- Haskell-Cafe - Haskell.org

[ANN] lxc-0.2 and bindings-0.2 – Haskell LXC API bindings
by Nickolay Kudasov 28 Sep '14

28 Sep '14

Greetings, I am pleased to announce the Haskell LXC API bindings. Bindings are represented by 2 cabal packages: lxc and bindings-lxc. LXC is a userspace interface for the Linux kernel containment features. For more information see https://linuxcontainers.org. http://hackage.haskell.org/package/lxc High level Haskell bindings to LXC (Linux containers). Git repository: https://github.com/fizruk/lxc http://hackage.haskell.org/package/bindings-lxc Direct Haskell bindings to LXC (Linux containers) C API. Git repository: https://github.com/fizruk/bindings-lxc === Before installation make sure you have LXC installed on your system with header files and static library. Packages have been tested with Ubuntu 12.04 and 14.04. On Ubuntu 14.04 LTS (Trusty Tahr): $ sudo apt-get install lxc-dev On previous Ubuntu versions (including 12.04 LTS Precise Pangolin) standard repositories do not contain liblxc1 package. You might want to use ppa:ubuntu-lxc/stable repository instead: $ sudo apt-get install software-properties-common python-software-properties $ sudo add-apt-repository ppa:ubuntu-lxc/stable $ sudo apt-get update $ sudo apt-get install lxc-dev === To get the latest stable version of both packages: $ cabal install lxc === While Haddock documentation is absent one can either build haddock documentation locally or view it online on GitHub Pages: lxc documentation: http://fizruk.github.io/lxc/docs/ bindings-lxc documentation: http://fizruk.github.io/bindings-lxc/docs/ See also Usage section of lxc package README file: https://github.com/fizruk/lxc/blob/master/README.md#usage === Feedback of any kind is welcome! Cheers, Nickolay Kudasov

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Debugging with ghci
by Dominick Samperi 28 Sep '14

28 Sep '14

For a long time I've worked under the assumption that a breakpoint can only be used once in a ghci session, because if you run the function (main say) again, it runs to completion, ignoring the breakpoint. Restarting ghci and resetting the break point resolves the problem, but does not explain why a breakpoint cannot be used more than once. How can this behavior be explained? Also, when I stop at a breakpoint :step works as expected, but :trace does not? It simply runs the rest of the function, and using :back subsequently results in the message "not stopped at a breakpoint." Any comments on this behavior? Thanks, Dominick

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Sum and Product do not respect the Monoid laws
by Jason Choy 27 Sep '14

27 Sep '14

The Monoid instances for Sum and Product do not respect the Monoid laws. The instances are: Num a => instance Monoid (Sum a) Num a => instance Monoid (Product a) Float and Double are instances of the Num typeclass, however, floating point addition and multiplication are not associative. Here's an example: > (Sum 1234.567 `mappend` Sum 45.67834) `mappend` Sum 0.0004 Sum {getSum = 1280.2457399999998} > Sum 1234.567 `mappend` (Sum 45.67834 `mappend` Sum 0.0004) Sum {getSum = 1280.24574} Shouldn't these instances be constrained to Integral types? Jason

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Munich Haskell Meeting
by Heinrich Hördegen 27 Sep '14

27 Sep '14

Dear all, our next Haskell meeting in Munich takes place at Cafe Puck at 19h30 on Mon, 29th of September. Please go to http://www.haskell-munich.de/dates and click the button if you plan to join. Everyone is welcome! Heinrich

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foreign ccall export / ghc 7.6.3
by Clint Adams 27 Sep '14

27 Sep '14

Are there examples of "foreign ccall export" that work with GHC 7.6.3?

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SIGVTALRM and Unbound threads
by Lane Seppala 26 Sep '14

26 Sep '14

Howdy, I've banged into this issue of FFI errors stemming from a C library improperly handling system call interrupts caused by the SIGVTALRM/SIGALRMs emitted by the Haskell runtime. Usually the 'proper' solution is to fix the C library—easier said than done for most C quagmires. The most popular workaround, described by Bryan O'Sullivan[1] and several others, is to block the culprit signals from reaching the FFI call. I found another workaround using unbound threads, and it seems to work generally for this problem. However, I'm not satisfied with my understanding about *why* it works given what I know about FFI, bound vs unbound threads, and sys calls. As the simplest example, compiled with the threaded runtime (and neglecting imports): main = void $ sleep 10 will be interrupted, whereas main = void $ runInUnboundThread $ sleep 10 will complete its 10 second sleep. How does running the FFI call on an unbound thread block SIGVTALRM or otherwise avoid the interrupt? My understanding is that these signals *should* reach the FFI process, since they're on the same OS thread the timers were set. Is there any danger to consider before saying that this is a viable workaround? Kindly, Lane [1]: http://www.serpentine.com/blog/2010/09/04/dealing-with-fragile-c-libraries-…

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Re: [Haskell-cafe] What happened to hugs?
by Doug McIlroy 26 Sep '14

26 Sep '14

> We know where the old contents of the hugs documentation and source are, > and are going to get them back online sometime soon. Welcome news! For my purposes, there is no substitute. GHCi's disregard for "default" is a fatal flaw. Doug McIlroy

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Hackage bad gateway?
by Dominick Samperi 26 Sep '14

26 Sep '14

Today (9/25/2014) Hackage is not reachable due to "bad gateway." Is there a status page? Thanks, Dominick

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api-tools questions
by Karsten Gebbert 25 Sep '14

25 Sep '14

Hi All, I have a question concerning the api-tools package. Its not clear from the tests, sources or the tutorial how I can use other date/time formats that the default `utc` type can handle. I'm trying to wrap a JSON API that is not under my control, so I have to adhere to whatever I get back from it. Could anybody with experience with the package point me to some example, relevant bits in the sources or a tip how to do it? I'm planning to create some more documentation around the package to contribute back once I figured out a few more details, because I think its quite a useful abstraction when dealing with (foreign) APIs IMO. Thanks already for any hints, karsten

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Replacement for Ptr () in FFI declaration?
by Howard B. Golden 25 Sep '14

25 Sep '14

Hi, Apparently, before GHC 7.8, it was possible to specify FFI declarations using Ptr (). However, it now gets an error message: This is from com-1.2.3.1 when I try to compile it: ------ message follows ----- [12 of 21] Compiling System.Win32.Com.Automation.TypeLib ( System\Win32\Com\Automation\TypeLib.hs, dist\build\System\Win 32\Com\Automation\TypeLib.o ) System\Win32\Com\Automation\TypeLib.hs:1379:1: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 ------ end of message ----- What is the proper declaration to use instead of Ptr () ? Thanks. Howard B. Golden

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Haskell Weekly News: Issue 307
by Daniel Santa Cruz 25 Sep '14

