- Haskell-Cafe - Haskell.org

Re[Haskell-cafe] duction Sequence of simple Fibonacci sequence implementation
by SimonK77 31 Aug '09

31 Aug '09

Hallo everyone, as Haskell uses the Lazy Evaluation reduction policy also known as Outermost Graph Reduction, I was wondering if the following simple implementation of the Fibonacci sequence would result in linear runtime behaviour. fib :: Integer -> Integer fib 0 = 0 fib 1 = 1 fib n = fib (n - 2) + fib (n - 1) Is the following reduction sequence realistic, or am I admitting to much intelligence to the Haskell Compiler? http://www.bilder-hochladen.net/files/bxlk-6-jpg.html http://www.bilder-hochladen.net/files/thumbs/bxlk-6.jpg I hope someone can help... -- View this message in context: http://www.nabble.com/Reduction-Sequence-of-simple-Fibonacci-sequence-imple… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Another thanks for inspiration to RWH - Bloom Filters
by Günther Schmidt 31 Aug '09

31 Aug '09

Hi all, My app has to use rather large amounts of static data in order to run. I have been using an SQLite database to hold this data so far. The largest chunk of data was a table with roughly 80k records. I only make use of this table to see if a particular key is present. Reading RWH over from time to time I came accross the Bloom Filter and finally realized that this would be a much better solution. So I could insert all the keys in the table into a suitable bloom filter instead and the just query the bloom filter instead of the database. I haven't figured out yet how to write a "literal" representation of the "filled" bloom filter yet though and could do with some tipps. I'm using the bloomfilter package from hackage. Günther

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ANN: moe html combinator
by Jinjing Wang 30 Aug '09

30 Aug '09

hand written html is fine, except for the closing tags: what if html' - do head' - do meta [http_equiv "Content-Type", content "text/html; charset-utf-8"] (/) title' - str "my title" link [rel "icon", _type "image/png", href "panda_icon.png"] (/) body' - do div [_class "container"] - do str "hello world" produces <html> <head> <meta http-equiv="Content-Type" content="text/html; charset-utf-8"> </meta> <title> my title </title> <link rel="icon" type="image/png" href="panda_icon.png"> </link> </head> <body> <div class="container"> hello world </div> </body> </html> That's moe. There's also extra dsl sugar / flavors for your convienence, including (currently available) markdown and kawaii. It's a new library, tries to bring some haskell fun back to web programming, so .. enjoy. - moe: http://hackage.haskell.org/package/moe -- jinjing

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[Haskell Cafe] ASCII Graphics Library wanted
by Paul Sujkov 30 Aug '09

30 Aug '09

Hi, is there any (by any means) portable ASCII-Graphics library for Haskell, such as NCurses or AALib? I see HaHa library on Hackage, but is it the only existing library for such a purpose (version 0.2 alerts me a bit)? Or maybe someone has any experience in developing some rogue-like gams in Haskell? :) -- Regards, Paul Sujkov

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Re: [Haskell-cafe] GUI library
by Michael Mossey 30 Aug '09

30 Aug '09

Hi Jean-Denis, Thanks for the information. Do you know how WxHaskell fits my needs? For example, does it have good docs and examples for a beginner? Does it have the ability to draw lines and characters on a surface? Does it have a type of "canvas" which usually refers to an optimized drawing surface? Thanks, Mike Jean-Denis Koeck wrote: > I began writing a commercial app with a GUI using Gtk2hs, > but it looked ugly on Windows. I'm switching to WxHaskell. > > 2009/8/29 Michael Mossey <mpm(a)alumni.caltech.edu > <mailto:mpm@alumni.caltech.edu>> > > I want to choose a GUI library for my project. Some background: I'm > a beginner to functional programming and have been working through > Haskell books for a few months now. I'm not just learning Haskell > for s**ts and giggles; my purpose is to write > music-composition-related code; in particular, I want to write a > graphical musical score editor. (Why write my own editor, you may > ask? Because I want to fully integrate it with > computer-assisted-composition algorithms that I plan to write, also > in Haskell.) I decided to use Haskell for its great features as a > functional programming language. > > Regarding a choice of GUI library, I want these factors: > > - it needs to provide at a minimum a drawing surface, a place I can > draw lines and insert characters, in addition to all the standard > widgets and layout capabilities we have to come to expect from a GUI > library. > > - This is a Windows application. > > - it needs to be non-confusing for an intermediate-beginner > Haskeller. Hopefully good documentation and examples will exist on > the web. > > - It might be nice to have advanced graphics capability such as Qt > provides, things like antialiasied shapes, and a canvas with > efficient refresh (refereshes only the area that was exposed, and if > your canvas items are only primitives, it can do refreshes from > within C++ (no need to touch your Haskell code at all). However I'm > wondering if qtHaskell fits my criteria "well-documented" and "lots > of examples aimed at beginners". > > Thanks, > Mike > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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How to use dyre relaunch/restore State of Gtk2hs Object?
by Andy Stewart 30 Aug '09

30 Aug '09

Hi all. I try to use dyre (http://hackage.haskell.org/packages/archive/dyre/0.7.2/doc/html/Config-Dyre…) to relaunch/restore State of TextView to make TextView's content can't lost when main program reboot. Below is source code for example: ------------------------------> source code start <------------------------------ module DyreExample where import Graphics.UI.Gtk hiding (get) import qualified Config.Dyre as Dyre import Config.Dyre.Relaunch import Config.Dyre.Compile import System.IO import Event import Test import Data.Binary import qualified Graphics.UI.Gtk.Gdk.Events as E data Config = Config { message :: String, errorMsg :: Maybe String } defaultConfig :: Config defaultConfig = Config "Dyre Example v0.1" Nothing showError :: Config -> String -> Config showError cfg msg = cfg { errorMsg = Just msg } realMain Config{message = message, errorMsg = errorMsg } = do initGUI textView <- textViewNew tn <- restoreTextState testName tv <- restoreBinaryState textView putStrLn tn rootWindow <- windowNew rootWindow `onDestroy` mainQuit windowFullscreen rootWindow -- rootWindow `containerAdd` textView rootWindow `containerAdd` tv widgetShowAll rootWindow rootWindow `onKeyPress` (\event -> dyreKeyTest event textView) mainGUI dyreExample = Dyre.wrapMain $ Dyre.defaultParams { Dyre.projectName = "Main" , Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/") , Dyre.cacheDir = Just (return "/test/Download/cache/") , Dyre.realMain = realMain , Dyre.showError = showError } dyreMainParams = Dyre.defaultParams { Dyre.projectName = "Main" , Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/") , Dyre.cacheDir = Just (return "/test/Download/cache/") , Dyre.realMain = realMain , Dyre.showError = showError } dyreTestParams = Dyre.defaultParams {Dyre.projectName = "Test" ,Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/")} dyreKeyTest :: E.Event -> TextView -> IO Bool dyreKeyTest ev textView = do case eventTransform ev of Nothing -> return False Just e -> do let eventName = eventGetName e case eventName of "M-m" -> do out2 <- customCompile dyreMainParams putStrLn $ show out2 relaunchWithTextState testName Nothing relaunchWithBinaryState textView Nothing return True _ -> return False -- instance Binary TextView where -- put a = put a -- get = get ------------------------------> source code end <------------------------------ On the surface, just add TextView's Binary instance can fix problem. But TextView is Haskell binding (by gtk2hs) for C struct in GTK+ library, i wonder how to write Binary instance for TextView. My aim is write GUI program (through gtk2hs) will keep GUI State when main program recompile/reboot (like XMonad). Example, above code, i add TextView widget in Window, and type "I love Haskell" in it, when this program recompile/reboot itself, the content "I love Haskell" still display in TextView widget, and not empty. So any ideas or suggestions? Thanks! -- Andy

