September 2013 - Haskell-Cafe

Package compatibility (SemVer, PVP, ECT)
by Thiago Negri 13 Sep '13

13 Sep '13

I've just read Semantic Versioning (SemVer) [1], Package Versioning Policy (PVP) [2] and Eternal Compatibility in Theory (ECT) [3]. Is PVP the one that every package on Hackage should use? SemVer seems to make more sense to me. Also, ECT looks very promising, but it is dated back to 2005 and I didn't see any package using it. Why? If Cabal could map the package version number to the versioned module name, there will be no need to "uglify" the client code. I mean, when I import module A and define in dependencies "a == 1.8.3", Cabal would map that to module A_7 and bind that to the "A" I've imported. We would be locked in with Cabal, but that seems to be a good tradeoff. [1] http://semver.org [2] http://www.haskell.org/haskellwiki/Pvp [3] http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue2/EternalCompatibi…

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Bytestring map/zipWith rationale
by Scott Lawrence 12 Sep '13

12 Sep '13

Hello all, Something's always bothered me about map and zipWith for ByteString. Why is it map :: (Word8 -> Word8) -> ByteString -> ByteString but zipWith :: (Word8 -> Word8 -> a) -> ByteString -> ByteString -> [a] ? Obviously they can be transformed into each other with pack/unpack, and as I understand it, the compiler performs sufficient optimizations so that there's no performance hit to doing things like (pack $ zipWith xor a b), but it still seems inconsistent. Is there a deep reason for this? -- Scott Lawrence Linux baidar 3.10.9-1-ARCH #1 SMP PREEMPT Wed Aug 21 13:49:35 CEST 2013 x86_64 GNU/Linux

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dns library version 1.0.0
by Kazu Yamamoto 12 Sep '13

12 Sep '13

Hi all, I have released dns library version 1.0.0. This version provides new APIs. Thus, version is now 1.0.0. The design and implementation was done by Michael Orlitzky based on his experience. Enjoy! --Kazu

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GLFW not working in Cabal 1.18
by Thiago Negri 12 Sep '13

12 Sep '13

The package GLFW is not building in Cabal 1.18. Setup.hs [1] depends on `rawSystemStdInOut` [2] that changed signature between 1.16 and 1.18. Is this considered a public API of Cabal? Cabal 1.16 rawSystemStdInOut :: Verbosity -> FilePath -> [String] -> Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode) Cabal 1.18 rawSystemStdInOut :: Verbosity -> FilePath -> [String] -> Maybe FilePath -- new arg -> Maybe [(String, String)] -- new arg -> Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode) Compilation output: [1 of 1] Compiling Main ( /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs, /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/dist/setup/Main.o ) /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs:167:33: Couldn't match expected type `IO (t0, t1, ExitCode)' with actual type `Maybe (String, Bool) -> Bool -> IO (String, String, ExitCode)' In the return type of a call of `rawSystemStdInOut' Probable cause: `rawSystemStdInOut' is applied to too few arguments In a stmt of a 'do' block: (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, "-o", objPath] ++ flags) Nothing False In the expression: do { hClose outHandle; hPutStr inHandle contents; hClose inHandle; (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, ....] ++ flags) Nothing False; .... } /tmp/GLFW-0.5.1.0-4035/GLFW-0.5.1.0/Setup.hs:167:113: Couldn't match expected type `Maybe [(String, String)]' with actual type `Bool' In the fifth argument of `rawSystemStdInOut', namely `False' In a stmt of a 'do' block: (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, "-o", objPath] ++ flags) Nothing False In the expression: do { hClose outHandle; hPutStr inHandle contents; hClose inHandle; (out, err, exitCode) <- rawSystemStdInOut verbosity "cc" (["-c", path, ....] ++ flags) Nothing False; .... } Failed to install GLFW-0.5.1.0 [1] http://code.haskell.org/GLFW/Setup.hs [2] https://github.com/haskell/cabal/blob/d16c307c33fb7af19d8f17a2ad8be4902a3af…

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Hackage 2 now available for beta testing
by Duncan Coutts 12 Sep '13

