May 2012 - Haskell-Cafe

Most Important GHC extensions to learn/use?
by Jonathan Geddes 03 Jun '12

03 Jun '12

Haskell Hackers, I'm pretty comfortable with all of Haskell 98 (and 2010, really). But I've always sort of avoided extensions. I realize that this is a bit silly and if I want to continue learning, it probably means delving into the extensions. Which ones are the most important to know from a practical point of view? And which ones from a {Language,Category,Math}-theoretical point of view? (Any other interesting/important points of view I'm missing? :D ) As always, thanks for the feedback. Cheers, --J Arthur

5 6

Need inputs for a Haskell awareness presentation
by C K Kashyap 03 Jun '12

03 Jun '12

Hi folks, I have the opportunity to make a presentation to folks (developers and managers) in my organization about Haskell - and why it's important - and why it's the only way forward. I request you to share your experiences/suggestions for the following - 1. Any thoughts around the outline of the presentation - target audience being seasoned imperative programmers who love and live at the pinnacle of object oriented bliss. 2. Handling questions/comments like these in witty/interesting ways - a) It looks good and mathematical but practically, what can we do with it, all our stuff is in C++ b) Wow, what do you mean you cannot reason about its space complexity? c) Where's my inheritance? d) Debugging looks like a nightmare - we cannot even put a print in the function? e) Static types - in this day and age - come on - productivity in X is so much more - and that's because they got rid of type mess. f) Is there anything "serious/large" written in it? [GHC will not qualify as a good answer I feel] g) Oh FP, as in Lisp, oh, that's AI stuff right ... we don't really do AI. h) Any other questions/comments that you may have heard. 3. Ideas about interesting problems that can be used so that it appeals to people. I mean, while fibonacci etc look good but showing those examples tend to send the signal that it's good for "those kind" of problems. 4. Is talking about or referring to Lambda calculus a good idea - I mean, showing that using its ultra simple constructs one could build up things like if/then etc I'm gonna do my bit to wear the limestone!!! Regards, Kashyap

9 9

Requesting Feedback: I Love Haskell, but can't find a place to use it
by Jonathan Geddes 01 Jun '12

01 Jun '12

I love Haskell. It is my absolute favorite language. But I have a very hard time finding places where I can actually use it! I had hoped that compiling Haskell to C with -fvia-C (or would it be just -C?) would allow Haskell to run in new, uncharted territory such as Android (with NDK), IOS, Google's NaCl, etc. But today I learned that GHC's C backend has been deprecated! Is it more difficult than I am imagining to get Haskell to work in these environments? Is it simply a matter of low interest in this kind of work? Or something more fundamental? Am I missing something? I'm hoping that the Haskell->JavaScript efforts will mature enough to make Haskell viable for client-side web apps. (I think the first sign of this will be a self-hosting Haskell->JavaScript compiler.) I use Haskell for Server-Side code with various web frameworks, but over the years more and more of the app logic is moved into client-side JavaScript, leaving the server-side code as little more than a simple validation and security layer over the database and other services. Haskell doesn't have any trouble with this, of course, but it's not exactly a role where it can shine. (Of course this is not true of ALL server-side code, just the kind of apps I have been writing.) So anyway I'd like to request feedback: where can I use Haskell besides simple CLI utilities, dull server code, or project Euler problems? Even if it's just to contribute to getting Haskell in the environments mentioned above, any feedback is welcome! Thanks for reading, --J Arthur

7 7

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 30 May '12

30 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 30 May '12

30 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 29 May '12

29 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