25 Sep '14

Welcome to issue 307 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from September 14 to 20, 2014 Quotes of the Week * Qfwfq: I seq what you mean. * trap_exit_: if haskell makes programmers 10x more productive, then why are there not better haskell ides? :-) Fuuzetsu: trap_exit_: because you don't need one when you're 10x more productive already ;P * joshcough: im just surprised when im looking for something and i dont find a package by ed kmett. Top Reddit Stories * Wayward Tide is Being Written in Haskell Domain: blog.chucklefish.org, Score: 131, Comments: 30 Original: [1] http://goo.gl/00xr22 On Reddit: [2] http://goo.gl/kJBqTe * Hython: The simplest possible language Domain: callcc.io, Score: 82, Comments: 16 Original: [3] http://goo.gl/T95xQ3 On Reddit: [4] http://goo.gl/LBKTks * Hey /r/haskell, we're hiring! Domain: blog.chucklefish.org, Score: 79, Comments: 49 Original: [5] http://goo.gl/93gW3N On Reddit: [6] http://goo.gl/zULSci * hindent: A Haskell indenter Domain: chrisdone.com, Score: 58, Comments: 18 Original: [7] http://goo.gl/4XlJpL On Reddit: [8] http://goo.gl/SCXxOS * Reflecting strictness in Haskell types Domain: h2.jaguarpaw.co.uk, Score: 51, Comments: 27 Original: [9] http://goo.gl/lfPkYD On Reddit: [10] http://goo.gl/hx6XtM * Towards Shake 1.0 Domain: neilmitchell.blogspot.com, Score: 49, Comments: 7 Original: [11] http://goo.gl/RPlddo On Reddit: [12] http://goo.gl/ckKD2m * Formatting in Haskell (Chris Done) Domain: chrisdone.com, Score: 49, Comments: 11 Original: [13] http://goo.gl/eq7hAF On Reddit: [14] http://goo.gl/f1Amec * Haskell Dice of Doom - Part 1 Domain: derekmcloughlin.github.io, Score: 39, Comments: 6 Original: [15] http://goo.gl/oF3ot3 On Reddit: [16] http://goo.gl/oLz2Fv * Beginner error messages in C++ vs Haskell Domain: izbicki.me, Score: 39, Comments: 65 Original: [17] http://goo.gl/j05AXH On Reddit: [18] http://goo.gl/ZOLvZW * Blog: GHC Weekly News - 2014/09/15 Domain: ghc.haskell.org, Score: 28, Comments: 0 Original: [19] http://goo.gl/8Dt0e8 On Reddit: [20] http://goo.gl/P4uz3y * Let's Build a Browser Engine in Haskell: part 3 Domain: hrothen.github.io, Score: 27, Comments: 8 Original: [21] http://goo.gl/0ckGRH On Reddit: [22] http://goo.gl/vZXZOy * ANN: best-haskell - haskell download ranking Domain: best-haskell.herokuapp.com, Score: 24, Comments: 11 Original: [23] http://goo.gl/9RtgrI On Reddit: [24] http://goo.gl/3zO7SN * ICFP "The Ghost of Church" - writeup by Olle Fredriksson who was the first to solve it! Domain: reddit.com, Score: 23, Comments: 3 Original: [25] http://goo.gl/9j7hG3 On Reddit: [26] http://goo.gl/nDxc4T * What is wrong with the Monoid instance for Maybe? Domain: self.haskell, Score: 21, Comments: 21 Original: [27] http://goo.gl/kgVlFf On Reddit: [28] http://goo.gl/kgVlFf * Inferring Algebraic Effects Domain: lmcs-online.org, Score: 20, Comments: 0 Original: [29] http://goo.gl/Wjklhq On Reddit: [30] http://goo.gl/PBJY7N * Are typeclasses essential? (includes Luke Palmer's view on it) Domain: stackoverflow.com, Score: 19, Comments: 30 Original: [31] http://goo.gl/5rh2dq On Reddit: [32] http://goo.gl/RuX5i7 * Built on Haskell: Findmelike - visual search for art Domain: findmelike.com, Score: 19, Comments: 1 Original: [33] http://goo.gl/MgM69t On Reddit: [34] http://goo.gl/Vzrc6R * Let's Build a Browser Engine in Haskell: setting up tests Domain: hrothen.github.io, Score: 17, Comments: 9 Original: [35] http://goo.gl/qwLXRq On Reddit: [36] http://goo.gl/mUqX6w Top StackOverflow Questions * Are typeclasses essential? votes: 20, answers: 4 Read on SO: [37] http://goo.gl/5rh2dq * Ambigous instance resolution in Haskell votes: 14, answers: 2 Read on SO: [38] http://goo.gl/KzV5jC * Understanding the casts involved in patterns matching a datatype that is indexed over a user defined kind votes: 11, answers: 1 Read on SO: [39] http://goo.gl/TppaAg * What does fixIO do? votes: 10, answers: 2 Read on SO: [40] http://goo.gl/yECrrk Until next time, [41]+Daniel Santa Cruz References 1. http://blog.chucklefish.org/?p=154 2. http://www.reddit.com/r/haskell/comments/2glk10/wayward_tide_is_being_writt… 3. http://callcc.io/hython-the-simplest-possible-language/ 4. http://www.reddit.com/r/haskell/comments/2go4v6/hython_the_simplest_possibl… 5. http://blog.chucklefish.org/?p=322 6. http://www.reddit.com/r/haskell/comments/2gyn1w/hey_rhaskell_were_hiring/ 7. http://chrisdone.com/posts/hindent 8. http://www.reddit.com/r/haskell/comments/2gyaw7/hindent_a_haskell_indenter/ 9. http://h2.jaguarpaw.co.uk/posts/strictness-in-types/ 10. http://www.reddit.com/r/haskell/comments/2gk51p/reflecting_strictness_in_ha… 11. http://neilmitchell.blogspot.com/2014/09/towards-shake-10.html 12. http://www.reddit.com/r/haskell/comments/2gnat7/towards_shake_10/ 13. http://chrisdone.com/posts/formatting 14. http://www.reddit.com/r/haskell/comments/2gyrvp/formatting_in_haskell_chris… 15. http://derekmcloughlin.github.io/2014/09/13/Haskell-Dice-Of-Doom-Part-1/ 16. http://www.reddit.com/r/haskell/comments/2gg3um/haskell_dice_of_doom_part_1/ 17. https://izbicki.me/blog/error-messages-in-ghc-vs-g++.html 18. http://www.reddit.com/r/haskell/comments/2go92u/beginner_error_messages_in_… 19. https://ghc.haskell.org/trac/ghc/blog/weekly20140915 20. http://www.reddit.com/r/haskell/comments/2gia13/blog_ghc_weekly_news_201409… 21. http://hrothen.github.io/2014/09/19/lets-build-a-browser-engine-in-haskell-… 22. http://www.reddit.com/r/haskell/comments/2gw2bw/lets_build_a_browser_engine… 23. http://best-haskell.herokuapp.com/ 24. http://www.reddit.com/r/haskell/comments/2gsjk0/ann_besthaskell_haskell_dow… 25. http://www.reddit.com/r/theghostofchurch/comments/2fccjw/i_found_this_trink… 26. http://www.reddit.com/r/haskell/comments/2gkjij/icfp_the_ghost_of_church_wr… 27. http://www.reddit.com/r/haskell/comments/2guo44/what_is_wrong_with_the_mono… 28. http://www.reddit.com/r/haskell/comments/2guo44/what_is_wrong_with_the_mono… 29. http://www.lmcs-online.org/ojs/viewarticle.php?id=1469 30. http://www.reddit.com/r/haskell/comments/2gdl31/inferring_algebraic_effects/ 31. http://stackoverflow.com/questions/25855507/are-typeclasses-essential 32. http://www.reddit.com/r/haskell/comments/2gjqop/are_typeclasses_essential_i… 33. http://findmelike.com/ 34. http://www.reddit.com/r/haskell/comments/2gn81y/built_on_haskell_findmelike… 35. http://hrothen.github.io/2014/09/14/lets-build-a-browser-engine-in-haskell-… 36. http://www.reddit.com/r/haskell/comments/2ge7om/lets_build_a_browser_engine… 37. http://stackoverflow.com/questions/25855507/are-typeclasses-essential 38. http://stackoverflow.com/questions/25854072/ambigous-instance-resolution-in… 39. http://stackoverflow.com/questions/25934209/understanding-the-casts-involve… 40. http://stackoverflow.com/questions/25876042/what-does-fixio-do 41. https://plus.google.com/105107667630152149014/about

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Taking over the thrift package.
by Sean Parsons 24 Sep '14

24 Sep '14

Hi there. As per the instructions on the Haskell wiki I'm posting here as I'm interested in either taking over the thrift package or helping out with getting more recent builds uploaded as it's wildly out of date: https://hackage.haskell.org/package/thrift Does anyone have any contact details for the current maintainer? Thanks, Sean.

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Combining LoggingT with another library's monad
by Joe Fiorini 24 Sep '14

24 Sep '14

As I've been learning Haskell, I've found I constantly get tripped up on combining monads with transformers to form stacks. This seems to be very common in web development, which is my primary use case for Haskell. I'm using the Wheb framework as it's been the simplest to understand of all the frameworks I've seen. To get a better understanding of monad transformers, I'm trying to implement logging in a small app I wrote. I'm attempting to use Control.Monad.Logger to add LoggingT to my monad stack, that way I can use runStdoutLoggingT to log straight to stdout (without having to deal with implementing my own monadLoggerLog method). I added LoggingT to the top of my monad stack, but ran into lots of problems and moved the LoggingT type to one of my route handlers (you can see my latest attempt & the compiler error here https://gist.github.com/joefiorini/9e89bdf86e28e6b98bf0) I'm at a point now where I believe my problem is either a) I'm just not understanding something about how monad transformers work, in which case I just need to do some more reading or b) the LoggingT transformer is simply not "compatible" with the WhebT transformer, in which case I need to completely change my approach. I'm not looking for someone to solve my problem for me; I'm just trying to get a little guidance. Am I heading in the right direction here or should I change up my approach? If the latter, can anyone give me a little direction as to where to look? Thanks!

