May 2014 - Haskell-Cafe

[ANN] codex 0.0.2 - generate tags file from dependencies
by Alois Cochard 04 May '14

04 May '14

Hi, Just to let you know that I released a new version of 'codex' (a tool which allow to generate a tags[1] file for a given cabal project using the sources of all the dependencies of that project). The main new feature is the support of "workspace resolution", which basically mean that if you have the following projects: ~/projects/foo ~/projects/bar Where 'foo' have a dependency on 'bar'... when you run `codex update` in 'foo' directory it will generate tags using the sources available in the sibling directory 'bar' instead of taking it from hackage (as long as versions match). It's specially useful when working on multiple modules, kudos to Edward Kmett for the original idea. [1]Those tags file basically contain references to functions/types definition in source code and allow "jump to definition" like functionality in text editors. source: http://github.com/aloiscochard/codex hackage: http://hackage.haskell.org/package/codex -- *Alois Cochard* http://twitter.com/aloiscochard http://github.com/aloiscochard

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cabal repl fails with GHC 7.8.2
by Omari Norman 04 May '14

04 May '14

I have a package for which "cabal repl" works fine with cabal version 1.20 and GHC 7.6.3. The package has a lot of FFI bindings and uses hsc2hs, so getting a REPL without "cabal repl" is a pain. The package also comes with the C sources that get linked in. But with GHC 7.8.2, "cabal repl" fails with: Loading object (static) dist/build/decnumber/src/decDouble.o ... /usr/bin/ld: dist/build/decnumber/src/decDouble.o: relocation R_X86_64_32S against `DPD2BIN' can not be used when making a shared object; recompile with -fPIC Any idea on how to fix this? "cabal build" and "cabal install" work fine and I can even build and execute a test binary against the library. It seems the problem is just with "cabal repl". Thanks. Omari

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Fwd: SBLP 2014 new deadline
by Simon Thompson 04 May '14

04 May '14

CALL FOR PAPERS - NEW DEADLINE: May 23rd 18th Brazilian Symposium on Programming Languages (SBLP 2014) A member of CBSoft joint conference http://www.ic.ufal.br/evento/cbsoft2014/ to be held in Maceio, Brazil. SBLP happens on Thursday, Oct 2nd, and Friday, Oct 3rd. CBSoft dates are September 28, 2014 to October 3, 2014 This CFP is also available at the webpage of the event: http://www.ic.ufal.br/evento/cbsoft2014/en/call-for-papers-sblp.html IMPORTANT DATES Paper abstract submission: May 23rd, 2014 Full paper submission: May 23rd, 2014 Notification of acceptance: June 27th, 2014 Final papers due: July 11th, 2014 INTRODUCTION The 18th Brazilian Symposium on Programming Languages, SBLP 2014, will be held in Maceio, a beautiful city in the Northeastern part of Brazil. The conference will happen on October 2nd, and October 3rd, 2014. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. SBLP 2014 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain-specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. Papers should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. Accepted papers written in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer. For this reason, all papers must be prepared using the LNCS template, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. We encourage the submission of short papers reporting partial results of on-going master dissertations or doctoral theses. Submissions should be done through the SBLP 2014 page at EasyChair, which is available at http://www.easychair.org/conferences/?conf=sblp2014. As in previous editions, a journal's special issue, with selected papers from accepted contributions, is anticipated. Selected papers from the 2003 to the 2008 editions of SBLP were published in special issues of the Journal of Universal Computer Science, by Springer. The post-proceedings of SBLP from 2009 to 2012, also with selected papers from the conference, have been published as special issues of Science of Computer Programming, by Elsevier. KEYNOTE SPEAKERS Louis-Noel Pouchet, University of California Los Angeles Fabrice Rastello, INRIA PROGRAMME CHAIR Fernando Magno Quintao Pereira, UFMG PROGRAMME COMMITTEE Alberto Pardo, Universidad de la Republica Alex Garcia, IME Alvaro Moreira, Federal University of Rio Grande do Sul Andre Rauber Du Bois, Federal University of Pelotas Carlos Camar√£o, Federal University of Minas Gerais Christiano Braga, Fluminense Federal University Fabio Mascarenhas, Federal University of Rio de Janeiro Fabrice Rastello, INRIA Fernando Pereira, Federal University of Minas Gerais Fernando Castor, Federal University of Pernambuco Francisco Carvalho-Junior, Federal University of Ceara Hans-Wolfgang Loidl, Heriot-Watt University Jeremy Singer University of Glasgow Jo√£o Saraiva, University of Minho Joao F. Ferreira, Teesside University Louis-Noel Pouchet, University of California, Los Angeles Lucilia Figueiredo, Federal University of Ouro Preto Luis Barbosa, University of Minho Manuel A. Martins, University of Aveiro Marcello Bonsangue, Leiden University Marcelo Maia, Federal University of Uberlandia Marcelo D'Amorim, Federal University of Pernambuco Mariza Bigonha, Federal University of Minas Gerais Martin Musicante, Federal University of Rio Grande do Norte Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Rafael Lins, Federal University of Pernambuco Renato Cerqueira, PUC-Rio Roberto Bigonha, Federal University of Minas Gerais Roberto Ierusalimschy PUC-Rio Rodrigo Geraldo, Federal University of Ouro Preto Sandro Rigo, State University of Campinas Sergio Medeiros, Federal University of Rio Grande do Norte Simon Thompson, University of Kent Varmo Vene, University of Tartu Zongyan Qiu Peking University Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Re: [Haskell-cafe] Haskell Programming Font with Ligatures
by Garrett Mitchener 03 May '14