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Problem with monadic formlets
by Colin Paul Adams 30 Aug '09

30 Aug '09

I'm trying to validate user input against a database (using HaskellDB, but that doesn't seem to be the problem, as replacing the database monadic code with return True gives the same problem. This is part of my code: register :: Database -> XForm Registration --register db = Registration <$> pure_user <*> passConfirmed register db = Registration <$> (user db) <*> passConfirmed user :: Database -> XForm String user db = pure_user `F.checkM` F.ensureM valid error where valid name = do let q = do t <- table user_table restrict (t!user_name .==. constant name) return t rs <- query db q return $ null rs error = "Username already exists in the database!" pure_user :: XForm String pure_user = input `F.check` F.ensure valid error where input = "Username" `label` F.input Nothing valid = (>= 3) . length error = "Username must be three characters or longer." passConfirmed :: XForm String passConfirmed = fst <$> passwords `F.check` F.ensure equal error where passwords = (,) <$> pass "Password" <*> pass "Password (confirm)" equal (a, b) = a == b error = "The entered passwords do not match!" pass :: String -> XForm String pass caption = input `F.check` F.ensure valid error where input = caption `label` F.password Nothing valid = (>=6) . length error = "Password must be six characters or longer." If I uncomment the commented line, and comment out the line after it (in register), then everything works as expected. However, using it as it is, one of the calls to pass gets the user's name for validation (and consequently either fails if the user name is only 5 characters, or the comparison of the two passwords fail (unless I type the user name as the password). I thought the applicative style meant the effects did not influence one another, but here there is clear contamination. What am i doing wrong? -- Colin Adams Preston Lancashire

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How to calculate de number of digits of an integer? (was: Is logBase right?)
by Roberto López 30 Aug '09

30 Aug '09

Ok. I wonder if someone could help me with this problem... I want to calculate the number of digits of a positive integer. I was thinking of ... numDigits n = truncate (logBase 10 n) + 1 But (logBase 10 1000) = 2.9999999999999996 so numDigits 1000 = 2. Maybe adding a small amount numDigits n = truncate ((logBase 10 n) + 0.0000000000000005) + 1 Prelude> numDigits 100 3 Prelude> numDigits 1000 4 Prelude> numDigits 10000 5 Prelude> numDigits 10000 5 Prelude> numDigits 100000 6 Prelude> numDigits 1000000 7 Prelude> numDigits 10000000 8 Prelude> numDigits 100000000 9 Prelude> numDigits 1000000000 9 <---- This is wrong!!!! Prelude> numDigits 10000000000 11 Is there a reliable way to calculate the number of digits by means of logBase? Regards!

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Re: [Haskell-cafe] How to calculate de number of digits of an integer?
by Henning Thielemann 29 Aug '09

29 Aug '09

Bulat Ziganshin schrieb: > Hello Henning, > > Tuesday, August 25, 2009, 7:01:24 PM, you wrote: > >> I hope that 'show' will not need quadratic time but will employ a more >> efficient algorithm > > yes, you are right I thought a little about it. If I had to implement that in GMP it could be done quite fast in many cases: Count the number of bits, say it is 'k' and multiply with logBase 10 2. If 2^k and 2^(k+1)-1 have the same number of decimal digits, we are done. Otherwise we have to process some of the most significant bits. If the number is between n*2^k and (n+1)*2^k-1 and both bounds have the same number of decimal digits (logBase 10 n + k * logBase 10 2), we are also done. Only for numbers close to powers of 10 we have to process the whole integer.

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Re: [Haskell-cafe] How to calculate the number of digits of an integer? (was: Is logBase right?)
by Uwe Hollerbach 29 Aug '09

29 Aug '09

Ouch! That is indeed an improvement... I don't recall all the details of this codelet, but I think I got the seed off the net somewhere (perhaps this list?), and it might well have been better originally. So, brightly brightly and with beauty, I probably executed a verschlimmbesserung. After a year and a half, I find I still have almost no intuition about performance issues in haskell... guess I have to practice more. Uwe On 8/29/09, Bertram Felgenhauer <bertram.felgenhauer(a)googlemail.com> wrote: > Uwe Hollerbach wrote: >> Here's my version... maybe not as elegant as some, but it seems to >> work. For base 2 (or 2^k), it's probably possible to make this even >> more efficient by just walking along the integer as stored in memory, >> but that difference probably won't show up until at least tens of >> thousands of digits. >> >> Uwe >> >> ilogb :: Integer -> Integer -> Integer >> ilogb b n | n < 0 = ilogb b (- n) >> | n < b = 0 >> | otherwise = (up 1) - 1 >> where up a = if n < (b ^ a) >> then bin (quot a 2) a >> else up (2*a) >> bin lo hi = if (hi - lo) <= 1 >> then hi >> else let av = quot (lo + hi) 2 >> in if n < (b ^ av) >> then bin lo av >> else bin av hi > > We can streamline this algorithm, avoiding the repeated iterated squaring > of the base that (^) does: > > -- numDigits b n | n < 0 = 1 + numDigits b (-n) > numDigits b n = 1 + fst (ilog b n) where > ilog b n > | n < b = (0, n) > | otherwise = let (e, r) = ilog (b*b) n > in if r < b then (2*e, r) else (2*e+1, r `div` b) > > It's a worthwhile optimization, as timings on n = 2^1000000 show: > > Prelude T> length (show n) > 301030 > (0.48 secs, 17531388 bytes) > Prelude T> numDigits 10 n > 301030 > (0.10 secs, 4233728 bytes) > Prelude T> ilogb 10 n > 301029 > (1.00 secs, 43026552 bytes) > > (Code compiled with -O2, but the interpreted version is just as fast; the > bulk of the time is spent in gmp anyway.) > > Regards, > > Bertram > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Hackage rankings : August 2009
by Don Stewart 29 Aug '09

