August 2013 - Haskell-Cafe

Compiler stops at SpecConstr optimization
by Daniel Díaz Casanueva 01 Sep '13

01 Sep '13

Hello. While hacking in one of my projects, one of my modules stopped to compile for apparently no reason. The compiler just freezes (like if it where in an infinite loop) while trying to compile that particular module. Since I had this problem I have been trying to reduce the problem as much as I could, and I came out with this small piece of code: > module Blah (foo) where > import Data.Vector (Vector) > import qualified Data.Vector as V > foo :: (a -> a) -> Vector a -> Vector a > foo f = V.fromList . V.foldl (\xs x -> f x : xs) [] The module I am importing comes from package vector-0.10.0.1. The compiler I am using is ghc-7.6.3. The problem arises when compiling using the following flags: ghc Blah.hs -O1 -fspec-constr If I compile it verbosely (with -v), I can see that the compiler stops in the SpecConstr optimization step. I have tried to reduce the -O1 flag to a particular set of flags, but I didn't manage yet to find the complete set of flags that -O1 translates to (optLevelFlags [1] seems to give not enough information). Since this problem can be OS-dependent, my system is Debian 7 and I didn't try yet to reproduce my problem in other systems (mainly because I don't have access to other systems at the moment). I am posting this here to, with the help of the Haskell Community, check first if this is indeed a bug and, in such case, to find out where that bug comes from (GHC, vector library, something else?). Then, if necessary, I would write a bug report where appropriate. Thank you, Daniel Díaz. -- References [1] - https://github.com/ghc/ghc/blob/ghc-7.6.3-release/compiler/main/DynFlags.hs…

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Hoogle vs Hayoo
by jabolopes＠google.com 01 Sep '13

01 Sep '13

Hi, I noticed Hayoo appears as a link in the toolbox of http://hackage.haskell.org and also that Hayoo seems to display better results than Hoogle. For example, if you search for 'PublicKey' in Hayoo, you will get several results from Hackage libraries, such as, 'crypto-pubkey' and 'crypto-api'. However, the same query in Hoogle displays no results. Is Hayoo the default Hackage search engine ? Is Hoogle deprecated ? What the status ? Thank you, Jose

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Tasty not compiling
by Thiago Negri 30 Aug '13

30 Aug '13

I can't install tasty with cabal. Anyone with the same issue or a fix? $ cabal install tasty ... Test\Tasty\Core.hs:147:11: Not in scope: `witness'

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Debugging ByteString and Data.Binary.Get memory usage
by Kyle Hanson 30 Aug '13

30 Aug '13

OK I have a bunch of BSON documents that I convert to ByteStrings, put in a Map, and write to a socket based on the response. I noticed some high memory usage (in the GBs) so I decided to investigate. I simplified my problem into a small program that demonstrates clearer what is happening. I wrote two versions, one with a Lazy Map and Lazy ByteStrings and one with a Strict Map and Strict ByteStrings. Both share the same memory behavior (except the lazy BS one is faster) Here is the strict version: http://lpaste.net/92298 And here is the lazy version: http://lpaste.net/92299 I wrote this and compared the memory and speed behavior of ByteStrings generated by converting it from a BSON document and ByteStrings generated more purely. The length of the ByteString from a BSON document is 68k and the length of the "pure" BS is 70k. This is my weird memory behavior, both BSON and "pure" methods use the same amount of memory after inserting 10k of them (90mb) However when I go to lookup a value, the BSON Map explodes the memory to over 250mb. Even if I lookup just 1 value. Looking up any number of values in the "pure BS" keeps the memory usage stable (90mb). I am hoping someone can help me understand this. I have read some posts about Temporary ByteStrings causing memory issues but I don't know how to get started debugging. -- Kyle Hanson

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Announcing GHC iOS
by Luke Iannini 29 Aug '13

29 Aug '13

Hi all! (in case you don't read /r/haskell : )) Stephen Blackheath and I are extremely happy to report that as of today, GHC can natively build binaries for iOS devices and the iOS Simulator. You'll find everything you need here: http://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS The short of it is you can use "ghc-ios myFiles.hs" to get a universal ARMv7/ARMv7s/i386 static library to drop straight into an Xcode project. Stephen's already working on some wonderful FRP-powered games, I'm working on a project called SpaceTime that I'll announce shortly, and you may now all begin creating insanely great iOS software using the finest language in the world : ). Stephen deserves the lion's share of the credit for this project — it's been over 3 years in the making! Many thanks to Maxwell Swadling for generalizing static library builds into a -staticlib flag for GHC, Carter Schonwald for compilation guidance, and the venerable Austin Seipp for reviewing and merging our patches.

