline: August 7 Late registration deadline: August 14 Submission deadline for pre-symposium proceedings: August 21 25th IFL Symposium: August 28-30 Submission deadline for post-symposium proceedings: November 11 Notification of acceptance for post-symposium proceedings: December 18 Camera-ready version for post-symposium proceedings: February 3 2014 Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must adhere to the standard ACM two columns conference format. For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. A suitable document template for LaTeX can be found at: http://www.acm.org/sigs/sigplan/authorInformation.htm Papers are to be submitted via the conference's EasyChair submission page: https://www.easychair.org/conferences/?conf=ifl2013 Topics ------ IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2013, please contact the PC chair at rinus(a)cs.ru.nl. Topics of interest include, but are not limited to: language concepts type systems, type checking, type inferencing compilation techniques staged compilation run-time function specialization run-time code generation partial evaluation (abstract) interpretation metaprogramming generic programming automatic program generation array processing concurrent/parallel programming concurrent/parallel program execution embedded systems web applications (embedded) domain specific languages security novel memory management techniques run-time profiling performance measurements debugging and tracing virtual/abstract machine architectures validation, verification of functional programs tools and programming techniques (industrial) applications Peter Landin Prize ------------------ The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honoured article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee ------------------- Thomas Arts, Quviq, Gothenburg, Sweden Andrew Butterfield, Trinity College, Dublin, Ireland Edwin Brady, University of St. Andrews, UK Clemens Grelck, University of Amsterdam, Netherlands Adam Granicz, IntelliFactory, Budapest, Hungary Jeremy Gibbons, University of Oxford, UK Fritz Henglein, University of Copenhagen, Denmark Stephan Herhut, Intel Labs, Santa Clara, US Ralf Hinze (co-chair), University of Oxford, UK Zoltán Horváth, Eötvös Loránd University, Budapest, Hungary Zhenjiang Hu, University of Tokyo, Japan Mauro Jaskelioff, Universidad Nacional de Rosario, Argentina Johan Jeuring, University of Utrecht, Netherlands Rita Loogen, University of Marburg, Germany Marco T. Morazán, Seton Hall University, New Jersey, US Dominic Orchard, University of Cambridge, UK Rinus Plasmeijer (chair), Radboud University Nijmegen, Netherlands Tim Sheard, Portland State University, US Sam Tobin-Hochstadt, Northeastern University / Indiana University, US Peter Thiemann, University of Freiburg, Germany Simon Thompson, University of Kent, UK Venue ----- The 25th IFL is organized by the Radboud University Nijmegen, Model Based Software Development Department at the Nijmegen Institute for Computing and Information Sciences. The event is held in the Landgoed Holthurnsche Hof, a rural estate in the woodlands surrounding Nijmegen. It can be reached quickly and easily by public transport.

1 0

Problems with GHC API and error handling
by Daniel F 16 Jun '13

16 Jun '13

Hello, everyone. I am in need of setting up custom exception handlers when using GHC API to compile modules. Right now I have the following piece of code: * Main.hs: -------------------------------------------------------------------------------------------------- import GHC import GHC.Paths import MonadUtils import Exception import Panic import Unsafe.Coerce import System.IO.Unsafe handleException :: (ExceptionMonad m, MonadIO m) => m a -> m (Either String a) handleException m = ghandle (\(ex :: SomeException) -> return (Left (show ex))) $ handleGhcException (\ge -> return (Left (showGhcException ge ""))) $ flip gfinally (liftIO restoreHandlers) $ m >>= return . Right initGhc :: Ghc () initGhc = do dfs <- getSessionDynFlags setSessionDynFlags $ dfs { hscTarget = HscInterpreted , ghcLink = LinkInMemory } return () test :: IO (Either String Int) test = handleException $ runGhc (Just libdir) $ do initGhc setTargets =<< sequence [ guessTarget "./test/file1.hs" Nothing ] graph <- depanal [] False loaded <- load LoadAllTargets -- when (failed loaded) $ throw LoadingException setContext (map (IIModule . moduleName . ms_mod) graph) let expr = "main" ty <- exprType expr -- throws exception if doesn't typecheck output ty res <- unsafePerformIO . unsafeCoerce <$> compileExpr expr return res -------------------------------------------------------------------------------------------------- * file1.hs: ---------------------------- module Main where main = do return x ---------------------------- The problem is when I run the 'test' function above I receive the following output: h> test test/file1.hs:4:10: Not in scope: `x' Left "Cannot add module Main to context: not a home module" it :: Either String Int So, if I understand this correctly, my exception handler does indeed catch an exception correctly, however, I still receive some output which I want to be captured. Is there a way to do this? -- Sincerely yours, -- Daniil Frumin

