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GHC 7.10 Prelude: we need your opinion
by Simon Peyton Jones 10 Feb '15

10 Feb '15

Haskell Friends This email asks for your help in deciding how to proceed with some Prelude changes in GHC 7.10. Please read on, but all the info is also at the survey link, here: http://goo.gl/forms/XP1W2JdfpX. Deadline is 21 Feb. The Core Libraries Committee (CLC) is responsible for developing the core libraries that ship with GHC. This is an important but painstaking task, and we owe the CLC a big vote of thanks for taking it on. For over a year the CLC has been working on integrating the Foldable and Traversable classes (shipped in base in GHC 7.8) into the core libraries, and into the Prelude in particular. Detailed planning for GHC 7.10 started in the autumn of 2014, and the CLC went ahead with this integration. Then we had a failure of communication. As these changes affect the Prelude, which is in scope for all users of Haskell, these changes should be held to a higher bar than the regular libraries@ review process. However, the Foldable/Traversable changes were not particularly well signposted. Many people have only recently woken up to them, and some have objected (both in principle and detail). This is an extremely unfortunate situation. On the one hand we are at RC2 for GHC 7.10, so library authors have invested effort in updating their libraries to the new Prelude. On the other, altering the Prelude is in effect altering the language, something we take pretty seriously. We should have had this debate back in 2014, but here we are, and it is unproductive to argue about whose fault it is. We all share responsibility. We need to decide what to do now. A small group of us met by Skype and we've decided to do this: · Push back GHC 7.10's release by at least a month, to late March. This delay also gives us breathing space to address an unrelated show-stopping bug, Trac #9858. · Invite input from the Haskell community on which of two approaches to adopt (this survey<http://goo.gl/forms/XP1W2JdfpX>). The main questions revolve around impact on the Haskell ecosystem (commercial applications, teaching, libraries, etc etc), so we want to ask your opinion rather than guess it. · Ask Simon Marlow and Simon Peyton Jones to decide which approach to follow for GHC 7.10. Wiki pages have been created summarizing these two primary alternatives, including many more points and counter-points and technical details: · Overall summary: https://ghc.haskell.org/trac/ghc/wiki/Prelude710<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> · Details of Plan List: https://ghc.haskell.org/trac/ghc/wiki/Prelude710/List<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> · Details of Plan FTP: https://ghc.haskell.org/trac/ghc/wiki/Prelude710/FTP<https://www.google.com/url?q=https%3A%2F%2Fghc.haskell.org%2Ftrac%2Fghc%2Fw…> This survey invites your input on which plan we should follow. Would you please · Read the details of the alternative plans on the three wiki pages above · Add your response to the survey<http://goo.gl/forms/XP1W2JdfpX> Please do read the background. Well-informed responses will help. Thank you! DEADLINE: 21 February 2015 Simon PJ

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How does RTS judge that a STM transaction is blocked?
by Magicloud Magiclouds 10 Feb '15

10 Feb '15

Hi, I met an issue while I was using xhb. When a certain property of root window being changed, it was supposed to give me an AccessError. But actually I got "thread blocked indefinitely in an STM transaction" when I waitForEvent. AFAIK, in xhb, it forks a thread to read what X server sends. Then parse and put it into a TChan. waitForEvent reads from the TChan. It is reasonable to me that if X server did not send anything, then waitForEvent would be blocked. But apparently there are some other policies in RTS. So what is that? And how should I debug it? -- 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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GHC-7.8.4 for Mac OS X
by Ozgur Akgun 09 Feb '15

09 Feb '15

Hi, Does anyone plan to put a binary release of GHC-7.8.4 for OS X? There isn't one here: https://www.haskell.org/ghc/download_ghc_7_8_4 Cheers! Ozgur

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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 07 Feb '15

07 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 07 Feb '15

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

06 Feb '15

Dmitry Olshansky schreef op 6-2-2015 om 16:25: > toDigits :: Integer -> [Integer] > toDigits n > | n < 0 = [] > | n < 10 = [n] > | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] > Thanks that worked, I have now to figure out why I see [0] on 0 and no zero when it reaches it end-point. isDigits 0 n< 10 so [0] IsDigits 1 n < 0 not true n < 10 not true. toDigits 1 ++ [0] toDigits 1 + [0] n< 0 not true n < 10 true so [1] ++ [0] [1] ++ [0] is [1.0] where the output is [1] So somehow I do not understand fully how this works . Roelof

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recursion problem part 2
by Roelof Wobben 06 Feb '15

