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Cabal config file Guide
by Daniel Díaz Casanueva 26 May '13

26 May '13

Hello Cafe! As you already know, cabal-install is configured in the file "config". It has a lot of fields, but I didn't find a single place where each field is explained with detail. Most of the options are trivial enough to understand what they do without previous explanation, but some of them aren't. Therefore, in order to know what these options do, you have to search info about that one specifically, when it would be much more convenient to have all the information together. So, is there any place like this out there in the web? Thank you, Daniel Díaz.

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ghc release schedule
by Rustom Mody 26 May '13

26 May '13

I give "ghc release schedule" to google and get at/near the top: http://hackage.haskell.org/trac/ghc/wiki/Status/Releases whose title is "Release plans for ghc 6.12"

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Debugging embedded ruby interpreter
by Simon Marechal 24 May '13

24 May '13

Hello, I am trying to embed a ruby interpreter into my Haskell library. It didn't seem complicated, and went flawlessly until I tried using it a lot. Then I got segfaults. Here is a test program that corrupts the Array object that is being created : https://github.com/bartavelle/hruby/blob/271d99e26a5df075b1011e4b30adf6addd… Changing line 46 to this makes it work: v <- pick (listOf arbitrary :: Gen [Double]) And this version makes it fail again : v <- pick (listOf subvalue :: Gen [Value]) As you can see, the "frequencies" of "subvalue" are set so that only JSON Values representing Doubles are generated. As the toRuby and fromRuby instances for Value are just pure wrappers over the instances for Double, the non-pure code that is executed should be the same. Am I wrong here ? Given that there is almost no documentation for doing this Ruby embedding, I am unsure that I am doing it right. I looked at the Hubris code and several other sources on the Internet, but can't find what is wrong with my implementation. I have no clue on how to debug this. Valgrind gives me a few "Conditional jump or move depends on uninitialised value(s)" during Ruby's gc, but I get more or less the same when just running the native interpreter. Also running this with the multithreaded runtime makes it fail immediately. Anyone has an idea on how I should approach this problem ?

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How not to force ghci debugger but still know the _result?
by haskell-cafe＠maartenfaddegon.nl 23 May '13

23 May '13

Hi, As the subject says, I'd like to use the GHCI debugger to inspect the results of some functions. I tried to set ":break"-points on these functions and ":step" through them, but unless I use ":force" I never get to see the result of these functions. Is it possible to somehow break when the result is evaluated? Please find below a log of my attempts to debug a small example, where during debugging none of the "_result" values is ever known/shown. Maarten Faddegon -- $ cat bla1.hs blabla :: [Int] -> Int bla :: Int -> Int papperlap :: Int -> Int -> Int bla x = x+x papperlap y x = ((y *) . bla) x blabla xs = foldl papperlap 0 xs $ ghci bla1.hs 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. [1 of 1] Compiling Main ( bla1.hs, interpreted ) Ok, modules loaded: Main. *Main> :break blabla Breakpoint 0 activated at bla1.hs:7:1-36 *Main> :break papperlap Breakpoint 1 activated at bla1.hs:6:1-31 *Main> :break bla Breakpoint 2 activated at bla1.hs:5:1-19 *Main> blabla [1,2,3] Stopped at bla1.hs:7:1-36 _result :: Int = _ [bla1.hs:7:1-36] *Main> :step Stopped at bla1.hs:7:17-36 _result :: Int = _ xs :: [Int] = [1,2,3] [bla1.hs:7:17-36] *Main> :step Stopped at bla1.hs:6:1-31 _result :: Int = _ [bla1.hs:6:1-31] *Main> :step Stopped at bla1.hs:6:17-31 _result :: Int = _ x :: Int = 3 y :: Int = _ [bla1.hs:6:17-31] *Main> :step Stopped at bla1.hs:6:1-31 _result :: Int = _ [bla1.hs:6:1-31] *Main> :step Stopped at bla1.hs:6:17-31 _result :: Int = _ x :: Int = 2 y :: Int = _ [bla1.hs:6:17-31] *Main> :step Stopped at bla1.hs:6:1-31 _result :: Int = _ [bla1.hs:6:1-31] *Main> :step Stopped at bla1.hs:6:17-31 _result :: Int = _ x :: Int = 1 y :: Int = 0 [bla1.hs:6:17-31] *Main> :step Stopped at bla1.hs:5:1-19 _result :: Int = _ [bla1.hs:5:1-19] *Main> :step Stopped at bla1.hs:5:17-19 _result :: Int = _ x :: Int = 1 [bla1.hs:5:17-19] *Main> :step Stopped at bla1.hs:5:1-19 _result :: Int = _ [bla1.hs:5:1-19] *Main> :step Stopped at bla1.hs:5:17-19 _result :: Int = _ x :: Int = 2 [bla1.hs:5:17-19] *Main> :step Stopped at bla1.hs:5:1-19 _result :: Int = _ [bla1.hs:5:1-19] *Main> :step Stopped at bla1.hs:5:17-19 _result :: Int = _ x :: Int = 3 [bla1.hs:5:17-19] *Main> :step 0 *Main>