1 1

Efficiency/Evaluation Question
by Christopher Howard 16 Jun '13

16 Jun '13

I'm working through some beginner-level "keyboard problems" I found at users.csc.calpoly.edu. One problem is the Saddle Points problem: quote: -------- Write a program to search for the "saddle points" in a 5 by 5 array of integers. A saddle point is a cell whose value is greater than or equal to any in its row, and less than or equal to any in its column. There may be more than one saddle point in the array. Print out the coordinates of any saddle points your program finds. Print out "No saddle points" if there are none. -------- Let's say I use a simple list grid like so: code: -------- array = Grid 5 [ [1,5,3,6,4] , [8,2,6,3,8] , [3,8,7,2,9] , [0,3,7,1,2] , [7,2,7,4,5] ] data Grid = Grid Int [[Int]] -------- And let's say I take a brute force approach, moving through each cell, checking to see if it is the greatest in its row and the least in its column. And say I have functions like so for getting rows and columns: code: -------- row (Grid s l) n = if (n >= s) then [] else l !! n col g@(Grid s l) n = if (n >= s) then [] else col_ g n 0 where col_ (Grid s l) n i = if (i >= s) then [] else (head l !! n) : col_ (Grid s (tail l)) n (i + 1) -------- My question: With the way Haskell works (thunks, lazy evaluation, and all that mystery), is it actually worth the trouble of /precalculating/ the maximum row values and minimum column values, to compare cell values against? Or will, for example, something like (smallest_list_value (col array 1)) definitely only evaluate once? -- frigidcode.com

4 5

Improved ghc-pkg cache warnings
by Andrew Pennebaker 16 Jun '13

16 Jun '13

When ghc-pkg observes your cache is out of date, it displays a helpful warning, recommending "ghc-pkg recache". However, sometimes running this command does not fix the problem, because it targets the wrong cache. For out of date global caches, "ghc-pkg --global recache" successfully clears the warning. For out of date user caches, "ghc-pkg --user recache" clears the warning. In the future, could ghc-pkg display the command more specific to the problem? -- Cheers, Andrew Pennebaker www.yellosoft.us

1 0

Haskell Platform 2013.2.0.0 64bit.pkg
by Richard A. O'Keefe 15 Jun '13

15 Jun '13

Today I cleared out everything, using uninstall-hs and rm -rf ~/.cabal ~/Library/Haskell I downloaded Haskell Platform 2013.2.0.0 64bit.pkg and installed it. I was unsuccessful in installing the packages I wanted using cabal install, which suggested running ghc-pkg check. So I cleared out everything again and reinstalled the HP. In the admin account, ghc-pkg check says Warning: haddock-interfaces: /Library/Haskell/ghc-7.6.3/lib/haskell-platform-2013.2.0.0/ doc/html/haskell-platform.haddock doesn't exist or isn't a file Warning: haddock-html: /Library/Haskell/ghc-7.6.3/lib/haskell-platform-2013.2.0.0/ doc/html doesn't exist or isn't a directory

4 4

Testing invasive proposals with Hackager
by Niklas Hambüchen 14 Jun '13

14 Jun '13

In many discussions we make guesses about how much code proposals like Functor => Monad would break. You can use https://github.com/dterei/Hackager to build all of Hackage (preferably in a VM). Of course many packages have external dependencies, so I'd like to share the following list of packages to save you some time. (These are the names of packages suited for a Ubuntu 13.04 VM, and with these installed, 2800 packages are successfully built and 1700 failing, so it's quite good coverage. If you find more packages that improve on this, please add them.) acl-dev attr-dev binutils-dev cfitsio-dev expect-dev freeglut3-dev libadns1-dev libalure-dev libasound-dev libaspell-dev libatlas-dev libaugeas-dev libavcodec-dev libavfilter-dev libavformat-dev libavutil-dev libbibutils-dev libbluetooth-dev libbz2-dev libcal3d12-dev libcmph-dev libcrack2-dev libcrypto++-dev libcsound64-dev libctemplate-dev libcurl-dev libcv-dev libcwiid-dev libdb-dev libdevil-dev libdpkg-dev libev-dev libevent-dev libexif-dev libfam-dev libfcgi-dev libfftw3-dev libfltk1.3-dev libfreenect-dev libftgl-dev libfuse-dev libgd2-xpm-dev libgeoip-dev libglfw-dev libglpk-dev libgmime-2.6-dev libgnome-keyring-dev libgnutls-dev libgnutls-dev libgpcl-dev libhighgui-dev libimlib2-dev libinsighttoolkit4-dev libjasper-dev libjudy-dev libkyotocabinet-dev liblapack-dev libldap2-dev libldap2-dev libleveldb-dev libmagic-dev libmarkdown2-dev libmecab-dev libmpfr-dev libmtp-dev libmx-dev libmysqlclient-dev libncurses-dev libncurses-dev libnotmuch-dev libobjc-4.7-dev libodbc1 libode-dev libogg-dev libois-dev libopenal-dev libpam0g-dev libpcap-dev libpoker-eval-dev libportaudio-dev libpulse-dev libqd-dev libqrencode-dev libraw1394-dev libreadline-dev libscsynth1 libselinux-dev libsnappy-dev libsndfile1-dev libsqlite3-dev libssl-dev libssl-dev libst-dev libsvm-dev libtalloc-dev libtbb-dev libtheora-dev libtiff-dev libtokyocabinet-dev libtokyotyrant-dev libtre-dev libuuid-dev libv4l-dev libvxl1-dev libxapian-dev libxen-dev libxerces-c-dev libxine-dev libxmmsclient-dev libxosd-dev libxqilla-dev libxss-dev libxtst-dev libyaml-dev libz-dev libzephyr-dev libzmq-dev linux-libc-dev tcl-dev wx2.8-headers xmms2-dev