06 Feb '15

Hello, I partly solved the problem of cis194 of turning 123 into [1,2,3] But I ran into a problem I cannot solve . When I have this : toDigits :: Integer -> [Integer] toDigits n | n < 0 = [] | n == 0 = [] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] toDigits 123 gives [ 1,2,3] but toDigits 0 gives [] where I expect to be [0] So I change the code to this : toDigits :: Integer -> [Integer] toDigits n | n < 0 = [] | n == 0 = [0] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] Now toDigits 0 gives [ 0] but toDigits 123 gives now [ 0, 1 ,2, 3] where I expect to be [1,2,3] Can someone give me a tip how to solve this I have asked this on the beginners ML but the only answer I get was someone who was using some if -then statement and I think I can be solved without. Roelof

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error compiling 24 Days of Hackage: heist
by hokum 06 Feb '15

06 Feb '15

Hello, I was trying to compile an example of heist (a template engine) usage from 24 Days of Hackage: heist, but ended up with an error message: src/Main.hs:60:7: `hcTemplateLocations' is not a record selector In the first argument of `initHeist', namely `mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}' In a stmt of a 'do' block: heist <- initHeist (mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}) In the second argument of `($)', namely `do { heist <- initHeist (mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}); Just (output, _) <- renderTemplate heist "billy"; liftIO . BS.putStrLn . toByteString $ output }' src/Main.hs:61:7: `hcInterpretedSplices' is not a record selector In the first argument of `initHeist', namely `mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}' In a stmt of a 'do' block: heist <- initHeist (mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}) In the second argument of `($)', namely `do { heist <- initHeist (mempty {hcTemplateLocations = [loadTemplates "templates"], hcInterpretedSplices = defaultInterpretedSplices}); Just (output, _) <- renderTemplate heist "billy"; liftIO . BS.putStrLn . toByteString $ output }' It's probably something to do with lenses, but I'm not very familiar with it. Four hours of googling didn't give me anything on the subject, so any help? Here is the code(in the state I left it): module Main ( main ) where import Blaze.ByteString.Builder (toByteString) import qualified Data.ByteString.Char8 as BS import Control.Monad.IO.Class (MonadIO(..)) import Control.Monad.Trans.Either import Data.Monoid (mempty) import Data.Foldable (forM_) import Heist import Heist.Interpreted import Text.XmlHtml (Node(TextNode), renderHtmlFragment, Encoding(UTF8)) billy :: IO () billy = eitherT (putStrLn . unlines) return $ do heist <- initHeist mempty { hcTemplateLocations = [ loadTemplates "templates" ] , hcInterpretedSplices = defaultInterpretedSplices } Just (output, _) <- renderTemplate heist "billy" liftIO . BS.putStrLn . toByteString $ output main = do putStrLn "---"

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Domain specific error messages
by Sean Seefried 06 Feb '15