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Numerals in input vs numerals in source
by Doug McIlroy 24 Sep '14

24 Sep '14

Consider e1 = read "1" :: Integer e2 = read "1" :: Rational e3 = read "1" :: Double e1 and e3 evaluate to 1 and 1.0 respectively e2 produces an error: "Prelude.read: no parse" Yet (1::Integer, 1::Rational, 1::Double) evaluates to (1, 1%1, 1.0) Is the inconsistent parsing defensible, or just an (irrational!) fact? I believe a reconciliation would break nothing. Doug McIlroy

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code.haskell.org down
by Henning Thielemann 24 Sep '14

24 Sep '14

code.haskell.org seems to be down :-(

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Re: [Haskell-cafe] [ANN] Declarative Game Programming & haskanoid (FRP, SDL, Android)
by Ivan Perez 24 Sep '14

24 Sep '14

Hi Mikolaj, (CC haskell-cafe) Thanks, Mikolaj! Indeed, the original links to the slides were broken (deployment env vs production env, yikes!). That has been fixed now. Bitbucket seeems to be taking some time to refresh the page, though. In the meantime: http://fplab.bitbucket.org/uploads/ppdp2014-tutorial.pdf http://fplab.bitbucket.org/uploads/ppdp2014-tutorial-4up.pdf Best Ivan On 24 September 2014 10:27, Mikolaj Konarski <mikolaj.konarski(a)gmail.com> wrote: > > * Announcement and slides: > > > http://fplab.bitbucket.org/posts/2014-09-23-declarative-game-programm.html > > Hi! Thank you for the post. Both slides links are dead for me, though. >

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Nottingham Research Fellowships (deadline 20th October)
by Graham Hutton 24 Sep '14

24 Sep '14

Dear all, The University of Nottingham is seeking applications for a number of Nottingham Research Fellowships: http://tinyurl.com/notts-fellows These are highly prestigious three-year fellowships, which are normally expected to lead into permenant academic positions. Candidates should have no more than 8 years postdoctoral experience, and fellowships normally start in October 2015. The deadline for submission of initial Expressions of Interest is ** 20th October 2014 **. Applicants in the area of the Functional Programming (FP) lab would be most welcome. The FP lab is keen to receive applications from outstanding candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from http://fp.cs.nott.ac.uk As an approximate guideline, candidates in the area of the FP lab would normally be expected to have at least 3 years postdoc experience, and a number of strong publications in leading international venues such as ICFP, POPL, LICS, JFP, Haskell Symposium, etc. Best wishes, Graham Hutton -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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[ANN] Declarative Game Programming & haskanoid (FRP, SDL, Android)
by Ivan Perez 24 Sep '14

24 Sep '14

Hi Café A few weeks ago Henrik Nilsson gave a tutorial at PPDP 14 on Declarative Game Programming, demoing an SDL game using the FRP implementation Yampa. Additional material has been made available now: * Announcement and slides: http://fplab.bitbucket.org/posts/2014-09-23-declarative-game-programm.html * Game: http://github.com/ivanperez-keera/haskanoid The game includes wiimote support, SDL graphics and sound. We have other versions working with a kinect, if anyone is particularly interested. This is, IMHO, very fun to play, since you have to actually run around the room. Also, Keera Studios (my company) is working on an Android version of the game and a video of an early demo running on a Galaxy Tab is available ( http://keera.co.uk/blog/2014/09/24/game-programming-videos-code/ <http://keera.co.uk/blog/?p=690&preview=true>). Our internal, latest version runs now much faster than the ones in either video (new videos will be released soon). I hope we can soon give you a great excuse to be playing games at the office ;) All the best Ivan

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Multicast receiver thread fails to receive last few messages
by Ben Gunton 23 Sep '14

23 Sep '14

I am trying to send multicast messages from one thread, and receive them on another. The sending thread reads lines from a file and sends them. However, the receiving thread does not receive all the messages - it seems to miss the last 10 message or so. If I run other programs to listen for the multicast message, they receive them fine. So I think the issue is something to do with receiving, not sending. I keep the program alive with a threadDelay, so it shouldn't be halting prematurely. If I add a small delay after each send, it works fine. And if I make the input file smaller, it works fine. On the provided data file, on my system (64-bit Ubuntu, GHC 7.8.2), the receiver fails to receive lines 125-140. Compiled with no optimizations. Thanks for any help! The relevant code snippets are below, and the sample data file and full program are attached. -- Loop to receive all the packets let receiveMulticast = do (msg, _) <- recvFrom recvSock 32628 putStrLn . BS.unpack $ BS.take 100 msg receiveMulticast _ <- forkIO receiveMulticast -- Send every line from a file as multicast message inputFile <- openFile "data" ReadMode fileLines <- BS.lines <$> BS.hGetContents inputFile let sendMulticast msg = do sendTo sendSock msg addr -- Receiver FAILS to receive last few messages unless this -- thread delay exists... why?! -- threadDelay (1) mapM_ sendMulticast fileLines hClose inputFile threadDelay (1000*1000*1000) -- Delay for 1000 seconds

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Copyright field vs. License file
by Joachim Breitner 23 Sep '14

23 Sep '14

Dear package maintainers, a lot of our LICENSE file start like tihs: ==> haskell-lens-4.1.2.1/LICENSE <== Copyright 2012-2014 Edward Kmett All rights reserved. ... ==> haskell-cryptocipher-0.6.2/LICENSE <== Copyright (c) 2010-2013 Vincent Hanquez <vincent(a)snarc.org> All rights reserved. ... ==> haskell-http-client-0.3.2.1/LICENSE <== The MIT License (MIT) Copyright (c) 2013 Michael Snoyman Permission is hereby granted, free of charge, to any person obtaining a copy of ... i.e. they mix the copyright information with the license. I know, of course, why that is: "cabal init" prepares it that way, and those not using cabal init probably copy it from an existing file. A notable exception are the GPL-licensed packages; these usually don’t have copyright information in the license. The problem is that this information is not structured, and hard to parse, i.e. to create license reports or (my use case) create distribution packages with much less manual labor. Cabal describes the fields as license-file: filename or license-files: filename list The name of a file(s) containing the precise copyright license for this package. The license file(s) will be installed with the package. If you have multiple license files then use the license-files field instead of (or in addition to) the license-file field. copyright: freeform The content of a copyright notice, typically the name of the holder of the copyright on the package and the year(s) from which copyright is claimed. For example: Copyright: (c) 2006-2007 Joe Bloggs which suggest to put the copyright data into the cabal file only and leave the license file alone. It would also have the advantage that after cabal init, you’d only have to modify one file, and the copyright information is easily visible on hackage. I know that there it is highly unlikely that a significant number of maintainers will change their existing files. But I’d still like to get feedback: Do you agree that this make sense? Should I try to make "cabal init" set it up this way? And would you accept pull requests for this? Thanks, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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Re: [Haskell-cafe] Bound and free variables
by Barak A. Pearlmutter 23 Sep '14

23 Sep '14

On Mon, 22 Sep 2014 19:30:33 -0400, Michael Orlitzky wrote: > It's hard to tell from your example, but if you want, you can > translate the let binding directly to lambda calculus and then go from > there where the rules are simple. > > In Haskell, > > let <name> = <argument> in <expression> > > translates to, > > (\<name> -> <expression>) <argument> Although this list might not be for homework problems, there is an element of terminological confusion and a quest for nomenclature in the question that don't seem entirely unreasonable. Even if we're not going to answer the question per se, we shouldn't give hints that are misleading, or true in Scheme and lambda calculus but false in Haskell. $ ghci Prelude> let b = 3:b in take 3 b [3,3,3] Prelude> (\b -> take 3 b) (3:b) <interactive>:3:21: Not in scope: `b'

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Re: [Haskell-cafe] usage of traversal
by Michael Snoyman 23 Sep '14