8 11

Haskell for Programmers Workshop: Denver, Colorado
by Peter Jones 14 Jun '13

14 Jun '13

Since this is shameless self-promotion I'll keep it short. I'm teaching a Haskell workshop for imperative programmers in Denver, Colorado, September 16-18. If you want more information please take a look at the workshop website: http://www.devalot.com/workshops/haskell/index.html -- Peter Jones --- Love to Develop Devalot: http://www.devalot.com

1 0

Automating Hackage accounts
by Andrew Pennebaker 14 Jun '13

14 Jun '13

Could we add an HTML form for creating new Hackage accounts? Right now, our community is small enough that emailing ross(a)soi.city.ac.uk and waiting for a manual response isn't too bad of a problem, but as we grow, it would be nice for these sorts of things to be handled by a server, like with RubyGems and NPM. -- Cheers, Andrew Pennebaker www.yellosoft.us

8 10

ANN: Angel 0.4.2
by Michael Xavier 13 Jun '13

13 Jun '13

I'm pleased to announce the release of Angel 0.4.2 and that I have officially taken over maintainership of this project. Thanks to Jamie Turner for starting such a great project and allowing me to take over this project. angel is a daemon that runs and monitors other processes. It is similar to djb's daemontools or the Ruby project god. It's goals are to keep a set of services running, and to facilitate the easy configuration and restart of those services. 0.4.1 added the "count" option to the config to control the number of instances of a particular process to start. 0.4.2 added the "pidfile" option to specify the path of a pidfile to generate when monitoring processes. -- Michael Xavier http://www.michaelxavier.net

2 1

Examples of MVars usage
by Francisco M. Soares Nt. 13 Jun '13

13 Jun '13

Hello, everyone. I am looking for packages on hackage which use MVars extensively. Those which create plenty of MVars -- not just one or two for conditional synchronization or to keep track of a value throughout the program. My purpose is to analyze usage patterns of MVars. Does anybody have any suggestions? So far I have analyzed a few packages: * conjure (0.1) * distributed-process (0.4.2) * distributed-process-p2p (0.1.1.0) * leksah (0.12.1.3) * manatee-core (0.1.1) * urlcheck (0.1.1) Nonetheless, I feel like I still haven't covered a good enough range of usage, since some examples are small, and some use MVars just for a small number of cases, while the heavy lifting is done with STM. And surely because there are only 6 of them. Any suggestion will be very much appreciated. []'s -- Francisco Soares Nt.