29 Aug '09

Monthly statistics on the most popular Haskell applications and libraries on Hackage. August 2009 edition now up: http://donsbot.wordpress.com/2009/08/29/haskell-popularity-rankings-septemb… -- Don

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Re: [Haskell-cafe] Problem with monadic formlets
by Jeremy Shaw 29 Aug '09

29 Aug '09

At Sat, 29 Aug 2009 15:46:42 +0100, Chris Eidhof (formlets) wrote: > > Confirmed. checkM is broken, thanks for noticing! I'll have a look > into it, I'm not sure whether it can be fixed. I was thinking of > removing all the monadic stuff from the formlets. I think this will > make for a much cleaner interface, monadic checking can then be done > afterwards. I am still a fan of (and use) this version of Form: newtype Form xml m a = Form { deform :: Env -> State FormState (Collector (m (Failing a)), xml, FormContentType) } not sure how I would feel about the removal of 'm' from the Collector. But 'xml' is nicer for me than 'm xml' because my collector and xml generator are often in different monads. I can, of course make them be in the same monad if I want: type MyForm a = Form (IO XML) IO a but I also have the option of just doing: type MyForm a = Form (IO XML) IO a or: type MyForm a = Form (HSP XML) IO a At present, I actually have my collector do all the validation and update the database. As a use case, let's assume that the form is creating a new user account. One possible error would be using a username that is already in use. Doing that check requires a database query. In fact, it seems best if it does a database update, so that there is no race condition between checking if the name is in use, and actually attempting to create the account with that username. If you remove the ability to do IO in the collector, then I believe I would need to: 1. run the collector to do the pure part of the validation. 2. if the pure part succeeds, use the returned value to do the impure validation 3. if that fails, then redisplay the form using the same environment that I used for #1, but passing in the impure validation errors. One potential drawback that I see with this is that it may make it difficult to the pass the error messages back to the specific formlet element that failed so that you can display the errors in-line. [Note: the following discussion reflects the pre-0.6 design]. Currently the environment we pass in is something like: type Env = [(String, Either String File)] The first component of the tuple is the name of the element. aka, input0, input1, etc. I would propose that we also pass in a Failures argument: type Failures = [(String, ErrorMsg)] where the first component of the tuple is the name of the element (input0, intpu1, etc) and the second element contains ErrorMsg. or perhaps modify Env to: type Env = [(String, (Maybe ErrorMsg, Either String File)] We would need to modify the Failing data type to: data Failing a = Failure [(String, ErrorMsg)] | Success a so that Failures would contain their location. Not all errors correspond to a specific form element. Let's say that you have 3 drop-down boxes that combine together to form a date selector. You want to validate the result of all three combined to make sure they picked a valid date, and if it is invalid, you produce an error message for that group, not a specific element. The problem then is that there is no 'location' that corresponds to that group, so what do you put in the Failure tuple? I think you can use freshName to generate an extra 'virtual' name that corresponds to the group as a whole. I have been meaning to prototype this in the near future and see if it actually works. I'll try to get something worked up in the next two weeks (my sister is getting married next week, so my schedule is pretty full). - jeremy

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Error during hlint install ?
by Dusan Kolar 29 Aug '09

29 Aug '09

Hello all, Am I doing something wrong if I get the following error during cabal installation of hlint? Is there any way how to solve it? I run on: Linux pc 2.6.30-ARCH #1 SMP PREEMPT Fri Jul 31 07:30:28 CEST 2009 x86_64 Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz GenuineIntel GNU/Linux The Glorious Glasgow Haskell Compilation System, version 6.10.4 Error: cabal install hlint Resolving dependencies... Configuring hlint-1.6.5... Preprocessing executables for hlint-1.6.5... Building hlint-1.6.5... [ 1 of 25] Compiling Paths_hlint ( dist/build/autogen/Paths_hlint.hs, dist/build/hlint/hlint-tmp/Paths_hlint.o ) [ 2 of 25] Compiling Parallel ( src/Parallel.hs, dist/build/hlint/hlint-tmp/Parallel.o ) [ 3 of 25] Compiling HSE.Generics ( src/HSE/Generics.hs, dist/build/hlint/hlint-tmp/HSE/Generics.o ) [ 4 of 25] Compiling HSE.NameMatch ( src/HSE/NameMatch.hs, dist/build/hlint/hlint-tmp/HSE/NameMatch.o ) [ 5 of 25] Compiling Util ( src/Util.hs, dist/build/hlint/hlint-tmp/Util.o ) [ 6 of 25] Compiling HSE.Util ( src/HSE/Util.hs, dist/build/hlint/hlint-tmp/HSE/Util.o ) [ 7 of 25] Compiling HSE.Match ( src/HSE/Match.hs, dist/build/hlint/hlint-tmp/HSE/Match.o ) [ 8 of 25] Compiling HSE.Bracket ( src/HSE/Bracket.hs, dist/build/hlint/hlint-tmp/HSE/Bracket.o ) [ 9 of 25] Compiling HSE.Evaluate ( src/HSE/Evaluate.hs, dist/build/hlint/hlint-tmp/HSE/Evaluate.o ) [10 of 25] Compiling HSE.All ( src/HSE/All.hs, dist/build/hlint/hlint-tmp/HSE/All.o ) [11 of 25] Compiling CmdLine ( src/CmdLine.hs, dist/build/hlint/hlint-tmp/CmdLine.o ) [12 of 25] Compiling Type ( src/Type.hs, dist/build/hlint/hlint-tmp/Type.o ) [13 of 25] Compiling Hint.Naming ( src/Hint/Naming.hs, dist/build/hlint/hlint-tmp/Hint/Naming.o ) [14 of 25] Compiling Hint.Bracket ( src/Hint/Bracket.hs, dist/build/hlint/hlint-tmp/Hint/Bracket.o ) [15 of 25] Compiling Hint.Lambda ( src/Hint/Lambda.hs, dist/build/hlint/hlint-tmp/Hint/Lambda.o ) [16 of 25] Compiling Hint.Monad ( src/Hint/Monad.hs, dist/build/hlint/hlint-tmp/Hint/Monad.o ) [17 of 25] Compiling Hint.ListRec ( src/Hint/ListRec.hs, dist/build/hlint/hlint-tmp/Hint/ListRec.o ) [18 of 25] Compiling Hint.List ( src/Hint/List.hs, dist/build/hlint/hlint-tmp/Hint/List.o ) [19 of 25] Compiling Hint.Match ( src/Hint/Match.hs, dist/build/hlint/hlint-tmp/Hint/Match.o ) [20 of 25] Compiling Settings ( src/Settings.hs, dist/build/hlint/hlint-tmp/Settings.o ) [21 of 25] Compiling Report ( src/Report.hs, dist/build/hlint/hlint-tmp/Report.o ) src/Report.hs:49:22: Couldn't match expected type `String' against inferred type `Bool' In the second argument of `hscolour', namely `True' In the expression: hscolour False True "" In the definition of `code': code = hscolour False True "" cabal: Error: some packages failed to install: hlint-1.6.5 failed during the building phase. The exception was: exit: ExitFailure 1 Regards Dušan P.S. Registered packages: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, cpphs-1.8, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), (dph-prim-interface-0.3), (dph-prim-par-0.3), (dph-prim-seq-0.3), (dph-seq-0.3), extensible-exceptions-0.1.1.0, filepath-1.1.0.2, (ghc-6.10.4), ghc-prim-0.1.0.0, haddock-2.4.2, haskell-src-1.0.1.3, haskell-src-exts-1.0.1, haskell98-1.0.1.0, hpc-0.5.0.3, hscolour-1.15, html-1.0.1.2, integer-0.1.0.1, mtl-1.1.0.2, network-2.2.1.2, old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, parallel-1.1.0.1, parsec-2.1.0.1, pretty-1.0.1.0, process-1.0.1.1, random-1.0.0.1, regex-base-0.72.0.2, regex-compat-0.71.0.1, regex-posix-0.72.0.3, rts-1.0, stm-2.1.1.2, syb-0.1.0.1, template-haskell-2.3.0.1, time-1.1.4, uniplate-1.2.0.3, unix-2.3.2.0, xhtml-3000.2.0.1

