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Haskell Weekly News: Issue 255
by Daniel Santa Cruz 24 Jan '13

24 Jan '13

Welcome to issue 255 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of January 06 to 19, 201. Quotes of the Week * ky3: Indexed monads are just categories in the category of endofunctors, what's the problem? * tac: I guess I'm just living in a fantasy world where the docs tell you how to use such a basic API :P * shachaf: 3.0 is an edwardk phase, not just a version number. It's like Picasso's Blue Period. * cmccann: Clearly the reason why edwardk uses Haskell now is because every C++ compiler has a restraining order against him. * shachaf: I'm not sure what your question is, but maybe lens is the answer. * cmccann: * cmccann has concluded that lenses are just a category whose objects are english prepositions and whose morphisms are all unsafeCoerce * shachaf: Some people, when asking about a problem in #haskell, get a link to okmij.org. Now they have two problems. * quicksilver: It returns undefined for convenience. I'm not really sure it's that convenient, but that's why. * edwardk: cmccann: let me get this straight you are saying that someone [mm_freak] who by their very nick admits to being a freak did something freakish? Top Reddit Stories * LiquidHaskell: Refinement Types in Haskell via SMT Domain: goto.ucsd.edu, Score: 71, Comments: 22 On Reddit: [1] http://goo.gl/PE2nf Original: [2] http://goo.gl/P3Gm9 * Hakyll 4.0 released Domain: jaspervdj.be, Score: 59, Comments: 19 On Reddit: [3] http://goo.gl/IQAXq Original: [4] http://goo.gl/AbLnM * When Haskell is faster than C Domain: paulspontifications.blogspot.co.uk, Score: 52, Comments: 23 On Reddit: [5] http://goo.gl/3rj8J Original: [6] http://goo.gl/N2cbA * I'm teaching an introductory 12-week class on Haskell. Join in for a journey! Domain: reddit.com, Score: 45, Comments: 15 On Reddit: [7] http://goo.gl/TsnQ5 Original: [8] http://goo.gl/EMMVR * Subtractable values are torsors Domain: ro-che.info, Score: 42, Comments: 15 On Reddit: [9] http://goo.gl/4IcMp Original: [10] http://goo.gl/6YJrY * Chordify: extract chord sequences from MP3's using HarmTrace Domain: chordify.net, Score: 42, Comments: 8 On Reddit: [11] http://goo.gl/8J7rA Original: [12] http://goo.gl/ySfR5 * Getting Started with Haskell Domain: bob.ippoli.to, Score: 41, Comments: 8 On Reddit: [13] http://goo.gl/4Evnb Original: [14] http://goo.gl/nzg9o * Using AngularJS with Haskell Snap Domain: nurpax.github.com, Score: 40, Comments: 19 On Reddit: [15] http://goo.gl/SH8e1 Original: [16] http://goo.gl/0lSyN * Down with kinds: adding dependent heterogeneous equality to to FC Domain: cis.upenn.edu, Score: 38, Comments: 14 On Reddit: [17] http://goo.gl/iEvyI Original: [18] http://goo.gl/cH0Eo * Fixing GADTs : Generic recursion schemes for GADTs using fixed points of higher-order functors Domain: timphilipwilliams.com, Score: 38, Comments: 25 On Reddit: [19] http://goo.gl/dEMq9 Original: [20] http://goo.gl/vnjhp * What's the importance of Category Theory in Haskell? Is it worth really studying? Domain: self.haskell, Score: 37, Comments: 60 On Reddit: [21] http://goo.gl/T430r Original: [22] http://goo.gl/T430r * Parsing Markdown with Parsec: How Pandoc does it Domain: youtube.com, Score: 35, Comments: 13 On Reddit: [23] http://goo.gl/TN9TJ Original: [24] http://goo.gl/zpX7f * Clckwrks, from the creator of Happstack, is developing into a clean alternative to Wordpress Domain: clckwrks.com, Score: 34, Comments: 7 On Reddit: [25] http://goo.gl/L9WXx Original: [26] http://goo.gl/FZRpU * pipes-safe-1.0 - Resource management and exception handling for pipes Domain: haskellforall.com, Score: 32, Comments: 27 On Reddit: [27] http://goo.gl/HeLWz Original: [28] http://goo.gl/1hTFc Top StackOverflow Questions * values, types, kinds,… as an infinite sequence? votes: 36, answers: 1 Read on SO: [29] http://goo.gl/lyyYc * Why is this a useful warning in Haskell when pattern matching? “Defined but not used” votes: 