February 2015 - Haskell-Cafe

Installing Package Candidates With Cabal ...
by aditya siram 09 Feb '15

09 Feb '15

Hi all, Is there a way to 'cabal install' a package candidate? The description at https://hackage.haskell.org/upload says it "let others install it before publishing it to the main index" which seems to indicate it can. Am I missing something? Thanks! -deech

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Building with some dependencies pre-installed in a sandbox
by Jakub Arnold 09 Feb '15

09 Feb '15

Hey guys, sorry for re-posting, but I feel that my original question is largely different from this one. I've managed to get my broken dependency to build (the mysql package), but unfortunately I had to manually change its source code to make this happen, (original post https://www.haskell.org/pipermail/haskell-cafe/2015-February/118064.html) but I do have the package installed locally (or in the sandbox of my app). The problem is that when I try to `cabal install` the whole application, it will re-download the `mysql` package from Hackage and try to build it again, instead of using the version I custom built and installed myself. Is it normal that cabal won't re-use the already existing packages, or did I somehow change it so that it won't recognize it? Also, is it possible to explicitly say where should cabal resolve some of its dependencies? Such as this case when one package is in a local path, and other should be built from hackage. I'm not really sure what the best approach is here. again sorry for posting twice in a row, but I just can't seem to find this information anywhere in the docs Thanks!

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MySQL + Windows + Haskell = ???
by Jakub Arnold 09 Feb '15

09 Feb '15

Hey guys, I've been trying to get my Yesod app working on Windows 8.1 using the Haskell Platform 2014.2.0.0, and everything seemes to work well, except for MySQL. The `mysql` package doesn't build with the following error setup.exe: The program mysql_config is required but it could not be found I've found a related GitHub issue (https://github.com/bos/mysql/issues/3) but there doesn't seem to be any solution. Is there anyone who's successfully running using Haskell and MySQL on Windows? I've also tried `hdbc-mysql` and `mysql-simple`, but they both depend on the `mysql` package, so that didn't really help. I can't really use a different database, or Linux. Thanks for any tips, Jakub

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Cabal: Missing field in record construction configProf
by Henk-Jan van Tuyl 09 Feb '15

09 Feb '15

L.S., I am trying to install a package with the new GHC 7.10 RC 2 (I recompiled cabal.exe with it, and now "cabal update" finally works again). The install fails, even after renaming the cabal config file: > cabal install Config file path source is default config file. Config file C:\Users\X\AppData\Roaming\cabal\config not found. Writing default configuration to C:\Users\X\AppData\Roaming\cabal\config Resolving dependencies... Configuring random-1.1... cabal: Distribution\Client\Config.hs:(246,37)-(299,9): Missing field in record construction configProf What does this message mean? How can I correct this? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Officially supported Android backend, GSoC?
by Ivan Perez 09 Feb '15

09 Feb '15

Hi Café For the past few years I have done quite a lot of work in functional languages targeting mobile platforms. I've tried every functional language I could find. GHC targets Android via Neurocyte's (and JoeyH's) backend [1], although, afaik, this is not officially supported. There have been recent discussions on strategies to improve the current state, especially wrt. TH. Nevertheless, the progress is amazing and the backend works very well. I haven't found any major bugs. We've successfully used this at Keera Studios to write multiple games [2,3,4], and I've also written small applications for Google Glass (yes, Haskell works on Glass!). Users are running the games on different Android devices (with different hardware and OS version), and none of them has reported any (ghc-related) bugs so far. Haskell's ability to 'write once, run anywhere' could be a major selling point. Soon Haskell might become one of the very few functional languages that can target any platform, be it web, desktop or mobile, with minimal changes to the codebase (F# can do all of this; but, apart from language differences, developing and deploying software for Android is not free as in beer). So, I'd like to ask: 1) Does officially supporting the Android backend sound like a (strategically) reasonable and viable idea? 2) What would it take? Could a Google Summer of Code project help get things up to speed? (The project proposal deadline is probably very soon.) Cheers Ivan [1] https://github.com/neurocyte/ghc-android [2] http://tiny.cc/w53rtx (Link to Facebook photo, visible without a FB account). [3] http://tiny.cc/l73rtx (Link to Facebook photo, visible without a FB account). [4] http://tiny.cc/e83rtx (Link to Facebook photo, visible without a FB account).

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Functional Programmers Sought
by Khushil Dep 07 Feb '15

07 Feb '15

I'm looking for experienced functional programmers who are happy to work with Haskell, Scala or Clojure. Please see https://hackerjobs.co.uk/jobs/2015/2/4/guardtime-experienced-functional-sof… I'm afraid I can't hire outside the UK or EU areas for this role. Very competitive salaries for the right person, occasional EU/US travel, working from home. Direct hire - I'm not an agency. You'll be working for me. Please ping me on here or privately at the e-mail address on the HackerJobs advert.

