- Haskell-Cafe - Haskell.org

ANN: Updates in the monadic regions family
by Bas van Dijk 10 Mar '11

10 Mar '11

Dear all, I released new versions of some of my packages. Here are the changelogs: http://hackage.haskell.org/package/regions-0.9 http://hackage.haskell.org/package/regions-mtl-0.3.1.5 http://hackage.haskell.org/package/regions-monadstf-0.3.1.5 * Switch from monad-peel to monad-control. * Removed Control.Monad.Trans.Region.Concurrent. The fork functions contained bugs which could not be fixed. I'm working on a way to copy handles from one thread to another but the design has not settled yet. * Removed Data.RegionRef. I always considered this module a bit of a wart. * Added support for local regions. Primarily needed for the alloca functions from the regional-pointers package. More on that below. http://hackage.haskell.org/package/regional-pointers-0.6 * Switch from monad-peel to monad-control. * Major API change: Use the new local regions. This means that instead of: alloca ∷ (Storable α, MonadPeelIO pr) ⇒ (∀ s. RegionalPtr α (RegionT s pr) → RegionT s pr β) → pr β I now have: alloca ∷ (Storable α, MonadControlIO pr) ⇒ (∀ sl. LocalPtr α (LocalRegion sl s) → RegionT (Local s) pr β) → RegionT s pr β This allows my alloca to use the native alloca which is more efficient than doing a manual malloc and free as I did before. The type also allows to open resources in the continuation and use them outside the continuation and visa versa. * Have separate types for the three different pointers: - RegionalPtr: for pointers created using malloc. - LocalPtr: for pointers created using alloca. - NullPtr: for the nullPtr. * Add the type classes Pointer and AllocatedPointer to classify them. http://hackage.haskell.org/package/safer-file-handles-0.10 http://hackage.haskell.org/package/safer-file-handles-bytestring-0.2 http://hackage.haskell.org/package/safer-file-handles-text-0.2 * Switch from monad-peel to monad-control. * Support regional-pointers-0.6 and use its overloaded pointers. * Add separate types for standard handles: StdFileHandle and normal handles: RegionalFileHandle. * Add the type class FileHandle to group them. * Overload the filehandle in operations. http://hackage.haskell.org/package/usb-0.8 * getStrDesc and getStrDescFirstLang now return a Text instead of a String. This is a more efficient and cleaner interface. http://hackage.haskell.org/package/usb-enumerator-0.3 * Switch from monad-peel to monad-control. * Support iteratee-0.8.*. * Support usb-0.9. http://hackage.haskell.org/package/usb-safe-0.12 * Switched from monad-peel to monad-control. * Exported the ReadEndpoint, WriteEndpoint and EnumReadEndpoint type classes so you can refer to them in type signatures. * Support usb-0.8: getStrDesc and getStrDescFirstLang now return a Text instead of a String. * Support regions-0.9. * Support usb-enumerator-0.3. * Support iteratee-0.8.*. Regards, Bas

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Re: [Haskell-cafe] Haskell mail server fail?
by wren ng thornton 10 Mar '11

10 Mar '11

Like Kenneth Hoste, I haven't been receiving mails from haskell-cafe@ nor libraries@ for a few days to a week now. What is the status of the mailing lists? (Please CC me off-list, for obvious reasons) -- Live well, ~wren

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Re: [Haskell-cafe] Tagless interpreter, expression problem and zipper
by oleg＠okmij.org 10 Mar '11

10 Mar '11

The Tagless final approach can do context-sensitive evaluation, using the well-known trick from the denotational semantics that explicating the context turns context-sensitive semantics to compositional. The trick isn't out of place given how much tagless-final approach is related to denotational semantics. The non-compositional processing has been discussed at length at lectures last year. Please see the explanation and the code at Non-compositionality: Fold-unlike processing http://okmij.org/ftp/tagless-final/course/course.html#non'compositionality Interestingly, one example FlatF.hs mentioned on the above page specifically talks about the context of being the left operand of the addition (the goal was to `flatten' additions, to re-associate them to the right). That is the same context as you are after, it seems. The detailed lecture notes will be eventually available.