3 3

Haskell Weekly News: Issue 270
by Daniel Santa Cruz 13 Jun '13

13 Jun '13

Welcome to issue 270 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 2 to 8, 2013. Quotes of the Week * shachaf: getLine :: IO String contains a String in the same way /bin/ls contains a list of files. Top Reddit Stories * Haskell's HLearn library cross-validates more than 400x faster than Java's Weka due to new monoid based algorithm Domain: izbicki.me, Score: 86, Comments: 13 On Reddit: [1] http://goo.gl/dAoe5 Original: [2] http://goo.gl/dLZJv * FP Complete Launches Haskell in Real World Competition with $1,000 Cash Prize Each Month Domain: fpcomplete.com, Score: 80, Comments: 10 On Reddit: [3] http://goo.gl/FthPX Original: [4] http://goo.gl/pzgQ2 * So I write compilers for a living now Domain: evincarofautumn.blogspot.it, Score: 76, Comments: 69 On Reddit: [5] http://goo.gl/hXp0G Original: [6] http://goo.gl/wMaab * Haskell for all: pipes-parse-1.0.0: Pushback, delimited parsers, resumable parsing, and lenses Domain: haskellforall.com, Score: 55, Comments: 77 On Reddit: [7] http://goo.gl/U3MzG Original: [8] http://goo.gl/3wEJ7 * "Thank you to the Haskell community for encouraging programmers to explore the relationship between algebra and programming." -- My accepted paper at the International Conference of Machine Learning (ICML13) Domain: jmlr.org, Score: 54, Comments: 3 On Reddit: [9] http://goo.gl/Ni0oF Original: [10] http://goo.gl/klOzM * GHCJS introduction – Concurrent Haskell in the browser, by Luite Stegeman Domain: weblog.luite.com, Score: 54, Comments: 27 On Reddit: [11] http://goo.gl/l9LDH Original: [12] http://goo.gl/avaez * Haskell from C: Where are the for Loops? Domain: fpcomplete.com, Score: 46, Comments: 28 On Reddit: [13] http://goo.gl/YqkSU Original: [14] http://goo.gl/pwYB3 * fficxx - FFI to C++ in Haskell Domain: ianwookim.org, Score: 45, Comments: 12 On Reddit: [15] http://goo.gl/y1awM Original: [16] http://goo.gl/wHF2U * Control.Category: now with kind polymorphism Domain: neocontra.blogspot.com, Score: 42, Comments: 7 On Reddit: [17] http://goo.gl/9gb6f Original: [18] http://goo.gl/ALvld * Pixel art editor in 600 lines of Haskell Domain: github.com, Score: 39, Comments: 5 On Reddit: [19] http://goo.gl/gmFOF Original: [20] http://goo.gl/w8yEg * Haskell for all: pipes-concurrency-1.2.0: Behaviors and broadcasts Domain: haskellforall.com, Score: 35, Comments: 35 On Reddit: [21] http://goo.gl/rojk1 Original: [22] http://goo.gl/aNK8l * Mastermind in Agda, Running in the Browser Domain: people.inf.elte.hu, Score: 28, Comments: 0 On Reddit: [23] http://goo.gl/N8yty Original: [24] http://goo.gl/7lhvT * Bittoll: A bitcoin transaction system written in Haskell. Domain: bittoll.com, Score: 25, Comments: 13 On Reddit: [25] http://goo.gl/9MFXB Original: [26] http://goo.gl/q39ps * standalone-haddock: host your Haskell documentation on your website Domain: feuerbach.github.io, Score: 22, Comments: 7 On Reddit: [27] http://goo.gl/dAsmP Original: [28] http://goo.gl/INyIe Top StackOverflow Questions * What is () in Haskell, exactly? votes: 18, answers: 4 Read on SO: [29] http://goo.gl/XkrXD * Haskell pre-monadic I/O votes: 15, answers: 1 Read on SO: [30] http://goo.gl/oryYA * Display function types in Haskell votes: 12, answers: 1 Read on SO: [31] http://goo.gl/qOtpV * Haskell Pattern Matching Fails on Negative Number votes: 12, answers: 2 Read on SO: [32] http://goo.gl/mw4WF * Debugging performance bottlenecks for longest common subsequence algorithm votes: 11, answers: 1 Read on SO: [33] http://goo.gl/mfa8S * What is a shrink, with regard to Haskell's QuickCheck? votes: 11, answers: 2 Read on SO: [34] http://goo.gl/eLlyO * Cross module optimizations in GHC votes: 10, answers: 1 Read on SO: [35] http://goo.gl/RyFQP * Functional composition with multi-valued functions in haskell? votes: 9, answers: 3 Read on SO: [36] http://goo.gl/10Xvd * A more succinct way to map functions onto fields of an algebraic datatype? votes: 9, answers: 1 Read on SO: [37] http://goo.gl/CfIOa * What's the history behind the Functor type class? votes: 8, answers: 1 Read on SO: [38] http://goo.gl/5BwZS Until next time, [39]+Daniel Santa Cruz References 1. http://izbicki.me/blog/hlearn-cross-validates-400x-faster-than-weka 2. http://www.reddit.com/r/haskell/comments/1fl33k/haskells_hlearn_library_cro… 3. http://www.fpcomplete.com/blog/2013/06/call-for-submissions 4. http://www.reddit.com/r/haskell/comments/1fvtbf/fp_complete_launches_haskel… 5. http://evincarofautumn.blogspot.it/2012/07/so-i-write-compilers-for-living-… 6. http://www.reddit.com/r/haskell/comments/1fsbs2/so_i_write_compilers_for_a_… 7. http://www.haskellforall.com/2013/06/pipes-parse-100-pushback-delimited.html 8. http://www.reddit.com/r/haskell/comments/1fjcpg/haskell_for_all_pipesparse1… 9. http://jmlr.org/proceedings/papers/v28/izbicki13.pdf 10. http://www.reddit.com/r/haskell/comments/1fk8fl/thank_you_to_the_haskell_co… 11. http://weblog.luite.com/wordpress/?p=14 12. http://www.reddit.com/r/haskell/comments/1fvv5q/ghcjs_introduction_concurre… 13. https://www.fpcomplete.com/blog/2013/06/haskell-from-c 14. http://www.reddit.com/r/haskell/comments/1fm55g/haskell_from_c_where_are_th… 15. http://ianwookim.org/fficxx/ 16. http://www.reddit.com/r/haskell/comments/1fy5gy/fficxx_ffi_to_c_in_haskell/ 17. http://neocontra.blogspot.com/2013/06/controlcategory-now-with-kind.html 18. http://www.reddit.com/r/haskell/comments/1fjjt0/controlcategory_now_with_ki… 19. https://github.com/mchakravarty/BigPixel 20. http://www.reddit.com/r/haskell/comments/1fqyrk/pixel_art_editor_in_600_lin… 21. http://www.haskellforall.com/2013/06/pipes-concurrency-120-behaviors-and.ht… 22. http://www.reddit.com/r/haskell/comments/1ft1sc/haskell_for_all_pipesconcur… 23. http://people.inf.elte.hu/divip/mastermind/index.html 24. http://www.reddit.com/r/haskell/comments/1fx4rb/mastermind_in_agda_running_… 25. https://www.bittoll.com/ 26. http://www.reddit.com/r/haskell/comments/1fqtmu/bittoll_a_bitcoin_transacti… 27. http://feuerbach.github.io/standalone-haddock/ 28. http://www.reddit.com/r/haskell/comments/1fuw5n/standalonehaddock_host_your… 29. http://stackoverflow.com/questions/16892570/what-is-in-haskell-exactly 30. http://stackoverflow.com/questions/17002119/haskell-pre-monadic-i-o 31. http://stackoverflow.com/questions/16971221/display-function-types-in-haske… 32. http://stackoverflow.com/questions/16976543/haskell-pattern-matching-fails-… 33. http://stackoverflow.com/questions/16952956/debugging-performance-bottlenec… 34. http://stackoverflow.com/questions/16968549/what-is-a-shrink-with-regard-to… 35. http://stackoverflow.com/questions/16908539/cross-module-optimizations-in-g… 36. http://stackoverflow.com/questions/16888222/functional-composition-with-mul… 37. http://stackoverflow.com/questions/16974868/a-more-succinct-way-to-map-func… 38. http://stackoverflow.com/questions/16880577/whats-the-history-behind-the-fu… 39. https://plus.google.com/105107667630152149014/about