17, answers: 3 Read on SO: [30] http://goo.gl/iQLFq * Why are getArgs and getProgName IO actions? votes: 16, answers: 3 Read on SO: [31] http://goo.gl/5ZPYO * Can GADTs be used to prove type inequalities in GHC? [duplicate] votes: 14, answers: 2 Read on SO: [32] http://goo.gl/fDdha * Applying Denotational Semantics to design of Programs [migrated] votes: 14, answers: 0 Read on SO: [33] http://goo.gl/KLOCX * Examples where compiler-optimized functional code performs better than imperative code votes: 14, answers: 1 Read on SO: [34] http://goo.gl/Fzqdx * Why is there only one non-strict function from Int to Int? votes: 12, answers: 2 Read on SO: [35] http://goo.gl/KXcAy * Dependency injection in Haskell: solving the task idiomatically votes: 12, answers: 3 Read on SO: [36] http://goo.gl/xTcYc * What does a Haskell comment starting with `{- |` generally mean? votes: 11, answers: 1 Read on SO: [37] http://goo.gl/zNikw * Is operational really isomorphic to a free monad? votes: 10, answers: 3 Read on SO: [38] http://goo.gl/AJlgH Until next time, [39]+Daniel Santa Cruz References 1. http://goto.ucsd.edu/~rjhala/liquid/haskell/blog/blog/2013/01/01/refinement… 2. http://www.reddit.com/r/haskell/comments/16w3hp/liquidhaskell_refinement_ty… 3. http://jaspervdj.be/posts/2013-01-16-hakyll-4.0.html 4. http://www.reddit.com/r/haskell/comments/16ojtv/hakyll_40_released/ 5. http://paulspontifications.blogspot.co.uk/2013/01/when-haskell-is-faster-th… 6. http://www.reddit.com/r/haskell/comments/16tryo/when_haskell_is_faster_than… 7. http://www.reddit.com/r/UniversityofReddit/comments/169jfe/class_introducti… 8. http://www.reddit.com/r/haskell/comments/169k67/im_teaching_an_introductory… 9. http://ro-che.info/articles/2013-01-08-torsors.html 10. http://www.reddit.com/r/haskell/comments/166iju/subtractable_values_are_tor… 11. http://chordify.net/pages/how-to-use-chordify/ 12. http://www.reddit.com/r/haskell/comments/16u44u/chordify_extract_chord_sequ… 13. http://bob.ippoli.to/archives/2013/01/11/getting-started-with-haskell/ 14. http://www.reddit.com/r/haskell/comments/16fegr/getting_started_with_haskel… 15. http://nurpax.github.com/posts/2013-01-13-angularjs-and-haskell.html 16. http://www.reddit.com/r/haskell/comments/16iaiq/using_angularjs_with_haskel… 17. http://www.cis.upenn.edu/~eir/papers/2013/jmnokinds/jmnokinds.pdf 18. http://www.reddit.com/r/haskell/comments/167c21/down_with_kinds_adding_depe… 19. http://www.timphilipwilliams.com/posts/2013-01-16-fixing-gadts.html 20. http://www.reddit.com/r/haskell/comments/16qr6q/fixing_gadts_generic_recurs… 21. http://www.reddit.com/r/haskell/comments/16i322/whats_the_importance_of_cat… 22. http://www.reddit.com/r/haskell/comments/16i322/whats_the_importance_of_cat… 23. http://www.youtube.com/watch?v=nCwG9ijQMuQ&t=01m22s 24. http://www.reddit.com/r/haskell/comments/16kf0j/parsing_markdown_with_parse… 25. http://www.clckwrks.com/ 26. http://www.reddit.com/r/haskell/comments/168j2w/clckwrks_from_the_creator_o… 27. http://www.haskellforall.com/2013/01/pipes-safe-10-resource-management-and.… 28. http://www.reddit.com/r/haskell/comments/16mban/pipessafe10_resource_manage… 29. http://stackoverflow.com/questions/14401689/values-types-kinds-as-an-infini… 30. http://stackoverflow.com/questions/14309396/why-is-this-a-useful-warning-in… 31. http://stackoverflow.com/questions/14295582/why-are-getargs-and-getprogname… 32. http://stackoverflow.com/questions/14273235/can-gadts-be-used-to-prove-type… 33. http://stackoverflow.com/questions/14349232/applying-denotational-semantics… 34. http://stackoverflow.com/questions/14357918/examples-where-compiler-optimiz… 35. http://stackoverflow.com/questions/14262014/why-is-there-only-one-non-stric… 36. http://stackoverflow.com/questions/14327327/dependency-injection-in-haskell… 37. http://stackoverflow.com/questions/14270500/what-does-a-haskell-comment-sta… 38. http://stackoverflow.com/questions/14263363/is-operational-really-isomorphi… 39. https://plus.google.com/105107667630152149014/about