12 Sep '13

Well-Typed and the Industrial Haskell Group (IHG) are very pleased to announce that Hackage 2 is now available for public beta testing. The plan is to do the final switchover in late September, to coincide with ICFP. http://beta.hackage.haskell.org/ Read on for details of how to help with the public beta, an overview of the new features and what the IHG has been doing to help. Support from the Industrial Haskell Group ========================================= The IHG is a consortium of companies that rely on Haskell. The IHG members have funded the effort to get Hackage 2 up to feature parity and get it ready for the switchover. The IHG funded this effort because while the volunteer effort got us the "first 90%" of the way there (including adding a number of new features) there was still the "last 90%" to do to get it production ready. The IHG members decided to fund Hackage 2 not just because they are good citizens, but out of enlightened self-interest. Hackage has over 5000 packages written by over 1000 people -- including the world's best Haskell developers. This is a massive resource. The IHG members recognise that improvements to the tools and infrastructure that the community uses helps the community to produce more and better code. This is a benefit to everyone in the community -- including the commercial users. The IHG is keen to increase its membership so that more resources can be dedicated to improving the Haskell development platform. If your organisation relies on Haskell in some way then you may want to consider joining. See the IHG website for more details or contact info(a)industry.haskell.org. [IHG website]: http://industry.haskell.org/ Despite the help of the IHG in getting to this point, Hackage is a community project, and its success depends on the community maintaining and further improving the new server. The code is now on github so it is easier to contribute, and now that the server is live there is more immediate gratification for volunteers contributing fixes and new features. Public beta =========== We would like to encourage you to take part in the public beta testing. We need help both from package authors as well as other users of the site. Please report any problems you find using the issue tracker on the hackage-server github site. [issue tracker]: https://github.com/haskell/hackage-server/issues [github site]: https://github.com/haskell/hackage-server We are mirroring packages from the old server (every 30min) so it is suitable to use as your main hackage server with some caveats: we are allowing package authors to upload (as well as doing the mirroring) so you may find a slightly different set of packages on this server. If you are a package author then you are welcome to poke about and upload packages. We have imported user accounts from the old server (except for a small number of early adopters of the original server who will need to contact an administrator). Note that before we do the final switchover we will *delete everything* from the beta instance and do a fresh import from the old hackage server. Configuring cabal-install ------------------------- Edit your ~/.cabal/config file. Comment-out the existing "remote-repo" line near the top of the file and add in a new one like this: --remote-repo: hackage.haskell.org:http://hackage.haskell.org/packages/archive remote-repo: beta.hackage.haskell.org:http://beta.hackage.haskell.org/ New features ============ Though our main priority has been feature parity so that we can switch over, volunteers have contributed several new features, including better package search, a new site theme, improved security, the ability to fix package dependencies after a release, changelogs, and a REST-style interface. See the beta site for more details on these new features, plus details of other features that are partially implemented or are in need of improvement. [new features]: http://beta.hackage.haskell.org/new-features Contributing to the development =============================== The code is on github and we welcome pull requests. There are open tickets describing existing bugs and features that we want or that are in need of improvement. Help on any of these would be greatly appreciated. There is some developer and user documentation on the github wiki, including a quick guide to getting your own server instance up and running. You can ask questions on the cabal-devel mailing list or on IRC in the #hackage channel on freenode. [code]: https://github.com/haskell/hackage-server [github wiki]: https://github.com/haskell/hackage-server/wiki [cabal-devel]: http://www.haskell.org/mailman/listinfo/cabal-devel -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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ANNOUNCE: HDBI-1.1.1 and friends
by Alexey Uimanov 12 Sep '13

12 Sep '13

Hello, haskellers! I am glad to announce new tested version of HDBI-1.1.1 (Haskell Database Independent interface). Now it becomes much more databae-independent than before because of SQlite3 driver in addition to PostrgreSQL driver. What changed from version 1.0.0: 1. removed `affectedRows` from `Statement`: sqlite does not support this feature directly. You can still use this feature of PostgreSQL with function `pgAffectedRows` 2. fixed default implementation of `executeMany`: perform reset after execute, statement will be in New state after `executeMany`. 3. "stm" is now a dependency 4. DriverUtils: ChildList implemented with STM, `closeAllChildren` now wait for all child statement finalizers and reduced the probability of blocking. 5. Tests improved. 6. Common tests moved to separate package http://hackage.haskell.org/package/hdbi-tests HDBI available on Hackage http://hackage.haskell.org/package/hdbi http://hackage.haskell.org/package/hdbi-postgresql http://hackage.haskell.org/package/hdbi-sqlite Here is github: https://github.com/s9gf4ult/hdbi https://github.com/s9gf4ult/hdbi-postgresql https://github.com/s9gf4ult/hdbi-sqlite https://github.com/s9gf4ult/hdbi-tests https://github.com/s9gf4ult/hdbi-svc Look at the issues list https://github.com/s9gf4ult/hdbi/issues?state=open Your issues and comments are welcome. Pull requests and contribution are welcome as well. Have a nice day.