1 0

Re: [Haskell-cafe] Question about Newtype "op()" function arguments.
by David Banas 12 Jun '13

12 Jun '13

Ah, so is the idea, then, to use *op()* when `n` wasn't actually constructed formally, but rather "assembled" by the user, so as to match the type of the accessor function normally supplied as the argument to the constructor? On 6/7/2013 4:51 PM, Tom Ellis wrote: > On Fri, Jun 07, 2013 at 04:05:09PM -0400, Joe Q wrote: >> The phantom parameter solves the same problem as scoped type variables. >> Granted, if you find yourself in that kind of polymorphic soup you have >> deeper problems... > I don't understand this. Scoped type variables are used when you want to > use a type variable from the top level within the body of a function. If > you use "op" and specify a particular constructor then you don't have a > variable but a concrete instance of a type. But maybe I'm missing some more > powerful way this can be used ... > > Tom You can use scoped type variables to correct an ambiguous type error. You can think of op as a variation on asTypeOf, as documented here on http://www.haskell.org/haskellwiki/Scoped_type_variables#Avoiding_Scoped_Ty… . If I tried to come up with an example that's specific to op, it would only be horribly contrived.

1 0

Wiki spam
by Stefan Holdermans 11 Jun '13

11 Jun '13

Hi all, The last days, some spam was added to the GHC developer wiki (http://hackage.haskell.org/trac/ghc) . I have removed some of it, but I could not get rid of the bits that were added through attachments; see for example the bottom of http://hackage.haskell.org/trac/ghc/wiki/Building. Can someone with wiki admin rights have a go at it? Thanks, Stefan

1 0

[ANN] Haskell-Paris meetup
by lucas di cioccio 11 Jun '13

11 Jun '13

Dear all, I'm happy to announce that the Haskell-Paris group will meet on June 25th in Paris. Please register (free) at http://www.meetup.com/haskell-paris/events/122515522/ . Program is not decided yet (contributions are welcome). We'll update once we know the lineup. I don't like to send spammy emails but I would like to take advantage of this announcement to invite any non-Parisian Haskellers to get in touch with us (email, twitter, or meetup page) if you are in town. We may an event scheduled close in time and we can anyway propose a get-together in a restaurant so that visitors can taste our great food&drink and everyone can chat about her favorite programming language. I did this type of semi-planned dinners a few times with some Ruby folks and it's a good way to build ties :). Regards, --Lucas DiCioccio

2 1

(no subject)
by Zed Becker 11 Jun '13

11 Jun '13

Hi all, Haskell, is arguably the best example of a design-by-committee language. The syntax is clean and most importantly, consistent. The essence of a purely functional programming is maintained, without disturbing its real world capacity. To all the people who revise the Haskell standard, and implement the language, 1. Promise to me, and the rest of the community, that you will keep up the good effort :) 2. Promise to me, and the rest of the community, that Haskell will always spiritually remain the same clean, consistent programming language as it is now! Regards, Zed Becker