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ANN: monad-bool 0.1
by John Wiegley 24 Jan '13

24 Jan '13

monad-bool implements a pair of Boolean monoids and monads, to support short-circuiting, value-returning computations similar to what Python and Ruby offer with their native && and || operators. For example, in Python you might see this: x = [1,2,3,0] print x[1] || x[3] -- prints "2" With this library, you can now mirror such code in Haskell: let x = [1,2,3,0] print $ (x !! 1) ||? (x !! 3) -- prints "Success 2" "Booleanness" is based on each type having an instance of the 'Control.Conditional.ToBool' type, for which only the basic types are covered (Bool, Int, Integer, Maybe a, Either a b, [a], Attempt a). If you wish to define a truth value for your own types, simply provide an instance for ToBool: instance ToBool MyType where toBool = ... The And/Or monoids use the Attempt library so that the actual type of the successful results depends on case analysis. It could be a list, a Maybe, an Either, or an exception in the IO Monad. The monad variants, AndM, AndMT, OrM and OrMT provide short-circuiting behavior in a Monad, which returns the last value returned before truth was determined. Here are two examples: Use 'onlyIf' with AndM and AndMT to guard later statements, which are only evaluated if every preceding 'onlyIf' evaluates to True. For example: foo :: AndM Int foo = do onlyIf (True == True) return 100 onlyIf (True == True) return 150 onlyIf (True == False) return 200 When run with `evalAndM foo (-1)` (where (-1) provides a default value), 'foo' returns 150. Use 'endIf' with OrM and OrMT to chain statements, which are only executed if every preceding 'endIf' evaluated to False. For example: bar :: OrM Int bar = do endIf (True == False) return 100 endIf (True == False) return 150 endIf (True == True) return 200 When run with `evalOrM bar (-1)` (where (-1) again provides a default value), 'bar' returns 150. And please, somebody let me know if this has already been done. A search for likely candidates did not turn up anything obvious in Hoogle, but as my knowledge of the whole of Hackage is minimal, I would appreciate any wiser minds that can inform me. Thank you, -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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Space leaks in function that uses Data.Vector.Mutable
by Andrey Yankin 24 Jan '13

24 Jan '13

Hi! I have a "low-level" function for insertion element into mutable vector. It looks like this: place :: (PrimMonad m) => MV.MVector (PrimState m) (Int, t) -> (Int, t) -> Int -> m () place v max@(val1,_) i = place' i where place' i = do let j = i - 1 if j < 0 then return () else do curr@(val2, _) <- MV.unsafeRead v j -- <<<<< if val2 > val1 then do MV.unsafeWrite v j max MV.unsafeWrite v i curr place' j else return () It searches a right place for the i-th element, moving it towards beginning . Surprisingly, it works, but is slow. Time profiling says this: COST CENTRE MODULE %time %alloc ticks bytes place.place' Main 40.1 77.1 9167 12169223232 And heap profiling says that most of allocations are for Int and (,). I think, binding of unsafeRead to curr and val might allocate my Precious memory more than, maybe, it is needed. Am I right? Is it some strictness that I need? Is there anything I can do in this situation? Or should I look outside of this function? PS Emergence of this piece of code is a long story. Originally I was trying to solve Project Euler's problem 411. But now it is a different issue.

