- Haskell-Cafe - Haskell.org

Job opportunities at Startup - London and Exeter
by Ben Moseley 08 Nov '13

08 Nov '13

If you are an experienced developer, have a keen interest in functional programming and would be interested in working at an ambitious, funded, consumer-focused UK startup, please get in touch with me. Our main server-side infra is Haskell-based but we also use Java and Objective-C for the mobile platforms we target. Beyond the obvious technical skills, successful candidates will need focus and determination to get the job done. In return we offer a hugely exciting and challenging working environment, competitive compensation, a great culture (we'll probably try and drag you away from your keyboard and onto a mountain bike) and an experienced management team with a proven track-record. We are looking for developers to be based in London and Exeter. -- Ben Moseley - Programmer ben(a)benmoseley.net

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QuickCheck
by Tony Morris 08 Nov '13

08 Nov '13

Hello, I have implemented a function for QuickCheck: untilJust :: Gen a -> (a -> Maybe b) -> Gen b I based it on the code for suchThat[1] and suchThatMaybe[2]. I am wondering if I have potentially re-implemented an existing function in part or full. In other words, is there an easier way of achieving this function without pulling the generator apart as much as I have in this implementation: http://lpaste.net/95317 [1] suchThat :: Gen a -> (a -> Bool) -> Gen a http://hackage.haskell.org/package/QuickCheck-2.6/docs/src/Test-QuickCheck-… [2] suchThatMaybe :: Gen a -> (a -> Bool) -> Gen (Maybe a) http://hackage.haskell.org/package/QuickCheck-2.6/docs/src/Test-QuickCheck-… -- Tony Morris http://tmorris.net/

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concatMap with state for Data.Vector?
by Evan Laforge 08 Nov '13

08 Nov '13

This is a Data.Vector question, hopefully here is a good place for those. Or would it be better to ask on stackoverflow? I want to generate a vector from another, where I interpolate between each point. So I need something like concatMap because the vector gets longer, but I need state because I need to remember the previous value (and I need to look at the next value to avoid overrunning, but that can be done with state and 'tails'). Stated imperatively, it would look like this: state = Nothing out = [] for (x, y) in signal: case state of Nothing -> out.append (x, y) Just (x0, y0) -> for (x, y) in interpolate ...: out.append (x, y) state = Just (x, y) The simplest option would be just doing the transformation on a list: `Vector.concat . snd . List.mapAccumL go Nothing . Vector.toList` But unless fusion is more powerful than I imagined, that builds a couple of lists and a bunch of intermediate vectors. I did try to read the core, but it's hard to follow when vector is involved. If they're large, it's probably no worse than a builder, but if they're small, well I'd want a builder to fill up each chunk efficiently... but there's no builder for Data.Vector (I did write one once, maybe I should pull it out and dust it off). unfoldr could also do it, but to emit multiple samples I'd need to make the state `Either ([a], state) state`, and some awkward plumbing, and constant tag checks. So it's looking like converting to a list and back is the most reasonable option... unless there's some way to express this that I'm missing?

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Re: [Haskell-cafe] Better integration with Xcode
by David Banas 07 Nov '13

07 Nov '13

Could the originator of this thread say something, as to what advantages his approach has over Leksah, please? Thanks, -db

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Haskell Weekly News: Issue 285
by Daniel Santa Cruz 07 Nov '13