9 10

writing a function to make a correspondance between type-level integers and value-level integers
by TP 11 Jun '13

11 Jun '13

Hi all, Following a discussion on Haskell-Cafe, Richard E. made the following proposition of a "Singleton" to make a correspondance between type-level integers and value-level integers: """ > data SNat :: Nat -> * where > SZero :: SNat 'Zero > SSucc :: SNat n -> SNat ('Succ n) """ (found at [1], and an example of application can be found at [2], also proposed by Richard E.) Now I asked myself if there is some means to write a function that would take any value of type e.g. `Succ (Succ Zero)`, and would return the value `SSucc (SSucc SZero)`. I have tried hard, but did not succeed (see below). I am constantly refrained by the fact a function or data constructor cannot take a value of type having a kind different from `*` (if I am right). Is there any solution to this problem? Below is my attempt, which yields a compilation error test_typeinteger_valueinteger_conversion.hs:18:14: Expected a type, but ‛n’ has kind ‛Nat’ In the type ‛(n :: Nat)’ In the definition of data constructor ‛P’ In the data declaration for ‛Proxy’ ---------------------- {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE StandaloneDeriving #-} -- type level integers data Nat = Zero | Succ Nat deriving ( Show, Eq, Ord ) -- Singleton allowing a correspondance between type-level and value-level -- integers. data SNat :: Nat -> * where SZero :: SNat Zero SSucc :: SNat n -> SNat (Succ n) deriving instance Show (SNat n) data Proxy :: Nat -> * where P :: (n::Nat) -> Proxy n class MkSNat (n::Nat) where mkSNat :: Proxy n -> SNat n instance MkSNat Zero where mkSNat _ = SZero instance MkSNat (Succ n) where mkSNat (P (Succ n)) = SSucc (mkSNat (P n)) main = do let one = undefined :: Proxy ('Succ 'Zero) print one print $ mkSNat (P one) ---------------------- Thanks, TP References: ----------- [1]: https://groups.google.com/forum/?fromgroups#!topic/haskell-cafe/mGDhPqHZAz8 [2]: https://groups.google.com/d/msg/haskell-cafe/Rh65kdPkX70/T2zZpV6ZpjoJ

2 4

Re: [Haskell-cafe] [Hackathon] ANN: ZuriHac 2013 & FP Afternoon with keynote by Simon Marlow
by Bas van Dijk 11 Jun '13

11 Jun '13

On 10 June 2013 19:38, Roman Cheplyaka <roma(a)ro-che.info> wrote: > Hi Bas, > >> When: Thursday 30 August - Friday 1 September >> Where: Erudify offices, Zurich, Switzerland > > Is this a mistake? 30 August is Friday, 1 September is Sunday. Oops! You're right, that's embarrassing :-) Thanks, Bas

