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Haskell Weekly News: Issue 241
by Daniel Santa Cruz 22 Aug '12

22 Aug '12

Welcome to issue 241 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 12 to 18, 2012. Quotes of the Week * monochrom: seq is a suggestion. pseq is an order. * johnw: Haskell is like using a finely crafted blade to whittle yourself a canoe, as your friends pass by on their cruise liner and wonder what you're up to Top Reddit Stories * Yesod gets financial backing. Domain: yesodweb.com, Score: 76, Comments: 23 On Reddit: [1] http://goo.gl/isVxZ Original: [2] http://goo.gl/aTtqC * The category design pattern Domain: haskellforall.com, Score: 65, Comments: 32 On Reddit: [3] http://goo.gl/Owrgh Original: [4] http://goo.gl/iDEen * I remember the time before monads | free variable Domain: free-variable.org, Score: 64, Comments: 5 On Reddit: [5] http://goo.gl/41Utn Original: [6] http://goo.gl/febU9 * GHC 7.6.1 Release Candidate 1 Domain: haskell.org, Score: 55, Comments: 33 On Reddit: [7] http://goo.gl/weAei Original: [8] http://goo.gl/QHtd7 * PVP: upper bounds are not our friends Domain: haskell.org, Score: 49, Comments: 94 On Reddit: [9] http://goo.gl/8ihwE Original: [10] http://goo.gl/0YbGl * MongoDB on the hunt for Haskell Evangelist Domain: haskellers.com, Score: 36, Comments: 17 On Reddit: [11] http://goo.gl/X1WLL Original: [12] http://goo.gl/TYDdE * Raspberry Pi in a Haskell Cloud Domain: alenribic.com, Score: 33, Comments: 6 On Reddit: [13] http://goo.gl/esAHP Original: [14] http://goo.gl/me7ob * Practical Foundations for Programming Languages (first impressions) Domain: blog.ezyang.com, Score: 30, Comments: On Reddit: [15] http://goo.gl/FjrY1 Original: [16] http://goo.gl/8AzpC * Scoutess GSoC project finished! Domain: self.haskell, Score: 28, Comments: 4 On Reddit: [17] http://goo.gl/hRzvC Original: [18] http://goo.gl/hRzvC Top StackOverflow Questions * Subtype polymorphism in Haskell votes: 17, answers: 2 Read on SO: [19] http://goo.gl/xERPq * Is it possible to use a bracketing syntactic sugar for an applicative functor? votes: 14, answers: 2 Read on SO: [20] http://goo.gl/3EH63 * Useful operations on free arrows votes: 11, answers: 0 Read on SO: [21] http://goo.gl/BQ6ry * Against cold boot attacks: how to restrain sensitive information in Haskell votes: 9, answers: 1 Read on SO: [22] http://goo.gl/kUOn2 * GHCi and compiled code seem to behave differently votes: 7, answers: 1 Read on SO: [23] http://goo.gl/q8yJi * Implementing Iota in Haskell votes: 7, answers: 