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Update Cabal
by Hauschild, Klaus (EXT) 10 Mar '11

10 Mar '11

Hallo, I'm using Haskell Platform 2010.2.0.0 on a Windows XP machine. This haskell platform includes cabal-1.8.0.6. Now I want to update cabal by "cabal install cabal". Installation works well. Call like "runhaskell ./Setup.hs" will use the updatetd cabal-1.10.0.0. But "cabal --version" says still 1.8.0.6 and I have to re-configure. How I update cabal in my current Haskel Platform? Thanks Klaus

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IPv6 issues for (code|community).haskell.org?
by Erlend Hamberg 10 Mar '11

10 Mar '11

Hi, When I wanted to get the newest xmonad code from darcs today¹ it was really, really slow. Running “host” revealed that code.haskell.org is an alias for community.haskell.org and that it has an IPv6 address associated with it. Being on an IPv6 network, and having been burnt by similar problems before, I tried adding IPv4 address for code.haskell.org in /etc/hosts in order to force the connection to happen over IPv4. This fixed the issue and “darcs get” now takes a few minutes. I just wanted to alert people in case the problem is at the server end. (And in case this isn’t just a temporary problem.) Connecting to code.haskell.org in a Web browser also takes minutes, while e.g. http://ipv6.google.com/ works perfectly. [1] With “darcs get http://code.haskell.org/xmonad” -- Erlend Hamberg ehamberg(a)gmail.com

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Custom monad using ST
by Yves Parès 10 Mar '11

10 Mar '11

Hello, I am trying to make a monad that uses ST internally. But even when reducing this to the simplest case I'm still cramped by the 's' phantom type : {-# LANGUAGE Rank2Types #-} newtype MyST a = MyST (forall s. ST s a) -- ^ I cannot use " deriving (Monad) " through GeneralizedNewtypeDeriving runMyST (MyST m) = runST m -- ^ works thanks to declaration of 's' at rank 2 in the definition of MyST -- It refuses to compile if MyST is declared as such: -- data MyST s a = MyST (ST s a) instance Monad MyST where return = MyST . return -- and this does not compile (MyST m) >>= f = MyST $ do x <- m case f x of (MyST m) -> m If you try it, GHC will complain: Simple.hs:13:20: Couldn't match expected type `forall s. ST s a' with actual type `ST s a' Expected type: a -> forall s1. ST s1 a Actual type: a -> ST s a In the second argument of `(.)', namely `(return :: a -> (forall s. ST s a))' In the expression: MyST . (return :: a -> (forall s. ST s a))

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Automated tests with cabal
by Hauschild, Klaus (EXT) 10 Mar '11

10 Mar '11

Hi Haskellers, I read about the cabal features for running test code from Setup.hs with "defaultMainWithHooks". I'm looking for more generic code that allows me to place any haskell in a subdirectory "test" or so and "cabal test" will run this test without any modification of my Setup.hs. Is there a possibility? Thanks Klaus

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Learn You a Haskell for Great Good - a few doubts
by Karthick Gururaj 10 Mar '11

10 Mar '11

Hello, I'm learning Haskell from the extremely well written (and well illustrated as well!) tutorial - http://learnyouahaskell.com/chapters. I have couple of questions from my readings so far. In "typeclasses - 101" (http://learnyouahaskell.com/types-and-typeclasses#typeclasses-101) there is a paragraph that reads: Enum members are sequentially ordered types - they can be enumerated. The main advantage of the Enum typeclass is that we can use its types in list ranges. They also have defined successors and predecesors, which you can get with the succ and pred functions. Types in this class: (), Bool, Char, Ordering, Int, Integer, Float and Double. What is the "()" type? Does it refer to a tuple? How can tuple be ordered, let alone be enum'd? I tried: Prelude> take 10 [(1,1) ..] <interactive>:1:8: No instance for (Enum (t, t1)) arising from the arithmetic sequence `(1, 1) .. ' at <interactive>:1:8-17 Possible fix: add an instance declaration for (Enum (t, t1)) In the second argument of `take', namely `[(1, 1) .. ]' In the expression: take 10 [(1, 1) .. ] In the definition of `it': it = take 10 [(1, 1) .. ] This is expected and is logical. But, surprise: Prelude> (1,1) > (1,2) False Prelude> (2,2) > (1,1) True Prelude> (1,2) > (2,1) False Prelude> (1,2) < (2,1) True So tuples are in "Ord" type class atleast. What is the ordering logic? Another question, on the curried functions - specifically for infix functions. Suppose I need a function that takes an argument and adds five to it. I can do: Prelude> let addFive = (+) 5 Prelude> addFive 4 9 The paragraph: "Infix functions can also be partially applied by using sections. To section an infix function, simply surround it with parentheses and only supply a parameter on one side. That creates a function that takes one parameter and then applies it to the side that's missing an operand": describes a different syntax. I tried that as well: Prelude> let addFive' = (+5) Prelude> addFive' 3 8 Ok. Works. But on a non-commutative operation like division, we get: Prelude> let x = (/) 20.0 Prelude> x 10 2.0 Prelude> let y = (/20.0) Prelude> y 10 0.5 So a curried infix operator fixes the first argument and a "sectioned infix" operator fixes the second argument? Regards, Karthick