06 Feb '15

If the list doesn't mind I'm reposting my reply to Alberto G. Corona in under the thread "Monads: external questions" as a new message since the topic has changed enough. ------------- Hi Alberto, I've been interested in domain specific error messages for years and I agree with you that it is one of the major things holding back the utility of DSLs to novice programmers. This is a shame since one of the touted benefits of DSLs is that they *can* be used by novices with a minimum of training, which is simply not true in the presence of error messages that require detailed knowledge of Haskell to understand. I'd caution against saying that there is a lack of interest in that ticket you linked to. It's still a research level problem despite the fact that some great work has been done on it already. Incidentally, the author of "Scripting the Type Inference Process" went on to do an entire PhD on the topic entitled "Top Quality Type Error Messages"[2]. I recently wrote him an email and he told me that the constraint-based type inference framework that he used in the thesis, TOP, is available on Hackage [3]. Recently I noticed that this problem is already being worked on in Idris. See "Reflect on your Mistakes!" [4] and some code on GitHub [5]. Perhaps we can get people interested in this feature again? Cheers, Sean [1] http://www.open.ou.nl/bhr/heeren-scripting.pdf [2] http://www.open.ou.nl/bhr/TopQuality.pdf [3] https://hackage.haskell.org/package/Top [4] http://www.itu.dk/people/drc/drafts/error-reflection-submission.pdf [5] https://gist.github.com/david-christiansen/8349698 On Sun Nov 23 2014 at 3:57:10 AM Alberto G. Corona <agocorona(a)gmail.com> wrote: > Michael: > > You are right, but these are minor problems I think, compared with the > huge potential advantages. > > I can not believe it when a slow immature language like Ruby could take > over web development just for one library, Rails and some buzzwords, when a > faster, safer language can do it millions of times better. Haskell can > revolutionize all the industry simply selling it not as one more language, > but as THE meta-language for building EDSLs for each domain problem. some > EDSLs so close to the domain problem that can be used by non-programmers. > > That lack of vision and effort in the side of the haskell community hurts > me. And the lack of interest in this ticket > > https://ghc.haskell.org/trac/ghc/ticket/7870 > > Is a clear display of this lack of interest. it is like the Aristocracy > of the Haskell Wondwerland fears to be hijacked by hordes of mediocre DSL > villains from the industry, so it is necessary to keep the walls high > > Haskell is a language dominated by academics that has no interest in the > success of Haskell. On the contrary. > > 2014-10-29 15:13 GMT+01:00 Michael Jones <mike(a)proclivis.com>: > >> When I took a Lambda Calculus class years ago in Silicon Valley, 90% of >> the students groaned and complained. They just wanted to learn Java and >> make money. Having a background in OO design and experience with Eiffel, I >> was intrigued and stuck with it, building some tools with ML, and later >> Haskell. >> >> In the workplace it was near impossible to avoid the .Net culture, and >> most of my code has been C#. But the factors that mattered were: >> >> - Continuity with past languages and tools >> - Availability of programmers >> - Third party libraries >> - Inter langage operability >> - Reuse of legacy code >> >> etc >> >> Best I can tell, there is no way to avoid the business context. I suggest >> that if you have freedom, you need to be multilingual. Many systems could >> benefit from applying the proper tool to the corresponding problem. >> >> But I will say this, becoming proficient at Haskell really improved my >> designs by providing an alternative conceptual framework. But, it had a >> very substantial learning curve. All I can say is trust that even if your >> core language is procedural, you will be better at that for learning a >> functional language. >> >> To make Haskell a first class citizen in the IT shops, I think focus >> would have to shift more to the business context and needs. And certainly >> more focus in the universities that are still dominated by procedural >> languages. Once that is drilled into ones head, it affects the way one >> thinks. >> >> To give an example, I have these problems: >> >> - Update to GHC 7.8.3 from 7.6 caused run time behavior changes breaking >> USB application >> - Sandboxes are not completely isolated from the core library and often >> builds break >> - Most new grads don’t even know what a functional language is >> - Documentation gets out of sync with releases (where documentation means >> Wiki and web) >> - FFI is difficult to use and debug >> - Lack of books, user groups, etc >> >> Mike >> >> On Oct 29, 2014, at 3:48 AM, Alberto G. Corona <agocorona(a)gmail.com> >> wrote: >> >> I know that I'm using a different language when talking about monads. The >> language of the IT industry. >> >> Many haskellers use the language for toy programming. Others are >> professional academics. The few that use the language for commercial >> purposes are too busy developing practical applications rather than >> thinking deep about how to apply the haskell concepts to their problems. >> As a result many of such problems remains essentially unsolved. These busy >> developers try to transcode solutions from other languages that lack the >> deep and expressiveness of Haskell. >> >> This lack of interest in one side and the lack of time in the other is >> disappointing. The symptoms are everywhere. Particularly, I find it in the >> lack of support and interests for this ticket: >> >> https://ghc.haskell.org/trac/ghc/ticket/7870 >> >> I though that there was definitively a shift from "avoid success at all >> costs" a few years ago, for a commitment for the success, but still there >> are many minds to change, especially the brilliant ones. >> >> 2014-10-26 2:02 GMT+01:00 Alberto G. Corona <agocorona(a)gmail.com>: >> >>> >>> >>> 2014-10-26 1:23 GMT+02:00 Jeffrey Brown <jeffbrown.the(a)gmail.com>: >>> >>>> As opposed to the internal logic of monads, how they work, I hope to >>>> start a discussion about their external logic: how and why to use monads. >>>> >>>> design >>>> ------ >>>> How do monads change the way one >>>> * thinks about a problem? >>>> * structures data? >>>> * refactors? >>>> * tests? >>>> Should I always be giving the monads a lot of cognitive bandwidth, >>>> because they reorder the way everything should be, or is it an investment >>>> with a high initial cognitive cost but requiring little maintenance >>>> thereafter? >>>> >>>> what is their common framework? >>>> ------------------------------- >>>> Monads let data reach farther than it otherwise would. Subjectively, >>>> they feel like a controlled way of violating encapsulation. >>>> >>>> Are there other, deeper or more specific, commonalities that explain >>>> why monads are a good way to implement exceptions, output, state, and >>>> perhaps other services? >>>> >>> >>> I made monads for execution state recovery, web navigation.. workflows, >>> long running transactions, backtracking, traceback and event chaining in >>> web browser applications. >>> >>> I´m confident that the perspectives for monads to solve real IT problems >>> are very promising. And when I mean monad I mean all the associated stuff : >>> applicative, alternative etc. >>> >>> I´m confident that there will be a cloud monad (for chaining jobs and >>> work distribution) an orchestration monad for orchestration of web services >>> etc. >>> >>> There are problems that are intrinsically procedural among them, almost >>> all problems in IT. instead of using ad-hoc data/control structures like >>> events, handlers, configurations, routes, exceptions, logs, transaction >>> compensations, promises ....the list goes on and on , the monad is the >>> common control structure that can subsume all of them inside his >>> programmable semicolon >>> >>> So, once the monad is set up, the user of the monad code the solution >>> for the domain problem in a clean EDSL with absolutely no plumbing, at the >>> level of the problem. so anyone that know the problem can understand the >>> code. >>> >>> Is the monad instance, and the applicative etc the ones that subsume >>> under the hood the special data/control structure necessary for the domain >>> problem. >>> >>> >>> Often if your code is general enough, it can be used in any monad. So >>> you benefit from this. I think that in th future there will be a lot of >>> surprises about the shareability of code between monads when the IT >>> industry start to use them seriously. I think that we are just at the >>> beginning. >>> >>> I hope that some others of your questions are also answered here >>> >> >> >> >> -- >> Alberto. >> >>