23 Sep '14

On Tue, Sep 23, 2014 at 10:12 AM, Kazu Yamamoto <kazu(a)iij.ad.jp> wrote: > Hi Michael, > > Probably, you forgot to Cc: to haskell-cafe. > > > I'd be -1 on an operator, I think having a named function for this is a > > good thing for readability of code. > > > > As far as good style: I personally think it is. In classy-prelude, I > > actually export the Foldable-based `mapM` by default, and will regularly > > use that (or forM) for this kind of code. > > With AMP, 'fmap' and 'liftM' are identical and we use <$> instead > recently. Likewise, 'traverse' and 'mapM' are identical. If we > introduce an operator, say <:>, we can forget 'traverse' and 'mapM' > when writing code and can write: > > getModificationTime <:> mfile > > This looks like function application like Applicative style. > > Just a thought. > > --Kazu > It's definitely true that with AMP the situation will be much improved, and it makes more sense to use an operator here since it will cover what is today two distinct functions. However, I think I'm overall still in favor of sticking to the named function, since both `mapM` and `traverse` are well known. This may just be a chicken-and-egg argument, however, and once <:> takes hold it's just as readable as `mapM`. Michael

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ANNOUNCE: bv-0.3.0
by Iago Abal 23 Sep '14

23 Sep '14

I am pleased to announce a new release of the bv package, a library for bit-vector arithmetic. http://hackage.haskell.org/package/bv This is essentially a maintenance release to support newest versions of GHC and base. Yet, I took the opportunity to do some code and documentation clean up. A changelog is available at https://bitbucket.org/iago/bv-haskell/src/v0.3.0/CHANGES.md. Iago

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usage of traversal
by Kazu Yamamoto 23 Sep '14

23 Sep '14

Hi, Recently, I need to write the following code: getModTime :: Maybe FilePath -> IO (Maybe UTCTime) getModTime mfile = case mfile of Nothing -> return Nothing Just file -> Just <$> getModificationTime file I feel that this is redundant. So, I used 'traverse' instead: getModTime :: Maybe FilePath -> IO (Maybe UTCTime) getModTime mfile = getModificationTime `traverse` mfile First, I would like to know whether or not this is a good coding style. Second, if this is acceptable, why don't we define an operator? For instance, (<:>) :: (Traversable t, Applicative f) => (a -> f b) -> t a -> f (t b) (<:>) = traverse getModTime :: Maybe FilePath -> IO (Maybe UTCTime) getModTime mfile = getModificationTime <:> mfile Is there such an operator already? Regards, --Kazu

2 1

Bound and free variables
by Jan Stolarek 22 Sep '14

22 Sep '14

Hi *, I have a simple question about terminology regarding bound and free variables. Assume I have: let f x = let g y = ... in g c in ... Now: - `c` is free in `g` and `f` - `y` is bound in `g` - `x` is free in `g`. - `x` is bound in `f` What about `y` in `f`? Is it also bound in `f`? If so then it certainly is bound in a different way that `x`. Is there a terminology that allows to distinguish these different forms of bound variables? Janek

5 6

[ANN] codex 0.2.0.0
by Alois Cochard 22 Sep '14

22 Sep '14

Hi Cafe, I released a new version of my tool `codex`[1] (a ctags file generator for cabal project dependencies). This new version include: - By default, the current project is now included in the generated `codex.tags` file - Resolve dependencies in the "benchmarks" of your cabal project - Fix tags file header to respect specification (to make them work in Sublime) - Cache eviction mechanism (no need of using `--force` in default use cases) - Add default configuration for Sublime Thanks to everyone for bug fixes and feature requests! Cheers [1] https://github.com/aloiscochard/codex -- *Λ\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

1 0

how to use hs_init_ghc and pass runtime params to a dll
by Kees Bleijenberg 22 Sep '14

22 Sep '14

Can someone provide a (pointer to a) working basic example of C code that uses a Windows dll created by ghc that calls hs_init_ghc? Can someone confirm that hs_init_ghc uses stdcall in a windows dll. Is ghc -no-hs-main -shared -threaded -O2 -o TBGlas.dll TBGlasDll.hs a correct way to create the dll? Do I have to use the ghc parameter -dynamic? if I use this parameter, the ghc tells me to install the 'dyn' libraries for package base. Sounds aweful. I'am using ghc version 7.8.3. and the 32 bits Haskell Platform on 64 bits Windows 7. Background: I create a dll with ghc. The program that uses the dll starts with a lookup for all functions I want to use from the dll. The result is a bunch of function pointers. I use a function pointer to call the real function. Then I call the defaultRtsConfig function from the dll. I get a pointer to a struct with a byte for the enumarable RTSOptEnabled followed by a pointer to a string. In the returned struct this pointer is nil. Then I set the pointer in the struct to the string "-N -s". After that I call hs_init_ghc(nil,nil,defaultRtsConcfigStruct). This works fine. I get a overview of the sparks created, the elapsed times, there is a speedup from the parallel code etc. But sometimes (1 out of 100) the program crashes. Any ideas what could be wrong? Kees

1 0

ghc-prof-options and libraries on Hackage
by Oliver Charles 22 Sep '14

22 Sep '14

Hi all, I'd like to start a conversation around the ghc-prof-options option that can be used in Cabal-ized libraries. As you may know, this option specifies extra GHC options that are used when --enable-library-profiling is enabled. This is very convenient for local development, but I argue that it can be counter-productive for releases onto Hackage. To provide an example, I'm currently working on a little game engine that uses JuicyPixels to load images. I have a problem in my code that needs optimizing, but the current state of things results in profiles that are very difficult to work with. JuicyPixels specifies -auto-all in its cabal file, which means I have no alternative but to profile JuicyPixels code. In this scenario, the bottleneck is actually within my FRP game loop and nothing to do with image loading! As a result, the profiles are fairly useless to me. Roman Cheplyaka also points out that by doing this, profiles are skewed - -auto-all is not free, so we actually pay in runtime performance every time a library does this. I would like to hear from others if we should consider managing this option a little more. My personal feeling is that this flag shouldn't be used unless in local development, so guarded by an off-by-default build flag. I think cabal should also warn authors who are using this flag, and encourage them to place a guard around this option. Thoughts? - ocharles

9 8

Haskell job opportunity in Santiago, Chile
by Anupam Jain 22 Sep '14

22 Sep '14

Hi all, We are looking to hire an experienced Haskell developer for our startup in Santiago, Chile. This will be a 6-8 week project involving web development using Yesod and you will be building a key component of our product. Part time is OK. We would prefer someone who can work out of our office, however, working remote is also a possibility. If remote, being in India would be a plus. Good english communication ability is essential. Please email me directly if interested. Thanks, Anupam Jain

1 0

DSL implementation
by felipe zapata 22 Sep '14

22 Sep '14

HI all, as a part of a project for physical simulation, I'm was given the task to help with the implementation of an Small DSL that is a minimal functional language. Point is that this small language contains some dependent typed theory, because we want to be able to check statically the shapes of vector and matrices, among other things. Haskell of course seems the tool for the job. But my boss insists that the implementation should be done in C, but I think that it is madness. Can someone please point me to relevant literature, discussions or general information that can help me to show my boss that Haskell is the right tool? any help would be appreciated. Felipe Z.