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Haskell Weekly News: Issue 279
by Daniel Santa Cruz 11 Sep '13

11 Sep '13

Welcome to issue 279 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 25 to September 7, 2013. Quotes of the Week * dmwit: I typically figure a parse error on line 500 means I have 500 lines to inspect. * bos: bah, all you category wonks spoil the atmosphere. we were simple peasants, and proud, once. * merijn: edwardk's delegation has reached the point where he can have the language implementor explain his libraries... * ReinH: haasn asks an offhand question. edwardk wrote a thesis on it. * JaffaCake: children are automatically killed if their parents die for any reason * geekosaur: if the platform is batteries included, lens is a full fledged transformer robot included :) * FreeFull: Mathematical laws are so much nicer than legal laws * twey: to edwardk, v. to be working on a larger problem and absent-mindedly spin out a package to satisfy dependencies, accidentally solving the problem more elegantly than existing solutions (but perhaps incomprehensibly to anyone who can't dualize in their head) * elliott: lens is the DLC expansion pack for Haskell * monochrom: new catchy thing to say: an IORef Int contains an Int in the same sense as an IO Int contains an Int Top Reddit Stories * Announcing GHC iOS Domain: self.haskell, Score: 259, Comments: 52 On Reddit: [1] http://goo.gl/28XFCH Original: [2] http://goo.gl/28XFCH * The Haskell Cast: Episode 1 - Edward Kmett on Lenses Domain: haskellcast.com, Score: 101, Comments: 35 On Reddit: [3] http://goo.gl/umnk1Y Original: [4] http://goo.gl/0vKtZ5 * The Haxl Project at Facebook (slides from my talk at ZuriHac) Domain: github.com, Score: 97, Comments: 76 On Reddit: [5] http://goo.gl/1nWvfs Original: [6] http://goo.gl/pWk1Ll * Haskell on the Daily WTF Domain: thedailywtf.com, Score: 96, Comments: 14 On Reddit: [7] http://goo.gl/kJs9FG Original: [8] http://goo.gl/qdM3sz * Cabal 1.18.0 released Domain: groups.google.com, Score: 92, Comments: 30 On Reddit: [9] http://goo.gl/QbNCmx Original: [10] http://goo.gl/OtYTZG * I've got functional curse. I lost my ability to write in C++. I have to ship C++. What do I do? Domain: self.haskell, Score: 69, Comments: 92 On Reddit: [11] http://goo.gl/oQBzBo Original: [12] http://goo.gl/oQBzBo * `pipes-4.0.0` release candidate Domain: self.haskell, Score: 58, Comments: 42 On Reddit: [13] http://goo.gl/NQDJyT Original: [14] http://goo.gl/NQDJyT * Lenses from Scratch Domain: fpcomplete.com, Score: 54, Comments: 29 On Reddit: [15] http://goo.gl/jYq0ob Original: [16] http://goo.gl/6aeyUP * Controlling Chromium in Haskell Domain: jaspervdj.be, Score: 51, Comments: 3 On Reddit: [17] http://goo.gl/W7VVPh Original: [18] http://goo.gl/WD32IC * Interactive Programming: Hot-swapping in Elm Domain: elm-lang.org, Score: 50, Comments: 6 On Reddit: [19] http://goo.gl/D7LYnM Original: [20] http://goo.gl/A9TwJ5 * Lens/Aeson Traversal/Prism Domain: fpcomplete.com, Score: 48, Comments: 10 On Reddit: [21] http://goo.gl/9o9r3R Original: [22] http://goo.gl/IuFJiC * Mandelbrot Domain: fpcomplete.com, Score: 42, Comments: 6 On Reddit: [23] http://goo.gl/pY3MHE Original: [24] http://goo.gl/gySnCb * ANNOUNCE: posix-paths, for faster file system operations Domain: permalink.gmane.org, Score: 41, Comments: 29 On Reddit: [25] http://goo.gl/vI3UCF Original: [26] http://goo.gl/PlQsAO * Control Text LCD with Haskell code. The code runs on mbed. The RAM size is 32kB. Using Ajhc Haskell compiler. Domain: youtube.com, Score: 41, Comments: 8 On Reddit: [27] http://goo.gl/jchNCd Original: [28] http://goo.gl/CqjxBp * DeepSeq Once Domain: fpcomplete.com, Score: 37, Comments: 57 On Reddit: [29] http://goo.gl/WBohP4 Original: [30] http://goo.gl/zRupez * DSLs and Towers of Abstraction with Gershom Bazerman at NY Haskell Domain: vimeo.com, Score: 36, Comments: 6 On Reddit: [31] http://goo.gl/737bji Original: [32] http://goo.gl/rzHY63 * Port Catch to GHC Domain: self.haskell, Score: 36, Comments: 10 On Reddit: [33] http://goo.gl/6mKcFZ Original: [34] http://goo.gl/6mKcFZ * Reflecting values to types and back Domain: fpcomplete.com, Score: 33, Comments: 11 On Reddit: [35] http://goo.gl/id2AiE Original: [36] http://goo.gl/LJHQfW * Haskell Game Programming Book? Domain: self.haskell, Score: 33, Comments: 25 On Reddit: [37] http://goo.gl/qJDSWv Original: [38] http://goo.gl/qJDSWv * ANN: Cabal v1.18.0 released Domain: haskell.org, Score: 33, Comments: 0 On Reddit: [39] http://goo.gl/w08cdI Original: [40] http://goo.gl/aYSkJf * How could little me make Haskell better? Domain: self.haskell, Score: 31, Comments: 31 On Reddit: [41] http://goo.gl/qtnee1 Original: [42] http://goo.gl/qtnee1 Top StackOverflow Questions * Examples of a monad whose Applicative part can be better optimized than the Monad part votes: 19, answers: 3 Read on SO: [43] http://goo.gl/06Awvz * Haskell: unnecessary reevaluations