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Composition of n-arity functions
by Bas van Dijk 29 Aug '09

29 Aug '09

Hello, In the levmar binding[1][2] me and my brother are working on, I need a function composition operator that is overloaded to work on functions of any arity. Basically its type needs to be something like the following: (.*) :: (b -> c) -> NFunction n a b -> NFunction n a c where 'NFunction n a b' represents the function 'a_0 -> a_1 -> ... -> a_n -> b' I have written the following implementation: (my question to this list is below) -- Begin ----------------------------------------------------------------------- {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE FlexibleContexts #-} module ComposeN where -------------------------------------------------------------------------------- -- Type-level naturals (from an idea by Ryan Ingram) -------------------------------------------------------------------------------- data Z = Z newtype S n = S n type N0 = Z type N1 = S N0 type N2 = S N1 type N3 = S N2 class Nat n where caseNat :: forall r. n -> (n ~ Z => r) -> (forall p. (n ~ S p, Nat p) => p -> r) -> r instance Nat Z where caseNat _ z _ = z instance Nat n => Nat (S n) where caseNat (S n) _ s = s n induction :: forall p n. Nat n => n -> p Z -> (forall x. Nat x => p x -> p (S x)) -> p n induction n z s = caseNat n isZ isS where isZ :: n ~ Z => p n isZ = z isS :: forall x. (n ~ S x, Nat x) => x -> p n isS x = s (induction x z s) newtype Witness x = Witness { unWitness :: x } witnessNat :: forall n. Nat n => n witnessNat = theWitness where theWitness = unWitness $ induction (undefined `asTypeOf` theWitness) (Witness Z) (Witness . S . unWitness) -------------------------------------------------------------------------------- -- N-arity functions -------------------------------------------------------------------------------- -- | A @NFunction n a b@ is a function which takes @n@ arguments of -- type @a@ and returns a @b@. -- For example: NFunction (S (S (S Z))) a b ~ (a -> a -> a -> b) type family NFunction n a b :: * type instance NFunction Z a b = b type instance NFunction (S n) a b = a -> NFunction n a b -- | @f .* g@ composes @f@ with the /n/-arity function @g@. (.*) :: forall n a b c. (ComposeN n) => (b -> c) -> NFunction n a b -> NFunction n a c (.*) = compose (witnessNat :: n) (undefined :: a) infixr 9 .* -- same as . class Nat n => ComposeN n where compose :: forall a b c. n -> a -> (b -> c) -> NFunction n a b -> NFunction n a c -- Note that the 'n' and 'a' arguments to 'compose' are needed so that the type -- checker has enough information to select the right 'compose' instance. instance ComposeN Z where compose Z _ = ($) instance ComposeN n => ComposeN (S n) where compose (S n) (_ :: a) f g = compose n (undefined :: a) f . g -------------------------------------------------------------------------------- -- Test -------------------------------------------------------------------------------- foo :: NFunction N3 Integer Integer foo x y z = x + y + z bar :: Integer -> Integer bar k = k - 1 test1 = compose (witnessNat :: N3) (undefined :: Integer) bar foo 1 2 3 test2 = (bar .* foo) 1 2 3 -- The End --------------------------------------------------------------------- The problem is test1 type checks and evaluates to 5 as expected but test2 gives the following type error: Couldn't match expected type `NFunction n a Integer' against inferred type `Integer -> Integer -> Integer -> Integer' In the second argument of `(.*)', namely `foo' However if I ask ghci to infer the type of (bar .* foo) I get: *ComposeN>:t (bar .* foo) (bar .* foo) :: (Integer -> Integer -> Integer -> Integer ~ NFunction n a Integer, ComposeN n) => NFunction n a Integer Here we see that the context contains the type equality: (Integer -> Integer -> Integer -> Integer ~ NFunction n a Integer So why is ghci unable to match the expected type `NFunction n a Integer' against the inferred type `Integer -> Integer -> Integer -> Integer' while the context contains just this equality? regards, Bas [1] http://code.haskell.org/~basvandijk/code/bindings-levmar/ [2] http://code.haskell.org/~basvandijk/code/levmar/

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Edinburgh Saturday Meetup - we are here! (Henderson's)
by Eric Y. Kow 29 Aug '09

29 Aug '09

Dear Haskellers, Just for info, we are currently at Henderson's Cafe on Hanover St (just around the corner from the RCPE). If anybody needs to get in touch: Eric - +44 75187 28483 Dougal - +44 7814 412539 More details on the wiki if we move: http://www.haskell.org/haskellwiki/Hac7 -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Re: rotate image
by h 29 Aug '09

29 Aug '09

Hello, > You could use Cairo. Load the image to a surface, then rotate the surface 10 degrees and paint it. The first steps aren’t problematic, but how can I paint it - in which widget - so that the background is transparent, with drawingArea it doesn't seem to work. -- best regards H.