6 6

Fwd: Pointers by using functions
by Holden Lee 21 Sep '14

21 Sep '14

What is the best way to implement data structures that use pointers in Haskell? My first thought is as follows. Suppose for simplicity the labels are just integers, and each object need to point to one other one. I would have a getObject:: Int -> a f:: Int -> Int But I don't understand how Haskell maintains functions in memory, and in particular whether lookup/update is efficient. So suppose I define a f f x = case x of 1 -> 2 2 -> 3 .... (etc.) Q: if there are n entries here, how long does looking up (f m) take? Suppose later I update, with something like this: update::Int->Int->(Int->Int)->(Int->Int) update x y f z = (if z==x then y else f x) Q: how is "update x y f" maintained? Is using a f::Int->Int like using a pointer in terms of time complexity, and if not, what data structure should I use instead? Thanks, Holden

3 2

ANN: vimus - An MPD client with vim-like key bindings
by Simon Hengel 21 Sep '14

21 Sep '14

Hi, I just uploaded vimus [1] to Hackage. vimus is an MPD client with vim-like key bindings. I've been using it as my exclusive music player since early 2010. It comes with it's own quick & dirty ncurses bindings, which is one of the reasons why I was reluctant to push it to Hackage so far. Check the README for installation and usage: https://github.com/vimus/vimus#readme vimus is not a masterpiece of coding beauty** , but it's incredibly useful to me and a couple of other people over at #vimus on Freenode. Patches are welcome as long as they - reduce code bloat and improve hackability - do not conflict with my use cases (in case of doubt we make things configurable) - do not require lots of work from my side (vimus already works incredibly well for me and I can not spend too many cycles on it) vimus would not be what it is today without the help of others. In particular I'm grateful to - Niklas Haas for being the second user of vimus and contributing many useful features - Joachim Fasting for his work on libmpd and contributing high quality patches to vimus - Markus Klinik for contributing documentation and features - Matvey Aksenov for adding even more features Cheers, Simon ** Indeed, vimus served me as a testing ground for experimenting with both Haskell and lean software development. [1] http://hackage.haskell.org/package/vimus [2] https://github.com/vimus/vimus#using-vimus

1 1

Linking/uploading a PDF file in HaskellWiki
by Henk-Jan van Tuyl 21 Sep '14

21 Sep '14

L.S., I am trying to link a PDF file, that is to be uploaded to the HaskellWiki environment, but I encounter several problems: - The link [[File:wxhaskell.pdf | wxHaskell - A Portable and Concise GUI Library for Haskell]] is displayed as File:wxhaskell.pdf instead of wxHaskell - A Portable and Concise GUI Library for Haskell If I leave out the vertical bar, the entire string between the brackets is displayed and used as filename - If I click the link, I am asked to upload the file; the resulting file should be http://www.haskell.org/haskellwiki/File:Wxhaskell.pdf but this URL results in a blank page - The list of uploaded files should be at http://www.haskell.org/haskellwiki/Special:ListFiles but this URL results in a blank page as well Am I doing something wrong, or are these Wiki bugs? 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 --

2 1

Chaining modifications using (>>>)
by martin 20 Sep '14

20 Sep '14

Hello all, I've been playing with Data.Graph.Inductive, which provides functions to modify a graph. I take these functions and build something more complex from them. To do so, I have to apply several such modifications one after the other. I can write this using ($) or (.), but I didn't like to write things backwards (last function to apply comes first), so I use (>>>) from the Arrows module to chain functions Graph -> Graph (aka GraphTransforms) and my complex function looks like f >>> g >>> h. To actually build a Graph I have to apply my function to some Graph (often "empty"). This works okay as long as the inidividual functions f,g,h do not need access to the graph. However, there is one function where I use "pre" (predecessors) and "suc" (succerssors), which need the Graph as a parameter. groupNodes :: DynGraph gr => (Label,[Node]) -> GraphTransform gr pl groupNodes (lbl,ids) gr = let id = head $ newNodes 1 gr oldEdgesTo = [(toOld, old) | old <- ids, toOld <- pre gr old] -- <== here "pre" oldEdgesFrom = [(old, fromOld) | old <- ids, fromOld <- suc gr old] oldEdges = oldEdgesTo ++ oldEdgesFrom oldEdgesWithin = [ (i,j) | (i,j) <- oldEdges, i `elem` ids, j `elem` ids] newEdges = map (uncurry e0) $ uniq $ map (setDest id) (oldEdgesTo \\ oldEdgesWithin) ++ map (setOrig id) (oldEdgesFrom \\ oldEdgesWithin) in --------------- look here: ----------------------- ( insNode (id, (NN lbl)) >>> delNodes ids >>> delEdges oldEdges >>> insEdges newEdges ) gr This works, but I don't like the "in" part and I'd rather omit the braces and the gr argument. Without the gr argument to the whole function this would work, however with the gr I have to kinda eta expand (?) it. Is there a way I can write this more point free and still have access to gr for pre and suc?

2 1

Haskell FFI Question
by ye yan 20 Sep '14

20 Sep '14

I have been play around with Haskell FFI. My question is for ByteString (Strict) is there a trailing null append to it when pass to it C functions?

2 4

Confused about AllowAmbiguousTypes
by Audun Skaugen 19 Sep '14

19 Sep '14

I am confused about the behaviour of the AllowAmbiguousTypes extension. I tried the basic example of the user manual: > class C a where > f :: C a => Int > f = 3 > main = print f Using ghc 7.8.3, without the extension, this will cause a type error at the type signature (line 2). Adding AllowAmbgiuousTypes causes an error at the call site instead (line 4). So far so good. However, if I replace the (C a) constraint with (Num a), no type error is reported, and the example runs fine! This is true with or without the extension. Using ghc 7.6.3, however, the example is still rejected with an "ambiguous constraint" error. Is this a bug in ghc 7.8? Is ghc somehow able to prove the Num constraint even though a is universally quantified? -- Audun Skaugen

2 1

Craig Gentry of IBM
by Vasili I. Galchin 19 Sep '14

19 Sep '14

Hi, Gentry is doing on homomorphic functions cryptography. I looked on Hackage but did see a homomorphic function library. Is there.any effort in this area in Haskell? If so by whom and is contact info? Vasya

1 0

Constructors on left and right hand sides of equation
by PATRICK BROWNE 19 Sep '14

19 Sep '14

Dear list In Eq 1 is (NewGlass i) on the LHS distinct from (NewGlass i) on the RHS? In Eq2 are the occurrences of the data constructor Fill on the LHS and RHS distinct? Thanks Pat data Glass = NewGlass Int | Fill Glass Int deriving Show drink :: Glass -> Int -> Glass drink (NewGlass i) j = (NewGlass i) -- Eq 1 drink (Fill m i) j | full (Fill m i) = Fill (NewGlass (size m)) ((size m) - j) -- Eq 2 | (i >= j) = Fill m (i-j) | otherwise = drink m (j-1) size :: Glass -> Int size (NewGlass i) = i size (Fill m i) = size m level :: Glass -> Int level (NewGlass i) = 0 level (Fill m i) | (level m) + i > (size m) = size m | otherwise = (level m) + i empty :: Glass -> Bool empty m = (level m) == 0 full :: Glass -> Bool full m = (level m) == (size m) {- drink (Fill (NewGlass 4) 5) 5 drink (Fill (NewGlass 4) 3) 3 -} -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

4 6

move sandboxes
by Corentin Dupont 19 Sep '14

19 Sep '14

Hi guys, is there a pratical way to move a sandbox? I see the absolute path are hard-coded in all the .conf files. I ask the question because I have an application that uses Hint. So it needs to run with the same object files that it was compiled with. If I want to deploy the application on a server, I need to send the whole sandbox there: this way Hint/GHC finds the database when I start with "cabal exec". But this forces me to deploy it on the same path it was compiled, which is not very practical. Cheers C

1 0

preventing space leaks?
by Baojun Wang 19 Sep '14

19 Sep '14

Hi Cafe, I'm very new to Haskell, and I'm confused about haskell space leaks, even I read some wiki pages from haskell.org. Below is the scenario I''m facing at: When I use listArray to create either IArray or UArray, I found it could lead to space leaks (very high GC time), is there a way to prevent this, and why there is no fusion when create IArray/UArray from the list? I don't need the list anyway after array is created. (I used this few times for DP, based on this link: http://jelv.is/blog/Lazy-Dynamic-Programming/) Further more, suppose if I create a IArray A from list 1, later I create UArray B based some computations based on A (also I need discard another list 2). After B is generated, I don't need A anymore (so this lead to space leaks, correct)? Another quetion, by the way, after the lazy IArray is created (say from a DP), is there a way convert it to UArray for better performance (IArray -> UArray)? How can I improve the space leak problems with above use case? Best Regards Baojun

