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Haskell Weekly News: Issue 225
by Daniel Santa Cruz 03 May '12

03 May '12

Welcome to issue 225 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of April 22 to 28, 2012. Announcements Matthias Fischmann made a call for participation to the Hong Kong Haskell hackathon, with tentative dates of May 11-13 or 25-27. Follow to link for more information. [1] http://goo.gl/9sRLF HacPhi, the hackathon in Philadelphia, Pennsylvania, USA, will be held between August 3-5, 2012. [2] http://goo.gl/Rm2qH Quotes of the Week * lambdabot was last seen on #haskell 1 hour, 37 minutes and 46 seconds ago, saying: <IO ()> * elliott: oh, it's not haskell-cafe? that explains the reasonable responses ["it" = GHC mailing list] * Morandat: "The astute reader will have noticed that the above example clashes with our claim that R is lexically scoped. As is often the case, R is lexically scoped up to the point it is not." * quicksilver: lisp doesn't have equational reasoning so there is nothing to lose :) * JoeyA: One thing I like about Haskell versus C++ is that abusing language features tends to give more satisfying results. monochrom: that seems to be a criterion for drugs rather than languages :) Top Reddit Stories * Darcs 2.8: faster, better annotate; patch stash (obliterate -O); revert hunk split; and more! Domain: wiki.darcs.net, Score: 63, Comments: 21 On Reddit: [3] http://goo.gl/4tj4i Original: [4] http://goo.gl/R7TpP * The Haskell / Snap ecosystem is as productive (or more) than Ruby/Rails. Domain: blog.dbpatterson.com, Score: 63, Comments: 46 On Reddit: [5] http://goo.gl/cP3NC Original: [6] http://goo.gl/23alh * Cabal is now hosted on GitHub Domain: github.com, Score: 44, Comments: 20 On Reddit: [7] http://goo.gl/yBqCC Original: [8] http://goo.gl/0ze9y * Eight Haskell Google Summer of Code projects accepted Domain: google-melange.com, Score: 40, Comments: 25 On Reddit: [9] http://goo.gl/c2KXg Original: [10] http://goo.gl/vu681 * “Live” code update for Haskell Domain: self.haskell, Score: 31, Comments: 17 On Reddit: [11] http://goo.gl/ZZDb6 Original: [12] http://goo.gl/ZZDb6 * Functional Programming in C++ Domain: altdevblogaday.com, Score: 30, Comments: 12 On Reddit: [13] http://goo.gl/pFuHk Original: [14] http://goo.gl/aG6iP * Why do we not define IO as a Free monad? Domain: self.haskell, Score: 28, Comments: 47 On Reddit: [15] http://goo.gl/jiSdb Original: [16] http://goo.gl/jiSdb * Elm: Source code and Compiler/Server now available. Write FRP code for the web! Domain: self.haskell, Score: 27, Comments: 11 On Reddit: [17] http://goo.gl/9hE6X Original: [18] http://goo.gl/9hE6X * An experiment with Bayesian Networks in Haskell Domain: alpheccar.org, Score: 27, Comments: On Reddit: [19] http://goo.gl/21VDw Original: [20] http://goo.gl/2piKL * More Client Side Yesod: todo sample Domain: yesodweb.com, Score: 25, Comments: 10 On Reddit: [21] http://goo.gl/LGO75 Original: [22] http://goo.gl/QGsrZ * C Style Haskell Domain: kormacode.blogspot.com, Score: 24, Comments: 21 On Reddit: [23] http://goo.gl/VsShj Original: [24] http://goo.gl/o75jn Top StackOverflow Questions * Avoiding IORefs in pure code votes: 19, answers: 1 Read on SO: [25] http://goo.gl/327fO * How can I get JHC to cooperate with the android ndk? votes: 11, answers: 1 Read on SO: [26] http://goo.gl/oGMQ9 * Is there a name for this function or pattern? votes: 11, answers: 2 