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SLD resolution code in Haskell
by Steffen Bock 29 Aug '09

29 Aug '09

Hi, is there any haskell code for SLD-resolution; I wanna learn it. Thanks a lot. Steffen.

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Unifcation and matching in Abelian groups
by John D. Ramsdell 29 Aug '09

29 Aug '09

I've been studying equational unification. I decided to test my understanding of it by implementing unification and matching in Abelian groups. I am quite surprised by how little code it takes. Let me share it with you. John Test cases: 2x+y=3z 2x=x+y 64x-41y=a Code: > -- Unification and matching in Abelian groups > -- John D. Ramsdell -- August 2009 > > module Main (main, test) where > > import Data.Char (isSpace, isAlpha, isAlphaNum, isDigit) > import Data.List (sort) > import System.IO (isEOF) > > -- Chapter 8, Section 5 of the Handbook of Automated Reasoning by > -- Franz Baader and Wayne Snyder describes unification and matching in > -- communtative/monoidal theories. This module refines the described > -- algorithms for the special case of Abelian groups. > > -- In this module, an Abelian group is a free algebra over a signature > -- with three function symbols, > -- > -- * the binary symbol +, the group operator, > -- * a constant 0, the identity element, and > -- * the unary symbol -, the inverse operator. > -- > -- The algebra is generated by a set of variables. Syntactically, a > -- variable is an identifer such as x and y. > > -- The axioms associated with the algebra are: > -- > -- * x + y = y + x Commutativity > -- * (x + y) + z = x + (y + z) Associativity > -- * x + 0 = x Group identity > -- * x + -x = 0 Cancellation > > -- A substitution maps variables to terms. A substitution s is > -- extended to a term as follows. > -- > -- s(0) = 0 > -- s(-t) = -s(t) > -- s(t + t') = s(t) + s(t') > > -- The unification problem is given the problem statement t =? t', > -- find a substitution s such that s(t) = s(t') modulo the axioms of > -- the algebra. The matching problem is to find substitution s such > -- that s(t) = t' modulo the axioms. > > -- A term is represented as the sum of factors, and a factor is the > -- product of an integer coeficient and a variable or the group > -- identity, zero. In this representation, every coeficient is > -- non-zero, and no variable occurs twice. > > -- A term can be represented by a finite map from variables to > -- non-negative integers. To make the code easier to understand, > -- association lists are used instead of Data.Map. > > newtype Lin = Lin [(String, Int)] > > -- Constructors > > -- Identity element (zero) > ide :: Lin > ide = Lin [] > > -- Factors > var :: Int -> String -> Lin > var 0 _ = Lin [] > var c x = Lin [(x, c)] > > -- Invert by negating coefficients. > neg :: Lin -> Lin > neg (Lin t) = > Lin $ map (\(x, c) -> (x, negate c)) t > > -- Join terms ensuring that coefficients are non-zero, and no variable > -- occurs twice. > add :: Lin -> Lin -> Lin > add (Lin t) (Lin t') = > Lin $ foldr f t' t > where > f (x, c) t = > case lookup x t of > Just c' | c + c' == 0 -> remove x t > | otherwise -> (x, c + c') : remove x t > Nothing -> (x, c) : t > > -- Remove the first pair in an association list that matches the key. > remove :: Eq a => a -> [(a, b)] -> [(a, b)] > remove _ [] = [] > remove x (y@(z, _) : ys) > | x == z = ys > | otherwise = y : remove x ys > > canonicalize :: Lin -> Lin > canonicalize (Lin t) = > Lin (sort t) > > -- Convert a linearized term into an association list. > assocs :: Lin -> [(String, Int)] > assocs (Lin t) = t > > term :: [(String, Int)] -> Lin > term assoc = > foldr f ide assoc > where > f (x, c) t = add t $ var c x > > -- Unification and Matching > > newtype Equation = Equation (Lin, Lin) > > newtype Maplet = Maplet (String, Lin) > > -- Unification is the same as matching when there are no constants > unify :: Monad m => Equation -> m [Maplet] > unify (Equation (t0, t1)) = > match $ Equation (add t0 (neg t1), ide) > > -- Matching in Abelian groups is performed by finding integer > -- solutions to linear equations, and then using the solutions to > -- construct a most general unifier. > match :: Monad m => Equation -> m [Maplet] > match (Equation (t0, t1)) = > case (assocs t0, assocs t1) of > ([], []) -> return [] > ([], _) -> fail "no solution" > (t0, t1) -> > do > subst <- intLinEq (map snd t0) (map snd t1) > return $ mgu (map fst t0) (map fst t1) subst > > -- Construct a most general unifier from a solution to a linear > -- equation. The function adds the variables back into terms, and > -- generates fresh variables as needed. > mgu :: [String] -> [String] -> Subst -> [Maplet] > mgu vars syms subst = > foldr f [] (zip vars [0..]) > where > f (x, n) maplets = > case lookup n subst of > Just (factors, consts) -> > Maplet (x, g factors consts) : maplets > Nothing -> > Maplet (x, var 1 $ genSym n) : maplets > g factors consts = > term (zip genSyms factors ++ zip syms consts) > genSyms = map genSym [0..] > > -- Generated variables start with this character. > genChar :: Char > genChar = 'g' > > genSym :: Int -> String > genSym i = genChar : show i > > -- So why solve linear equations? Consider the matching problem > -- > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] =? > -- d[0]*a[0] + d[1]*a[1] + ... + d[m-1]*a[m-1] > -- > -- with n variables and m constants. We seek a most general unifier s > -- such that > -- > -- s(c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1]) = > -- d[0]*a[0] + d[1]*a[1] + ... + d[m-1]*a[m-1] > -- > -- which is the same as > -- > -- c[0]*s(x[0]) + c[1]*s(x[1]) + ... + c[n-1]*s(x[n-1]) = > -- d[0]*a[0] + d[1]*a[1] + ... + d[m-1]*a[m-1] > -- > -- Notice that the number of occurrences of constant a[0] in s(x[0]) > -- plus s(x[1]) ... s(x[n-1]) must equal d[0]. Thus the mappings of > -- the unifier that involve constant a[0] respect integer solutions of > -- the following linear equation. > -- > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] = d[0] > -- > -- To compute a most general unifier, a most general integer solution > -- to a linear equation must be found. > > -- Integer Solutions of Linear Inhomogeneous Equations > > type LinEq = ([Int], [Int]) > > -- A linear equation with integer coefficients is represented as a > -- pair of lists of integers, the coefficients and the constants. If > -- there are no constants, the linear equation represented by (c, []) > -- is the homogeneous equation: > -- > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] = 0 > -- > -- where n is the length of c. Otherwise, (c, d) represents a > -- sequence of inhomogeneous linear equations with the same > -- left-hand-side: > -- > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] = d[0] > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] = d[1] > -- ... > -- c[0]*x[0] + c[1]*x[1] + ... + c[n-1]*x[n-1] = d[m-1] > -- > -- where m is the length of d. > > type Subst = [(Int, LinEq)] > > -- A solution is a