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Template Haskell and Unit
by Jose A. Lopes 29 Aug '13

29 Aug '13

Hi, I am positive about the following situation, but I can't find any concrete answer on the Web. Can anyone confirm this ? In template-haskell-2.7.0, the following quote [t| () |] appears as a (ConT name), where "name" is the name for unit. However, in template-haskell-2.8.0, the same quote appears as (TupleT 0). Is this the expected behaviour ? Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

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Building recent Cabal/cabal-install
by Mateusz Kowalczyk 29 Aug '13

29 Aug '13

Greetings café, There are some problems in Haddock to do with Template Haskell that I believe are being caused by Cabal. These were apparently addressed in 1.18 which came out recently. ‘Great!’, I thought. My problem is that I'm unsure how to use 1.18. I'm using GHC HEAD (well, 3 days old now) which is meant to come with Cabal 1.18 (the library) and in fact it seems to ship with it. Running ‘cabal --version’ however: > cabal-install version 1.17.0 > using version 1.17.0 of the Cabal library I'm unsure how to get it to use 1.18. I tried to build cabal-install from git but that just complains about dependencies not being possible to resolve: it depends on HTTP which depends on base <4.7 but 4.7 comes with GHC HEAD! I am baffled as to how I should go about using 1.18 and would love to hear from some people who have this up and running. -- Mateusz K.

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enumerators: exception that can't be catched
by Yuras Shumovich 29 Aug '13

29 Aug '13

Hello, I'm debugging an issue in "websockets" package, https://github.com/jaspervdj/websockets/issues/42 I'm not familiar with "enumerator" package (websockets are based on it), so I'm looking for help. The exception is throws inside "enumSocket" enumerator using "throwError" ( http://hackage.haskell.org/packages/archive/network-enumerator/0.1.5/doc/ht… ), but I can't catch it with "catchError". It is propagated to "run" function: <interactive>: recv: resource vanished (Connection reset by peer) The question is: how is it possible? could it be a bug in "enumerator" package? Thanks, Yuras

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Haskell Weekly News: Issue 278
by Daniel Santa Cruz 28 Aug '13