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How to throw an error if using "cabal-install" < version XYZ?
by Ryan Newton 23 May '13

23 May '13

A cabal-install bug <https://github.com/haskell/cabal/issues/1284> was fixed recently that pertains to building C libraries with profiling. As a result, I want a certain package<http://hackage.haskell.org/package/atomic-primops-0.1.0.2>to test if cabal-install < 0.17.0 is used, and throw a preemptive error. Otherwise this package fails in weird ways at runtime (it's a nasty one). I noticed with some surprise the following sequence: * $ cabal --version* * cabal-install version 1.16.0.2* * using version 1.16.0.3 of the Cabal library* * $ cabal clean* * $ cabal install* * $ cat dist/build/autogen/cabal_macros.h | grep VERSION_Cabal* * #define VERSION_Cabal "1.17.0"* Alright, so that, in retrospect, makes sense. The version is which *my* library is linked with is the relevant one, not the one cabal-install was linked with [1]. So the natural next thought is to move the MIN_VERSION_Cabal test into Setup.hs, and force cabal to use it by setting the build type to Custom. But... I just learned from this ticket that the cabal macros are not available in Setup.hs: http://hackage.haskell.org/trac/hackage/ticket/326 Uh oh, what's left? -Ryan [1] P.S. Personally I'm now using a bash function like below, to force the two versions to be the same: function safe_cabal_install () { VER=`cabal --version | tail -n1 | awk '{ print $3 }'` cabal install --constraint="Cabal==$VER" $* }

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question about type constructors
by TP 23 May '13

23 May '13

Hi, In the program I am trying to write, I have a problem that can be reduced to the following dummy example: -------------------------- {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE IncoherentInstances #-} class PrettyPrint a where prettify :: a -> String data Gender = Male | Female | Gender3 | Gender4 data Person :: Gender -> * where Person :: String -> Person b Child :: String -> Person a -> Person b -> Person c instance PrettyPrint (Person a) instance PrettyPrint (Person Male) where prettify (Person name) = "My name is " ++ (show name) ++ " and I am a male" prettify (Child name person1 person2) = "My name is " ++ (show name) ++ " and my parents are:" ++ (prettify person1) ++ ", " ++ (prettify person2) main = do let p1 = Person "Jim" :: Person Male let p2 = Person "Joe" :: Person Male let p3 = Child "Jack" p1 p2 print $ prettify p1 print $ prettify p2 print $ prettify p3 -------------------------- The idea is that I want to implement PrettyPrint only for a subset of the possible types in Gender (through promotion process). Why? It would be longer to explain (it is a bit more complicated in my real program). Anyway, in the program above, I have found that with IncoherentInstances (and the empty instance definition for (Person a)), it is working, it is able to use the most specific instance corresponding to the current type (I don't know exactly why). For example, p1 and p2 are correctly printed above, because they are of type (Person Male) and because I have implemented PrettyPrint for (Person Male). But it does not work for p3, I obtain an error at runtime: ----- $ runghc test.hs "My name is \"Jim\" and I am a male" "My name is \"Joe\" and I am a male" test_typelost.hs: test_typelost.hs:16:10-31: No instance nor default method for class operation Main.prettify ----- The reason is that the information that p1 and p2 are Male seems to be "lost" when we construct the child "Child "Jack" p1 p2", probably because GHC only sees that in the type signature of Child, we have a more general (Person a) -> (Person b). So he tries to find an implementation of prettify in PrettyPrint (Person a), but there is none. Is there any workaround? Thanks, TP