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Are conditional changes in library interfaces problematic?
by Adam Bergmark 06 Feb '15

06 Feb '15

transformers 0.4 deprecated ErrorT in favor of ExceptT. I've noticed that several packages use CPP to conditionally add ExceptT instances, like this: ``` #if MIN_VERSION_transformers(0,4,0) instance C m => C (ExceptT e m) where #endif ``` The problem I've found with this is that I will get a different interface depending on which transformers version is picked. There is a transformers-compat[1] library that provides ExceptT when using older versions of transformers, when combining these I end up with a compilation failure since the ExceptT instances are no longer included due to CPP. The alternatives I see are: 1. Use transformers-compat (and as of today mtl-compat[2]) 2. Drop support of older versions of dependencies 3. Advise against compat packages The first alternative seems preferable from a user's standpoint. For transformers and mtl I argue it's the most pragmatic option. I think the compat packages are a good thing, especially for core packages, since they let us maintain backwards compatibility. Do you agree with this line of thought? I started working on pull requests to "fix" this but I'd appreciate feedback on the issue first. Along these lines, does conditionally altering the interface of a package like this violate the PVP? Cheers, Adam [1] http://hackage.haskell.org/package/transformers-compat [2] http://hackage.haskell.org/package/mtl-compat

1 0

ghci on ARM m3?
by Mike Meyer 06 Feb '15

06 Feb '15

Lately, I've not been doing much Haskell because I've buried in embedded systems work. The code is all C, with only a smidgen of C++ (for which I am grateful). One of the things I've been toying with is turning part of the on-chip configuration information a haskel functions. They'd even be pure code, and would be a lot more expressive than what's there now. The target platforms are running Cortex M3 CPUs. I know that jhc/ajhc will generate code for those CPUs, but I really need at least a subset of the compiler running on the target processor with no pre-installed RTOS. Has that been done? Is it likely to even be possible? As part of the project, I'll need to install both a cross-compiler on my Unix box, and the the compiler (or parts of it) on the M3. Can someone provide pointers to tutorials on that? My google-foo wasn't powerful enough to find any. I'm thinking about using either an Aruino Due (as it has the M3 CPU on it) as a testbed platform. So tutorials targeted for that platform would be particularly appreciated. Since I hopefully have the eyes of people experienced with these things, should I be considering a raspberry pi or something similar instead of the Due? Basically, will I be able to run code built for the M3 CPU on the pi CPUs? thanks, Mike

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ANNOUNCE: bash script for haskell
by James Crayne 06 Feb '15

06 Feb '15

I'm sure variations of this exist, but for some reason I didn't find any while searching, so I made my own. The idea is for using haskell on the command line instead of perl or awk etc. https://github.com/jimcrayne/hk

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ANNOUNCE: UISF Release
by Daniel Winograd-Cort 05 Feb '15

05 Feb '15

I'm happy to announce the release of UISF on Hackage! http://hackage.haskell.org/package/UISF UISF provides an arrowized FRP library for graphical user interfaces. It stems from work done on Euterpea (http://haskell.cs.yale.edu/euterpea/) and is built atop GLFW. One of the neat things about UISF is how it behaves with ArrowChoice. Specifically, one can create dynamic arrows so long as they are *predictably* dynamic. This means that it is possible to have widgets activate, deactivate, and even replicate without resorting to switch. If you're interested in playing around with it, feel free to take a look at the example files, some of which are pretty well documented (note that there are more than just the two displayed on the hackage page). Do let us know about comments you have and bugs you find. -Daniel

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Re: [Haskell-cafe] stuck at the first part of chapter 1 of the CIS course
by Roelof Wobben 05 Feb '15