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Haskell Weekly News: Issue 316
by Daniel Santa Cruz 06 Feb '15

06 Feb '15

Welcome to issue 316 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from January 18 to 31, 2015 After doing more than 150 of these, I have come to the conclusion that it is time for me to take a permanent break from HWN. I realize that "we could do better", and have something a bit more edited than a collection of links, and some quotes. I wish I had more time to devote to such endeavor, but my time is currently being taken up by trying to help my 1 year old explore her little world. If you'd like to be the one to continue the long tradition of HWN, drop me a line. Thanks for following along for the last 3 years! Quotes of the Week * johnw: "Sir, what weapon did the assailant use against you?" "All I know is that it was done in IO, officer." * monochrom: $ can't buy you love, but it can buy you function application * hiptobecubic: benzrf, well sure. I'm not suggesting that lens has actually left any operators available for anything Top Reddit Stories * I think I've nailed it! I've solved the records problem! Domain: nikita-volkov.github.io, Score: 348, Comments: 193 Original: [1] http://goo.gl/PhJo6K On Reddit: [2] http://goo.gl/vWPeOj * Use Haskell for shell scripting Domain: haskellforall.com, Score: 121, Comments: 62 Original: [3] http://goo.gl/Fx4HeT On Reddit: [4] http://goo.gl/2g83KR * Major Prelude changes proposed Domain: haskell.org, Score: 119, Comments: 252 Original: [5] http://goo.gl/wtFWt3 On Reddit: [6] http://goo.gl/kaegj2 * Slides from Don Stewart's Google tech talk Domain: code.haskell.org, Score: 104, Comments: 75 Original: [7] http://goo.gl/ejfoFp On Reddit: [8] http://goo.gl/9cK9dB * Haskell Design Patterns: .Extended Modules Domain: jaspervdj.be, Score: 86, Comments: 29 Original: [9] http://goo.gl/TvwGCF On Reddit: [10] http://goo.gl/wkNx38 * IO Monad Considered Harmful Domain: blog.jle.im, Score: 86, Comments: 157 Original: [11] http://goo.gl/z9Elq7 On Reddit: [12] http://goo.gl/uehgF9 * We are making great efforts to spread Haskell around our local programming groups. Today, I finally saw a Haskell post, so I'm glad it is kinda working. Kinda. Domain: i.imgur.com, Score: 86, Comments: 41 Original: [13] http://goo.gl/0SsbT7 On Reddit: [14] http://goo.gl/gyBJ1o * My Haskell tooling wishlist by Chris Done Domain: chrisdone.com, Score: 77, Comments: 27 Original: [15] http://goo.gl/VIRJgM On Reddit: [16] http://goo.gl/ChmItY * Categories for Programmers: Functors Domain: bartoszmilewski.com, Score: 70, Comments: 14 Original: [17] http://goo.gl/6M64jC On Reddit: [18] http://goo.gl/tW6ubH * I wrote another monad tutorial. Sorry. Domain: kevinmahoney.co.uk, Score: 63, Comments: 22 Original: [19] http://goo.gl/zEQIgS On Reddit: [20] http://goo.gl/XXeZN3 * Introducing SJS, a type inferer and checker for JavaScript (written in Haskell) Domain: noamlewis.wordpress.com, Score: 61, Comments: 33 Original: [21] http://goo.gl/oQRSNo On Reddit: [22] http://goo.gl/1LfY1u * Solving the expression problem with Object Algebras and Tagless Interpreters Domain: oleksandrmanzyuk.wordpress.com, Score: 59, Comments: 30 Original: [23] http://goo.gl/daJWGy On Reddit: [24] http://goo.gl/fKy9zV Top StackOverflow Questions * Why do we need monads? votes: 142, answers: 5 Read on SO: [25] http://goo.gl/Ioviv9 * Why compiling this Haskell program with -fllvm produces a different result? votes: 19, answers: 0 Read on SO: [26] http://goo.gl/twid2N * What is the difference between traits in Rust and typeclasses in Haskell? votes: 18, answers: 0 Read on SO: [27] http://goo.gl/KJdBqh * Motivation behind Phantom Types? votes: 17, answers: 3 Read on SO: [28] http://goo.gl/q7IQQC * What is the precedence of ~, and why? votes: 17, answers: 1 Read on SO: [29] http://goo.gl/iZHxLy * Using GHC API to compile Haskell sources to CORE and CORE to binary votes: 13, answers: 2 Read on SO: [30] http://goo.gl/Qto29m * How to ship an executable with Language.Haskell.Interpreter? votes: 13, answers: 0 Read on SO: [31] http://goo.gl/JptPyG Until next time, [32]+Daniel Santa Cruz References 1. http://nikita-volkov.github.io/record/ 2. http://www.reddit.com/r/haskell/comments/2svayz/i_think_ive_nailed_it_ive_s… 3. http://www.haskellforall.com/2015/01/use-haskell-for-shell-scripting.html 4. http://www.reddit.com/r/haskell/comments/2u6b8m/use_haskell_for_shell_scrip… 5. https://www.haskell.org/pipermail/libraries/2015-January/024777.html 6. http://www.reddit.com/r/haskell/comments/2ttvsj/major_prelude_changes_propo… 7. http://code.haskell.org/~dons/talks/dons-google-2015-01-27.pdf 8. http://www.reddit.com/r/haskell/comments/2u0380/slides_from_don_stewarts_go… 9. http://jaspervdj.be/posts/2015-01-20-haskell-design-patterns-extended-modul… 10. http://www.reddit.com/r/haskell/comments/2t4lba/haskell_design_patterns_ext… 11. http://blog.jle.im/entry/io-monad-considered-harmful 12. http://www.reddit.com/r/haskell/comments/2tbbxh/io_monad_considered_harmful/ 13. http://i.imgur.com/Rw8mmad.jpg 14. http://www.reddit.com/r/haskell/comments/2tg9gs/we_are_making_great_efforts… 15. http://chrisdone.com/posts/haskell-wishlist 16. http://www.reddit.com/r/haskell/comments/2tpwim/my_haskell_tooling_wishlist… 17. http://bartoszmilewski.com/2015/01/20/functors/ 18. http://www.reddit.com/r/haskell/comments/2t2g5a/categories_for_programmers_… 19. http://kevinmahoney.co.uk/articles/haskell-io/ 20. http://www.reddit.com/r/haskell/comments/2trmyx/i_wrote_another_monad_tutor… 21. https://noamlewis.wordpress.com/2015/01/20/introducing-sjs-a-type-inferer-a… 22. http://www.reddit.com/r/haskell/comments/2szt1u/introducing_sjs_a_type_infe… 23. https://oleksandrmanzyuk.wordpress.com/2014/06/18/from-object-algebras-to-f… 24. http://www.reddit.com/r/haskell/comments/2stmt6/solving_the_expression_prob… 25. http://stackoverflow.com/questions/28139259/why-do-we-need-monads 26. http://stackoverflow.com/questions/28053448/why-compiling-this-haskell-prog… 27. http://stackoverflow.com/questions/28123453/what-is-the-difference-between-… 28. http://stackoverflow.com/questions/28247543/motivation-behind-phantom-types 29. http://stackoverflow.com/questions/28257511/what-is-the-precedence-of-and-w… 30. http://stackoverflow.com/questions/28059669/using-ghc-api-to-compile-haskel… 31. http://stackoverflow.com/questions/28204973/how-to-ship-an-executable-with-… 32. https://plus.google.com/105107667630152149014/about