of constant expressions votes: 17, answers: 2 Read on SO: [44] http://goo.gl/82hb99 * equal sign with haskell literals votes: 16, answers: 3 Read on SO: [45] http://goo.gl/F08H9Z * Haskell standalone desktop application authentication (using google / facebook / twitter / etc accounts) votes: 15, answers: 1 Read on SO: [46] http://goo.gl/JQaIfD * Continue debugging in GHC after interrupt votes: 12, answers: 0 Read on SO: [47] http://goo.gl/ZpQms7 * How to automatically generate (and see the code for) Read and Show in Haskell votes: 12, answers: 1 Read on SO: [48] http://goo.gl/dET9H6 * Killing a Haskell binary votes: 11, answers: 2 Read on SO: [49] http://goo.gl/FDnsvz * Haskell: What does ((->) t) mean? votes: 10, answers: 3 Read on SO: [50] http://goo.gl/HAV96m * What does the term “reason about” mean in computer science? votes: 10, answers: 5 Read on SO: [51] http://goo.gl/CV6EE5 * Idiomatic bidirectional Pipes with downstream state without loss votes: 9, answers: 3 Read on SO: [52] http://goo.gl/rjX2uK Until next time, [53]+Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/1lboh4/announcing_ghc_ios/ 2. http://www.reddit.com/r/haskell/comments/1lboh4/announcing_ghc_ios/ 3. http://www.haskellcast.com/episode/001-edward-kmett-on-lenses/ 4. http://www.reddit.com/r/haskell/comments/1l45c3/the_haskell_cast_episode_1_… 5. https://github.com/meiersi/HaskellerZ/blob/master/meetups/20130829-FPAftern… 6. http://www.reddit.com/r/haskell/comments/1le4y5/the_haxl_project_at_faceboo… 7. http://thedailywtf.com/Articles/OMGWTF2-The-Honorable-Mentions.aspx 8. http://www.reddit.com/r/haskell/comments/1locjo/haskell_on_the_daily_wtf/ 9. https://groups.google.com/forum/#!topic/haskell-cafe/SFoNwaq8wdc 10. http://www.reddit.com/r/haskell/comments/1lqo13/cabal_1180_released/ 11. http://www.reddit.com/r/haskell/comments/1lrrhk/ive_got_functional_curse_i_… 12. http://www.reddit.com/r/haskell/comments/1lrrhk/ive_got_functional_curse_i_… 13. http://www.reddit.com/r/haskell/comments/1lh5u7/pipes400_release_candidate/ 14. http://www.reddit.com/r/haskell/comments/1lh5u7/pipes400_release_candidate/ 15. https://www.fpcomplete.com/user/tel/lenses-from-scratch 16. http://www.reddit.com/r/haskell/comments/1l8ld7/lenses_from_scratch/ 17. http://jaspervdj.be/posts/2013-09-01-controlling-chromium-in-haskell.html 18. http://www.reddit.com/r/haskell/comments/1liux8/controlling_chromium_in_has… 19. http://elm-lang.org/blog/Interactive-Programming.elm 20. http://www.reddit.com/r/haskell/comments/1ln75a/interactive_programming_hot… 21. https://www.fpcomplete.com/user/tel/lens-aeson-traversals-prisms 22. http://www.reddit.com/r/haskell/comments/1l64oe/lensaeson_traversalprism/ 23. https://www.fpcomplete.com/user/edwardk/snippets/mandelbrot 24. http://www.reddit.com/r/haskell/comments/1lkkb0/mandelbrot/ 25. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19946 26. http://www.reddit.com/r/haskell/comments/1liahv/announce_posixpaths_for_fas… 27. http://www.youtube.com/watch?v=fOsjEfNmTkY 28. http://www.reddit.com/r/haskell/comments/1lkboo/control_text_lcd_with_haske… 29. https://www.fpcomplete.com/user/edwardk/snippets/once 30. http://www.reddit.com/r/haskell/comments/1lo15w/deepseq_once/ 31. https://vimeo.com/72870861 32. http://www.reddit.com/r/haskell/comments/1l4ph3/dsls_and_towers_of_abstract… 33. http://www.reddit.com/r/haskell/comments/1lj39q/port_catch_to_ghc/ 34. http://www.reddit.com/r/haskell/comments/1lj39q/port_catch_to_ghc/ 35. https://www.fpcomplete.com/user/thoughtpolice/using-reflection 36. http://www.reddit.com/r/haskell/comments/1l9qzw/reflecting_values_to_types_… 37. http://www.reddit.com/r/haskell/comments/1lgano/haskell_game_programming_bo… 38. http://www.reddit.com/r/haskell/comments/1lgano/haskell_game_programming_bo… 39. http://www.haskell.org/pipermail/haskell-cafe/2013-September/108722.html 40. http://www.reddit.com/r/haskell/comments/1lqo00/ann_cabal_v1180_released/ 41. http://www.reddit.com/r/haskell/comments/1lf6z5/how_could_little_me_make_ha… 42. http://www.reddit.com/r/haskell/comments/1lf6z5/how_could_little_me_make_ha… 43. http://stackoverflow.com/questions/18590639/examples-of-a-monad-whose-appli… 44. http://stackoverflow.com/questions/18655561/haskell-unnecessary-reevaluatio… 45. http://stackoverflow.com/questions/18598365/equal-sign-with-haskell-literals 46. http://stackoverflow.com/questions/18472448/haskell-standalone-desktop-appl… 47. http://stackoverflow.com/questions/18504432/continue-debugging-in-ghc-after… 48. http://stackoverflow.com/questions/18538055/how-to-automatically-generate-a… 49. http://stackoverflow.com/questions/18430748/killing-a-haskell-binary 50. http://stackoverflow.com/questions/18561859/haskell-what-does-t-mean 51. http://stackoverflow.com/questions/18666821/what-does-the-term-reason-about… 52. http://stackoverflow.com/questions/18471706/idiomatic-bidirectional-pipes-w… 53. https://plus.google.com/105107667630152149014/about