1 Read on SO: [24] http://goo.gl/KonKf * Understanding STG votes: 6, answers: 1 Read on SO: [25] http://goo.gl/uQmKn * Quickcheck for non-boolean tests votes: 6, answers: 0 Read on SO: [26] http://goo.gl/on6jt * Screen capture in Haskell? votes: 6, answers: 3 Read on SO: [27] http://goo.gl/7xIX5 * typeclass challenge: having both variadic arguments and results votes: 6, answers: 2 Read on SO: [28] http://goo.gl/8aA6Z Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/08/joining-forces-advance-haskell 2. http://www.reddit.com/r/haskell/comments/y62j3/yesod_gets_financial_backing/ 3. http://www.haskellforall.com/2012/08/the-category-design-pattern.html 4. http://www.reddit.com/r/haskell/comments/yfhem/the_category_design_pattern/ 5. http://www.free-variable.org/2012/08/i-remember-the-time-before-monads/ 6. http://www.reddit.com/r/haskell/comments/y87rh/i_remember_the_time_before_m… 7. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-August/022671.h… 8. http://www.reddit.com/r/haskell/comments/y3vzd/ghc_761_release_candidate_1/ 9. http://www.haskell.org/pipermail/haskell-cafe/2012-August/102885.html 10. http://www.reddit.com/r/haskell/comments/ydkcq/pvp_upper_bounds_are_not_our… 11. http://www.haskellers.com/jobs/18 12. http://www.reddit.com/r/haskell/comments/yc1no/mongodb_on_the_hunt_for_hask… 13. http://alenribic.com/writings/post/raspberry-pi-in-a-haskell-cloud 14. http://www.reddit.com/r/haskell/comments/ydvn2/raspberry_pi_in_a_haskell_cl… 15. http://blog.ezyang.com/2012/08/practical-foundations-for-programming-langua… 16. http://www.reddit.com/r/haskell/comments/yda6c/practical_foundations_for_pr… 17. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 18. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 19. http://stackoverflow.com/questions/12002979/subtype-polymorphism-in-haskell 20. http://stackoverflow.com/questions/12014524/is-it-possible-to-use-a-bracket… 21. http://stackoverflow.com/questions/12001350/useful-operations-on-free-arrows 22. http://stackoverflow.com/questions/11932246/against-cold-boot-attacks-how-t… 23. http://stackoverflow.com/questions/11926398/ghci-and-compiled-code-seem-to-… 24. http://stackoverflow.com/questions/11960809/implementing-iota-in-haskell 25. http://stackoverflow.com/questions/11921683/understanding-stg 26. http://stackoverflow.com/questions/11942554/quickcheck-for-non-boolean-tests 27. http://stackoverflow.com/questions/11963561/screen-capture-in-haskell 28. http://stackoverflow.com/questions/12014420/typeclass-challenge-having-both…