03 May '14

I just tried this in gedit and it looks *really* nice! Thanks for putting this together. Doesn't work in emacs though :-(

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Parsec lookAhead with choice (<|>) doesn't work as expected?
by Baojun Wang 03 May '14

03 May '14

Hi List, There is my first email on haskell-cafe, I must admit haskell is a amazing language even though I found it hard to program. I'm trying to use Parsec to parse a text file something like: format 1) HEADER BODY format 2) BODY BODY = many DATA_FIELDS I must distinguish the format is either 1) or 2), because I need process them differently. So I'm trying to use lookAhead & <|>, something like: format1Header = do HEADER_parser1 HEADER_parser2 return .. format2Header = do -- whatever return .. headerVersion = do version <- (lookAhead format1Header) <|> (lookAhead format2Header) return version Howerver if format1Header parser failed headerVersion just return failure without trying format2Header parser. Full source code is attached, you can load the source then just run ``main'' I searched seems similar issue had been reported: https://groups.google.com/forum/#!topic/fa.haskell/NGjQqR05-TA Is there anything I'm fundamentally wrong with lookAhead and <|> in Parsec? How can I get headerVersion works as expected? Thank you Baojun

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Prevent replicateM_ from repeating expensive (pure) computations
by suhorng Y 01 May '14

01 May '14

Hi all, I've been writing loops in a monad using `replicateM_`. However, recently I discovered that it caused my program become extremely slow. For example, > module Main where > > import Control.Monad (replicateM_) > > fib :: Int -> Int > fib 0 = 0 > fib 1 = 1 > fib n = fib (n-1) + fib (n-2) > > main = do > n <- readLn > let fn = fib n > replicateM_ 20 (print fn) The program ran fine if no optimization is turned on. However, when I used `-O` or `-O2` with GHC (7.4/7.6), the program became extremely slow, and if I replaced `replicateM_` by `mapM_`, the problem disappeared. I suspected that GHC inlined `fn`, causing `replicateM_` to recalculate the value in every loop. Though the problem could be solved by either using `mapM_`, lifting `print fn` to a global definition or manually demanding it be evaluated strictly, I would expected `fn` not to be computed multiple times. Any suggestions around this? Thanks! Sincerely, suhorng

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forLoop + strict State monad is much faster than foldl'
by Niklas Hambüchen 01 May '14

01 May '14

This is just a short notice that using foldl' (+) 0 [0..100000::Int] is over 10 times slower than using flip execState 0 $ forLoop (0 :: Int) (< n) (+1) $ \i -> do x <- get put $! x + i with `loopFor` as on https://github.com/nh2/loop. Even using an IORef is twice as fast as the pure foldl' (but still 5 times slower than strict State). The benchmark is at http://htmlpreview.github.io/?https://github.com/nh2/loop/blob/master/resul…. (All benchmarks are linked from https://github.com/nh2/loop.) You can see there that the problem is gone when using Vector.foldl', but only for Int - for Word32 it persists. It seems that manual looping is beneficial even when dealing with prime examples of pure code that GHC ought to optimize well. Niklas