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[ANN] Hac Phi 2013
by Daniel Wagner 23 May '13

23 May '13

Hac Phi 2013, a Haskell hackathon/get-together, will be held June 7-9 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac Phi was a lot of fun, drawing more than 30 Haskellers, and many people have already expressed interest in coming back this year. Everyone is welcome - you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac Phi wiki [2]. This year, we are making a public list of attendees' names and nicks - please let us know if you'd prefer not to be listed when you register. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game - a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! - The Hac Phi team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://www.haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://www.haskell.org/haskellwiki/Hac_%CF%86 [3] http://www.haskell.org/haskellwiki/Hac_%CF%86/Talks

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Second Call for Papers: Workshop on Functional Art, Music, Modeling, and Design (colocated with ICFP)
by Brent Yorgey 23 May '13

23 May '13

Please submit something to FARM --- the deadline is fast approaching! ====================================================================== FARM 2013: Call for Papers ACM SIGPLAN Workshop on Functional Art, Music, Modeling and Design Boston, Massachusetts, USA 28th September, 2013 (directly after ICFP) http://www.cis.upenn.edu/~byorgey/farm13/ ====================================================================== Scope ----- The functional programming community is largely interested in writing beautiful programs. This workshop is intended to gather researchers and practitioners interested in writing beautiful programs that generate beautiful artifacts. Such artifacts may include visual art, music, 3D sculptures, animations, GUIs, video games, physical models, architectural models, choreographies for dance, poetry, and even physical objects such as VLSI layouts, GPU configurations, or mechanical engineering designs. The framework used need not be purely functional (“mostly functional” is fine); may be based on abstractions such as higher-order functions, monads, arrows, or streams; and may be manifested as a domain specific language or tool. Theoretical foundations, language design, implementation issues, and applications are all within the scope of the workshop. Important dates --------------- - Submissions due: Friday, 14 June 2013, 23:59 UTC-11 (American Samoa time [1]) - Author notification: Thursday, 11 July 2013 - Final papers due: Thursday, 25 July 2013 - Workshop: Saturday, 28 September 2013 (the day after ICFP) [1] http://www.timeanddate.com/worldclock/fixedtime.html?msg=FARM+submissions+d… Proceedings ----------- There will be a formal proceedings published by ACM Press. Accepted papers will be included in the ACM Digital Library; see http://authors.acm.org/main.cfm for information on the options available to authors. Authors are also encouraged to submit auxiliary material for publication along with their paper (source code, data, videos, images, etc.); authors retain all rights to the auxiliary material. Submission details ------------------ We solicit submissions in two categories: - Full papers. A full paper is limited to 12 pages, and will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions, identify what has been accomplished, explain why it is significant, and compare it with previous work. - "Aesthetic applications". An aesthetic application paper is limited to 6 pages, and will be judged primarily on the effective and elegant use of FP to generate aesthetic artifacts (though the criteria for full papers are still relevant). Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines. The text should be in a 9-point font in two columns. Submissions can be made via EasyChair at https://www.easychair.org/conferences/?conf=farm2013 Organizers ---------- Workshop Chair: Paul Hudak Program Chair: Conal Elliott Publicity Chair: Brent Yorgey Program Committee ----------------- Daniel Cukier (University of São Paulo) Conal Elliott (Tabula) (chair) Kathleen Fisher (Tufts University) Richard Gabriel (IBM Research) George Giorgidze (University of Tübingen) Paul Hudak (Yale University) José Pedro Magalhães (University of Oxford) Alex McLean (University of Leeds) John Peterson (Western State Colorado University) Michael Sperber (Active Group) Henning Thielemann Brent Yorgey (University of Pennsylvania)