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regex-pcre is not working with UTF-8
by José Romildo Malaquias 21 Aug '12

21 Aug '12

Hello. It seems that the regex-pcre has a bug dealing with utf-8: Prelude> :m + Text.Regex.PCRE Prelude Text.Regex.PCRE> "país:Brasil" =~ "país:(.*)" :: (String,String,String,[String]) ("","pa\237s:Brasil","",["rasil"]) Notice the missing 'B' in the result of the regex matching. With regex-posix this does not happen: Prelude> :m + Text.Regex.Posix Prelude Text.Regex.Posix> "país:Brasil" =~ "país:(.*)" ::(String,String,String,[String]) ("","pa\237s:Brasil","",["Brasil"]) I hope this bug can be fixed soon. Is there a bug tracker to report the bug? If so, what is it? Romildo

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salvia
by Sergey Mironov 21 Aug '12

21 Aug '12

Hi. Does anybody know anything about Sebastiaan Visser, the maintainer of Salvia-* packages (web server) ? Looks like his email is dead. Sergey

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Galois internships available
by Lee Pike 21 Aug '12

21 Aug '12

**************************************** * ANNOUNCING: * INTERNSHIP AT GALOIS, Inc. **************************************** Galois, Inc. <www.galois.com> has an internship available in Portland, Oregon, USA. PROJECT OVERVIEW: The project is a 4+ year research project on build high-assurance autonomous vehicles. Galois will be working on three aspects of this problem: * Synthesizing software components from Haskell-based embedded DSLs. * Building/porting a hardware platform for testing our prototypes. * Performing static/dynamic analysis on C/C++ code to detect vulnerabilities. We intend to release open-source most (if not all) source code developed on the project and we will be publishing on our research results. Being an intern for repeated terms is a possibility. LOGISTICS: The length and start date of the internship are all negotiable: anytime from this fall through next fall is acceptable. Ideally, you can be at Galois for at least 3 continuous months. The internship is paid competitively, and the intern will be responsible for her own living arrangements (although we can certainly help you find arrangements). Galois is located in the heart of downtown, with multiple public transportation options available and world-class bicycle infrastructure, so living here without an automobile is a viable option. QUALIFICATIONS: * MUST-HAVES: * The ability to be geographically located in Portland during the internship. * Good knowledge of C/C++. * Some experience with low-level/embedded design. * Excellent software engineering ability and aptitude. * NICE-TO-HAVES (not necessary, but let me know if you have these qualifications!): * Experience with Haskell and particularly embedded DSLs. * Experience with microcontrollers (particularly ARM Cortex M). * Experience with control systems. * Interest in/experience with real-time systems and RTOSes. * Interest in/experience with software security. * Interest in/experience with static analysis. * Experience with ArduPilot <http://code.google.com/p/ardupilot-mega/> or other autopilot systems. * Good writing skills/experience in writing technical papers. * DO NOT NEED: * A specific degree (we're interested in hearing from anyone from post-docs to undergrads). ABOUT GALOIS: Galois, Inc. is located in Portland, Oregon with a mission to create trustworthiness in critical systems. We're in the business of taking blue-sky ideas and turning them into real-world technology solutions. We've been developing real-world systems for over 10 years using Haskell. To get a sense of life at Galois, one of our previous interns documented his internship here: <http://blog.ezyang.com/2010/08/day-in-the-life-of-a-galois-intern/>. TO APPLY: In one email, * Please attach a C.V. (PDF, plain text, or markdown only). * In the body of the email, include a brief non-HTML note stating your interest and experience and any other relevant details. Send this to Lee Pike (leepike at galois.com) with the subject line "Internship 2012". If you follow these directions, you'll get a confirmation from me.

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fclabels > 0.5
by Sergey Mironov 21 Aug '12

21 Aug '12

Hi. I'm porting old code, which uses fclabels <0.5. Old fclabels define Iso typeclass as follows: class Iso f where iso :: a :<->: b -> f a -> f b iso (Lens a b) = osi (b <-> a) osi :: a :<->: b -> f b -> f a osi (Lens a b) = iso (b <-> a) Newer one defines iso: class Iso (~>) f where iso :: Bijection (~>) a b -> f a ~> f b instance Arrow (~>) => Iso (~>) (Lens (~>) f) where iso bi = arr ((\a -> lens (fw bi . _get a) (_set a . first (bw bi))) . unLens) instance Arrow (~>) => Iso (~>) (Bijection (~>) a) where iso = arr . (.) but no osi. I'm not a guru in categories, can you help me define osi? Thanks Sergey.

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Pure and monadic functions using the Repa arrays
by felipe zapata 21 Aug '12

21 Aug '12

Hi Haskellers, I have been playing with the Repa functions and trying the Repa-examples. In order to gain experience with the Repa functions I have written some small linear algebra utilities and import this module to a bigger project. In the beginning of my project I used the mmultP function from the repa-examples to calculate a big matrix, therefore I have and array of type: arr :: Monad m => m (Array U DIM2 Double) Then I carried this array in a lot of functions which become Monadic function and then it is necessary to introduce the monadic machinery for manipulating this functions . The Question is then if there is the possibility to work with a pure function in place of the monadic version? There is something like a runRepa function? runRepa :: Monad m => m (Array U DIM2 Double) - > Array U DIM2 Double or could I used the unsafePerformIO function ? or the evaluation of the parallel arrays must be postponed until the Repa.Array is called in the main function? Thanks in Advance, Felipe.

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phantom types
by TP 21 Aug '12

