- Haskell-Cafe - Haskell.org

Tests by properties: origin?
by Yves Parès 01 Jun '12

01 Jun '12

Out of curiosity, does someone know if QuickCheck was the first test framework working through test by properties associated with random generation or if it drew the idea from something else? Because the idea has be retaken by a lot of frameworks in several languages (see http://en.wikipedia.org/wiki/Quickcheck) but I can't find what was QuickCheck inspiration. (This question pop up when arguing with a Java user about what Haskell brings to the field of development, he pretends it would be astonishing that QuickCheck is the first one using this testing paradigm).

1 0

Confused by ghci output
by Clark Gaebel 01 Jun '12

01 Jun '12

*X> 3^40 `mod` 3 == modexp2 3 40 3 False *X> modexp2 3 40 3 0 *X> 3^40 `mod` 3 0 I'm confused. Last I checked, 0 == 0. Using GHC 7.4.1, and the file x.hs (which has been loaded in ghci) can be found here: http://hpaste.org/69342 I noticed this after prop_sanemodexp was failing. Any help would be appreciated, - Clark

8 9

Re: [Haskell-cafe] How to write Source for TChan working with LC.take?
by Hiromi ISHII 31 May '12

31 May '12

Oops, sorry. The last case's behaviour was not as I expected... A correct log is below: ~~~~ ghci> sourceTChanRaw ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanState ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanYield ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () *blocks* ~~~~ So again, sourceTChanYield blocks here even if it is already supplied with enough values! -- Hiromi ISHII konn.jinro(a)gmail.com

2 3

Finding the average in constant space
by Chris Wong 31 May '12

31 May '12

Hello all I just came up with a way of executing multiple folds in a single pass. In short, we can write code like this: average = foldLeft $ (/) <$> sumF <*> lengthF and it will only traverse the input list once. The code is at: https://gist.github.com/2802644 My question is: has anyone done this already? If not, I might release this on Hackage -- it seems quite useful. Chris