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ANN: hgdbmi 0.2, GDB Machine Interface
by Alexander Bernauer 23 Jan '13

23 Jan '13

Hi I am happy to announce the second release of hgdbmi, a Haskell implementation of the GDB Machine Interface. The Machine Interface is a machine-readable variant of the stdin/stdout-based interface of the GNU debugger. With hgdbmi you can control the execution of a GDB instance to drive debugging of arbitrary targets. Version 0.2 of this library is a substantial, non-compatible extension of version 0.1. The API provides a thread-safe and synchronous command-response function and supports user-defined call-backs for asynchronous notifications. All commands, responses and notifications are supported in this version. Also, for some response and notification types, we additionally provide semantic data types and corresponding translation functions. The package: http://hackage.haskell.org/package/hgdbmi-0.2 Greetings, Alex PS: This is my first serious Haskell package. If you see something that you consider odd, I might have done this simply because I didn't know better. I would appreciate if you would let me know in such a case.

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The Coverage Condition of functional dependencies
by Petr P 23 Jan '13

23 Jan '13

Hi, trying to understand UndecidableInstances (and to find and answer to < http://stackoverflow.com/q/14476230/1333025>), I was trying to find out why "mtl" needs UndecidableInstances. The reason is that instances like > instance MonadState s m => MonadState s (ContT r m) where don't satisfy the Coverage Condition: "The Coverage Condition. For each functional dependency, tvsleft -> tvsright, of the class, every type variable in S(tvsright) must appear in S(tvsleft), where S is the substitution mapping each type variable in the class declaration to the corresponding type in the instance declaration. " (See http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/type-class-extension… ) In other words, "s" isn't expressed using type variables in "ContT r m". But in these cases, it's actually possible. Because of the assertion "MonadState s m" and its dependency "m -> s" we know that "s" will be always deducible from "m". I wonder, would it be possible to augment the type checker to realize this? It seems reasonable: Before comparing if S(tvsright) is a subset of S(tvsleft), we'd add every type variable to S(tvsleft) that is determined from it using functional dependencies in the assertion of the instance. Best regards, Petr

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Re: [Haskell-cafe] Mobile app development?
by Andrew Pennebaker 23 Jan '13

23 Jan '13

Can we un-deprecate GHC's ability to compile to C code? C may be the best option to bridge to mobile, as Android, iOS, and Windows RT do support C/C++ apps. On Jan 22, 2013 2:14 PM, "Dan Choi" <dhchoi(a)gmail.com> wrote: > > What about the option of using Haskell's Parsec or AttoParsec to implement > a Haskell-ish language that compiles down not to machine code but to > Objective C or Android Java? Like how CoffeeScript compiles down to > JavaScript. > > > > On Saturday, January 19, 2013 1:58:03 PM UTC-5, Casey Hawthorne wrote: >> >> You would need native compilers for all the platforms and/or virtual >> machine technology. >> >> Might be easier to have the browser connect to a Haskell app. >> >> >> On Sat, Jan 19, 2013 at 10:42 AM, Andrew Pennebaker >> <andrew.p...@gmail.com> wrote: >> > There are currently very few options, especially free and open source >> > options, when it comes to developing cross-platform mobile >> applications. >> > It's basically web apps with JavaScript, or C++. If Haskell supported >> app >> > development on Android, iOS, and Windows RT, that alone would bring in >> more >> > developers. >> > >> > Similarly, there are very few languages for mobile development that >> take >> > advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an >> > amazing selling point. Can we please prioritize mobile support? I'd >> much >> > rather write everything in ML than PhoneGap. >> > >> > -- >> > Cheers, >> > >> > Andrew Pennebaker >> > www.yellosoft.us >> > >> > ______________________________**_________________ >> > Haskell-Cafe mailing list >> > Haskel...@haskell.org >> > http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >> > >> >> >> >> -- >> -- >> Regards, >> KC >> >> ______________________________**_________________ >> Haskell-Cafe mailing list >> Haskel...@haskell.org >> http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >> >

