January 2013 - Haskell-Cafe

Data.Sequence and replicateM
by Daniel Díaz Casanueva 25 Jan '13

25 Jan '13

Hi Cafe, I was coding this morning when I suddenly found something that surprised me. It's been a short time since I am really caring about the performance of my programs. Before, I was just caring about their correctness. So I am trying different things and profiling to see differences. One difference I have found surprising is that the function f is MUCH faster and less space consuming than the function g: import Control.Monad import qualified Data.Sequence as Seq type Seq = Seq.Seq f :: Monad m => Int -> m a -> m (Seq a) f n = fmap Seq.fromList . replicateM n g :: Monad m => Int -> m a -> m (Seq a) g = Seq.replicateM Maybe is just in my test case, where the Int argument is big and the monadic action short, but it looks to me that Data.Sequence.replicateM can be faster than it is right now. Regards, Daniel Díaz. -- E-mail sent by Daniel Díaz Casanueva let f x = x in x

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ANNOUNCE: hslogstash (library for working with Logstash and related tools)
by Simon Marechal 25 Jan '13

25 Jan '13

This is a library for sysadmins and/or tool writers. It provides proper types and correct serialization for Logstash messages, along with small utilities for working with ElasticSearch or Redis (using hedis). This library focus will be on safety (no messages lost). Right now, it can be used to: * send properly formated messages to Logstash * read Logstash messages from Redis or any socket * write Logstash messages to ElasticSearch or any socket This is the work of a sysadmin, thus very likely to be horrible to read. Suggestions are very welcome.

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ANNOUNCE: creatur (framework for artificial life)
by Amy de Buitléir 25 Jan '13

25 Jan '13

I'm happy to announce a new package called Créatúr (creatur) http://hackage.haskell.org/package/creatur https://github.com/mhwombat/creatur-examples/raw/master/Tutorial.pdf (tutorial) https://github.com/mhwombat/creatur (github) ABOUT CRÉATÚR: Créatúr is a software framework for automating experiments with artificial life (ALife) or other evolutionary algorithms (EA). It provides a daemon which ensures that each agent gets its turn to use the CPU. You can use other applications on the computer at the same time without fear of interfering with experiments; they will run normally (although perhaps more slowly). Créatúr also provides a library of modules to help you implement your own ALife species. Even if you aren't using the Créatúr framework, you may find some of these modules useful. A tutorial on how to use Créatúr, including lots of examples, is available at https://github.com/mhwombat/creatur-examples/raw/master/Tutorial.pdf. Créatúr (pronounced kray-toor) is an irish word meaning animal, creature, or unfortunate person.

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ANN: gray-extended (tools for working with Gray codes)
by Amy de Buitléir 25 Jan '13

25 Jan '13

I'm happy to announce a new package called gray-extended http://hackage.haskell.org/package/gray-extended-1.2 https://github.com/mhwombat/gray-extended (github) ABOUT GRAY-EXTENDED: Gray codes satisfy the property that two successive values differ in only one digit. Usually the term \"Gray code\" refers to the Binary Reflected Gray code (BRGC), but non-binary Gray codes have also been discovered.

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Manually instantiating Typeable w/DataKinds
by Uri Braun 25 Jan '13

25 Jan '13

I've read the recent posting titled "Non-derivable Typeable" (http://www.mail-archive.com/haskell-cafe@haskell.org/msg103616.html) which explains that Typeable cannot be automatically derived for cases where the kind is constrained. I'm very impressed that a solution is imminent. In the interim, can somebody kindly suggest a workaround? I'm okay with a manual instance, but I'd appreciate some help as to how to write one. I'm looking for a Typeable instance for TaggedVar for the following example below (extracted from my code). Thank you in advance! +Uri {-# LANGUAGE DeriveDataTypeable, DataKinds, KindSignatures #-} module Example where import Data.Typeable data Tag = TagV | TagE | TagA | TagL deriving Typeable newtype TaggedVar (t :: Tag) = TaggedVar Int

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hxt pickling question
by briand＠aracnet.com 24 Jan '13

24 Jan '13

Hi, Trying to understand how to write a document using a pickler but I can't make sense of the types. From the example: runX ( xunpickleDocument xpSeason [ withValidate no , withTrace 1 , withRemoveWS yes , withPreserveComment no ] "simple2.xml" >>> processSeason >>> xpickleDocument xpSeason [ withIndent yes ] "new-simple2.xml" ) So all I want to do is pickle a value directly instead of reading the value from a document. I expected to do something like: runX (someHXTPicklingFunction myValue >>> xpickleDocument ...) but I can't seem to figure out what someHXTPicklingFunction should be, it's certainly nothing obvious like pickleDoc, because that generates the wrong value. Seems like I probably have a much more fundamental problem in that I really don't understand how the arrow part of this little example really works, but I was kind of hoping that doing something "simple" like this might shed some light on that. And then was immediately stuck in type hell. Thanks, Brian

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Video - Simon Marlow on High Performance Concurrency - Skills Matter's Haskell eXchange
by Theo 24 Jan '13

24 Jan '13

Simon Marlow took the floor at the first annual Haskell eXchange last year to talk about high performance concurrency. Simon observes that the haskell community has been spending inordinate amounts of time designing beautiful programming models and APIs, whilst spending relatively less time building applications that solve real problems. In this talk Simon wants to start from an application that they want to build -- a simple concurrent chat server---and use that to inform some library design. Along the way we’ll learn useful techniques for building concurrent network applications, and show how the power of Software Transactional Memory can be brought to bear. Yet this application demonstrates some holes in the abstractions provided by Haskell as it stands, Simon shows a simple layer called “async” that fills the gap and should be widely useful for concurrent programming. To watch the podcast in full follow this link - http://bit.ly/10Bzhr1

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Haskell Weekly News: Issue 255
by Daniel Santa Cruz 23 Jan '13

23 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 23 Jan '13

23 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 23 Jan '13

23 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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