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ANN: hledger-0.23, hledger-web-0.23
by Simon Michael 01 May '14

01 May '14

I'm pleased to announce hledger and hledger-web 0.23! This release includes command-line fixes and polish, a new accounts command, and a number of changes to the balance command relating to --depth, --flat, and multicolumn mode, which I find has made it much more useful. Release notes: http://hledger.org/release-notes#hledger-023-201451 . Contributors to this release: Peter Simons and Marko Kocić. hledger (http://hledger.org) is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. It can also read CSV or timelog files. It provides useful reports, and can also help you record new transactions interactively. Add-on commands include hledger-web (a web interface), hledger-irr (for calculating internal rate of return) and hledger-interest (for generating interest transactions). hledger is inspired by and largely compatible with Ledger. Installation: cabal update; cabal install hledger [hledger-web] (see http://hledger.org/installing for help) or sponsor a binary at http://hledger.org/download Best! -Simon

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Re: [Haskell-cafe] Stupid newbie question of the day: why is newMVar in the IO monad
by Ryan Yates 01 May '14

01 May '14

There is extra work done to make this safe. See the following: https://github.com/ghc/ghc/blob/af6746fb6b5adb5ba5be6e0f647c4ebe767ce084/rt… https://ghc.haskell.org/trac/ghc/ticket/5558 Ryan On Thu, May 1, 2014 at 8:25 AM, Mikulas Patocka < mikulas(a)artax.karlin.mff.cuni.cz> wrote: > > Also note linked from the global variables page on the wiki is: > > http://www.haskell.org/haskellwiki/Top_level_mutable_state > > > > Important for the `unsafePerformIO` option is the NOINLINE pragma to > ensure > > that only one global variable exists. In the STM case it is also > important > > to use `newTVarIO` rather then `unsafePerformIO $ atomically newTVar` > which > > does not work. > > > > Ryan > > Hi > > I think it isn't correct even with NOINLINE. > > > Read the paper "Haskell on a Shared-Memory Multiprocessor" > http://research.microsoft.com/pubs/67424/2005-haskell.pdf > > Among other things, the paper presents these ideas: > > (section 3.1) When multiple threads start to evaluate the same thunk > concurrently, we could use interlocked compare-and-swap instruction to > make sure that only one thread starts to evaluate it. However, the > compare-and-swap instruction is slow and executing it at every thunk > evaluation decreases performance. So, the designers avoid the > compare-and-swap instruction (section 3.2) and consequently there is a > race condition - there is a small window where two or more threads may > evaluate the same thunk concurrently. > > Normally, this race condition doesn't matter, because Haskell is purely > functional language, so when multiple threads evaluate the same thunk they > end up with the same result - but in this particular example, where one is > using unsafePerformIO to construct a unique variable, it can hurt, because > you end up executing "unsafePerformIO $ newMVar 0" concurrently (section > 3.5 of the paper). > > > So - code like this isn't correct, the numbers returned by "uniq" may not > be unique if we hit the race condition when evaluating "global": > > global :: MVar Integer > global = unsafePerformIO $ newMVar 0 > {-# NOINLINE global #-} > uniq = modifyMVar global (\i -> return (i+1,i)) > > > Has anything changed since that time, so that this construct is safe? > > Mikulas >

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Dealing with GHC 7.8
by Ivan Lazar Miljenovic 01 May '14

01 May '14

I've been trying to update the various Haskell packages for Exherbo, but for some packages I keep getting errors like this (taken from vector-th-unbox, which is ultimately needed by criterion): runhaskell Setup copy --destdir=/var/tmp/paludis/build/dev-haskell-vector-th-unbox-0.2.0.2/image/ Installing library in /var/tmp/paludis/build/dev-haskell-vector-th-unbox-0.2.0.2/image/usr/lib64/x86_64-linux-ghc-7.8.2/vector-th-unbox-0.2.0.2 Setup: Error: Could not find module: Data.Vector.Unboxed.Deriving with any suffix: ["dyn_hi"] in the search path: ["dist/build"] When trying to find solutions for this, every result happens to be for Windows... which this isn't. Has some way of dealing with building packages using just runhaskell Setup changed? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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