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GHC API + Cabal API + Cabal version checks: is there a way out?
by JP Moresmau 11 Sep '13

11 Sep '13

Hello all, I have an issue that has been nagging me for a while, and I'd like to make sure I haven't missed a solution to it. I'm the maintainer for the buildwrapper package. This package has dependencies on the Cabal and GHC libraries, and also uses the cabal-install executable. For example it will run cabal configure by invoking the executable, then it will load the LocalBuildInfo via the Cabal API, and extract the relevant information to start a GHC session with the proper flags, etc. Now yesterday some great news were announced, a new version of Cabal! However, I can't use it. I can of course install the Cabal library and rebuild the cabal executable but - buildwrapper will only use the version of the Cabal library that was bundled with GHC, since the GHC library has a dependency on that version of Cabal - so the library access will be using Cabal 1.16, but the executable will be 1.18 - and the library checks when reading the setup-config file that the Cabal versions match. Hence, running configure with 1.18 create build information I can't read back in 1.16. So I'm stuck. People can easily install cabal 1.18 and build their projects with it, but I can't use it in conjunction with buildwrapper. Some solutions I've considered - do not use the Cabal API and write my own code to read the local build info, and to replace all the Cabal library code I use - do not use the cabal-install executable but do everything by the API Both solutions reek of madness, and involve rewriting code that others have already written, and a maintenance nightmare with each Cabal release. So, do I have to wait a new release of GHC/Haskell Platform or is there a better solution? I suspect ghc-mod and other similar packages have the same issues. Thanks for any help! -- JP Moresmau http://jpmoresmau.blogspot.com/