Read on SO: [27] http://goo.gl/HJixe * Haskell rank two polymorphism compile error votes: 11, answers: 2 Read on SO: [28] http://goo.gl/hYdRX * Python faster than compiled Haskell? votes: 11, answers: 5 Read on SO: [29] http://goo.gl/sUXMa * How does one release a *nix project? votes: 10, answers: 2 Read on SO: [30] http://goo.gl/JTgCz * Optimizing partial computation in Haskell votes: 8, answers: 1 Read on SO: [31] http://goo.gl/Dncn7 * Proper way to treat global flags in Haskell votes: 8, answers: 2 Read on SO: [32] http://goo.gl/di2Ld * Generating code for another language in Haskell votes: 7, answers: 1 Read on SO: [33] http://goo.gl/OlRCI * Why does haskell enumerator based IO call sigprocmask so often? votes: 7, answers: 4 Read on SO: [34] http://goo.gl/kfTSy * confused about function as instance of Functor in haskell votes: 7, answers: 5 Read on SO: [35] http://goo.gl/aX9JA Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/98038 2. http://www.haskell.org/haskellwiki/Hac_%CF%86 3. http://wiki.darcs.net/Releases/2.8 4. http://www.reddit.com/r/haskell/comments/snyvh/darcs_28_faster_better_annot… 5. http://blog.dbpatterson.com/post/21885034168 6. http://www.reddit.com/r/haskell/comments/suj5s/the_haskell_snap_ecosystem_i… 7. https://github.com/haskell/cabal 8. http://www.reddit.com/r/haskell/comments/smwft/cabal_is_now_hosted_on_githu… 9. http://www.google-melange.com/gsoc/org/google/gsoc2012/haskell 10. http://www.reddit.com/r/haskell/comments/sqr4r/eight_haskell_google_summer_… 11. http://www.reddit.com/r/haskell/comments/stcwz/live_code_update_for_haskell/ 12. http://www.reddit.com/r/haskell/comments/stcwz/live_code_update_for_haskell/ 13. http://www.altdevblogaday.com/2012/04/26/functional-programming-in-c/ 14. http://www.reddit.com/r/haskell/comments/suxv8/functional_programming_in_c/ 15. http://www.reddit.com/r/haskell/comments/swffy/why_do_we_not_define_io_as_a… 16. http://www.reddit.com/r/haskell/comments/swffy/why_do_we_not_define_io_as_a… 17. http://www.reddit.com/r/haskell/comments/subvk/elm_source_code_and_compiler… 18. http://www.reddit.com/r/haskell/comments/subvk/elm_source_code_and_compiler… 19. http://www.alpheccar.org/content/98.html 20. http://www.reddit.com/r/haskell/comments/swfud/an_experiment_with_bayesian_… 21. http://www.yesodweb.com/blog/2012/04/yesod-js-todo 22. http://www.reddit.com/r/haskell/comments/snzsa/more_client_side_yesod_todo_… 23. http://kormacode.blogspot.com/2011/11/c-style-haskell_10.html 24. http://www.reddit.com/r/haskell/comments/sxoof/c_style_haskell/ 25. http://stackoverflow.com/questions/10298664/avoiding-iorefs-in-pure-code 26. http://stackoverflow.com/questions/10305909/how-can-i-get-jhc-to-cooperate-… 27. http://stackoverflow.com/questions/10319659/is-there-a-name-for-this-functi… 28. http://stackoverflow.com/questions/10345809/haskell-rank-two-polymorphism-c… 29. http://stackoverflow.com/questions/10357663/python-faster-than-compiled-has… 30. http://stackoverflow.com/questions/10279508/how-does-one-release-a-nix-proj… 31. http://stackoverflow.com/questions/10266954/optimizing-partial-computation-… 32. http://stackoverflow.com/questions/10285454/proper-way-to-treat-global-flag… 33. http://stackoverflow.com/questions/10268052/generating-code-for-another-lan… 34. http://stackoverflow.com/questions/10273696/why-does-haskell-enumerator-bas… 35. http://stackoverflow.com/questions/10294272/confused-about-function-as-inst…