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Re: [Haskell-cafe] recursion problem part 2
by Roelof Wobben 06 Feb '15

06 Feb '15

Jerzy Karczmarczuk schreef op 6-2-2015 om 16:57: > > Le 06/02/2015 16:48, Roelof Wobben a écrit : >> Im only sure that somehow I take the wrong turn somewhere because I >> cannot figure out why I do not get the right answer. >> >> I think I have the value of n wrong. > Did you read my answer entirely? Do so, find the red fat line > > You take n=1, and you write: n<10 not True. > You don't need to be a specialist on recursion, to see the error. > > Jerzy > oke, Another try : toDigits :: Integer -> [Integer] toDigits n | n < 0 = [] | n < 10 = [n] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] isDigits 1 n < 0 not true. n < 10 true so [1] IsDigits 12 n < 0 not true n < 10 not true toDigits 1 + [2] toDigits 1 + [2] n < 0 not true n < 10 true [1] ++ [2] 1 ++2 = [1,2] I think I understand it finnaly. Roelof

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hflags
by Maurizio Vitale 06 Feb '15

06 Feb '15

with ghc 7.8.4 and hflags 0.4 the following program: {-# LANGUAGE TemplateHaskell #-} module Main where import HFlags defineFlag "f" True "A flag." --defineFlag "g" True "Another flag." main = do _ <- $initHFlags "flags demo" print Main.flags_f gives me a runtime error about the flag f not being found, unless I add the declaration of a second flag. It seems like the last flag definition is not seen. Any idea/suggestion? Thanks, Maurizio

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Re: [Haskell-cafe] recursion problem part 2
by Roelof Wobben 06 Feb '15

06 Feb '15

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