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Re: [Haskell-cafe] ANN: Haskell bindings for the igraph C library
by George Giorgidze 23 Jan '13

23 Jan '13

Hi Jason, > How does this compare with fgl? http://hackage.haskell.org/package/fgl A couple of months ago, I was working on a project where I had to do some graph analysis. I encountered two problems with the fgl library: (1) The fgl library did not work for large graphs, even graph construction would fail with stack overflow. (2) The fgl library did not provide the graph algorithms needed in the project: closeness centrality, Brandes betweenness centrality, eigenvector centrality, pagerank and some more. The igraph library featured all functions that I needed and also worked for large graphs without problems. I was also very pleased with its performance and started writing Haskell bindings to igraph. Cheers, George

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: MFlow 0.2
by Alberto G. Corona 22 Jan '13

22 Jan '13

The template look is very simple but it uses a lot of dynamic code behind I changed the template to something more light. http://haskell-web.blogspot.com.es/ 2013/1/20 <briand(a)aracnet.com> > On Thu, 17 Jan 2013 16:07:41 +0100 > "Alberto G. Corona " <agocorona(a)gmail.com> wrote: > > > > > The entry in my blog, with the announcement and the philosophy behind > > http://haskell-web.blogspot.com.es/2013/01/announce-mflow-02.html > > > > FYI > That page never comes up for me. I just see spinning gears. > iceweasel 10.0.7 > > Brian > > -- Alberto.

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Re: [Haskell-cafe] Mobile app development?
by KC 22 Jan '13

22 Jan '13

Then it looks as if the easier implementation would be small Haskell VM's for the various platforms with a byte code compiler. I do not believe the JVM supports all the optimizations GHC can do. Oh wait! Can the LLVM be easily ported to do this? On Sat, Jan 19, 2013 at 11:40 AM, Andrew Pennebaker <andrew.pennebaker(a)gmail.com> wrote: > >> Might be easier to have the browser connect to a Haskell app. > > > Not all apps can be run as thin clients. 3D video games and other intensive > programs aren't easily done as thin clients. Mobile Haskell would be very > powerful, because concurrency and parallelism aren't something the C-family > languages are supporting that well. > >> >> >> >> On Sat, Jan 19, 2013 at 10:42 AM, Andrew Pennebaker >> <andrew.pennebaker(a)gmail.com> wrote: >> > There are currently very few options, especially free and open source >> > options, when it comes to developing cross-platform mobile applications. >> > It's basically web apps with JavaScript, or C++. If Haskell supported >> > app >> > development on Android, iOS, and Windows RT, that alone would bring in >> > more >> > developers. >> > >> > Similarly, there are very few languages for mobile development that take >> > advantage of multiple cores and multiple CPUs. Haskell's `parmap` is an >> > amazing selling point. Can we please prioritize mobile support? I'd much >> > rather write everything in ML than PhoneGap. >> > >> > -- >> > Cheers, >> > >> > Andrew Pennebaker >> > www.yellosoft.us >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >> >> >> -- >> -- >> Regards, >> KC > > > > > -- > Cheers, > > Andrew Pennebaker > www.yellosoft.us -- -- Regards, KC

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Non-derivable Typeable
by Roman Cheplyaka 22 Jan '13

22 Jan '13

I need to declare a Typeable instance for a type which has an argument of kind * -> *. GHC refuses to derive it. What is a recommended way to go about it? In particular, if I write the instance by hand, how important is the fingerprint, and how could I generate it? Roman

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