05 Feb '15

1 0

ghc --make Vs runhaskell
by Madhu Babu 05 Feb '15

05 Feb '15

Hi !, I have the following program from “Learn You a Haskell” ( http://learnyouahaskell.com/input-and-output#randomness ). When i run this using runhaskell, code works perfectly fine. However, when i compile and run the binary file, code behaves differently. Basically, in the following code, we print a line first and the read the line from stdin. This behavior works perfectly fine when using “runhaskell guess.hs”. But when i say “./guess”, i had to type in a number directly ( i.e. first prompt is skipped ). btw.. i am using mac yosomite command terminal. Any help greatly appreciated.. Thanks a lot in advance.. import System.Random import Control.Monad(when) main = do ranGen <- getStdGen let (rand,_) = randomR (1,10) ranGen :: (Int,StdGen) putStr "Guess a number between 1 and 10 : " numStr <- getLine when(not $ null numStr) $ do let num = read numStr if rand == num then putStrLn "Yuhaa!! you are right baby!" else putStrLn $ "Sorry dude!! it was " ++ show rand newStdGen main

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Re: [Haskell-cafe] learn you a good haskell exercises
by Roelof Wobben 04 Feb '15

04 Feb '15

1 0

learn you a good haskell exercises
by Roelof Wobben 04 Feb '15

04 Feb '15

Hello, If I want to look if I understand the contents of the book , are these good exercises : https://github.com/noelmarkham/learn-you-a-haskell-exercises Roelof

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Haskell opportunities at Facebook
by Bryan O'Sullivan 04 Feb '15

04 Feb '15

Hi, friends – I have a number of very interesting openings at Facebook HQ in Menlo Park, California. There are two different teams hiring. The first set of positions are for an entirely new team. This project involves distributed systems, data mining, and machine learning. There may be roles on this team for less experienced candidates in a few months, but right now we are looking for people who have written a reasonable amount of Haskell, have built real production systems in some language or other (and have the scars to prove it), and can contribute in major ways to the design and construction of a demanding new system that we're building from scratch. The second set of positions are for a cousin team, which is building on the success of our Haxl project to extend our capability to fight spam and malware. For these roles, we're open to a broader range of experience levels. If you're interested, please drop me an email with a current CV. Cheers, Bryan.

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hiring fp engineers for information security systems
by Khushil Dep 04 Feb '15

04 Feb '15

About Guardtime We are team of US and Estonian scientists, network architects, software developers and security specialists who have decades of experience building cyber security solutions in our nations' military, supporting national objectives for mission and information assurance. In 2007 we invented Keyless Signature Infrastructure (KSI), a block-chain security technology designed for real-time detection and mitigation of cyber attack. About The Role We're looking for experienced functional software engineers to work on real-time backend systems dealing with high volume events. We need people who want to build systems to apply the power of workflow processing to security events to build deep intelligence platforms. We want people who understand the benefits of converged understanding over imperative understanding. About You - You should have an appetite for tackling complex and interesting software challenges. - Want to build systems that handle data on a global scale in a timely manner. - Have a strong understanding of Linux/UNIX kernel operations - Understand the value of documentation. - Be able to disseminate complex ideas to a wider audience. - Prepared to work in an agile but structured manner. - Prepared to travel in the E.U. - Prepared to travel to the U.S.A. on a quarterly basis. Minimum Qualifications - BS in Computer Science, Mathematics or Physics or equivalent work and life experience. - Clojure, Scala, Java, C/C++ Programming experience. - Large systems software design and development experience, with knowledge of Unix/Linux. Preferred Qualifications - MS or PhD in Computer Science, Mathematics or Physics. - Strong FP skills. - Strong competencies in data structures, algorithms, and software design. - Haskell, Clojure, Scala, Java, C/C++, NodeJS, Python programming experience, database design, knowledge of TCP/IP and network programming. Application Process In the first instance please send your C.V. and GitHub username (if you have one) to khushil.dep(a)guardtime.com. We have a two stage interview process and we hire quickly.

1 0

GHC API - list all exports of a module
by Jochen Keil 02 Feb '15

02 Feb '15

Dear Cafe, currently I'm working on a small program for which I need a list of all *exported* function and data types. In order to achieve this I've been toying around with the GHC API. My observations so far: * Using the pure `parser` function yields a `HsModule` object, which gives me a list of all exports and all declarations (internal & exported). I cannot or don't know how to use the list of exports (just `RdrName`s) to filter the declarations. * Using `parseModule` (which is essentially the same as the `parser` function) and `typecheckModule` on a `ModuleGraph` yields a `TypecheckedModule` object which contains a list of exported `Name`s which in turn can be used to get additional information about them. So, the second approach seems more promising, but I'm not too happy with it. The reason is that calling `typecheckModule` results in a compilation of the module and all of its dependencies, which I think is not necessary for what I want to achieve. This is especially cumbersome for large modules like `GHC.hs` where I'm struggling to figure out all of the dependencies without the build system. My main goal would be to have a program which takes two modules of different versions, compare them and highlight the differences between them. I think that'd be helpful for upgrading dependant packages or for writing compatibility layers. Thank you and all the best, Jochen