partial map from variables to terms, and a term is > -- a pair of lists of integers, the variable part of the term followed > -- by the constant part. The variable part may specify variables not > -- in the input. For example, the solution of > -- > -- 64x = 41y + 1 > -- > -- is x = -41z - 16 and y = -64z - 25. The computed solution is read > -- off the list returned as an answer. > -- > -- intLinEq [64,-41] [1] = > -- [(0,([0,0,0,0,0,0,-41],[-16])), > -- (1,([0,0,0,0,0,0,-64],[-25]))] > > -- Find integer solutions to linear equations > intLinEq :: Monad m => [Int] -> [Int] -> m Subst > intLinEq coefficients constants = > intLinEqLoop (length coefficients) (coefficients, constants) [] > > -- The algorithm used to find solutions is described in Vol. 2 of The > -- Art of Computer Programming / Seminumerical Alorithms, 2nd Ed., > -- 1981, by Donald E. Knuth, pg. 327. > > -- On input, n is the number of variables in the original problem, c > -- is the coefficients, d is the constants, and subst is a list of > -- eliminated variables. > intLinEqLoop :: Monad m => Int -> LinEq -> Subst -> m Subst > intLinEqLoop n (c, d) subst = > -- Find the smallest coefficient in absolute value > let (i, ci) = smallest c in > case () of > _ | ci < 0 -> intLinEqLoop n (invert c, invert d) subst > -- Ensure the smallest coefficient is positive > | ci == 0 -> fail "bad problem" > -- Lack of non-zero coefficients is an error > | ci == 1 -> > -- A general solution of the following form has been found: > -- x[i] = sum[j] -c'[j]*x[j] + d[k] for all k > -- where c' is c with c'[i] = 0. > return $ eliminate n (i, (invert (zero i c), d)) subst > | divisible ci c -> > -- If all the coefficients are divisible by c[i], a solution is > -- immediate if all the constants are divisible by c[i], > -- otherwise there is no solution. > if divisible ci d then > let c' = divide ci c > d' = divide ci d in > return $ eliminate n (i, (invert (zero i c'), d')) subst > else > fail "no solution" > | otherwise -> > -- Eliminate x[i] in favor of freshly created variable x[n], > -- where n is the length of c. > -- x[n] = sum[j] (c[j] div c[i] * x[j]) > -- The new equation to be solved is: > -- c[i]*x[n] + sum[j] c'[j]*x[j] = d[k] for all k > -- where c'[j] = c[j] mod c[i] for j /= i and c'[i] = 0. > let c' = map (\x -> mod x ci) (zero i c) > c'' = divide ci (zero i c) > subst' = eliminate n (i, (invert c'' ++ [1], [])) subst in > intLinEqLoop n (c' ++ [ci], d) subst' > > -- Find the smallest coefficient in absolute value > smallest :: [Int] -> (Int, Int) > smallest xs = > foldl f (-1, 0) (zip [0..] xs) > where > f (i, n) (j, x) > | n == 0 = (j, x) > | x == 0 || abs n <= abs x = (i, n) > | otherwise = (j, x) > > invert :: [Int] -> [Int] > invert t = map negate t > > -- Zero the ith position in a list > zero :: Int -> [Int] -> [Int] > zero _ [] = [] > zero 0 (_:xs) = 0 : xs > zero i (x:xs) = x : zero (i - 1) xs > > -- Eliminate a variable from the existing substitution. If the > -- variable is in the original problem, add it to the substitution. > eliminate :: Int -> (Int, LinEq) -> Subst -> Subst > eliminate n m@(i, (c, d)) subst = > if i < n then > m : map f subst > else > map f subst > where > f m'@(i', (c', d')) = -- Eliminate i in c' if it occurs in c' > case get i c' of > 0 -> m' -- i is not in c' > ci -> (i', (addmul ci (zero i c') c, addmul ci d' d)) > -- Find ith coefficient > get _ [] = 0 > get 0 (x:_) = x > get i (_:xs) = get (i - 1) xs > -- addnum n xs ys sums xs and ys after multiplying ys by n > addmul 1 [] ys = ys > addmul n [] ys = map (* n) ys > addmul _ xs [] = xs > addmul n (x:xs) (y:ys) = (x + n * y) : addmul n xs ys > > divisible :: Int -> [Int] -> Bool > divisible small t = > all (\x -> mod x small == 0) t > > divide :: Int -> [Int] -> [Int] > divide small t = > map (\x -> div x small) t > > -- Input and Output > > instance Show Lin where > showsPrec _ (Lin []) = > showString "0" > showsPrec _ x = > showFactor t . showl ts > where > Lin (t:ts) = canonicalize x > showFactor (x, 1) = showString x > showFactor (x, -1) = showChar '-' . showString x > showFactor (x, c) = shows c . showString x > showl [] = id > showl ((s,n):ts) > | n < 0 = > showString " - " . showFactor (s, negate n) . showl ts > showl (t:ts) = showString " + " . showFactor t . showl ts > > instance Read Lin where > readsPrec _ s0 = > [ (t1, s2) | (t0, s1) <- readFactor s0, > (t1, s2) <- readRest t0 s1 ] > where > readPrimary s0 = > [ (t0, s1) | (x, s1) <- scan s0, isVar x, > let t0 = var 1 x ] ++ > [ (t0, s1) | ("0", s1) <- scan s0, > (s, _) <- scan s1, not (isVar s), > let t0 = ide ] ++ > [ (t0, s2) | (n, s1) <- scan s0, isNum n, > (x, s2) <- scan s1, isVar x, > let t0 = var (read n) x ] ++ > [ (t0, s3) | ("(", s1) <- scan s0, > (t0, s2) <- reads s1, > (")", s3) <- scan s2 ] > readFactor s0 = > [ (t1, s2) | ("-", s1) <- scan s0, > (t0, s2) <- readPrimary s1, > let t1 = neg t0 ] ++ > [ (t0, s1) | (s, _) <- scan s0, s /= "-", > (t0, s1) <- readPrimary s0 ] > readRest t0 s0 = > [ (t2, s3) | ("+", s1) <- scan s0, > (t1, s2) <- readFactor s1, > (t2, s3) <- readRest (add t0 t1) s2 ] ++ > [ (t2, s3) | ("-", s1) <- scan s0, > (t1, s2) <- readPrimary s1, > (t2, s3) <- readRest (add t0 (neg t1)) s2 ] ++ > [ (t0, s0) | (s, _) <- scan s0, s /= "+" && s /= "-" ] > > isNum :: String -> Bool > isNum (c:_) = isDigit c > isNum _ = False > > isVar :: String -> Bool > isVar (c:_) = isAlpha c && c /= genChar > isVar _ = False > > scan :: ReadS String > scan "" = [("", "")] > scan (c:s) > | isSpace c = scan s > | isAlpha c = [ (c:part, t) | (part,t) <- [span isAlphaNum s] ] > | isDigit c = [ (c:part, t) | (part,t) <- [span isDigit s] ] > | otherwise = [([c], s)] > > instance Show Equation where > showsPrec _ (Equation (t0, t1)) = > shows t0 . showString " = " . shows t1 > > instance Read Equation where > readsPrec _ s0 = > [ (Equation (t0, t1), s3) | (t0, s1) <- reads s0, > ("=", s2) <- scan s1, > (t1, s3) <- reads s2 ] > > instance Show Maplet where > showsPrec _ (Maplet (x, t)) = > showString x . showString " -> " . shows t > > -- Test Routine > > -- Given an equation, display a unifier and a matcher. > test :: String -> IO () > test prob = > case readM prob of > Err err -> putStrLn err > Ans (Equation (t0, t1)) -> > do > putStr "Problem: " > print $ Equation (canonicalize t0, canonicalize t1) > subst <- unify $ Equation (t0, t1) > putStr "Unifier: " > print subst > putStr "Matcher: " > case match $ Equation (t0, t1) of > Err err -> putStrLn err > Ans subst -> print subst > putStrLn "" > > readM :: (Read a, Monad m) => String -> m a > readM s = > case [ x | (x, t) <- reads s, ("", "") <- lex t ] of > [x] -> return x > [] -> fail "no parse" > _ -> fail "ambiguous parse" > > data AnsErr a > = Ans a > | Err String > > instance Monad AnsErr where > (Ans x) >>= k = k x > (Err s) >>= _ = Err s > return = Ans > fail = Err > > main :: IO () > main = > do > done <- isEOF > case done of > True -> return () > False -> > do > prob <- getLine > test prob > main