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Re: [Haskell-cafe] heterogeneous environment
by oleg＠okmij.org 03 May '12

03 May '12

Ben wrote: > - use Data.Unique to identify Refs, and use existential quantification > or Data.Dynamic to create a heterogenous Map from uid to log. for > example, to represent a log of compare-and-swaps we might do something > like > data Ref a = Ref (IORef a) Unique > data OpaqueCAS = forall a . > OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } > type CASLog = Map Unique OpaqueCAS > logCAS r@(Ref _ uid) o n log = insert uid (OpaqueCAS r o n) log... > but what if the transaction wants to perform a second CAS on a > reference we've already CAS'd? then we should create a combined > OpaqueCAS record which expects t he first oldVal and swaps in the > second newVal. unfortunately the type checker balks at this, as it > can't prove that the type variable 'a from the first record is the > same as the 'a in the new record; i suppose there might be some fancy > type shenanigans which might solve this... It seems you actually prefer this solution, if it worked. This solution will entail some run-time check one way or another, because we `erase' types when we store them in the log and we have to recover them later. If the problem is of merging two CASLog entries into one, your code above already solves it. There are no type shenanigans are necessary. I will make slight modification though by introducing an extra IORef for `ephemeral' communication. Values written to that IORef almost immediately retrieved. It is not strictly necessary since your Ref already has one IORef; I assume that it can't be garbled, even for a brief moment. The complete code is enclosed at the end of the message. If one wants to live a bit dangerously but efficiently, one may wish to create a module of reference cells indexed by unique numbers, like your Ref. The data constructor Ref is not exported. If the only way to create Refs is to use the function newRef of the module, and that function assuredly creates a Ref with a unique label, one is justified in writing the function cast :: Ref a -> Ref b -> Maybe (Ref b) cast r1@(Ref _ u1) (Ref _ u2) | u1 == u2 -> Just (unsafeCoerce r1) cast _ _ = Nothing because the same labels correspond to references of the same type. The _safe_ solution, similar to that in the enclosed code below, was used in HANSEI, which is the embedded DSL for probabilistic programming. Here is the relevant code describing first-class memory (it is OCaml). (* We often use mutable variables as `communication channel', to appease the type-checker. The variable stores the `option' value -- most of the time, None. One function writes a Some x value, and another function almost immediately reads the value -- exactly once. The protocol of using such a variable is a sequence of one write almost immediately followed by one read. We use the following helpers to access our `communication channel'. *) let from_option = function Some x -> x | None -> failwith "fromoption";; let read_answer r = let v = from_option !r in r := None; v (* for safety *) (* First-class storage: for the implementation of `thread-local' memory *) module Memory = struct type 'a loc = int * 'a option ref type cell = unit -> unit module M = Map.Make(struct type t = int let compare x y = x - y end) type mem = cell M.t let newm = M.empty let genid : unit -> int = (* generating fresh locations *) let counter = ref 0 in fun () -> let v = !counter in counter := succ v; v let new_loc () = (genid (), ref None) let new_cell (_,chan) x = fun () -> chan := Some x let mref (id,chan) m = let cell = M.find id m in cell (); read_answer chan let mset ((id,chan) as loc) x m = M.add id (new_cell loc x) m end;; Enclosed Haskell code: {-# LANGUAGE ExistentialQuantification #-} module Ref where import Data.IORef import Data.Unique import Data.Map as M data Ref a = Ref (IORef a) (IORef a) Unique data OpaqueCAS = forall a . OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } type CASLog = M.Map Unique OpaqueCAS logCAS r@(Ref _ _ uid) o n log = M.insert uid (OpaqueCAS r o n) log -- If one is bothered with undefined below, use Nothing newRef :: a -> IO (Ref a) newRef x = do r1 <- newIORef x r2 <- newIORef undefined -- auxiliary u <- newUnique return (Ref r1 r2 u) writeOld :: OpaqueCAS -> IO () writeOld (OpaqueCAS (Ref _ ra _) o n) = writeIORef ra o readOld :: OpaqueCAS -> IO OpaqueCAS readOld (OpaqueCAS r@(Ref _ ra _) _ n) = do o <- readIORef ra -- guard against errors and memory leaks writeIORef ra undefined return (OpaqueCAS r o n) writeNew :: OpaqueCAS -> IO () writeNew (OpaqueCAS (Ref _ ra _) o n) = writeIORef ra n readNew :: OpaqueCAS -> IO OpaqueCAS readNew (OpaqueCAS r@(Ref _ ra _) o _) = do n <- readIORef ra -- guard against errors and memory leaks writeIORef ra undefined return (OpaqueCAS r o n) -- Create CAS record with the old value from the first CAS and -- the new value from the new CAS. -- First we assume that the two OpaqueCAS correspond to the same -- variable. A run-time error occurs otherwise. -- Second, we make no checks for the relation between the new -- value of the first CAS and the old value of the new CAS. Probably -- they are meant to be the same. Anyway, we don't even look at them. combineCAS:: OpaqueCAS -> OpaqueCAS -> IO OpaqueCAS combineCAS cas1 cas2 = do writeNew cas2 readNew cas1