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Cabal config file - default install flags?
by Lucas Paul 02 Feb '15

02 Feb '15

I'd like to have cabal automatically use -j8 when I cabal install something. Is there something I can put into the .cabal/config file that will accomplish this? More generally, where can I find documentation on that config file? - Lucas Paul

2 2

Template Haskell attempts to load mingw32.dll during build
by Darren Grant 02 Feb '15

02 Feb '15

Hi all, Windows GHC 7.8.4 build issue here: I've noticed that GHC will attempt to link mingw32 if a related dependency (in my case sdl2) is required in a TH pass. Unsurprisingly, the OS fails to find the imaginary mingw32.dll file. Are there any known resolutions to this quirk? Cheers, Darren

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What could be the possible reason of linking errors on "_info" and "_closure"?
by Magicloud Magiclouds 02 Feb '15

02 Feb '15

Hi, I am making a cabal project including a library and a executable using the library. Building the library is fine. But when linking src/main, I got "undefined reference to someFunction1_info" and "someFunction1_closure". -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

1 1

ANNOUNCE: monad-levels
by Ivan Lazar Miljenovic 02 Feb '15

02 Feb '15

I'm pleased to announce the first release of my new monad-levels library (aka Yet Another Monad Transformer Library ;-) http://hackage.haskell.org/package/monad-levels I've written more about the motivations of the library here: https://ivanmiljenovic.wordpress.com/2015/02/02/monadic-yak-shaving/ -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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parallelism and purity
by Kazu Yamamoto 02 Feb '15

02 Feb '15

Hi cafe, I received questions about parallelism in Haskell. I'm familiar with concurrency but not with parallelism. So, let me ask: Q1) Are there any *applications* using Haskell parallelism? Q2) What kind of parallelism is used in the application? - Task parallel - Data parallel - Data-flow paralle? - etc Q3) Does Haskell purity contribute for the application in the parallelism point of view? I'm one of the translators of "Parallel and Concurrent Programming in Haskell" into Japanese. So, please don't say read the book. :-) --Kazu

1 0

cpp question
by S. Doaitse Swierstra 01 Feb '15

01 Feb '15

With the new version of GHC (7.8.3) I am running into problems with cpp. The header of one of the files of my uu-parsinglib library reads as follows: {-# LANGUAGE NoMonomorphismRestriction, RankNTypes, FlexibleContexts, CPP #-} #define DEMO(p,i) demo "p" i p #define DEMOG(p,i) demo "p" i (mkP (p)) module Text.ParserCombinators.UU.Demo.MergeAndPermute where ... However when I try to compile this file I get error messages like: Text/ParserCombinators/UU/Demo/Demo.hs:88:17: Not in scope: data constructor ‘DEMOG’ So it seems that the cpp is no longer called at all. Any hints? I am running: MacBook-Doaitse:src doaitse$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.3 MacBook-Doaitse:src doaitse$ gcc --version Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) Target: x86_64-apple-darwin14.1.0 Thread model: posix MacBook-Doaitse:src doaitse$ Any help is appreciated, Doaitse Swierstra

3 2

Side conditions on Data.Profunctor.Choice?
by David Turner 31 Jan '15

31 Jan '15

Hi, I was expecting to see coherence conditions for instances of Data.Profunctor.Choice along the lines of: dimap (either (Left . f) Right) id . right' === dimap id (either (Left . f) Right) . right' and similarly for left', but there's nothing like that in the docs. Does anyone know whether (a) those conditions aren't true in general, or (b) they are true but provable from other theorems so not worth mentioning? Many thanks, David

1 0

ANN: processor-creative-kit
by Takenobu Tani 31 Jan '15

31 Jan '15

Dear Haskellers, I am pleased to announce the first release of processor-creative-kit package for playing processors. You can create your processors with your own instruction set. It simulates a simple microprocessor(cpu) with some development tools. https://hackage.haskell.org/package/processor-creative-kit https://github.com/takenobu-hs/processor-creative-kit feature: * easy try, easy modify * a purely functional CPU core (without IO) * including a very simple prototype assembler * including a very simple prototype profiler * including a very simple prototype debugger Thank you, Takenobu

1 0

Using m4 as a preprocessor
by Francesco Ariis 30 Jan '15

30 Jan '15

Hello, today I tried to compile a Haskell program using m4 as preprocessor, with the {-# OPTIONS_GHC -F -pgmF m4 #-} pragma on top of a .hs file. I didn't add anything else, there are no ''' or '`' in the source. Upon compiling, ghc (7.8.3) complains: m4: cannot open `/tmp/ghc10655_0/ghc10655_1.hspp': No such file or directory I searched the net for a solution, but no dice (apparently there are no Haskell programs using -F -pgmF m4?), so I am writing here. Any ideas on what is wrong?