28 Aug '13

Welcome to issue 278 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 18 to 24, 2013. Quotes of the Week * johnw: finger trees must be related to palm trees somehow * monochrom: "do, or undo. there is no redo." :) * danharaj: arrows are just strong monads in the 2-category of profunctors. what's the problem? * BMeph: If only they knew the POWER...of the darcs-IDE! * geekosaur: "tekmo and the pipes list" sounds like some kind of postmodernist band Top Reddit Stories * Building an Asteroids Clone in Haskell using Netwire Domain: ocharles.org.uk, Score: 88, Comments: 22 On Reddit: [1] http://goo.gl/HZBtcK Original: [2] http://goo.gl/Gp8TVw * What's new in Cabal 1.18 - sandboxes, REPL, cross-compilation and more! Domain: coldwa.st, Score: 75, Comments: 18 On Reddit: [3] http://goo.gl/8eSChe Original: [4] http://goo.gl/VrHkMY * An Introduction to Cabal sandboxes Domain: coldwa.st, Score: 64, Comments: 33 On Reddit: [5] http://goo.gl/oFGzfK Original: [6] http://goo.gl/ByA6Ru * Reasoning about space leaks with space invariants Domain: apfelmus.nfshost.com, Score: 59, Comments: 48 On Reddit: [7] http://goo.gl/cKlPrM Original: [8] http://goo.gl/E4kIjD * Bound: Making de Bruijn Succ Less on School of Haskell by Edward Kmett Domain: fpcomplete.com, Score: 48, Comments: 14 On Reddit: [9] http://goo.gl/v7tgWO Original: [10] http://goo.gl/glTA3N * PatternSynonyms - ghc trac Domain: ghc.haskell.org, Score: 35, Comments: 14 On Reddit: [11] http://goo.gl/8zuy1T Original: [12] http://goo.gl/5Bw6FN * Revisiting Matrix Multiplication, Part III: Customizing Vector by Edward Kmett Domain: fpcomplete.com, Score: 33, Comments: 1 On Reddit: [13] http://goo.gl/hfoJJ7 Original: [14] http://goo.gl/yLUSWP * Great new platform to increase *practical* code literacy (with Haskell support). Domain: mdswanson.com, Score: 29, Comments: 13 On Reddit: [15] http://goo.gl/sFLBQm Original: [16] http://goo.gl/TIJPTi * Bartosz' basic Haskell: Error Handling Domain: fpcomplete.com, Score: 28, Comments: 4 On Reddit: [17] http://goo.gl/1jBaWG Original: [18] http://goo.gl/zZrPxH * A Pragmatic Case for Static Typing with Brian Hurt on Vimeo (at NY-Haskell) Domain: vimeo.com, Score: 27, Comments: 11 On Reddit: [19] http://goo.gl/uECaaB Original: [20] http://goo.gl/59c8Wo * Write yourself a Brainfuck in an hour Domain: github.com, Score: 26, Comments: 13 On Reddit: [21] http://goo.gl/lAfzRf Original: [22] http://goo.gl/cbJZ7t Top StackOverflow Questions * Is it possible to lazily traverse a recursive data-structure with O(1) memory usage, tail-call optimized? votes: 10, answers: 4 Read on SO: [23] http://goo.gl/1qsBKq * pattern matching of the form: Option{..} <- votes: 10, answers: 1 Read on SO: [24] http://goo.gl/HY3FWY Until next time, [25]+Daniel Santa Cruz References 1. http://ocharles.org.uk/blog/posts/2013-08-18-asteroids-in-netwire.html 2. http://www.reddit.com/r/haskell/comments/1kmes7/building_an_asteroids_clone… 3. http://coldwa.st/e/blog/2013-08-21-Cabal-1-18.html 4. http://www.reddit.com/r/haskell/comments/1kslyi/whats_new_in_cabal_118_sand… 5. http://coldwa.st/e/blog/2013-08-20-Cabal-sandbox.html 6. http://www.reddit.com/r/haskell/comments/1kpynl/an_introduction_to_cabal_sa… 7. http://apfelmus.nfshost.com/blog/2013/08/21-space-invariants.html 8. http://www.reddit.com/r/haskell/comments/1ksu0v/reasoning_about_space_leaks… 9. https://www.fpcomplete.com/user/edwardk/bound 10. http://www.reddit.com/r/haskell/comments/1knvz2/bound_making_de_bruijn_succ… 11. http://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms 12. http://www.reddit.com/r/haskell/comments/1kmods/patternsynonyms_ghc_trac/ 13. https://www.fpcomplete.com/user/edwardk/revisiting-matrix-multiplication-pa… 14. http://www.reddit.com/r/haskell/comments/1kmvs2/revisiting_matrix_multiplic… 15. http://mdswanson.com/blog/2013/08/19/how-to-practice-code-reviews.html 16. http://www.reddit.com/r/haskell/comments/1krc6t/great_new_platform_to_incre… 17. https://www.fpcomplete.com/user/bartosz/basics-of-haskell/10_Error_Handling 18. http://www.reddit.com/r/haskell/comments/1ksjt4/bartosz_basic_haskell_error… 19. https://vimeo.com/72870631 20. http://www.reddit.com/r/haskell/comments/1kusbt/a_pragmatic_case_for_static… 21. https://github.com/quchen/articles/blob/master/write_yourself_a_brainfuck.md 22. http://www.reddit.com/r/haskell/comments/1kz39d/write_yourself_a_brainfuck_… 23. http://stackoverflow.com/questions/18345734/is-it-possible-to-lazily-traver… 24. http://stackoverflow.com/questions/18372188/pattern-matching-of-the-form-op… 25. https://plus.google.com/105107667630152149014/about

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cpphs calls error when it finds an #error declaration
by Niklas Hambüchen 28 Aug '13

28 Aug '13

Hi, after some debugging of a higher-level tool I found out that when I use cpphs as a library and the `runCpphs` function that is to produce the preprocessed output, when it comes across the #error directive it will terminate my program. This is because handling #error is implemented with Haskell's `error`. I find that slightly unfortunate since it means I cannot distinguish betwen an #error written in the input file and a programming error inside cpphs. @Malcolm, would you mind a change towards throwing an exception that is different from error so that it can be easily caught, or even better, a change from runCpphs :: ... -> IO String to runCpphs :: ... -> IO (Either String String) or similar? If an exception based interface is kept, it would be nice to add some haddock to `runCpphs`; not knowing about the existence of #error, it is easy to assume that the IO is only used for accessing the FilePath passed in.

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