07 Nov '13

Welcome to issue 285 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of October 20 to November 02, 2013. Quotes of the Week * roconnor: edwardk: is comonad Haskell 1.4? I liked Haskell 1.4. * Peaker: Don't trust the @src, luke * dmwit: If it fits in a tweet, you haven't included enough information to debug it yet. Top Reddit Stories * The Haskell Cast #3 - Simon Peyton Jones on GHC Domain: haskellcast.com, Score: 88, Comments: 11 On Reddit: [1] http://goo.gl/0sGzlg Original: [2] http://goo.gl/wQ4FKi * Elm 0.10 released: faster strings, nice colors, bug fixes, and searchable documentation Domain: elm-lang.org, Score: 74, Comments: 9 On Reddit: [3] http://goo.gl/HJ2fc7 Original: [4] http://goo.gl/xFJJMN * Procrustean Mathematics by Edward Kmett Domain: fpcomplete.com, Score: 71, Comments: 17 On Reddit: [5] http://goo.gl/AsB0CM Original: [6] http://goo.gl/E1iDNM * Haskell hurdle: uninteresting world Domain: gundersen.net, Score: 68, Comments: 58 On Reddit: [7] http://goo.gl/W8l3T1 Original: [8] http://goo.gl/uBVp1f * websockets-0.8 released: with browser shell example + bonus section on pipes vs. conduit vs. io-streams Domain: jaspervdj.be, Score: 65, Comments: 9 On Reddit: [9] http://goo.gl/0XvXZa Original: [10] http://goo.gl/eUDMn1 * IHaskell: Haskell for Interactive Computing Domain: gibiansky.github.io, Score: 61, Comments: 26 On Reddit: [11] http://goo.gl/ZpNpq8 Original: [12] http://goo.gl/A8iccI * ShellCheck – static analysis and linting tool for sh/bash script Domain: shellcheck.net, Score: 55, Comments: 10 On Reddit: [13] http://goo.gl/z0giaP Original: [14] http://goo.gl/zF3DLB * Micro-tutorial: liftM by accident Domain: self.haskell, Score: 54, Comments: 29 On Reddit: [15] http://goo.gl/d6ReRs Original: [16] http://goo.gl/d6ReRs * Barclays are hiring: Haskell developers in London and Kiev Domain: self.haskell, Score: 52, Comments: 14 On Reddit: [17] http://goo.gl/Fq7JPE Original: [18] http://goo.gl/Fq7JPE * License monads Domain: joeyh.name, Score: 52, Comments: 18 On Reddit: [19] http://goo.gl/oZ9La1 Original: [20] http://goo.gl/9C1Stp * Show Reddit: My weekend project, PureScript Domain: functorial.com, Score: 49, Comments: 61 On Reddit: [21] http://goo.gl/sRx1Eh Original: [22] http://goo.gl/TJBCYN * Deamortized ST by Edward Kmett Domain: fpcomplete.com, Score: 49, Comments: 57 On Reddit: [23] http://goo.gl/Vck3Ij Original: [24] http://goo.gl/wtaueO * HaLVM 2.0 Developer's Release (trans: buggy but API-stable pre-release) now available for folks to try. GHC 7.8, threaded RTS, SMP HaLVM support, 0-copy networking, and more. Domain: community.galois.com, Score: 44, Comments: 13 On Reddit: [25] http://goo.gl/5Psg7B Original: [26] http://goo.gl/xQFUA2 * Could you explain why complexity of monadic IO is worth it? Domain: self.haskell, Score: 43, Comments: 128 On Reddit: [27] http://goo.gl/esFCc4 Original: [28] http://goo.gl/esFCc4 * London Haskell User Group 18/09/2013: Oliver Charles on pipes [video] Domain: ocharles.org.uk, Score: 42, Comments: 14 On Reddit: [29] http://goo.gl/8kDKnw Original: [30] http://goo.gl/hi3L3P * C-- as a project separate from GHC Domain: cminusminus.org, Score: 42, Comments: 14 On Reddit: [31] http://goo.gl/fvUjIk Original: [32] http://goo.gl/VueAA * How to start a new Haskell project Domain: jabberwocky.eu, Score: 42, Comments: 26 On Reddit: [33] http://goo.gl/6dRRXn Original: [34] http://goo.gl/btWJYS * The let-no-escape optimization Domain: lambda.jstolarek.com, Score: 41, Comments: 1 On Reddit: [35] http://goo.gl/l8xhuP Original: [36] http://goo.gl/J8uekI * Improving Applicative do-notation Domain: tmorris.net, Score: 40, Comments: 150 On Reddit: [37] http://goo.gl/MXoPdz Original: [38] http://goo.gl/y1I8Tx * Odersky: The Trouble with Types - Strange Loop 2013 [InfoQ] Domain: infoq.com, Score: 39, Comments: 55 On Reddit: [39] http://goo.gl/5QEJfz Original: [40] http://goo.gl/Ir4kwG Top StackOverflow Questions * Haskell Thrift library 300x slower than C++ in performance test votes: 34, answers: 5 Read on SO: [41] http://goo.gl/timvYf * Creating methods bound to records in Haskell votes: 10, answers: 1 Read on SO: [42] http://goo.gl/4E4Sgh * Installing & Building GHC with OSX Mavericks GHC votes: 10, answers: 1 Read on SO: [43] http://goo.gl/44PIaj * How to properly communicate compile-time information to Template Haskell functions? votes: 10, answers: 0 Read on SO: [44] http://goo.gl/YQkGbl * Data.Text vs String votes: 9, answers: 2 Read on SO: [45] http://goo.gl/ttjnJ1 * Why is matrix multiplication faster with Repa than with hmatrix? votes: 9, answers: 1 Read on SO: [46] http://goo.gl/e36toD * Efficient functional algorithm for computing closure under an operator votes: 8, answers: 1 Read on SO: [47] http://goo.gl/538Yyr * Why doesn't Haskell sequence these IO actions properly? votes: 8, answers: 2 Read on SO: [48] http://goo.gl/82PZYb * Is there a way to make my word counting program faster without using impure tricks? votes: 8, answers: 1 Read on SO: [49] http://goo.gl/F3OiUk * Is it better to define Functor in terms of Applicative in terms of Monad, or vice versa? votes: 8, answers: 3 Read on SO: [50] http://goo.gl/xEkVIf * Standard Haskell function :: (a -> Maybe b) -> [a] -> Maybe b votes: 8, answers: 3 Read on SO: [51] http://goo.gl/JdA5Xf * Why are cabal reinstalls “always dangerous”? votes: 8, answers: 1 Read on SO: [52] http://goo.gl/kCdUsL * How to use the maybe monoid and combine values with a custom operation, easily? votes: 8, answers: 3 Read on SO: [53] http://goo.gl/BpL8NK Until next time, [54]+Daniel Santa Cruz References 1. http://www.haskellcast.com/episode/003-simon-peyton-jones-on-ghc/ 2. http://www.reddit.com/r/haskell/comments/1pe8tj/the_haskell_cast_3_simon_pe… 3. http://elm-lang.org/blog/announce/0.10.elm 4. http://www.reddit.com/r/haskell/comments/1p43xx/elm_010_released_faster_str… 5. https://www.fpcomplete.com/user/edwardk/editorial/procrustean-mathematics 6. http://www.reddit.com/r/haskell/comments/1p85al/procrustean_mathematics_by_… 7. http://www.gundersen.net/functional-programming-hurdle-uninteresting-progra… 8. http://www.reddit.com/r/haskell/comments/1oyy54/haskell_hurdle_uninterestin… 9. http://jaspervdj.be/posts/2013-10-22-websockets-0.8.html 10. http://www.reddit.com/r/haskell/comments/1p0bey/websockets08_released_with_… 11. http://gibiansky.github.io/IHaskell/ 12. http://www.reddit.com/r/haskell/comments/1oxj6k/ihaskell_haskell_for_intera… 13. http://www.shellcheck.net/about.html 14. http://www.reddit.com/r/haskell/comments/1pg7ym/shellcheck_static_analysis_… 15. http://www.reddit.com/r/haskell/comments/1pd1ep/microtutorial_liftm_by_acci… 16. http://www.reddit.com/r/haskell/comments/1pd1ep/microtutorial_liftm_by_acci… 17. http://www.reddit.com/r/haskell/comments/1p70l3/barclays_are_hiring_haskell… 18. http://www.reddit.com/r/haskell/comments/1p70l3/barclays_are_hiring_haskell… 19. http://joeyh.name/blog/entry/license_monads/ 20. http://www.reddit.com/r/haskell/comments/1plql7/license_monads/ 21. http://functorial.com/purescript/ 22. http://www.reddit.com/r/haskell/comments/1pkzd0/show_reddit_my_weekend_proj… 23. https://www.fpcomplete.com/user/edwardk/oblivious/deamortized-st 24. http://www.reddit.com/r/haskell/comments/1pnrqn/deamortized_st_by_edward_km… 25. http://community.galois.com/pipermail/halvm-devel/2013-October/000041.html 26. http://www.reddit.com/r/haskell/comments/1ot4ep/halvm_20_developers_release… 27. http://www.reddit.com/r/haskell/comments/1owtvs/could_you_explain_why_compl… 28. http://www.reddit.com/r/haskell/comments/1owtvs/could_you_explain_why_compl… 29. http://ocharles.org.uk/blog/posts/2013-10-23-pipes-talk.html 30. http://www.reddit.com/r/haskell/comments/1p1wrt/london_haskell_user_group_1… 31. http://www.cminusminus.org/ 32. http://www.reddit.com/r/haskell/comments/1pbbon/c_as_a_project_separate_fro… 33. http://jabberwocky.eu/2013/10/24/how-to-start-a-new-haskell-project/ 34. http://www.reddit.com/r/haskell/comments/1pdxcr/how_to_start_a_new_haskell_… 35. http://lambda.jstolarek.com/2013/10/let-no-escape/ 36. http://www.reddit.com/r/haskell/comments/1pi6fz/the_letnoescape_optimizatio… 37. http://tmorris.net/posts/applicative-do/index.html 38. http://www.reddit.com/r/haskell/comments/1ou06l/improving_applicative_donot… 39. http://www.infoq.com/presentations/data-types-issues 40. http://www.reddit.com/r/haskell/comments/1pjjy5/odersky_the_trouble_with_ty… 41. http://stackoverflow.com/questions/19512952/haskell-thrift-library-300x-slo… 42. http://stackoverflow.com/questions/19484232/creating-methods-bound-to-recor… 43. http://stackoverflow.com/questions/19579577/installing-building-ghc-with-os… 44. http://stackoverflow.com/questions/19679024/how-to-properly-communicate-com… 45. http://stackoverflow.com/questions/19608745/data-text-vs-string 46. http://stackoverflow.com/questions/19733382/why-is-matrix-multiplication-fa… 47. http://stackoverflow.com/questions/19485082/efficient-functional-algorithm-… 48. http://stackoverflow.com/questions/19513660/why-doesnt-haskell-sequence-the… 49. http://stackoverflow.com/questions/19535688/is-there-a-way-to-make-my-word-… 50. http://stackoverflow.com/questions/19635265/is-it-better-to-define-functor-… 51. http://stackoverflow.com/questions/19643664/standard-haskell-function-a-may… 52. http://stackoverflow.com/questions/19692644/why-are-cabal-reinstalls-always… 53. http://stackoverflow.com/questions/19706501/how-to-use-the-maybe-monoid-and… 54. https://plus.google.com/105107667630152149014/about