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"Haskell from N00b to Real World Programmer"
by Mihai Maruseac 22 May '13

22 May '13

Hello, As hinted in the previous HCAR, this year we will organize an workshop entitled like the subject of this mail[1]. It will be organized by ROSEdu (Romanian Open Source Education) [2] and the newly founded Haskell-Romania group (no link yet, working on that). It is one workshop in a series of summer workshops organized by ROSEdu (go one level up in the wiki, some of them are in Romanian and I don't want to blindly give links to Romanian pages). The entire schedule is given on the wiki page at [1] and detailed below: Day 1 (8th of July) Introduction. History and Syntax. -- basic introduction to the language. Including @lambdabot usage as an example Programming with Static Typing as An Ally. -- designing with types and using GHCI for rapid prototyping and testing type of expressions. Possible to include @djinn Programming with State and Style. -- gentle introduction to monads in the style of "You could have invented monads" [3] Day 2 (9th of July) Packaging and Distributing Code. -- hackage, cabal, cabal-dev and, maybe, stackage The Awesome xmonad Window Manager -- we'll let them tweak configuration files in xmonad and use them. In this way, they get to use real world apps and change them to suit their needs. Maybe attract some developers. Compare and Contrast. Blow Your Mind with Haskell and Compare with Python. -- Starting with a comparison between some Python code and the equivalent Haskell one we get to cover topics in [4] Day 3 (11th of July) Lenses for Updating Details. -- you've guessed it, lenses Creating Graphical Interfaces. -- GTK2hs or lGTK. I'd like to test the second one Static Site Generation. -- Hakyll, config tweaking, generating first site, etc. Day 4 (12th of July) Beautiful Diagrams -- diagrams package Pipes and Conduits for Faster I/O -- pipes Creating Real-World Web Applications. -- Yesod (because that's what we know) Day 5 (13th of July) Hackathon Day If you have some suggestions, please make them. Lastly, for the hackathon day we have considered making some bots for I.GameBot[5] and an IRC bot with features that we'd like on #rosedu and #haskell-ro (grab, karma, resource location, statistics). If you'd have some other suggestions which could be implemented in a single day after the previous courses, please, do share them with us. Thanks and sorry for the length of the mail. [1]: http://workshop.rosedu.org/2013/sesiuni/haskell [2]: http://rosedu.org/ [3]: http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html [4]: http://www.haskell.org/haskellwiki/Blow_your_mind [5]: https://github.com/rosedu/I.GameBot -- MM "All we have to decide is what we do with the time that is given to us"

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Functional JavaScript Opportunity in NYC
by Ryan Trinkle 22 May '13

22 May '13

Hi everyone, Our company, skedge.me, is looking for a full-time front-end developer in the NYC area. Our front-end is written in JavaScript, but our back-end is written in Haskell, so we're looking for people with solid JavaScript experience but also a strong interest in functional programming. No work experience in Haskell is required, but we do require all applicants to submit a small code sample - something on the scale of a Project Euler problem - in a statically-typed functional language in addition to a more traditional JavaScript code sample. We write all of our JavaScript in a functional style, and we use the Google Closure compiler to provide some type checking for us, so we're looking for people who enjoy working in an environment with a bit more structure than the average JavaScript project. For more information or to apply, visit http://skedge.me/careers or contact me at ryan(a)skedge.me. Ryan Trinkle

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