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12th GhentFPG meeting: Tuesday, May 8th, 2012: final program
by Andy Georges 02 May '12

02 May '12

Dear GhentFPG members and functional programming enthousiasts, After consulting the doodle, we have decided on holding the 12th GhentFPG meeting at Tuesday, May 8th, 2012 starting at 19:30. The venue will be the same as for the previous meeting. We will convene at Ghent University, campus De Sterre, Krijgslaan 281 (main entrance), building S9, see also the attached image. The program will be as follows: 1. Tom Schrijvers - A functional approach to solving challenging programming problems - Flemish programming contest solutions. You can find the contest challenges here (in Dutch): http://www.vlaamseprogrammeerwedstrijd.be/current/opgaves.php 2. Jasper Van der Jeugt - A short tutorial on using semi-implicit parallelism in Haskell programs using the available annotations. We hope to see you all there. Regards, The GhentFPG Organizing Committee.

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Edmonton Functional Programming Users Group (EFPUG)
by Kevin Charter 02 May '12

02 May '12

Hi Cafe, If you live in the Edmonton, Canada area, and you're interested in functional programming, EFPUG is an informal discussion group you might find interesting. Most of our talks have been on Haskell topics, but we're interested in functional languages in general. Our Google Group is https://groups.google.com/forum/?fromgroups&hl=en#!forum/edmonton-functiona… We meet on the first Tuesday of (almost) each month at the Computing Science building on the University of Alberta north campus. New participants, and new ideas for talks, hack nights, or other FP-related activities, are always welcome. Cheers, Kevin -- Kevin Charter kevin.charter(a)acm.org

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Threads and hGetLine
by H. M. 02 May '12

02 May '12

Hello, The simplified problem: There are two threads, one which is waits on input via hGetLine and another, which should terminate this thread or close this handle. hClose as well as killThread doesn't seem to work, caused by the fact, that the thread is blocked until input is availiable. Hopefully you have some solution how to kill the thread and/or close the handle and/or have some other idea to get the lined input in the other thread. -- Thanks in advance H.

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ANN: HacPhi 2012
by Chris Casinghino 02 May '12

02 May '12

Greetings, I am very pleased to officially announce Hac Phi 2012, a Haskell hackathon/get-together to be held August 3-5 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Last year's Hac Phi was a lot of fun, drawing more than 30 Haskellers, and many people have already expressed interest in coming back this year. Come meet your Haskelly comrades-in-arms! I want to stress that everyone is welcome - you do not have to be a Haskell guru. Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1]. Registration, travel, lodging and many other details can be found on the Hac Phi wiki [2]. This year, we are making a public list of attendees' names and nicks - please let us know if you'd prefer not to be listed when you register. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game - a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hac Phi 2012 is supported by contributions from Amgen and Jane Street. Hope to see you in Philadelphia! - The Hac Phi team Brent Yorgey (byorgey) Daniel Wagner (dmwit) Chris Casinghino (ccasin) [1] http://www.haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://www.haskell.org/haskellwiki/Hac_%CF%86 [3] http://www.haskell.org/haskellwiki/Hac_%CF%86/Talks

