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Haskell Weekly News: Issue 231
by Daniel Santa Cruz 14 Jun '12

14 Jun '12

Welcome to issue 231 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 03 to 09, 2012. Announcements Paolo Capriotti announced the patchlevel release of GHC 7.4.2. "This release contains a number of bugfixes relative to 7.4.1, so we recommend upgrading." [1] http://goo.gl/X3Z85 Quotes of the Week * roconnor: there's a right way, and a wrong way, and then there is the ekmett way. Top Reddit Stories * Haskell Platform 2012.2.0.0 released Domain: hackage.haskell.org, Score: 104, Comments: 33 On Reddit: [2] http://goo.gl/ZNa9h Original: [3] http://goo.gl/JjJg2 * Why free monads matter Domain: haskellforall.com, Score: 61, Comments: 47 On Reddit: [4] http://goo.gl/hfKlc Original: [5] http://goo.gl/JPpsp * 3 Flavors of MVar: Lock, Var, Barrier Domain: neilmitchell.blogspot.com, Score: 43, Comments: 20 On Reddit: [6] http://goo.gl/3eEkO Original: [7] http://goo.gl/ek7Qq * Introducing the HERMIT Equational Reasoning Framework Domain: ittc.ku.edu, Score: 36, Comments: 20 On Reddit: [8] http://goo.gl/UC0sF Original: [9] http://goo.gl/ifS7f * Complicating conduit? Domain: yesodweb.com, Score: 35, Comments: 45 On Reddit: [10] http://goo.gl/W4rfE Original: [11] http://goo.gl/rU0vK * Beautiful Code, Compelling Evidence: OpenGL, Cairo tutorial [PDF, 33 pages] Domain: renci.org, Score: 33, Comments: 2 On Reddit: [12] http://goo.gl/qKY8w Original: [13] http://goo.gl/r5SVj * Forklift - a pattern for performing monadic actions in a worker thread Domain: apfelmus.nfshost.com, Score: 30, Comments: 7 On Reddit: [14] http://goo.gl/B6brV Original: [15] http://goo.gl/mWVAv * Dependently typed programming with singletons (Eisenberg, Weirich)[PDF] Domain: cis.upenn.edu, Score: 29, Comments: 3 On Reddit: [16] http://goo.gl/y3fYv Original: [17] http://goo.gl/RqM5m * Continuation-based relative-time FRP Domain: paolocapriotti.com, Score: 22, Comments: 6 On Reddit: [18] http://goo.gl/CFSK7 Original: [19] http://goo.gl/cP0cJ * Explaining the Prompt monad with a clearer representation Domain: joeysmandatory.blogspot.com, Score: 21, Comments: 42 On Reddit: [20] http://goo.gl/ZZM93 Original: [21] http://goo.gl/Im7SF * Lazily Evaluate length of a list Domain: self.haskell, Score: 21, Comments: 29 On Reddit: [22] http://goo.gl/cKUOt Original: [23] http://goo.gl/cKUOt Top StackOverflow Questions * Killing a thread when MVar is garbage collected votes: 22, answers: 4 Read on SO: [24] http://goo.gl/cvoqw * How does one write efficient Dynamic Programming algorithms in Haskell? votes: 16, answers: 3 Read on SO: [25] http://goo.gl/mkMTp * How to apply a polymorphic function to a Dynamic value votes: 11, answers: 1 Read on SO: [26] http://goo.gl/3jIVr * Haskell Typeclass for Tuples votes: 11, answers: 3 Read on SO: [27] http://goo.gl/pvDah * Monadic expressions in conditionals - GHC compiles, cabal refuses votes: 10, answers: 2 Read on SO: [28] http://goo.gl/QXMXh * GHC Optimization: Collatz conjecture votes: 10, answers: 2 Read on SO: [29] http://goo.gl/AfiEC * Haskell: Lists vs Streams votes: 10, answers: 3 Read on SO: [30] http://goo.gl/qn1dE * Is my program Turing-complete? votes: 9, answers: 2 Read on SO: [31] http://goo.gl/PLf5q * How to find the optimal processing order? votes: 9, answers: 1 Read on SO: [32] http://goo.gl/P3XZ2 * How to do fast data deserialization in Haskell votes: 9, answers: 1 Read on SO: [33] http://goo.gl/Synm2 * What's so bad about OverlappingInstances? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/M17Rp Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/22181 2. http://hackage.haskell.org/platform/?2012.2.0.0 3. http://www.reddit.com/r/haskell/comments/uipvz/haskell_platform_2012200_rel… 4. http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.ht… 5. http://www.reddit.com/r/haskell/comments/utxq2/why_free_monads_matter/ 6. http://neilmitchell.blogspot.com/2012/06/flavours-of-mvar_04.html 7. http://www.reddit.com/r/haskell/comments/uk9wu/3_flavors_of_mvar_lock_var_b… 8. http://www.ittc.ku.edu/csdlblog/?p=105 9. http://www.reddit.com/r/haskell/comments/ujdet/introducing_the_hermit_equat… 10. http://www.yesodweb.com/blog/2012/06/complicating-conduit 11. http://www.reddit.com/r/haskell/comments/urc75/complicating_conduit/ 12. http://www.renci.org/wp-content/pub/tutorials/BeautifulCode.pdf 13. http://www.reddit.com/r/haskell/comments/uqc33/beautiful_code_compelling_ev… 14. http://apfelmus.nfshost.com/blog/2012/06/07-forklift.html 15. http://www.reddit.com/r/haskell/comments/uq44h/forklift_a_pattern_for_perfo… 16. http://www.cis.upenn.edu/~eir/papers/2012/singletons/paper.pdf 17. http://www.reddit.com/r/haskell/comments/uic9g/dependently_typed_programmin… 18. http://paolocapriotti.com/blog/2012/06/04/continuation-based-relative-time-… 19. http://www.reddit.com/r/haskell/comments/uklsq/continuationbased_relativeti… 20. http://joeysmandatory.blogspot.com/2012/06/explaining-prompt-monad-with-sim… 21. http://www.reddit.com/r/haskell/comments/unpaz/explaining_the_prompt_monad_… 22. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 23. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 24. http://stackoverflow.com/questions/10871303/killing-a-thread-when-mvar-is-g… 25. http://stackoverflow.com/questions/10906053/how-does-one-write-efficient-dy… 26. http://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-func… 27. http://stackoverflow.com/questions/10899804/haskell-typeclass-for-tuples 28. http://stackoverflow.com/questions/10867300/monadic-expressions-in-conditio… 29. http://stackoverflow.com/questions/10876110/ghc-optimization-collatz-conjec… 30. http://stackoverflow.com/questions/10942450/haskell-lists-vs-streams 31. http://stackoverflow.com/questions/10878846/is-my-program-turing-complete 32. http://stackoverflow.com/questions/10879838/how-to-find-the-optimal-process… 33. http://stackoverflow.com/questions/10902405/how-to-do-fast-data-deserializa… 34. http://stackoverflow.com/questions/10942136/whats-so-bad-about-overlappingi…