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Better integration with Xcode
by haskell cafe 06 Nov '13

06 Nov '13

I'm pretty new to Haskell, but have been having fun learning it. Being a Mac and Windows programmer most of my life, I'm used to using GUIs and IDEs to program and don't really care for using command-line tools. As such, I've been working on getting Xcode to build Haskell files directly and I have something working that I thought others might be interested in. Looking through the "Building/Cross-Compiling/iOS" instructions, it looks like the standard way of building Haskell files is to build them on the command line into a library and then link your C/C++/Objective-C sources against the library. But you don't need to do this. You can directly edit Haskell in Xcode (with very basic syntax highlighting), and build it from Xcode all without ever going to the command-line. Would people be interested in working this way? If so, I'll be happy to post steps to do it. -DC

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Stackage Package List
by Daniel Díaz Casanueva 06 Nov '13

06 Nov '13

Hello Haskellers. I was looking for a complete list of the packages included in Stackage, the vetted (sub)set of packages from Hackage. I couldn't find one (except by looking at the source code of Stackage [1] directly) so I made one. http://daniel-diaz.github.io/stackagelist It is automatically updated regularly, based on the GitHub repo of Stackage. As a next step, I will include also the list of packages that are not included directly on Stackage, but as a dependency of an included package. This way, it will be possible to check easily which libraries will be available after building Stackage, also having access to their documentation via links to Hackage. I was doing this for me, but I thought it could be useful for someone else, so I share it. Best regards, Daniel Díaz. -- References [1] https://github.com/fpco/stackage/blob/master/Stackage/Config.hs