2 2

Injective type families for GHC - syntax proposals
by Jan Stolarek 19 Sep '14

19 Sep '14

Haskellers, there is some discussion going on at the moment about implementing injective type families in GHC [1]. I'd like to hear your opinions on proposed syntax for this new feature. == Problem == Currently GHC assumes that type families are not injective. This is of course conservative and the idea behind implementing injective type families is to allow the programmer to explicitly declare that a type family has the injectivity property (which GHC will of course verify). == Forms of injectivity == Injectivity can take forms more complicated than just determining the LHS from RHS: A. RHS determines all arguments on the LHS. Example: type family Id a where Id a = a Here the RHS uniquely determines LHS. B. RHS determines some but not all arguments on the LHS. Example (contrieved): type family G a b c where G Int Char Bool = Bool G Int Char Int = Bool G Bool Int Int = Int Here RHS uniquely determines `a` and `b` but not `c`. C. RHS and some LHS arguments uniquely determine some LHS arguments. Example: type family Plus a b where Plus Z m = m Plus (S n) m = S (Plus n m) Here knowing the RHS and one of LHS arguments uniqely determines the other LHS argument. At the moment we've only seen use cases for A, which means that we will most likely only implement A. If someone comes up with compelling examples of either B or C we will revise and consider implementing these more involved forms of injectivity. == Syntax == So the most important decission to make is what syntax do we want for this new feature :-) One of the features we would like is extensibility, ie. if we only implement injectivity of type A we still want to be able to add injectivities B and C sometime in the future without breaking A. When thinking about syntax an important (and open) question is whether injectivity becomes part of TypeFamilies language extension or do we add a new InjectiveTypeFamilies extension for this feature. The problem with incorporating injectivity into existing TypeFamilies is that we would have to come up with syntax that is fully backwards compatible with what we have now. Below is a list of proposals that have come up until now in the discussion between developers. Note that all proposals are given for closed type families. For open type families things would be identical except that the `where` keyword would be skipped. === Proposal 1 === Use syntax similar to functional dependencies. The injectivity declaration begins with `|` following type family declaration head. `|` is followed by a list of comma-separated injectivity conditions. Each injectivity condition has the form: {{{ result A -> B }}} where `A` is a possibly-empty list of type variables declared in type family head and `B` is non-empty list of said type variables. Things on the left and right of `->` are called LHS and RHS of an injectivity condition, respectively. `result` becomes a restricted word that cannot be used as a type variable's identifier in a type family head. Examples: type family Id a | result -> a where type family G a b c | result -> a b where type family Plus a b | result a -> b, result b -> a where Pros: * has natural reading: `result a -> b` reads as "knowing the result and the type variable a determines b". * extensible for the future Cons: * steals `result` identifier in the type family head. This means it would be illegal to have a type variable named `result` in a type family. * the above also makes this proposal backwards incompatible with TypeFamilies extension. Further proposals are based on this one in an attempt to solve the problem of stealing syntax and backwards incompatibility. === Proposal 2 === Use `Result` instead of `result`: type family Id a | Result -> a where type family G a b c | Result -> a b where type family Plus a b | Result a -> b, Result b -> a where Pros: * has natural reading * extensible for the future * allows to have type variables named `result` (does not steal syntax) Cons: * all other keywords in Haskell are lowercase. This would be the first one that is capitalized. * confusion could arise if we have a type `Result` and use it in the type family head. Unknowns (for me): * not sure if it would be possible to avoid name clash between `Result` and type of the same name. If not then this proposal would also be backwards incompatible with TypeFamilies. === Proposal 3 === Use `type` instead of `result`: type family Id a | type -> a where type family G a b c | type -> a b where type family Plus a b | type a -> b, type b -> a where Pros: * extensible for the future * no syntax stealing Cons: * no natural reading * usage of `type` here might seem unintuitive === Proposal 4 === Use name of the type family instead of `result`: type family Id a | Id -> a where type family G a b c | G -> a b where type family Plus a b | Plus a -> b, Plus b -> a where Pros: * extensible for the future * no syntax stealing Cons: * writing something like `Plus a` might be confusing. It looks as if Plus could be partially applied. == Further discussion == I'd like to hear what you think about these proposals. Neither of them is perfect so perhaps someone can come with something better. You can also check out the GHC Trac ticket [1] and the GHC Wiki page [2] (which repeats most of this but has some more details as well). [1] https://ghc.haskell.org/trac/ghc/ticket/6018 [2] https://ghc.haskell.org/trac/ghc/wiki/InjectiveTypeFamilies Janek

8 14

optional ghc-options based on GHC version?
by Mateusz Kowalczyk 19 Sep '14

19 Sep '14

Hi, I have a project here where GHC's option -flate-dmd-anal gets out some extra speed but the flag is not available on 7.6.3. How can I specify in the cabal file that I only want to pass this through when building with 7.8.3 or newer? It'd be a shame to not take advantage of the extra speed on 7.8.3 just because we want to support 7.6.3 too for a bit longer. I was unable to find the info online. Thanks. -- Mateusz K.

2 2

Re: [Haskell-cafe] Hello and type-level SKI
by oleg＠okmij.org 18 Sep '14

18 Sep '14

Yet another demonstration of Turing completeness of the type checker (given the unrestricted context reduction stack) was lambda-calculator in types, posted back in 2006. http://okmij.org/ftp/Computation/lambda-calc.html#haskell-type-level The examples included the Fibonacci function (encoded using the fix-point combinator) and the SKI calculator (expressing combinators as lambda-terms).

2 1

Improving ghc error messages
by Toby Goodwin 18 Sep '14

18 Sep '14

Interesting article: https://izbicki.me/blog/error-messages-in-ghc-vs-g++.html I do hope some of these changes make it into a future ghc. I'd particularly like to highlight this one: No instance for (Show ([a0] -> Int)) arising from a use of `show' Possible fix: add an instance declaration for (Show ([a0] -> Int)) The second line looks like it was added by someone who'd read an article about making error messages helpful: "suggest a possible way to fix the error". Like creating a Show instance for a function type!?! I must have seen that "possible fix" 1000 times, and 999 of them it was nothing like the right fix. Now I know just to ignore it. Simply removing the second line would be an improvement: it doesn't add anything to the first line. Toby.

3 2

cryptography and homomorphic library support?
by Vasili I. Galchin 18 Sep '14

18 Sep '14

Hi, I just perused the crypto section of Hackage and I didn't see any support for homomorphc functions. I didn't see anything .. maybe I missed. If no support, I wouldn't mind giving a shot at working on this as long as no one has already implemented. If someone is working on this, please inform me of contact info so perhaps I can join the team. Kind regards, Vasya

