- Haskell-Cafe - Haskell.org

Haskell source AST zipper with state
by Tony Morris 03 May '12

03 May '12

Is there a library to traverse a source AST keeping state? -- Tony Morris http://tmorris.net/

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Tutorial on how to use multirec
by j.romildo＠gmail.com 03 May '12

03 May '12

Hello. I am writing a compiler in Haskell and need some tools to work with abstract syntax trees (like annotating them with source positions and other things). I have been pointed out to the multirec package. I have found some information about the package, but it is too dense to read and understand, as the ideas are presented from a theoretical point of view. So I am looking for something more light, as tutorials or simple applications that illustrate the use of the package. Any help? Romildo

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Specify compile error
by Ismael Figueroa Palet 03 May '12

03 May '12

Hi, I'm writing a program like this: data B = B Int data A = Safe Int | Unsafe Int createB :: A -> B createB (Safe i) = B i createB (Unsafe i) = error "This is not allowed" Unfortunately, the situation when createB is called with an Unsafe value is only checked at runtime. If I omit the second case, it is not an error to not be exhaustive :-( Is there a way to make it a compile time error?? Thanks! -- Ismael

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twitter election on favorite programming language
by Kazu Yamamoto 03 May '12

03 May '12

Hello, A twitter election on favorite programming language was held in Japan and it appeared that Heskell is No. 10 loved language in Japan. :-) http://twisen.com/election/index/654 Regards, --Kazu

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OpenGL: No instance for Random GLfloat
by Mark Spezzano 03 May '12

03 May '12

Hi Haskellers, I'm trying to generate a random vertex in OpenGL as follows. genPosition :: IO (Vertex3 GLfloat) genPosition = do x <- getStdRandom $ randomR (-1.6,1.6) y <- getStdRandom $ randomR (-1.0,1.0) return (Vertex3 x y (-1)) Unfortunately the compiler complains about me having to implement an instance of Random for GLfloat. How do I do this (or avoid having to do this)? Cheers, Mark

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Haskell Weekly News: Issue 225
by Daniel Santa Cruz 02 May '12

02 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 02 May '12

02 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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