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Perth Functional Programmers meetup group launched
by Todd Owen 13 Jun '12

13 Jun '12

We are pleased to announce that a functional programming user group has recently been formed in Perth, Australia. Being a small community, we aim to keep the group as inclusive as possible, and welcome new members from all levels of experience and language backgrounds. (That said, Haskellers currently account for about half our membership, followed by F#, and then a heap of other languages). As this is a global mailing list, I would also like to invite our colleagues from around Australia, and indeed anywhere in the world, to get in touch if you are ever passing through Perth. We're always happy to share a beer and talk about programming! For more information, visit: http://www.meetup.com/PerthFP/

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Warning when attempting to install cabal-dev on the latest HP
by Edward Amsden 13 Jun '12

13 Jun '12

http://hpaste.org/69885 The warning in that paste occurs when I attempt $ cabal update $ cabal install cabal-dev This is curious because it wants to install a previous version of mtl. I can't figure out why. There doesn't seem to be any dependency between cabal-dev and mtl. According to hackage, the dependencies for cabal-dev are: cabal-dev -> (base, Cabal) Cabal -> (base, filepath) filepath -> (base) base -> () The dependencies for mtl are (just to show no conflicts) mtl -> (base, transformers) transformers -> (base) base -> () And there's not a versioning conflict with base. Why is cabal insisting on backdating mtl then?

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RawFilePath vs System.FilePath
by Kazu Yamamoto 13 Jun '12

13 Jun '12

Hello, After releaseing new Haskell Platform, many people can now use RawFilePath (e.g. ByteString) for System.*. However, there is no System.FilePath.ByteString which manipulates RawFilePath. How do you guys manipulate RawFilePath as file path? Is there a plan to implement System.FilePath.ByteString? # I know the system-filepath package. But I would like to know # answers to the questions above. Regards, --Kazu

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[Meetup] Haskell web development - Munich - this Friday
by Vincent Ambo 13 Jun '12

13 Jun '12

Hej, would anyone be interested in a short dated meetup in Munich this Friday? (June 15th) Michael Snoyman of Yesod will be giving a talk at the Big Tech Day and a meeting afterwards would be possible on Friday afternoon. I'd like if this meeting were somewhat focused on web development in Haskell (Yesod/WAI, Happstack, Snap, clckwrks, hakyll, Elm, .... (please don't be angry if I forgot your specific project, our web development tools are growing at an out of control speed!)), complaining about how annoying Javascript's type 'system' is and generally having a bit of fun (definitely nothing formal or with a strict schedule). We've also not decided on a location yet, so if you have any ideas for that please bring them up here :-) Cheers, Vincent

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Derived Read for records
by Conal Elliott 13 Jun '12

13 Jun '12

Someone at work just asked me about the inflexibility of the derived Read instances for records -- specifically that they require fields to be given in the same order as in the type definition and that fields cannot be omitted. I hadn't been aware of these restrictions. A few questions: * Are there known work-arounds? * Is it particularly difficult to synthesize more flexible Read instances? * Do people just not mind the restrictions? Thanks, -- Conal

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ANN: clckwrks - a Haskell blogging and CMS framework
by Jeremy Shaw 12 Jun '12