1 0

Haskell Weekly News: Issue 306
by Daniel Santa Cruz 18 Sep '14

18 Sep '14

Welcome to issue 306 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from September 07 to 13, 2014 Quotes of the Week * prophile: at this point ekmett is more part of the language than a practitioner of it * Qfwfq: Simon is a title, not a forename. Top Reddit Stories * Haskell for all: Morte: an intermediate language for super-optimizing functional programs Domain: haskellforall.com, Score: 110, Comments: 83 Original: [1] http://goo.gl/MjPyyr On Reddit: [2] http://goo.gl/v2btl4 * AMP has landed in GHC Main! Domain: haskell.org, Score: 88, Comments: 71 Original: [3] http://goo.gl/giu1Xh On Reddit: [4] http://goo.gl/ZGJ76S * [Meta] What's with all these downvotes on beginner questions? Domain: self.haskell, Score: 69, Comments: 67 Original: [5] http://goo.gl/ik2uu2 On Reddit: [6] http://goo.gl/ik2uu2 * Haskell Implementors Workshop 2014 videos [youtube] Domain: youtube.com, Score: 59, Comments: 11 Original: [7] http://goo.gl/ijM6hs On Reddit: [8] http://goo.gl/48GOSr * FP Complete is hiring: Haskell web UI developer Domain: fpcomplete.com, Score: 59, Comments: 14 Original: [9] http://goo.gl/0kPlFl On Reddit: [10] http://goo.gl/WtivSo * How to get (approx) stack traces with profiled builds Domain: self.haskell, Score: 46, Comments: 15 Original: [11] http://goo.gl/ZAurK6 On Reddit: [12] http://goo.gl/ZAurK6 * Hakaru: An embedded probabilistic programming language Domain: indiana.edu, Score: 40, Comments: 10 Original: [13] http://goo.gl/PLcCGW On Reddit: [14] http://goo.gl/aP7giY * Why does David Turner say type classes were a bad idea? Domain: self.haskell, Score: 37, Comments: 96 Original: [15] http://goo.gl/WqRLZQ On Reddit: [16] http://goo.gl/WqRLZQ * Let's Build a Browser Engine in Haskell: Part 2 Domain: hrothen.github.io, Score: 33, Comments: 14 Original: [17] http://goo.gl/PyxMOy On Reddit: [18] http://goo.gl/45L7hs * Proposal for copatterns in Idris Domain: github.com, Score: 30, Comments: 38 Original: [19] http://goo.gl/7Zv7U1 On Reddit: [20] http://goo.gl/pPIq8p * GHC Blog: Static pointers and serialisation (by SPJ) Domain: ghc.haskell.org, Score: 28, Comments: 2 Original: [21] http://goo.gl/HJrm4t On Reddit: [22] http://goo.gl/kPxJj5 * introduction to the basic lens operators Domain: intolerable.me, Score: 26, Comments: 12 Original: [23] http://goo.gl/VXlKnm On Reddit: [24] http://goo.gl/b0bLfE * Prisms lead to typeclasses for subtractive types Domain: gelisam.blogspot.ca, Score: 22, Comments: 6 Original: [25] http://goo.gl/l9QIIm On Reddit: [26] http://goo.gl/GMD5Cj * Preventing memoization in (AI) search problems Domain: okmij.org, Score: 22, Comments: 16 Original: [27] http://goo.gl/oEb6WD On Reddit: [28] http://goo.gl/jqVKPS * A shallow survey of formal methods for C code Domain: imperialviolet.org, Score: 21, Comments: 7 Original: [29] http://goo.gl/rJgycy On Reddit: [30] http://goo.gl/RrXwns * Haskell Lectures Domain: shuklan.com, Score: 21, Comments: 1 Original: [31] http://goo.gl/Ef0E4 On Reddit: [32] http://goo.gl/VASdCX * A Netwire 5 Tutorial Domain: self.haskell, Score: 20, Comments: 5 Original: [33] http://goo.gl/BmcKWr On Reddit: [34] http://goo.gl/BmcKWr * Haskell Symposium talk on inline Objective-C in Haskell Domain: speakerdeck.com, Score: 20, Comments: 0 Original: [35] http://goo.gl/VDHOiP On Reddit: [36] http://goo.gl/TyL90G * HaLVM / Unikernel talk at the Xen User Summit on Monday, September 15th Domain: sched.co, Score: 19, Comments: 8 Original: [37] http://goo.gl/HoVwWT On Reddit: [38] http://goo.gl/9lqCE0 * Fun with (Extended Kalman) Filters Domain: idontgetoutmuch.wordpress.com, Score: 18, Comments: 2 Original: [39] http://goo.gl/8EGSQR On Reddit: [40] http://goo.gl/aTASRU Top StackOverflow Questions * Why does OCaml sometimes require eta expansion? votes: 12, answers: 1 Read on SO: [41] http://goo.gl/TsBuz5 * Can Haskell evaluate and not garbage collect random indexes in a list? votes: 11, answers: 2 Read on SO: [42] http://goo.gl/zXsBT1 * How do I install dependencies when cross compiling haskell code? votes: 10, answers: 1 Read on SO: [43] http://goo.gl/3qH6zK * Haskell lenses: how to make view play nicely with traverse? votes: 10, answers: 3 Read on SO: [44] http://goo.gl/EWrR5j * Is there an instance of Monad but not of MonadFix? votes: 10, answers: 2 Read on SO: [45] http://goo.gl/EAh0qc * How can the continuation monad be expressed using the free monad? votes: 10, answers: 3 Read on SO: [46] http://goo.gl/ymMoFw Until next time, [47]+Daniel Santa Cruz References 1. http://www.haskellforall.com/2014/09/morte-intermediate-language-for-super.… 2. http://www.reddit.com/r/haskell/comments/2g6wsx/haskell_for_all_morte_an_in… 3. http://www.haskell.org/pipermail/ghc-devs/2014-September/006265.html 4. http://www.reddit.com/r/haskell/comments/2fxqts/amp_has_landed_in_ghc_main/ 5. http://www.reddit.com/r/haskell/comments/2g3em9/meta_whats_with_all_these_d… 6. http://www.reddit.com/r/haskell/comments/2g3em9/meta_whats_with_all_these_d… 7. https://www.youtube.com/playlist?list=PL4UWOFngo5DW6nKDjK0UB5Oy9zmdWdo7K 8. http://www.reddit.com/r/haskell/comments/2frhfn/haskell_implementors_worksh… 9. https://www.fpcomplete.com/blog/2014/09/hiring-haskell-web-ui-dev 10. http://www.reddit.com/r/haskell/comments/2g4mdu/fp_complete_is_hiring_haske… 11. http://www.reddit.com/r/haskell/comments/2fwwx3/how_to_get_approx_stack_tra… 12. http://www.reddit.com/r/haskell/comments/2fwwx3/how_to_get_approx_stack_tra… 13. http://indiana.edu/~ppaml/ 14. http://www.reddit.com/r/haskell/comments/2g4q81/hakaru_an_embedded_probabil… 15. http://www.reddit.com/r/haskell/comments/2foggq/why_does_david_turner_say_t… 16. http://www.reddit.com/r/haskell/comments/2foggq/why_does_david_turner_say_t… 17. http://hrothen.github.io/2014/09/08/lets-build-a-browser-engine-in-haskell-… 18. http://www.reddit.com/r/haskell/comments/2fu2l5/lets_build_a_browser_engine… 19. https://github.com/idris-lang/Idris-dev/wiki/Copatterns 20. http://www.reddit.com/r/haskell/comments/2fy1hj/proposal_for_copatterns_in_… 21. https://ghc.haskell.org/trac/ghc/blog/simonpj/StaticPointers 22. http://www.reddit.com/r/haskell/comments/2g4vvf/ghc_blog_static_pointers_an… 23. http://intolerable.me/lens-operators-intro/ 24. http://www.reddit.com/r/haskell/comments/2fud8k/introduction_to_the_basic_l… 25. http://gelisam.blogspot.ca/2014/09/prisms-lead-to-typeclasses-for.html 26. http://www.reddit.com/r/haskell/comments/2foz4a/prisms_lead_to_typeclasses_… 27. http://okmij.org/ftp/Haskell/#memo-off 28. http://www.reddit.com/r/haskell/comments/2g9akh/preventing_memoization_in_a… 29. https://www.imperialviolet.org/2014/09/07/provers.html 30. http://www.reddit.com/r/haskell/comments/2fsvns/a_shallow_survey_of_formal_… 31. http://shuklan.com/haskell/ 32. http://www.reddit.com/r/haskell/comments/2g672e/haskell_lectures/ 33. http://www.reddit.com/r/haskell/comments/2fs4mv/a_netwire_5_tutorial/ 34. http://www.reddit.com/r/haskell/comments/2fs4mv/a_netwire_5_tutorial/ 35. https://speakerdeck.com/mchakravarty/foreign-inline-code-in-haskell-haskell… 36. http://www.reddit.com/r/haskell/comments/2fve6k/haskell_symposium_talk_on_i… 37. http://sched.co/1rFBMed 38. http://www.reddit.com/r/haskell/comments/2g2sa0/halvm_unikernel_talk_at_the… 39. http://idontgetoutmuch.wordpress.com/2014/09/09/fun-with-extended-kalman-fi… 40. http://www.reddit.com/r/haskell/comments/2fwbhd/fun_with_extended_kalman_fi… 41. http://stackoverflow.com/questions/25763412/why-does-ocaml-sometimes-requir… 42. http://stackoverflow.com/questions/25751194/can-haskell-evaluate-and-not-ga… 43. http://stackoverflow.com/questions/25765893/how-do-i-install-dependencies-w… 44. http://stackoverflow.com/questions/25773444/haskell-lenses-how-to-make-view… 45. http://stackoverflow.com/questions/25814489/is-there-an-instance-of-monad-b… 46. http://stackoverflow.com/questions/25827271/how-can-the-continuation-monad-… 47. https://plus.google.com/105107667630152149014/about

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Boolean formula: part2
by Dmitry Bogatov 17 Sep '14

17 Sep '14

Hello! I have following code: buildFormulas :: [Variable] -> Int -> [Formula] buildFormulas vars 0 = map Var vars buildFormulas vars n = join $ forM [0 .. n - 1 ] $ \i -> do x <- buildFormulas vars i y <- buildFormulas vars (n - i - 1) z <- buildFormulas vars (n - 1) let t1 = case z of (Negation _) -> [] _ -> [Negation z] t1 ++ [conj x y, impl x y, disj x y] It is correct, typechecks, but it is insanely slow (calculating take 35 (buildFormulas [P, Q, R] 5) hangs my computer). My guess is that it builds too much before returning first results. Can you suggest more efficent way enumerate formulas of set of variables? -- Best regards, Dmitry Bogatov <KAction(a)gnu.org>, Free Software supporter, esperantisto and netiquette guardian. GPG: 54B7F00D

