June 2013 - Haskell-Cafe

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

13 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

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Automating Hackage accounts
by Andrew Pennebaker 13 Jun '13

13 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

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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

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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.

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Haskell Weekly News: Issue 270
by Daniel Santa Cruz 12 Jun '13

12 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

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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.

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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

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[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

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(no subject)
by Zed Becker 10 Jun '13

10 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

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writing a function to make a correspondance between type-level integers and value-level integers
by TP 10 Jun '13

10 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

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