2 1

Workshop on Generic Programming, Second CFP
by Jacques Carette 10 Jun '13

10 Jun '13

====================================================================== CALL FOR PAPERS WGP 2013 9th ACM SIGPLAN Workshop on Generic Programming Boston, Massachusetts, USA Saturday, September 29th, 2013 http://www.wgp-sigplan.org/2013 Co-located with the International Conference on Functional Programming (ICFP 2013) ====================================================================== Goals of the workshop --------------------- Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic program are often quite rich in structure; for example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, and, for at least 20 years, generic programming techniques have been a specific focus of research in the functional and object-oriented programming communities. Generic programming has gradually spread to more and more mainstream languages, and today is widely used in industry. This workshop brings together leading researchers and practitioners in generic programming from around the world, and features papers capturing the state of the art in this important area. We welcome contributions on all aspects, theoretical as well as practical, of * generic programming, * programming with (C++) concepts, * meta-programming, * programming with type classes, * programming with modules, * programming with dependent types, * type systems for generic programming, * polytypic programming, * adaptive object-oriented programming, * component-based programming, * strategic programming, * aspect-oriented programming, * family polymorphism, * object-oriented generic programming, * implementation of generic programming languages, * static and dynamic analyses of generic programs, * and so on. Program Committee ----------------- Jeremiah Willcock (co-chair), Indiana University Jacques Carette (co-chair), McMaster University Florian Rabe, Jacobs University Bremen Emilie Balland, INRIA Bordeaux Jeremy Siek, University of Colorado, Boulder Gabriel Dos Reis, Texas A&M University Christophe Raffalli, Savoie University Anya Helene Bagge, Universitetet i Bergen Tiark Rompf, Ecole Polytechnique Federale de Lausanne Andreas Abel, Ludwig-Maximilians-Universitat Munchen Edward Kmett, S&P Capital IQ William Cook, University of Texas, Austin Proceedings and Copyright ------------------------- We plan to have formal proceedings, published by the ACM. Authors must transfer copyright to ACM upon acceptance (for government work, to the extent transferable), but retain various rights (http://www.acm.org/publications/policies/copyright_policy) Authors are encouraged to publish auxiliary material with their paper (source code, test data, etc.); they retain copyright of auxiliary material. Submission details ------------------ Deadline for submission: Friday 2013-06-14 Notification of acceptance: Wednesday 2013-07-11 Final submission due: Tuesday 2013-07-25 Workshop: Sunday 2013-09-28 Papers should be submitted via EasyChair at https://www.easychair.org/conferences/?conf=wgp2013 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines (two-column, 9pt). The length is restricted to 12 pages. Travel Support -------------- Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC program, see its web page (http://www.sigplan.org/PAC.htm) History of the Workshop on Generic Programming ---------------------------------------------- Earlier Workshops on Generic Programming have been held in * Copenhagen, Denmark 2012 (affiliated with ICFP12), * Tokyo, Japan 2011 (affiliated with ICFP11), * Baltimore, Maryland, US 2010 (affiliated with ICFP10), * Edinburgh, UK 2009 (affiliated with ICFP09), * Victoria, BC, Canada 2008 (affiliated with ICFP), * Portland 2006 (affiliated with ICFP), * Ponte de Lima 2000 (affiliated with MPC), * Marstrand 1998 (affiliated with MPC). Furthermore, there were a few informal workshops * Utrecht 2005 (informal workshop), * Dagstuhl 2002 (IFIP WG2.1 Working Conference), * Nottingham 2001 (informal workshop). There were also (closely related) DGP workshops in Oxford (June 3-4 2004), and a Spring School on DGP in Nottingham (April 24-27 2006, which had a half-day workshop attached). WGP Steering Committee ---------------------- Marcin Zalewski (chair) Bruno Oliveira Jaako Järvi Shin-Cheng Mu Andres Löh Ronald Garcia Magne Haveraaen Tim Sheard Stephanie Weirich

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ANN: ZuriHac 2013 & FP Afternoon with keynote by Simon Marlow
by Bas van Dijk 10 Jun '13

10 Jun '13

Dear Haskellers, we are delighted to announce ZuriHac 2013 and a Zurich FP Afternoon. Zurich FP Afternoon 2013 ------------------------ When: Thursday 29 August Where: ETH, Zurich, Switzerland Half a day of talks by FP practitioners, with Simon Marlow giving the keynote speech. ZuriHac 2013 ------------ When: Thursday 30 August - Friday 1 September Where: Erudify offices, Zurich, Switzerland An international, grassroots, collaborative coding festival with a simple focus, to build and improve Haskell libraries, tools, and infrastructure. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools, or even start new ones! Registration ------------ If you wish to attend one or both events, please register by filling in this form: http://bit.ly/ZuriHac2013Reg Please note that we have a limited number of places for the Hackathon -- first come, first served. Arrangements ------------ Full details, including accommodation arrangements, can be found on the wiki page: http://www.haskell.org/haskellwiki/ZuriHac2013 We look forward to seeing you there! -- The organisers of ZuriHac 2013 ZuriHac 2013 is sponsored by Erudify and ETH

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Non-deterministic behaviour of aeson's parser
by Roman Cheplyaka 10 Jun '13

10 Jun '13

I am observing a non-deterministic behaviour of aeson's parser. I'm writing here in addition to filing a bug report [1] to draw attention to this (pretty scary) problem. To try to reproduce this problem, do this: git clone https://gist.github.com/5604887.git aeson cd aeson ghc aeson.hs ./aeson | sort | uniq -c This is my result: 32 Left "key \"module\" not present" 55 Left "When parsing the record SymValue of type Main.SymValueInfo the key fixity was not present." 1913 Right () Can others reproduce this in their environments? Does anyone have ideas about where the bug may lie? Many aeson's dependencies do unsafe IO stuff that could lead to such consequences. Roman [1]: https://github.com/bos/aeson/issues/125

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[ANNOUNCE] rainbow - print colored text on UNIX-like systems
by Omari Norman 09 Jun '13

09 Jun '13

Hi all, rainbow is a simple package to help you print colored text on UNIX-like systems. It's different from packages like terminfo (upon which it is based) and ansi-terminal in two ways. First, rainbow assumes you print text one "chunk" at a time. The properties of the terminal are reset with every chunk. That way you do not need to remember what the terminal's properties were before you print a new chunk. One chunk does not affect the next chunk. It's a state-free model that fits in well with functional programming. Second, rainbow makes it easy to use both 8- and 256-color terminals. Some sample things you can do in ghci: :set -XOverloadedStrings :m +System.Console.Rainbow putChunkLn $ "Hello bold green world!" <> f_green <> bold putChunkLn $ "Red on 8-color, pink on 256-color" <> f_red <> c256_f_201 mapM_ putChunk ["green on blue" <> f_green <> b_blue, " ", "blue on green" <> f_blue <> b_green, "\n"] So, add some color to things: http://hackage.haskell.org/package/rainbow