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Finalizer registered with System.Mem.Weak.addFinalizer is called surprisingly early
by balodja 10 Mar '11

10 Mar '11

Hello, I use addFinalizer from System.Mem.Weak for performing some actions on data just before that is reclaimed by GC. Registered in this way finalizer is called surprisingly early for me, much earlier the data is tend to be collected. I've attached source code of simple two-threaded program. Source code also can be found at [1]. It is pretty simple and should be readable enough. Program is intentioned to pass messages through transactional variable (TVar) for printing by worker thread. Messages consist of formatted to String Int values from 1 to 1000000. So the output should look like this: > 1 > 2 > ... > 1000000 But running the program gives something like that: > 1 > 2 > ... > 283 > finalizing > Test: thread blocked indefinitely in an STM transaction As far as I see, finalizer is called too early. If someone could point me out how to do the right thing, that would be helpful. Operaton system: Debian Squeeze Compiler: GHC 6.12.1 Compiling options: none, just "ghc --make Test.hs" Thanks, balodja [1] http://hpaste.org/44648

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Haskell Weekly News: Issue 172
by Daniel Santa Cruz 10 Mar '11

10 Mar '11

Welcome to issue 172 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of February 27 to March 05, 2011. Announcements Jasper Van der Jeugt [2]announced the release of version 3 of Hakyll. He informs us that this is a complete re-write, and not backwards compatible. Horvath Zoltan [3]informed us about the 4th Central European Functional Programming School (CEFP 2011), which is to take place in Eotvos Lorand University, Budapest, Hungary, from June 14-24, 2011. "The invited lecturers are the most prominent researchers in the field in Europe, and they will present state-of-the-art functional programming techniques." Janis Voigtlander [4]issued a call for papers for the 20th International Workshop on Functional and (Constraint) Logic Programming (WFLP 2011) to be held in Odense, Denmark, on July 19, 2011. Michael Hanus [5]issued a call for papers for the 13th International ACM SIGPLAN Symposium on Principles and Practice of Declarative Programming (PPDP 2011) to be held in Odense, Denmark, on July 20-22, 2011. Trystan Spangler [6]released a new version (0.3.0) of hspec, with "improvements including fixing a base dependency problem, and better reporting of failed examples." Magnus Therming [7]released new versions of dataenc (0.14), and omnicodec (0.5). "It contains a large change to the API. The old, rather simplistic, lazy API has been removed. It has been replaced by an API based on incremental encoding/decoding. This should make the library easier to use together with left-fold enumerators..." Rogan Creswick [8]announced that cabal-dev 0.7.4.0 has been uploaded to hackage. "We strongly suggest that everyone upgrade to this release, since this release specifically addresses changes in Cabal-1.10 and newer, which the latest cabal-install new uses." Frederick Ross [9]announced a minor upgrade to haskell-pgm (0.1.2), with a minor upgrade to the library to make it work with GHC 7. Uwe Schmidt [10]announced an update to hxt to version 9.1. Eric Kow [11]announced that the date and venue for the sixth Darcs Hacking sprint has been confirmed for Paris, April 1-3, 2011. Ian Lynagh and the GHC Team [12]announced a new patchlevel release of GHC. "This release contains a number of bugfixes relative to 7.0.1, so we recommend upgrading." Denis Bueno [13]announced new releases of funsat (0.6.2) and bitset (1.1). Michal Konecny [14]announced version 0.3.2 of hmpfr, bindings to the MPFR arbitrary precision floating point arithmetic library. "The changes in this version are quite small but significant." Jonathan Daugherty [15]announced version 1.0 of the vty-ui terminal user interface library. "The vty-ui