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heterogeneous environment
by Ben 02 May '12

02 May '12

dear static typing masters -- while working on an STM-like library, i ran into the issue of trying to create a transaction log for reads / writes of heterogeneous reference types. i don't have a good answer to this problem. the problem is twofold : first, the general heterogeneous collection problem, and second, connecting a reference to it's log. there are some solutions i've tried but each has drawbacks : - store the transaction log inside of the reference itself. essentially, each reference has an IORef (Map ThreadId a) associated to it. this is the approach used by [1]. unfortunately this creates a point of concurrency contention at each reference for each read / write, and a lot of repeated computation (the transaction log is spread out in a lot of pieces.) - use Data.Unique to identify Refs, and use existential quantification or Data.Dynamic to create a heterogenous Map from uid to log. for example, to represent a log of compare-and-swaps we might do something like data Ref a = Ref (IORef a) Unique data OpaqueCAS = forall a . OpaqueCAS { casRef :: Ref a, oldVal :: a, newVal :: a } type CASLog = Map Unique OpaqueCAS logCAS r@(Ref _ uid) o n log = insert uid (OpaqueCAS r o n) log... but what if the transaction wants to perform a second CAS on a reference we've already CAS'd? then we should create a combined OpaqueCAS record which expects the first oldVal and swaps in the second newVal. unfortunately the type checker balks at this, as it can't prove that the type variable 'a from the first record is the same as the 'a in the new record; i suppose there might be some fancy type shenanigans which might solve this... Data.Dynamic works but puts a Typeable constraint on the Ref type, so requires the user to modify their data types, and requires a run-time type check (and while performance isn't paramount now it will become important to me later.) also it doesn't feel right... - tupling and functional representations. a monadic function that does a read on an reference can be thought of as a pure function with an extra argument. a monadic function that does a write can be thought of as a pure function with an extra return value. combining all the reads and writes into a transaction log is a big network / switchboard connecting problem, e.g. creating the extra inputs / outputs to the various functions and then stitching them together. running the monad then just is connecting up the final composed function to the actual input / output references. in a sense this amounts to tupling (or currying) the heterogeneous types. (is this is a kind of "final" representation, in the "finally tagless" sense?) i looked at this there were two problems : 1 - the representation is very difficult to manipulate, at least the way i was trying (using the arrow operations); the switchboard problem is extremely verbose, and 2 - it is hard to reconcile identity and position in the tuples -- the repeated read / write problem again. i also experimented with HList but got stuck on similar issues. it strikes me this is just the problem of keeping an environment for an interpreter of a language with mutable heterogeneous reference types. this must be a problem that has either been solved or else there is a haskell point of view on it i'm not grasping which avoids the need for this data structure. maybe there is a way of writing this as an interpreter or using some existing monad, like the ST monad? best, ben [1] - Frank Huch and Frank Kupke, A High-Level Implementation of Composable Memory Transactions in Concurrent Haskell

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using ResourceT with MVars
by Warren Harris 02 May '12