4 4

Haskell Weekly News: Issue 229
by Daniel Santa Cruz 31 May '12

31 May '12

Welcome to issue 229 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 20 to 26, 2012. Quotes of the Week * Peaker: Harrop taught me that I am a team of experts * applicative: Even if no one else understands me, still, the Glasgow Haskell Compiler understands me * JoeyA: Answers in #haskell are automatically sorted by readability descending. * monochrom: monochromism is a graph morphism that maps graphs to 1-colorable graphs :) * sipa: can we write a TutorialMonad t ? which is used to build and combine monad tutorials? Top Reddit Stories * my Haskell project was 100% funded on Kickstarter Domain: self.haskell, Score: 125, Comments: 22 On Reddit: [1] http://goo.gl/E2wkY Original: [2] http://goo.gl/E2wkY * Building intuition for monads, without mentioning monads (Ertugrul Söylemez) Domain: haskell.org, Score: 56, Comments: 32 On Reddit: [3] http://goo.gl/2leRd Original: [4] http://goo.gl/kMbJR * A gamified tutorial on the sequent calculus Domain: logitext.ezyang.scripts.mit.edu, Score: 51, Comments: 23 On Reddit: [5] http://goo.gl/tD8fZ Original: [6] http://goo.gl/E6zBl * pipes 2.0 - Pipe Finalization Domain: haskellforall.com, Score: 45, Comments: 32 On Reddit: [7] http://goo.gl/boq03 Original: [8] http://goo.gl/AABwV * Haste: Haskell to Javascript Compiler Domain: github.com, Score: 40, Comments: 45 On Reddit: [9] http://goo.gl/sJCcJ Original: [10] http://goo.gl/ZhYbP * Monad Reification in Haskell and the Sunroof Javascript compiler Domain: ittc.ku.edu, Score: 37, Comments: 26 On Reddit: [11] http://goo.gl/LVSVO Original: [12] http://goo.gl/s0PVp * HaskSymb, An Experiment in Haskell Symbolic Algebra. Feedback appreciated! Domain: github.com, Score: 36, Comments: 34 On Reddit: [13] http://goo.gl/d6Xo8 Original: [14] http://goo.gl/TUiJt * Using Ragel, Clang, LLVM and GHC to produce really fast stockmarket data parsers Domain: breaks.for.alienz.org, Score: 35, Comments: 8 On Reddit: [15] http://goo.gl/qEmKb Original: [16] http://goo.gl/ABzsA * Looks like Bryan finished importing Cabal issues to Github — thanks! Domain: github.com, Score: 32, Comments: 6 On Reddit: [17] http://goo.gl/9O8Sa Original: [18] http://goo.gl/bWrLg * What if haskellwiki were run by a Haskell wiki? Domain: haskellwiki.gitit.net, Score: 29, Comments: 14 On Reddit: [19] http://goo.gl/713Yb Original: [20] http://goo.gl/chSPM * New release of Elm available. Looking for feedback on the language so far. Domain: self.haskell, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/ejS9p Original: [22] http://goo.gl/ejS9p * Generics and Protocol Buffers Domain: breaks.for.alienz.org, Score: 17, Comments: On Reddit: [23] http://goo.gl/z0obj Original: [24] http://goo.gl/R5b3l Top StackOverflow Questions * What's the theoretical basis for existential types? votes: 32, answers: 3 Read on SO: [25] http://goo.gl/ZdAJb * How can a gc find out about object references done from the stack? votes: 20, answers: 4 Read on SO: [26] http://goo.gl/AtXUr * Why is GHC complaining about non-exhaustive patterns? votes: 14, answers: 3 Read on SO: [27] http://goo.gl/9oIsB * Evolving data structure votes: 13, answers: 2 Read on SO: [28] http://goo.gl/gbeLN * What makes a good name for a helper function? votes: 11, answers: 5 Read on SO: [29] http://goo.gl/b6B7o * Haskell “Apply”? [closed] votes: 11, answers: 3 Read on SO: [30] http://goo.gl/EBwsy * Round to nearest integer votes: 10, answers: 1 Read on SO: [31] http://goo.gl/1RaVk * Haskell: lazy versus eager evaluation for insertion sort votes: 6, answers: 1 Read on SO: [32] http://goo.gl/zGrie * Loss of polymorphism after pattern matching votes: 6, answers: 1 Read on SO: [33] http://goo.gl/YmA1y * Church-Rosser Theorem Example in a Functional Programming Language votes: 6, answers: 1 Read on SO: [34] http://goo.gl/FmSMh Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 2. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 3. http://www.haskell.org/pipermail/haskell-cafe/2012-May/101338.html 4. http://www.reddit.com/r/haskell/comments/u04vp/building_intuition_for_monad… 5. http://logitext.ezyang.scripts.mit.edu/logitext.fcgi/tutorial 6. http://www.reddit.com/r/haskell/comments/tzp7f/a_gamified_tutorial_on_the_s… 7. http://www.haskellforall.com/2012/05/pipes-20-pipe-finalization.html 8. http://www.reddit.com/r/haskell/comments/txkb0/pipes_20_pipe_finalization/ 9. https://github.com/valderman/haste-compiler#haste 10. http://www.reddit.com/r/haskell/comments/tw997/haste_haskell_to_javascript_… 11. http://www.ittc.ku.edu/csdlblog/?p=88 12. http://www.reddit.com/r/haskell/comments/tvwn7/monad_reification_in_haskell… 13. https://github.com/colah/HaskSymb 14. http://www.reddit.com/r/haskell/comments/u1t8n/hasksymb_an_experiment_in_ha… 15. http://breaks.for.alienz.org/blog/2012/02/09/parsing-market-data-feeds-with… 16. http://www.reddit.com/r/haskell/comments/txfhq/using_ragel_clang_llvm_and_g… 17. https://github.com/haskell/cabal/issues 18. http://www.reddit.com/r/haskell/comments/u2faj/looks_like_bryan_finished_im… 19. http://haskellwiki.gitit.net/ 20. http://www.reddit.com/r/haskell/comments/txn1p/what_if_haskellwiki_were_run… 21. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 22. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 23. http://breaks.for.alienz.org/blog/2012/05/20/generics-and-protocol-buffers/ 24. http://www.reddit.com/r/haskell/comments/tw6dq/generics_and_protocol_buffer… 25. http://stackoverflow.com/questions/10753073/whats-the-theoretical-basis-for… 26. http://stackoverflow.com/questions/10710603/how-can-a-gc-find-out-about-obj… 27. http://stackoverflow.com/questions/10699532/why-is-ghc-complaining-about-no… 28. http://stackoverflow.com/questions/10761583/evolving-data-structure 29. http://stackoverflow.com/questions/10740515/what-makes-a-good-name-for-a-he… 30. http://stackoverflow.com/questions/10767407/haskell-apply 31. http://stackoverflow.com/questions/10738569/round-to-nearest-integer 32. http://stackoverflow.com/questions/10681507/haskell-lazy-versus-eager-evalu… 33. http://stackoverflow.com/questions/10728897/loss-of-polymorphism-after-patt… 34. http://stackoverflow.com/questions/10728997/church-rosser-theorem-example-i…

1 0

Unification of PolyTypeable TypeReps ?
by Ismael Figueroa Palet 30 May '12

30 May '12

Hi haskell-cafe In my current project I'm working with type representations created with PolyTypeable, and it turns out that given two type representations I need to find out if they are unifiable. I've looked at polytypeable-utils and see that they have some functions for substituting/applying/free vars, but it doesn't seem like a full solution for what I need. So my question is: is there a library to do this? is there a known way to transform the TypeRep's to make them more amenable to other unification libraries. I noticed that in GHC-api there is an unification library, and that there is also unification-fd. I wanted to ask here first in case someone can point me to the right direction. Thanks! -- Ismael