6 20

Re: Is logBase right?
by Steve 28 Aug '09

28 Aug '09

On Sat, 2009-08-22 at 13:03 -0400, haskell-cafe-request(a)haskell.org wrote: > Message: 10 > Date: Sat, 22 Aug 2009 11:24:21 +0200 > From: Roberto L?pez <plastermoso(a)hotmail.com> > Subject: [Haskell-cafe] Re: Is logBase right? > To: haskell-cafe(a)haskell.org > Message-ID: <h6odg8%2493j%241(a)ger.gmane.org> > Content-Type: text/plain; charset="ISO-8859-1" > > If 4.0 / 2.0 was 1.9999999999999999999998, it would be ok? > > The real value of log10 1000 is 3 (3.0). It can be represented with > accuracy > and it should be. > > You get the accuracy value in Perl, but there is the same problem in > Python. > It's a bit discouraging. > There is *not* the same problem in Python: $ python Python 2.6.2 (r262:71600, Jul 9 2009, 23:16:53) [GCC 4.4.0 20090506 (Red Hat 4.4.0-4)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import math >>> math.log10(1000) 3.0 Recent work in Python 3 (and Python 2.6) has improved the handling of floating point numbers, and addresses exactly the problem that Roberto has raised. I see no reason why Haskell could not improve its handling of floating point numbers by using similar techniques. Steve

9 13

Time constrained computation
by Mitar 28 Aug '09

28 Aug '09

Hi! I am not really sure if this is correct term for it but I am open to better (search) terms. pure

4 4

strictness analysis on algebraic data types
by Peter Verswyvelen 28 Aug '09

28 Aug '09

I'm reading the book "Modern Compiler Design<http://www.cs.vu.nl/~dick/MCD.html>", and in the chapter about functional programming it is stated that strictness analysis in the presence of ADTs (aka records) is a very hard problem to tackle. I'm not sure what the current state of art is here (e.g GHC or JHC), and don't understand what the problem is. Does it mean that it's a good idea to carefully consider when to make data constructor arguments (aka fields) strict, since the compiler can't do good strictness analysis when it encounters say a record with many lazy fields?

1 0

Arrows extentions and ArrowApplicable
by Maciej Piechotka 28 Aug '09

28 Aug '09

Arrows syntax supports Arrow, ArrowChoice(if, case etc.) and ArrowLoop(rec) - but not ArrowApplicable. Therefore it is not possible to write: proc x -> do a <- someArrow -< x a -< x but proc x -> do a <- someArrow -< x app -< (a, x) and proc x -> do a <- someArrow -< x app -< (otherArrow a, x) instead of proc x -> do a <- someArrow -< x otherArrow a -< x Such approach adds some syntax sugar but without adding any additional keyword and IMHO it can simplify writing ArrowApply (especially Kleisli IO). Regards

2 1

Mapping over multiple values of a list at once?
by haskell＠kudling.de 28 Aug '09

28 Aug '09

Hi, Imagine you have a list with n-values. You are asked to iterate over the list and calculate the average value of each 3 neighbouring values. For example, starting from [4,3,2,6,7] you need to find the averages of 4,3,2 and 3,2,6 and 2,6,7 resulting in [3,4,5] What is the most elegant way to do that? The naive ansatz to use "(!!") excessively sounds pretty inefficient. Bye, Lenny

8 8

Uninstall a cabal package?
by Colin Paul Adams 28 Aug '09

28 Aug '09

What is the procedure to uninstall a cabal package? -- Colin Adams Preston Lancashire

2 2

tomorrow: Edinburgh Meetup (Sat 29 Aug) and Hack Day (Sun 30 Aug)
by Eric Y. Kow 28 Aug '09