library is a complete interactive user interface development toolkit for terminal applications." Jason Dusek [16]announced his now bash-0.0.0, which has "types and functions for generation of Bash scripts, with safe escaping and composition of large subsets of Bash statements and expressions." Quotes of the Week * lpsmith: I didn't take the time to figure out what should be done in finite cases. * <companion_cube> edwardk, you mean it's 2 phD/nick in average ? :) <edwardk> clearly this is why SPJ stays away, he doesn't want to dilute that metric. * edwardk: Haskell 98 is a laudable goal. * douglas-adams: I love deadlines, I especially like the swooshing sound they make as they fly past * kmc: the way to convert imperative code to functional is to think about what your code actually means, and then throw out most of the irrelevant details that imperative programming has forced on you * MonadsSuck: man, I hate it when an adjoint pair of functors get together and compose. So damn annoying * bwright: I don't get it everyone keeps telling me java is a safe language but it melted the plastic on my eeepc900 * danka: you could be a monadic vampire. I hear they bleed semicolons. Top Reddit Stories * ANNOUNCE: GHC version 7.0.2: From (haskell.org) scored 46 with comments. Read on [17]reddit or the [18]original post. * Greenfield Haskell: From (self.haskell), scored 36 with 188 comments. Read on [19]reddit or the [20]original post. * Patches for monad comprehensions are in and will be in the upcoming GHC 7.2 release: From (hackage.haskell.org) scored 34 with 15 comments. Read on [21]reddit or the [22]original post. * GHC 7.0.2 released!: From (haskell.org) scored 32 with 10 comments. Read on [23]reddit or the [24]original post. * Haskell on Android: From (self.haskell), scored 31 with 22 comments. Read on [25]reddit or the [26]original post. * Do you think Haskell will ever catch on?: From (self.haskell), scored 26 with 63 comments. Read on [27]reddit or the [28]original post. * hakyll 3.0 released (static site generator): From (jaspervdj.be), scored 25 with 1 comments. Read on [29]reddit or the [30]original post. * New release of cabal-dev: sandboxed, reproducible build environments for Haskell: From (haskell.org) scored 24 with 9 comments. Read on [31]reddit or the [32]original post. * News on Well-Typed, the Parallel GHC Project, and Eric Kow joins the team.: From (well-typed.com) scored 22 with 3 comments. Read on [33]reddit or the [34]original post. * Build Yourself a Bluetooth Controlled Six-Legged Robot : A Neighborhood of Infinity: From (blog.sigfpe.com) scored 21 with comments. Read on [35]reddit or the [36]original post. Top StackOverflow Answers * [37]Goto in Haskell: Can anyone explain this seemingly insane effect of continuation monad usage? votes: 14 Here's a somewhat informal answer, but hopefully useful. getCC' returns a continuation to the current point of execution; you can think of it as saving a stack frame. The continuation returned by getCC' has not only ContT's state at the point of the call, but also the state of any monad above ContT on the stack. When you restore that state by calling the ... * [38]How do Haskell compilers decide whether to allocate on the heap or the stack? votes: 13 When you call a function like this f 42 (g x y) then the runtime behaviour is something like the following: p1 = malloc(2 * sizeof(Word)) p1[0] = &Tag_for_Int p1[1] = 42 p2 = malloc(3 * sizeof(Word)) p2[0] = &Code_for_g_x_y p2[1] = x p2[2] = y f(p1, p2) That is, arguments are usually passed as pointers to objects on the heap like in Java, but ... * [39]Quick question about Arrow operators votes: 11 (***) :: (Arrow a) => a b c -> a b' c' -> a (b, b') (c, c') So specialize a to -> and we get: (***) :: (Arrow a) => (b -> c) -> (b' -> c') -> (b, b') -> (c, c') And that's great, except we want to, for whatever reason, take the first two arguments as a single pair instead. But that's easy, we just uncurry. Prelude ... * [40]QuickCheck: defining Arbitrary instance in terms of other Arbitraries votes: 9 I'd do it like this: instance Arbitrary C where arbitrary = do a <- arbitrary b <- arbitrary return (C a b) Although sclv's idea of using liftM2 from Control.Monad is probably better: instance Arbitrary C where arbitrary = liftM2 C arbitrary arbitrary Top StackOverflow Questions * Running a Haskell program on the Android OS (votes: 21, answers: 1) [41]read * Goto in Haskell: Can anyone explain this seemingly insane effect of continuation monad usage? (votes: 18, answers: 2) [42]read * Proving equality of streams (votes: 15, answers: 1) [43]read * Rotate the first argument to a function to become nth (votes: 12, answers: 7) [44]read * How do Haskell compilers decide whether to allocate on the heap or the stack? (votes: 9, answers: 2) [45]read About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.hakyll/128 3. http://article.gmane.org/gmane.comp.lang.haskell.general/18527 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18528 5. http://article.gmane.org/gmane.comp.lang.haskell.general/18530 6. http://article.gmane.org/gmane.comp.lang.haskell.general/18533 7. http://article.gmane.org/gmane.comp.lang.haskell.libraries/15426 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86897 9. http://article.gmane.org/gmane.comp.lang.haskell.general/18542 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86922 11. http://article.gmane.org/gmane.comp.version-control.darcs.user/25916 12. http://article.gmane.org/gmane.comp.lang.haskell.general/18535 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86937 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86882 15. http://article.gmane.org/gmane.comp.lang.haskell.general/18540 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/86975 17. http://www.reddit.com/r/haskell/comments/fwk78/announce_ghc_version_702/ 18. http://www.haskell.org/pipermail/haskell/2011-March/022610.html 19. http://www.reddit.com/r/haskell/comments/fvabx/greenfield_haskell/ 20. file://localhost/r/haskell/comments/fvabx/greenfield_haskell/ 21. http://www.reddit.com/r/haskell/comments/fxjcl/patches_for_monad_comprehens… 22. http://hackage.haskell.org/trac/ghc/ticket/4370#comment:48 23. http://www.reddit.com/r/haskell/comments/fwkdh/ghc_702_released/ 24. http://www.haskell.org/ghc/download_ghc_7_0_2 25. http://www.reddit.com/r/haskell/comments/ful84/haskell_on_android/ 26. http://www.reddit.com/r/haskell/comments/ful84/haskell_on_android/ 27. http://www.reddit.com/r/haskell/comments/ftxc7/do_you_think_haskell_will_ev… 28. http://www.reddit.com/r/haskell/comments/ftxc7/do_you_think_haskell_will_ev… 29. http://www.reddit.com/r/haskell/comments/fusve/hakyll_30_released_static_si… 30. http://jaspervdj.be/hakyll/tutorial.html 31. http://www.reddit.com/r/haskell/comments/fx47b/new_release_of_cabaldev_sand… 32. http://www.haskell.org/pipermail/haskell-cafe/2011-March/089831.html 33. http://www.reddit.com/r/haskell/comments/fw311/news_on_welltyped_the_parall… 34. http://www.well-typed.com/blog/51 35. http://www.reddit.com/r/haskell/comments/ftxzx/build_yourself_a_bluetooth_c… 36. http://blog.sigfpe.com/2011/02/build-yourself-bluetooth-controlled-six.html 37. http://stackoverflow.com/questions/5193876/goto-in-haskell-can-anyone-expla… 38. http://stackoverflow.com/questions/5132350/how-do-haskell-compilers-decide-… 39. http://stackoverflow.com/questions/5200462/quick-question-about-arrow-opera… 40. http://stackoverflow.com/questions/5134259/quickcheck-defining-arbitrary-in… 41. http://stackoverflow.com/questions/5151858/running-a-haskell-program-on-the… 42. http://stackoverflow.com/questions/5193876/goto-in-haskell-can-anyone-expla… 43. http://stackoverflow.com/questions/5179821/proving-equality-of-streams 44. http://stackoverflow.com/questions/5188212/rotate-the-first-argument-to-a-f… 45. http://stackoverflow.com/questions/5132350/how-do-haskell-compilers-decide-…

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