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Deprecation of MonadCatchIO-transformers
by Arie Peterson 06 Nov '13

06 Nov '13

Hi all, As discussed before <http://www.mail-archive.com/haskell-cafe@haskell.org/msg104498.html>, the interface of the MonadCatchIO packages is effectively deprecated by the removal of the 'block' and 'unblock' functions in base-4.7. I would like to deprecate MonadCatchIO-transformers now. As there are quite a few reverse dependencies, and as there is at least one use case that apparently cannot be migrated easily <http://www.mail-archive.com/haskell-cafe@haskell.org/msg104501.html>, I will try to keep the package functioning, for now. In order to do this, I followed Edward Z. Yang's suggestion, and re- implemented the IO instances of block and unblock, in terms of GHC primitives. The instance now reads > import qualified Control.Exception.Extensible as E > import GHC.Base (maskAsyncExceptions#) > import GHC.IO (unsafeUnmask,IO(IO)) > > instance MonadCatchIO IO where > catch = E.catch > block = \ (IO io) -> IO $ maskAsyncExceptions# io > unblock = unsafeUnmask You can get a release candidate for a new version 0.3.1.0, by darcs get http://patch-tag.com/r/AriePeterson/MonadCatchIO-transformers I would be most grateful if someone could review and/or test this under GHC 7.8! Regards, Arie

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Unused Arrow commands do not give warning
by Tom Ellis 06 Nov '13

06 Nov '13

GHC can give you a warning when you fail to use the result of a monadic action in a do block. The same does not hold for arrow commands in arrow notation. (See demonstration below signature). What would it take to add a warning for this? I am writing arrow-heavy code at the moment and I think such a warning would be a nice feature. Tom % cat Test.hs {-# LANGUAGE Arrows #-} module Test where import Control.Arrow (returnA, arr) import Control.Monad.Identity (Identity(..)) foo :: Identity () foo = do Identity True return () bar :: () -> () bar = proc () -> do arr (const True) -< () returnA -< () % ghc -fforce-recomp -Wall Test.hs [1 of 1] Compiling Test ( Test.hs, Test.o ) Test.hs:10:5: Warning: A do-notation statement discarded a result of type Bool. Suppress this warning by saying "_ <- Data.Functor.Identity.Identity GHC.Types.True", or by using the flag -fno-warn-unused-do-bind

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Soostone Hiring In Turkey
by Ozgun Ataman 05 Nov '13

05 Nov '13

Dear Haskellers, Soostone is hiring Haskell programmers for its Turkey R&D group located in METU research park in Ankara. These position(s) are only available for local candidates with no possibility for remote work at this point in time. Interested fellow Haskellers may respond directly to me or to jobs(a)soostone.com. Soostone is an enterprise analytics company with a technology stack built entirely on Haskell. We work on interesting engineering challenges that span a multitude of topics in our area of expertise, including big-data crunching, predictive models, machine learning, data visualization and web-app development - all predominantly using Haskell. As a full-time member of our team in Ankara, you will work collaboratively with our HQ team in New York and get a chance to put your Haskell skills to use in real-world research, products and services. Soostone is also very involved with the open source community. Our team members maintain several packages on Hackage, contribute significantly to the Snap Web Framework, co-organize the New York Haskell Meetup and frequently participate in various community debates. We encourage all our team members to play an active role in the community in ways they find interesting and rewarding. Ideal candidates for this position will have: - A solid grasp of the Haskell language with an ability to produce working code from from day one (monad transformers, concurrency, streaming IO, dealing with space leaks, etc.) - Significant experience crafting software products, in academia and/or the industry - Strong presence in the OSS community with released packages in Hackage considered a big plus - Good communication skills and comfort in working as part of a team - Significant experience in a relevant application area such as compilers, distributed computing, numerics, machine learning, simulations or graphics - Experience in web application development and familiarity with web technologies - Willingness and enthusiasm to learn quickly and extend capabilities in new directions as necessary We look forward to hearing from interested candidates. The Soostone Team

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