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ANNOUNCE: standalone-haddock-1.0
by Roman Cheplyaka 09 Jun '13

09 Jun '13

I am happy to announce the first release of standalone-haddock. http://feuerbach.github.io/standalone-haddock/ standalone-haddock generates standalone haddock Haskell documentation. When you simply run `cabal haddock`, the resulting HTML documentation contains hyperlinks to other packages on your system. As a result, you cannot publish it on the internet (well, you can, but the links will be broken). standalone-haddock takes several packages for which you want to publish documentation. It generates documentation for them with proper links: * links to identifiers inside this package set are relative * links to identifiers from external packages lead to hackage Thus the resulting directory with HTML is relocatable and publishable. **TL;DR**: it just works. See the [haskell-suite][] documentation for an example output. [haskell-suite]: http://haskell-suite.github.io/docs Usage ----- Usage: standalone-haddock [--package-db DB-PATH] -o OUTPUT-PATH [PACKAGE-PATH] Available options: -h,--help Show this help text --package-db DB-PATH Additional package database -o OUTPUT-PATH Directory where html files will be placed `PACKAGE-PATH` is the path to the (unpacked) package — i.e. a directory with a `.cabal` file. For example: standalone-haddock -o doc haskell-names haskell-packages haskell-src-exts hse-cpp cabal/Cabal **NOTE**: dependencies of every package need to be already installed in the system with documentation (even those dependencies that themselves belong to the current package set). If they are installed in a non-standard package database (e.g. if you use sandboxes), use the `--package-db` option. Cabal dependency ---------------- The program only builds with (unreleased) Cabal 1.17 which you can get from [github](https://github.com/haskell/cabal). I spent some time trying to make it compatible with Cabal 1.16 (see [Cabal-1.16][] branch), but the API seems to have changed too much. If you seriously care about this, feel free to send a patch, but it's really easier just to install Cabal HEAD. [Cabal-1.16]: https://github.com/feuerbach/standalone-haddock/tree/Cabal-1.16 Roman

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Getting a Hackage account
by Kevin Backhouse 08 Jun '13

08 Jun '13

Please could someone on this list help me to get an account on Hackage? I followed the instructions the webpage and emailed Ross Paterson, but that was two weeks ago and I have not received a response. Thank you, Kevin Backhouse

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ANNOUNCE: ig: bindings to Instagram Web Services API
by JP Moresmau 08 Jun '13

08 Jun '13

Hello all, It's my pleasure to announce the initial release of ig, a library providing bindings to the Instagram Web Services API (http://instagram.com/developer/#) I'm indebted to Felipe Lessa (and all the folks at Loyful) as I've reused a lot of ideas from his code on the Facebook library. I've tried to implement much of the API, please reports on the Github repository ( https://github.com/loyful/ig) if something is missing and of course if you find issues. At https://github.com/loyful/ig-testapp you can find a yesod test application that shows authentication, real-time subscriptions and some other calls. It's very crude but can help you started. Grab it from Hackage, of course! Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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Question about Newtype "op()" function arguments.
by David Banas 08 Jun '13

08 Jun '13

Hi All, Referring to the following, which is taken from the *Control.Newtype *documentation page: op :: Newtype<http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/Control-Ne…> n o => (o -> n) -> n -> oSource<http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/src/Contro…> This function serves two purposes: 1. Giving you the unpack of a newtype without you needing to remember the name. 2. Showing that the first parameter is *completely ignored* on the value level, meaning the only reason you pass in the constructor is to provide type information. Typeclasses sure are neat. As point #2, above, emphasizes, the only purpose for the first argument to the function (i.e. - the constructor "(o -> n)") is to specify the type of 'n'. However, being a *newtype*, 'n' can have only one constructor. So, why is it necessary to pass in the constructor to this function, when we're already passing in 'n'? Thanks, -db

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ANNOUNCE: Ajhc 0.8.0.5 Release
by Kiwamu Okabe 07 Jun '13

07 Jun '13

We are happy to announce Ajhc 0.8.0.5. Major change on this release is supporting haddock. You can see Ajhc's internal using web browser. Ajhc's project web site is found at http://ajhc.metasepi.org/. You can get the source code from https://github.com/ajhc/ajhc/tags. ## Changes * Support haddock. You can read haddock on http://hackage.haskell.org/package/ajhc, hopefully. * Split library from Ajhc program. * Better NAIVEGC. * Add arity "arena" to all functions compiled by Ajhc. For supporting reentrant in future. * Run SelfTest and rtstest on travis-ci. * Add jahm command to download file from Hackage DB, because sometime perl script occurs error. Enjoy. :) - - - Metasepi team

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