12 Jun '12

Hello! I am pleased to announce the launch of clckwrks.com. clckwrks (pronounced, 'clockworks') is a new Haskell-based content management (CMS) and blogging framework: http://www.clckwrks.com/ It is freely available under the BSD3 license. clckwrks is still in early alpha development. We are looking for developers who want to contribute or just like to try bleeding edge software. clckwrks is already being used to power happstack.com and clckwrks.com -- so it is already useful. There are a lot of things we are not happy with yet ranging from code formatting to design issues. But, at the same time, there are a lot of things we are very happy with. If you would like to help get us to clckwrks 1.0, we'd love to have your help! For more information, check out the website and consider joining the mailing list. Thanks! - jeremy

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Troubles understanding Parsec Error Handling
by Matthias Hörmann 12 Jun '12

12 Jun '12

I recently started writing my first application at work in Haskell and it deals with a lot of parsing. Among other things I often have to check for a lot of alternatives for fixed strings (parsing natural language text and people have a lot of ways to abbreviate the same thing in labels). So far I have been doing this basically via choice $ map (try . string) [ "foo", "bar", ... ] This works fine but has two disadvantages, it isn't very fast, in particular when many of the strings start with the same prefix and it also is a bit error prone since it breaks when you place a prefix of another string earlier in the list. My attempt at a solution was to use the bytestring-trie package for a little utility function that basically parses one character at a time, checks if the string parsed so far is in the trie and then calls itself recursively with the trie starting with that string. My attempt at that so far looks like this: (dependencies bytestring-trie, utf8-string and parsec 3) import qualified Data.ByteString.UTF8 as UTF8 import qualified Data.Trie as Trie import Text.Parsec import Text.Parsec.Text (GenParser) anyOf :: [String] -> GenParser u String anyOf l = try $ anyOf' t "" where t = Trie.fromList $ zip (map UTF8.fromString l) (repeat ()) anyOf' :: Trie.Trie () -> String -> GenParser u String anyOf' t s = try $ do c <- lookAhead $ anyChar let newS = s ++ [ c ] in case Trie.submap (UTF8.fromString newS) t of emptyT | Trie.null emptyT -> case Trie.member (UTF8.fromString s) t of True -> return s False -> unexpected $ show newS++", expecting one of "++show l restT -> do _ <- anyChar try $ anyOf' restT newS A successful example usage would be: parseTest (do; r1 <- anyOf ["Hello", "Hallo", "Foo", "HallofFame"]; r2 <- string "bla"; return (r1, r2)) "Hallobla" which results in ("Hallo","bla") (the extra string parser is there so errors in parsing too much are not hidden). An error would result .e.g. from parseTest (do; r1 <- anyOf ["Hello", "Hallo", "Foo", "HallofFame"]; r2 <- string "bla"; return (r1, r2)) "Hallofbla" which prints this: parse error at (line 1, column 8):unknown parse error And my question about this is made up of two parts 1. Why doesn't it print my "unexpected" message but instead says unknown parse error 2. Why is the location in the text off (I would expect it to fail at column 6 (first character beyond the result it could return) or 7 (first character that makes the string no prefix of any acceptable string) I am afraid my knowledge of Parsec internals is a bit too limited, some Google queries showed no similar problems and no obvious places in the Parsec source code to check for the answer to the first question in particular and I suspect the second is closely related to the first. Thanks for reading through my question and I hope someone knows the answer or at least some clues on where i might find it. Matthias Hoermann P.S.: I am hoping this time this works, last time it was rejected because google sends with @googlemail.com instead of @gmail.com for some reason.

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vector operations
by Evan Laforge 11 Jun '12

11 Jun '12

So I wanted to find the first index in a vector whose running sum is greater than a given number. The straightforward way is to create the running sum and then search: Vector.findIndex (>=target) (Vector.scanl' (+) 0 vector) But vectors are strict so it could do extra work, and what if I don't want to generate garbage? I could do it with a fold, but it would have to have the ability to abort early. Of course I could write such a fold myself using indexing: import qualified Data.Vector.Generic as Vector fold_abort :: (Vector.Vector v a) => (accum -> a -> Maybe accum) -> accum -> v a -> accum fold_abort f accum vec = go 0 accum where go i accum = maybe accum (go (i+1)) $ f accum =<< vec Vector.!? i find_before :: (Vector.Vector v a, Num a, Ord a) => a -> v a -> Int find_before n = fst . fold_abort go (0, 0) where go (i, total) a | total + a >= n = Nothing | otherwise = Just (i+1, total+a) So it's bigger and clunkier, but I would think it would be much more efficient (provided using Data.Vector.Generic won't inhibit inlining and unboxing). But I'm a bit surprised there isn't already something like fold_abort... or is there?

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GHCi Loop Detection
by John Van Enk 11 Jun '12

11 Jun '12

Hi Cafe, Is there a reason that the GHCi interpreter doesn't detect and report infinite loops in statements like this (like compiled programs do) even though no CPU time appears to be used? My (admittedly weak) searching for an answer didn't turn much up. let s | not $ null s = [] in s GHCi v7.0.4 on OSX Thanks, John

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