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Re: [Haskell-cafe] Hackage package "synopsis" sections
by Mike Izbicki 17 Sep '14

17 Sep '14

One problem with using the README file as the synopsis is that a lot of the standard information in a README file would be out of place when displayed on hackage. For example, many READMEs have: * installation instructions * a list of contributors * a list of known bugs * how to contribute bug reports * licensing information Some of this information is very out of place on hackage, some of it is duplication of the standard fields in the cabal file, and some of it is just not things that a new user needs to see on their first visit to the package documentation. Maybe this could be solved by having the name of the synopsis file as a variable the author can specify. It can default to README(.md) but can be overridden to something else for authors who choose to do so. On Tue, Sep 16, 2014 at 11:20 PM, Mike Izbicki <mike.izbicki(a)gmail.com> wrote: > One problem with using the README file as the synopsis is that a lot > of the standard information in a README file would be out of place > when displayed on hackage. For example, many READMEs have: > > * installation instructions > * a list of contributors > * a list of known bugs > * how to contribute bug reports > * licensing information > > Some of this information is very out of place on hackage, some of it > is duplication of the standard fields in the cabal file, and some of > it is just not things that a new user needs to see on their first > visit to the package documentation. > > Maybe this could be solved by having the name of the synopsis file as > a variable the author can specify. It can default to README(.md) but > can be overridden to something else for authors who choose to do so. > > On Tue, Sep 16, 2014 at 8:58 PM, Michael Snoyman <michael(a)snoyman.com> wrote: >> >> >> On Wed, Sep 17, 2014 at 12:10 AM, Richard Lewis <richard(a)rjlewis.me.uk> >> wrote: >>> >>> So there's clearly an important technical hurdle to be overcome for >>> this: what markup language is to be used?; and where should the >>> synposis be stored? >>> >>> But I can't help thinking that the real work is in getting the >>> synopses written. Encouraging package authors/maintainers to add them >>> to their own packages is one possible way forward. >>> >> >> Actually, if we get the README support in place, I think it should be much >> easier to get package authors to write synopses. Most projects have a README >> file already, and getting it included in Hackage would now be a one-line >> pull request (adding `extra-source-files: README.extension`). It will >> hopefully be easier to get authors to elaborate on their READMEs then >> because: >> >> 1. They're serving double duty, both in the project repository and on >> Hackage. >> 2. It's trivial for outside developers to send pull requests against a >> simple text file. I've received quite a few such contributions on projects I >> maintain, even from people not ready to work on the code itself. >> >>> >>> At Mon, 15 Sep 2014 11:28:38 -0700, >>> Tikhon Jelvis wrote: >>> >>> > Maybe we could have a guerrilla campaign of pull requests adding >>> > examples and a bit of explanation to every package you like that >>> > doesn't have them... That could also be a good way for beginners who >>> > want to contribute to start. >>> >>> Another, as Tikhon suggests, would be for others to write them and >>> send pull requests (or whatever) to the maintainers. >>> >>> At Mon, 15 Sep 2014 20:10:18 -0400, >>> Dominick Samperi wrote: >>> >>> > I think this is a great idea, but it probably needs a complementary >>> > "nudge" if it is going to have a significant impact. This could be >>> > incorporated into the package submission process where the submitter >>> > runs a final check and is warned when examples are not provided for >>> > exported functions (unless an "opt out" flag is turned on for >>> > functions that have "obvious" semantics). >>> >>> And yet another, as Dominick suggests, is effectively to require a >>> synposis when a package is submitted. In CPAN, it's just become part >>> of the culture, but it's also not required and you do find packages >>> without a synposis. >>> >>> If there's interest, I'd like to solicit some discussion on this part >>> of the proposal on this thread... >>> >>> There may also be a potentially significant difference between >>> Hackage/Haskell and CPAN/Perl: most CPAN packages tend to be quite >>> small and specific in their purpose and consequently have just a few, >>> simple common use cases which suit a synposis very well. Hackage >>> packages, on the other hand, are quite often more broad in their >>> scope, often comprising many modules. Also, Perl has only one >>> semantics for organising code at the finest level: sequential, >>> imperative statements. In Haskell, some packages actually define whole >>> coding styles. As a result, it's always pretty obvious how to write a >>> few isolated lines of Perl code, but not necessarily so with Haskell >>> code. Anyway, I'm sure all this can be overcome, and/or argued >>> against. >>> >>> Richard >>> -- >>> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- >>> Richard Lewis >>> j: ironchicken(a)jabber.earth.li >>> @: lewisrichard >>> http://www.richardlewis.me.uk/ >>> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >>

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Mapping over Type Level Literals
by Dominic Steinitz 17 Sep '14

17 Sep '14

I was playing around with Haar functions and realised I could encode the constraints in the types > > {-# LANGUAGE DataKinds #-} > > {-# LANGUAGE TypeOperators #-} > > {-# LANGUAGE KindSignatures #-} > > {-# LANGUAGE TypeFamilies #-} > > {-# LANGUAGE TypeOperators #-} > > {-# LANGUAGE Rank2Types #-} > > {-# LANGUAGE ScopedTypeVariables #-} > > > import GHC.TypeLits > > import Data.Proxy > > > data Haar (a :: Nat) (b :: Nat) = Haar { unHaar :: Double -> Double } > > > haar :: forall n k . > > (KnownNat n, KnownNat k, (2 * k - 1 <=? 2^n - 1) ~ 'True) => > > Haar n (2 * k - 1) > > haar = Haar g where > > g t | (k' - 1) * 2 ** (-n') < t && t <= k' * 2 ** (-n') = 2 ** ((n' - 1) / 2) > > | k' * 2 ** (-n') < t && t <= (k' + 1) * 2 ** (-n') = -2 ** ((n' - 1) / 2) > > | otherwise = 0 > > where > > n' = fromIntegral (natVal (Proxy :: Proxy n)) > > k' = 2 * (fromIntegral (natVal (Proxy :: Proxy k))) - 1 So now I can do e.g. > *Main> unHaar (haar :: Haar 3 7) 0.05 > 0.0 > *Main> unHaar (haar :: Haar 3 6) 0.05 > > <interactive>:359:9: > Couldn't match type ‘2 * k0’ with ‘7’ > The type variable ‘k0’ is ambiguous > Expected type: 'True > Actual type: ((2 * k0) - 1) <=? ((2 ^ 3) - 1) > In the first argument of ‘unHaar’, namely ‘(haar :: Haar 3 6)’ > In the expression: unHaar (haar :: Haar 3 6) 0.05 > In an equation for ‘it’: it = unHaar (haar :: Haar 3 6) 0.05 > *Main> unHaar (haar :: Haar 3 9) 0.05 > > <interactive>:360:9: > Couldn't match type ‘'False’ with ‘'True’ > Expected type: 'True > Actual type: ((2 * 5) - 1) <=? ((2 ^ 3) - 1) > In the first argument of ‘unHaar’, namely ‘(haar :: Haar 3 9)’ > In the expression: unHaar (haar :: Haar 3 9) 0.05 > In an equation for ‘it’: it = unHaar (haar :: Haar 3 9) 0.05 > *Main> So errors get rejected at compilation time as one would like and with moderately informative error messages. But now of course I would like to map over n and k but these are at the type level. Can this be done? I imagine unsafeCoerce would have to come into it somewhere. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Re: [Haskell-cafe] Type union
by oleg＠okmij.org 17 Sep '14

17 Sep '14

Type unions are also necessary for extensible effects. In fact, they are the foundation of extensible effects. One implementation http://okmij.org/ftp/Haskell/extensible/TList.hs uses only common and uncontroversial extensions (No Typeable). It could be converted to closed type families. The other, more optimal implementation, relies on Typeable, so that injection and projection become constant time, regardless of the size of the union. The Typeable constraint can be dispensed with; see the HList implementation of type-indexed coproducts (TIC), which are essentially open unions.

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