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GHC API question: Getting access to cpp processed source
by AlanKim Zimmerman 11 Sep '13

11 Sep '13

Hi Cafe I have just discovered that GHC.getTokenStream fails if it is used on a module with CPP directives in it. This is reported in http://ghc.haskell.org/trac/ghc/ticket/8265 Is there an easy way to get access to the pre-processed source, without having to explicitly write it to an output file in a temporary location? In other words an equivalent to getModuleSourceAndFlags that does the right thing. This currently prevents HaRe from processing files with preprocessor directives in them, I would like to come up with a workaround for current GHC versions, rather than having to wait for a future one. Regards Alan

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Reasoning about performance
by Scott Pakin 10 Sep '13

10 Sep '13

I'm a Haskell beginner, and I'm baffled trying to reason about code performance, at least with GHC. For a program I'm writing I needed to find all pairs of elements of a list. That is, given the list "ABCD" I wanted to wind up with the list [('A','B'),('A','C'),('A','D'),('B','C'),('B','D'),('C','D')], where the order of elements in the resulting list is immaterial, but I don't want both ('A', 'B') and ('B', 'A'), for example. My first attempt looked like this: allPairs1 :: [a] -> [(a, a)] allPairs1 [] = [] allPairs1 (x:xs) = map (\a -> (x, a)) xs ++ allPairs1 xs Benchmarking with ghci's ":set +s" reveals the following performance (median of three runs shown): $ ghci GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :!ghc -c -O2 allpairs.hs Prelude> :load allpairs Ok, modules loaded: AllPairs. Prelude AllPairs> :m +Control.DeepSeq Prelude Control.DeepSeq AllPairs> :show modules AllPairs ( allpairs.hs, allpairs.o ) Prelude Control.DeepSeq AllPairs> :set +s Prelude Control.DeepSeq AllPairs> deepseq (allPairs1 [1..10000]) True True (4.85 secs, 4004173984 bytes) A colleague suggested getting rid of the list append as follows: allPairs2 :: [a] -> [(a, a)] allPairs2 [] = [] allPairs2 (x:xs) = allPairs2' x xs xs where allPairs2' x [] [] = [] allPairs2' x (y:ys) zs = (x,y) : allPairs2' x ys zs allPairs2' x [] (z:zs) = allPairs2' z zs zs Not surprisingly, that runs faster: Prelude Control.DeepSeq AllPairs> deepseq (allPairs2 [1..10000]) True True (2.14 secs, 4403686376 bytes) I then figured I could do even better by rewriting the code tail-recursively: allPairs3 :: [a] -> [(a, a)] allPairs3 [] = [] allPairs3 (x:xs) = allPairs3' x xs xs [] where allPairs3' :: a -> [a] -> [a] -> [(a, a)] -> [(a, a)] allPairs3' h (x:xs) ys result = allPairs3' h xs ys ((h, x):result) allPairs3' _ [] (y:ys) result = allPairs3' y ys ys result allPairs3' _ [] [] result = result This takes half the memory as the above (yay!) but runs substantially *slower* (and appears to be thrashing my computer's memory system): Prelude Control.DeepSeq AllPairs> deepseq (allPairs3 [1..10000]) True True (10.58 secs, 2403820152 bytes) What gives? Why does my tail-recursive implementation run even slower and presumably produce more garbage than my initial, naive implementation? Is there some optimization GHC is performing for allPairs1 and allPairs2 that it can't apply to allPairs3? Similarly, how should I, as a newcomer who wants to write efficient Haskell code, reason about whether a code change is likely to improve rather than degrade performance? Guidelines like "per-iteration appends = bad" and "tail recursion = good" are great, but the preceding data indicate that something subtler is taking precedence over those guidelines with respect to code performance. Thanks in advance, -- Scott

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