3 3

Creating a list with do notation
by Cody Goodman 30 Jan '15

30 Jan '15

List is a monad, does that mean i can create a list with do notation? My intuition led me to believe this would work: main = do x <- return $ do 1 2 3 print (x :: [Int]) However it didn't. Is this possible?

4 3

Compiling Setup.hs with -threaded
by Michael Orlitzky 29 Jan '15

29 Jan '15

I found a weird bug at, https://bugs.gentoo.org/show_bug.cgi?id=537500 There is an awesome tool called shellcheck: git clone https://github.com/koalaman/shellcheck.git and we want to run its test suite. Basically: * If you run the test suite using runghc, then it works fine. * If you compile Setup.hs to setup, then the test suite hangs. * But if you compile Setup.hs to setup using -threaded, it works? Normally I would blabber on about what I think the problem could be, but the WTF factor here is just too high. Should we (always?) be compiling Setup.hs with -threaded? Or is this a bug in the runtime or a library or REALITY ITSELF?

2 1

Pure functional and pure logical language at the same time
by Timotej Tomandl 29 Jan '15

29 Jan '15

This question was bugging me for quite a long time. Can we have a language which uses the functional logic while being both pure functional and pure logical? Do we get any advantages from maintaining both both of this purities at the same time? P.S.: I have feeling the answer is no, but I am not sure.

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Fwd: Announcing a solution to the records problem
by Christopher Done 29 Jan '15

29 Jan '15

Forwarding to Haskell-cafe because not as many people read libraries@. ---------- Forwarded message ---------- From: Nikita Volkov <nikita.y.volkov(a)gmail.com> Date: 19 January 2015 at 05:54 Subject: Announcing a solution to the records problem To: Haskell Libraries <libraries(a)haskell.org> This thing seems to be going viral on social networks already, nonetheless here's a link for those of you, who aren't yet informed: http://nikita-volkov.github.io/record/ _______________________________________________ Libraries mailing list Libraries(a)haskell.org http://www.haskell.org/mailman/listinfo/libraries

2 1

Representation of lenses
by David Turner 28 Jan '15

28 Jan '15

Hi, I'm planning on discussing lenses with some colleagues shortly so I'm consolidating some of my notes into a coherent story. At one point I found myself describing lenses as a way of packaging up a getter and a setter into a single thing. However, this raises the question: why not just use a pair (getter, setter)? More precisely, I believe that the types (a -> s, s -> a -> a) and (forall f. Functor f => (s -> f s) -> a -> f a) are isomorphic. Is that right? I see that one advantage of the lens type is that you can use (.) to compose them since they're just functions, but that doesn't bother me much and it seems I could define another operator to compose (getter, setter) lenses and the rest of the machinery would work either way. It's also possible that you can't get the full generality of (forall f. (s -> f t) -> a -> f b) lenses with getter/setter pairs, although I haven't worked through the details yet so don't know either way. So, my question is: what is the advantage of representing lenses in the way that they are? Many thanks, David

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Using GHC API to compile Haskell sources to CORE and CORE to binary
by Wojciech Danilo 28 Jan '15

28 Jan '15

Hello All! :) Recently I've came across a problem and hopefully you could help me with it. Basically I and some people here are running a startup and we have created a language for processing images. This language has 2 interchangeable representations - textual and visual, dataflow one. Right now we are compiling it to haskell sources, but we want to move over GHC CORE. Getting into GHC has a high learning curve, as far as we see, and we would be very thankfull for any help with a minimal working example of compiling haskell sources to CORE and then CORE to binaries. I've posted couple days ago a detailed question on StackOverflow, here: http://stackoverflow.com/questions/28059669/using-ghc-api-to-compile-haskel… I would be very thankful for any help or hints! All the best, Wojciech

4 6

Contracting Haskell dev role at Standard Chartered
by Don Stewart 28 Jan '15

28 Jan '15

The Strats team at Standard Chartered is hiring a developer for a 1 year contracting role in London. The role is to develop and extend our parsing and validation library for FpML, using the FpML Haskell library to parse and build financial product data into our internal Haskell data types. You will extend our library to support more products, as well as building testing and validation tools (e.g. in QuickCheck) to confirm the soundness and completeness of the implementation. This is a rare entry level Haskell role, and you'll join a very experienced team. More details at: https://donsbot.wordpress.com/2015/01/28/contracting-haskell-development-ro…