21 Aug '12

Hi, I am currently reading documentation on Generalized Algebraic Data Types: http://en.wikibooks.org/wiki/Haskell/GADT I have a question concerning this page. Let us consider the following code proposed in the page: ---------------------------------- -- Phantom type variable a (does not appear in any Expr: it is just a -- dummy variable). data Expr a = I Int | B Bool | Add (Expr a) (Expr a) | Mul (Expr a) (Expr a) | Eq (Expr a) (Expr a) deriving (Show) -- Smart constructors add :: Expr Int -> Expr Int -> Expr Int add = Add i :: Int -> Expr Int i = I b :: Bool -> Expr Bool b = B eval :: Expr a -> a eval (I n) = n ---------------------------------- I obtain the following error: Phantom.hs:27:14: Couldn't match type `a' with `Int' `a' is a rigid type variable bound by the type signature for eval :: Expr a -> a at Phantom.hs:27:1 In the expression: n In an equation for `eval': eval (I n) = n The wiki page explains: """ But alas, this does not work: how would the compiler know that encountering the constructor I means that a = Int? """ I don't understand. When we write "eval (I n) = n", as I is a constructor which takes an Int as argument, we are able to deduce that the type of n is Int; so the type of eval should be in this case "Expr Int -> Int". What do I miss? Thanks, TP

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Re: [Haskell-cafe] [Haskell] Compositional Compiler Construction, Oberon0 examples available
by S. Doaitse Swierstra 21 Aug '12

21 Aug '12

On Aug 21, 2012, at 13:46 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Heinrich Apfelmus wrote: >>> I have a small question: Last I remember, you've mainly been using >>> your UUAGC preprocessor to write attribute grammars in Haskell, >>> especially for UHC. Now that you have first-class attribute >>> grammars in Haskell ("achievement unlocked"), what do you intend to >>> do with the preprocessor? How do these two approaches compare at >>> the moment and where would you like to take them? >> On the page http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo there is >> a link (http://www.fing.edu.uy/~mviera/papers/VSM12.pdf) to a paper >> we presented at LDTA (one of the ETAPS events) this spring. It >> explains how UUAGC can be used to generate first class compiler >> modules. >> We have also a facility for grouping attributes, so one can trade >> flexibility for speed. The first class approach stores list of >> attributes as nested cartesian products, access to which a clever >> compiler might be able to optimize. This however would correspond a >> form of specialisation, so you can hardly say that we have really >> independent modules; as always global optimisation is never >> compositional). From the point of view of the first class approach >> such grouped non-termionals are seen as a single composite >> non-terminal. > > Ah, I see. So the custom syntax offered by UUAGC is still appreciated, but you now intend to compile it to first-class attribute grammars instead of "bare metal Haskell". Makes sense. Thanks! It is not much that it is our intention, but it is an easy way to make an existing compiler extensible. The main (fixed) part of the compiler is constructed in the "old" way from an UUAGC description, and those attributes are grouped (and quite a bit more efficient). On top of this you can define extra attributes and computations, which plug in to the old system. Notice that there is a main difference between the two approaches is that the uuagc route gives you fast compilers, because we can analyse the grammar, and generate efficient tree walking evaluators, whereas the first-class approach gives you great flexibility and the possibility to abstract from common patterns for which others prefer to get lost in stacks of monads, or find out that monads do not work at all since they cannot feed back information into a computation easily. Doaitse > > > Best regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Lazy decoding of a list with Data.Binary
by Niklas Hambüchen 21 Aug '12

21 Aug '12

I just encountered the following problem: http://stackoverflow.com/questions/11695373/lazy-decoding-of-a-list-with-da… If I get that correctly, there once was a discussion about adding a Data.Binary.Lazy to binary: http://www.haskell.org/pipermail/haskell-cafe/2007-November/034758.html What happened to this? And if binary is strict by default now, shouldn't the binary-strict package be deprecated? Is there an alternative these days how to lazily write/read binary? Thanks Niklas

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Wanted: Haskell binding for libbdd (buddy)
by Johannes Waldmann 21 Aug '12

21 Aug '12

Are there any Haskell bindings for BDD libraries (reduced ordered binary decision diagrams)? E.g., it seems "buddy" is commonly used http://packages.debian.org/squeeze/libbdd-dev and it has an Ocaml binding. Yes, there is http://hackage.haskell.org/package/obdd but I need better performance (with the same API, ideally). Thanks - J.W. PS: I wonder where performance goes out the window ... I suspect "Map (Int,Int) whatever" should really be a hashtable but I don't like it in IO, it should be in ST?

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