02 May '12

I would like to use LevelDB in my code, and the HEAD version of leveldb-haskell now uses runResourceT to manage open db connections and iterators. I would also like to run multiple threads, and coordinate them for transaction-like behavior, i.e. read and writing data without interruption. LevelDB doesn't support transactions itself, so the alternative would seem to be using MVars for mutual exclusion. With the 0.0.3 version of leveldb-haskell, the get and put operations were in the IO monad, which made it trivial to write something like this: withMVar state $ \db -> do maybeValue <- get db rdOpts key put db wrOpts key $ maybe init incr maybeValue Now with the HEAD version, the get and put operations have types: get :: MonadResource m => DB -> ReadOptions -> ByteString -> m (Maybe ByteString) put :: MonadResource m => DB -> WriteOptions -> ByteString -> ByteString -> m () I don't think it makes sense in general to lift the withMVar into MonadResource and use runResourceT to run its body in IO. This would not satisfy the intention behind using ResourceT -- resource allocated inside the withMVar body would be deallocated when the withMVar scope is exited, rather than being managed by the outermost ResourceT. (Although, in this case we aren't allocating any resources, so maybe it's a moot point for my simple example.) I also don't think it makes sense to use takeMVar and putMVar instead of withMVars as this would subvert the exception handling that withMVars provides. And my attempts to define a withMVarR (in the ResourceT IO monad) also seem to require runResourceT: withMVarR :: MVar a -> (a -> ResourceT IO b) -> ResourceT IO b withMVarR m io = do -- mask $ \restore -> do a <- lift $ takeMVar m b <- {-restore-} (io a) `onExceptionR` (lift $ putMVar m a) lift $ putMVar m a return b onExceptionR :: ResourceT IO a -> ResourceT IO b -> ResourceT IO a onExceptionR a b = do runResourceT $ transResourceT (\a -> transResourceT (onException a) b) a Maybe I need to create a "HasMVar" class analogous to HasRef? (http://hackage.haskell.org/packages/archive/conduit/0.0.0/doc/html/Control-…) I'd appreciate any suggestions on how to make these fit together. Warren

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Haskell Hackathon in Munich
by Heinrich Hördegen 02 May '12

02 May '12

Dear all, the Munich Haskell Meeting is glad to announce a local one day Hackathon in Munich. It will take place the 12th of May 2012 from 10am to 6pm. Please checkout the details at: http://www.haskell-munich.de/dates Cordially yours, Heinrich -- -- hoerdegen(a)funktional.info www.funktional.info --

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Reader monad, refactoring and missing the point all at once
by Eugene Dzhurinsky 02 May '12

02 May '12

Hi all! Last day I was trying to fix idiii library, because it uses utf8 for parsing non-unicode content. I found the functions > -- | Parses one value and returns it as a 'String' > parseString :: CharEncoding -> TagParser String > parseString enc = do > v <- case enc of > 0x01 -> parseUntilWord16Null -- UTF-16 > 0x02 -> parseUntilWord16Null -- UTF-16 BOM > _ -> parseUntilWord8Null -- ISO-8859-1 or UTF-8 > return $ encPack enc v > > encPack :: CharEncoding -> [Token] -> String > encPack 0x00 s = Text.unpack $ decodeASCII $ BS.pack s > encPack 0x01 (0xFF:0xFE:s) = Text.unpack $ decodeUtf16LE $ BS.pack s > encPack 0x01 (0xFE:0xFF:s) = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack 0x02 s = Text.unpack $ decodeUtf16BE $ BS.pack s > encPack _ s = Text.unpack $ decodeUtf8 $ BS.pack s updated the dependency from > import Data.Text.Encoding (decodeASCII, decodeUtf16LE, decodeUtf16BE, decodeUtf8) to > import Data.Text.ICU.Convert and added implementation for decoding functions: > decodeAny :: String -> BS.ByteString -> Text.Text > decodeAny charset src = unsafePerformIO $ ((flip toUnicode) src) `fmap` open charset (Just True) > > decodeASCII :: BS.ByteString -> Text.Text > decodeASCII = decodeAny "latin1" > > decodeUtf16LE = decodeAny "utf-16le" > > decodeUtf16BE = decodeAny "utf-16be" > > decodeUtf8 = decodeAny "utf-8" Now I want to add possibility to specify encoding to yse with decodeASCII. I was thinking of adding Reader monad and providing some sort of charset configuration there - but it will lead up to complicating the code, which uses this parseString function. And this code is used inside Parser of Text.ParserCombinators.Poly.State - so I will need to update all usages of this parser. Another approach might be to use IORef with encoding stored there, but I don't really like this solution. What would be the best way of refactoring of such kind? Thanks! -- Eugene N Dzhurinsky

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