1 0

runSubStateT
by Christopher Done 28 Jan '15

28 Jan '15

runSubStateT :: Monad m => (s -> s') -> (s' -> s) -> StateT s' m a -> StateT s m a runSubStateT to from m = StateT (\s -> liftM (\(a,s') -> (a,from s')) (runStateT m (to s))) Anyone ever needed something like this? Does it already exist in some form in the standard libraries? Ciao

3 2

Tooling for equational reasoning in Haskell
by Christopher Done 28 Jan '15

28 Jan '15

Hi I quite enjoy doing equational reasoning to prove that my functions satisfy some laws, I like to type out each substitution step until I get back what I started with. The only trouble is that it's rather manual and possibly error prone. Is there any tooling anywhere out there that can take in a Haskell expression and reduce it by one step? I only know of stepeval: http://bm380.user.srcf.net/cgi-bin/stepeval.cgi?expr=foldr+%28%2B%29+0+%5B1… But it's just a prototype and works on a teeny weeny subset of Haskell. As much as I like doing tooling, my bandwidth for this area is already full. It seems quite hard to implement such a tool with existing tooling. Real compilers and interpreters tend to be distinctly far away from a simple substitution model or retaining the original source code and being able to print valid source back out. If such a tool existed, though, it'd be super handy and you could probably include it as another check for your build process like your type checking, your quickcheck properties and your unit tests. I would personally invest a little bit of time to add interactive Emacs support for such a tool. Ciao

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Re: [Haskell-cafe] Drastic Prelude changes imminent
by adam vogt 27 Jan '15

27 Jan '15

On Tue, Jan 27, 2015 at 11:03 AM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > Looking at the generalizations in Data.List I find them pretty odd now when > I think about it. I'd expect Data.List functions to be monomorphic to lists, > just like I expect functions in Data.Map to be monomorphic to maps. Now > there might be generalized versions of these functions in e.g. the Prelude, > but generalizing Data.List means that I don't even have the monomorphic > versions available if I want to resolve ambiguity by using them*. > > * It turns out resolving ambiguity is pretty annoying. The type signatures > are awkward to write so I end having to write something akin to > > mymap :: (a -> b) -> [a] -> [b] > mymap = map Unless you're using -XOverloadedLists, you can throw in a (`asTypeOf` []) to disambiguate. Or is that the annoyance you were talking about?

1 0

Error trying to re-upload to the Haskell wiki
by Henk-Jan van Tuyl 27 Jan '15

27 Jan '15

L.S., I am trying to upload a file to the Haskell wiki again, after I deleted it (the description was wrong and there is no option to edit the description). Every time I try this, I get the message: Remote server or file not found You tried to access the address https://wiki.haskell.org/Special:Upload, which is currently unavailable. The file is not visible in the upload log. How can I upload this file again? 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 --

1 0

Help finding one-sided ordered list implementation
by David Feuer 27 Jan '15

27 Jan '15

In "Numerical Representations as Higher-Order Nested Datatypes", Ralf Hinze describes an implementation of one-sided ordered lists as 2-3 search trees under the left spine view. I'm wondering if anyone's actually coded those up somewhere I could download them, or if I'll have to dig into all the details in the paper and write them myself. Thanks, David

1 0

Wrapping a C++ library
by Richard Lewis 27 Jan '15

27 Jan '15

Hi there, There is a C++ library for which I'm working on an FFI wrapper. I've written a translation unit which defines several functions with an extern "C" API. Each of the functions does a little bit of C++ to operate the third party library, but the point is that I now have a C API that I can wrap with the FFI. The next thing I did was to try and get Cabal to compile this translation unit for me. I eventually hacked together a Setup.hs that I barely understand that calls gcc to compile the .cpp file and then calls `ar` to, um, well do whatever it is that ar does with .o files, make some sort of archive, or something. Next, I need to get my hsc file (in which I define some imported functions and some data types with Storable instances for shovelling data through the FFI) to compile. This file uses the hsc2hs #include macro to include the .h file of my translation unit. In turn, that .h includes some of the C++ header files from the third party, and eventually an STL header: vector. At this point, the compiler, gcc, complains that it can't find vector. The gcc command that hsc2hs (I think?) is creating does not include -lstdc++, and nethier does it use g++, either of which would, I think, result in a successful compilation of the hsc2hs .c file. In my cabal file I have: extra-libraries: stdc++ ghc-options: -pgml g++ -Wall But these don't seem to fix anything. Perhaps they don't have any effect on how hsc2hs operates? There's a specific question here which goes something like: how do I get hsc2hs to compile code that uses STL headers? But there's also a more general question as to whether my whole approach of creating an extern "C" wrapper is actually going to work? Any answers would be very appreciated. Richard

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