1 0

What is the difference between runhaskell and compile?
by Magicloud Magiclouds 30 May '12

30 May '12

Hi, I am writing a small program using LDAP hackage. A weird problem occured. When the code was run by runhaskell, things were fine, worked as expected. But when ghc compiled (no any args), and ran, I got this: LDAP error: ldapSimpleBind: LDAPException LdapServerDown(-1): Can't contact LDAP server. There is sure no problem with the server. So I am confused. I thought the two supposed to be the same. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

6 17

A functional programming solution for Mr and Mrs Hollingberry
by Andreas Pauley 29 May '12

29 May '12

Hi all, I'm in the process of learning how to approach problems from a functional perspective, coming from an Object Oriented background (mostly Smalltalk). One of the general concerns/questions raised when talking to people in a similar position is: "How do I design/model a problem when I don't have my trusted classes and objects available?" With this in mind I've created a programming exercise where I imagine an OO programmer would use an object hierarchy with subtype polymorphism as part of the solution. And then I'd like to compare functional implementations of the same problem: https://github.com/apauley/HollingBerries I want to see how elegant a solution I can get in a functional language, given that the problem description is not really elegant at all. It has a few annoying exceptions to the normal rules, typical of what one might get in a real specification from some client. Currently there are 3 implementations: - one in Erlang, my attempt at implementing a functional solution - one in Ruby, my attempt to see how an object hierarchy could be used - one in Clojure, done by one of the people in our FP user group [1] I would love to include some Haskell implementations as well, if any of you are interested. Kind regards, Andreas Pauley 1. http://www.meetup.com/lambda-luminaries/ -- http://pauley.org.za/ http://twitter.com/apauley

7 11

ANN: hledger 0.18
by Simon Michael 29 May '12

29 May '12

I'm pleased to announce version 0.18 of hledger! hledger (http://hledger.org) is a haskell library, command-line tool and web application for working with financial data (or anything that can be tracked numerically in an accounting journal). It is inspired by and compatible with John Wiegley's Ledger. hledger reads a plain text file describing transactions and lets you query account balances or transaction details by various criteria. It can also help you record new transactions, or convert CSV data from your bank. Release notes (http://hledger.org/NEWS.html) * web: hledger-web is now based on yesod 1.0 * web: fix js error breaking second use of add form (#72) * web: make `yesod devel` work * the command-line now supports a more powerful query language, consistent with the web UI * hledger now fully supports tags (aka metadata) on both transactions and postings, and querying by tag or tag value * new commands `incomestatement`, `balancesheet`, and `cashflow` provide basic financial statements under certain conditions * format conversion is now done on demand, and the convert command has been dropped. So instead of `hledger convert FILE.csv` just do `hledger -f FILE.csv print` or any other command. You can also pipe any supported format into `hledger -f- CMD` and hledger will try to do the right thing. * support for GHC 6.12 has been dropped; this release has been tested with GHC 7.0.4, 7.2.2, and 7.4.1 * unicode is now handled properly on all supported GHC versions * extensive API, test and internal cleanups Stats: - Release contributors: Simon Michael, xiaoruoruo - 87 days, 155 commits, 6 end-user features and 3 end-user bug fixes since last major release - 216 unit & functional tests (hledger-lib & hledger) - 7836 lines of code (hledger-lib, hledger & hledger-web) Install it: cabal update; cabal install hledger [hledger-web] If you have trouble, see http://hledger.org/MANUAL.html#installing . You can fund a ready-to-run binary for your platform with a donation at http://hledger.org/DOWNLOAD.html . Prosperity and clarity, -Simon

1 0

Estágio com Haskell em SP (Haskell internship at São Paulo, Brazil)
by Felipe Almeida Lessa 28 May '12

28 May '12

(English message follows after the break.) Olá! Estamos procurando um estagiário para trabalhar conosco em São Paulo capital. Nós somos uma startup apaixonada por Haskell com uma boa participação na comunidade e grandes projetos e desafios! Se você está interessado, basta me mandar um e-mail (felipe.lessa(a)gmail.com) com um link para sua conta do GitHub (ou para projetos em que você já tenha trabalhado). Você não precisa ser experiente em Haskell nem ter nenhum código Haskell publicado, mas isso seria um diferencial =). Como a startup está em stealth mode, não posso divulgar o nome da empresa e por isso estou usando meu e-mail pessoal. Até mais! === Hey! We're looking for an intern to work with us in São Paulo, Brazil. We're a startup passionate about using Haskell with great projects and challenges! If you're interested, just send me an e-mail (felipe.lessa(a)gmail.com) with a link to your GitHub account (or to any projects you've worked at). You don't need to be an experienced Haskeller nor have any published Haskell code, but that would be a plus =). As the startup is in stealth mode, I can't disclose the company's name nor use my work e-mail (that's why I'm using my personal one). Cheers, -- Felipe.

1 0