28 Aug '09

Dear Haskellers, Last minute reminder: We will be having a Hack Day in Edinburgh on Sunday 30 August (ICFP venue). That's this weekend! For the interested, we will also be meeting up the day before 09:30 Saturday 29 August just outside the RCPE (ie. the ICFP venue again). We'll have a quick wander and hopefully find some nice places to sit, chat and hack. On Sunday the real Hack Day begins at the ICFP venue, rooms 4/5. http://www.haskell.org/haskellwiki/Hac7 And for the interested, Brent says: > I'll be there Saturday morning---looking forward to meeting people! > > Also, I have purchased a ticket to hear a concert of some 16th-century > a capella 16th-century music at the Canongate Kirk at 5pm on Saturday: > > http://www.edfringe.com/ticketing/detail.php?id=13649 > > Just thought I'd mention it, so if there's anyone else coming to the > meetup day and 16th-century a capella sounds like your cup of tea, you > can get a ticket too and we can go together. Looking forward to seeing you, Eric PS. Bring a name tag if you've got one. There are about 15 people registered so far. It's not too late to register! (yes, you could also just drop by...) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

1 0

Applicative and Monad transformers
by Jeremy Shaw 28 Aug '09

28 Aug '09

Hello, I have seen it said that all Monads are Applicative Functors because you can just do: instance (Monad f, Applicative f) => Applicative (ReaderT r f) where pure = return (<*>) = ap However, that instance for ReaderT does not exhibit the properties I was hoping for. By substitution the definition of ap: ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 id liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) } we see that it becomes: instance (Monad f, Applicative f) => Applicative (ReaderT r f) where pure = return f <*> x = do { f' <- f; x' <- x; return (f' x') } What I would prefer is: instance (Monad f, Applicative f) => Applicative (ReaderT r f) where pure a = ReaderT $ const (pure a) f <*> a = ReaderT $ \r -> ((runReaderT f r) <*> (runReaderT a r)) I assume that only one version is correct, but I am having a hard time figuring out which one. I have attached a file which shows my motivation for prefering the second variation. There is a 'looker' function which does three lookups and combines the results using the Applicative Functor. With the first Applicative instance for ReaderT you will only get a failure message for the first lookup that fails -- which is what you expect with monadic behaviour. With the second instance, you get back a list of all the lookups that failed, which seems like what I would expect with Applicative Functor behaviour. Thanks! - jeremy {-# LANGUAGE FlexibleContexts, FlexibleInstances #-} module Main where import Control.Applicative (Applicative((<*>), pure), (<$>)) import Control.Monad (Monad((>>=), return), ap) import Control.Monad.Reader (MonadReader(ask, local), ReaderT(ReaderT, runReaderT)) import Data.Monoid(Monoid(mappend)) instance (Monad f, Applicative f) => Applicative (ReaderT r f) where pure = return (<*>) = ap {- instance (Monad f, Applicative f) => Applicative (ReaderT r f) where pure a = ReaderT $ const (pure a) f <*> a = ReaderT $ \r -> ((runReaderT f r) <*> (runReaderT a r)) -} instance (Monoid e) => Applicative (Either e) where pure = Right (Left errF) <*> (Left errA) = Left (errF `mappend` errA) (Left err) <*> _ = Left err _ <*> (Left err) = Left err (Right f) <*> (Right a) = Right (f a) instance Monad (Either [String]) where return = Right (Right a) >>= f = f a (Left e) >>= f = (Left e) fail str = Left [str] lookupE :: (Eq a) => a -> [(a,b)] -> (Either a b) lookupE a env = case lookup a env of Just b -> Right b Nothing -> Left a look :: String -> ReaderT [(String,b)] (Either [String]) b look a = do env <- ask case lookup a env of Just b -> return b Nothing -> fail a looker :: ReaderT [(String, Int)] (Either [String]) (Int, Int, Int) looker = ((,,) <$> look "foo" <*> look "bar" <*> look "baz") test :: Either [String] (Int, Int, Int) test = runReaderT looker [("bar", 1)]

6 8

ANNOUNCE: jhc 0.7.1
by John Meacham 27 Aug '09

27 Aug '09

Hi, I am happy to announce the jhc optimizing haskell compiler version 0.7.1. Information on installing jhc is here: http://repetae.net/computer/jhc/building.shtml And the Main page is here: http://repetae.net/computer/jhc There have been a lot of changes since the last public release, Some notable ones are: * The use of a general compiler cache by default rather than object files. This means work done by jhc is shared between projects, jhc uses cryptographic hashes internally to never compile the same piece of code more than once. This has numerous benefits, a notable one being speed. * Reworked library support. Jhc libraries are now much more general, when linking only the bits needed are loaded from the hl file, libraries are allowed to re-export modules from other libraries, making versioning or providing multiple interfaces to the same functionality a lot simpler. Library conflicts are 'lazy', like ambiguity errors now. * Updated Manual, clearer build instructions * Support for writing pure C libraries in Haskell. * numerous library updates, filled out many IO routines that were stubs before * Smart progress meters when compiler for a better user experience * performs all typechecking before compilation, for a faster edit-compile loop when writing code with jhc. * various bug fixes * Cross Compilation improvements, for instance you can compile for windows transparently on a linux box. Or for an embedded target that is independent of the host. * Better Mac OSX Support, as both a host and target. If you are wondering about the large version number bump since the last release, It is because several versions were released only internally to the jhc list for testing. If you are interested in jhc, join the list at: http://www.haskell.org/mailman/listinfo/jhc John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

5 5

Killing a thread blocking on a foreign call
by John Van Enk 27 Aug '09

27 Aug '09

Hi List, I'm trying to kill a thread (during program cleanup) that is almost always going to be perma-blocking on a foreign call (read). It seems that there's no facility to do this built into Haskell at this point, or is there? /jve

1 0

Inexplicable allocation
by Rohan Lean 27 Aug '09

27 Aug '09

Hello, this is my first posting to this List, I hope everything goes well. Seeing that the haskell is currently poorly represented in the fasta benchmark at http://shootout.alioth.debian.org/u64q/benchmark.php?test=fasta&lang=all , I decided to help out the contributors a bit. My draft already performs considerably better than the current program, but while optimizing I ran into a weird problem. I already reported this as a ghc bug, but on IRC I was encouraged to also describe my problem here. Source of the program: http://rohanlean.de/pub/ghc_alloc/fasta.hs Core: http://rohanlean.de/pub/ghc_alloc/fasta.core Relevant port of the core (`gen' with inlined `pick'): http://rohanlean.de/pub/ghc_alloc/fasta_snip.core Running with the -p option suggested that `gen' and `pick' allocate a lot of memory throughout the run of the program (that memory is not retained). I managed to confirm that for non-profiling builds by observing how the the output of -sstderr changed when I modified `gen' to call itself fewer times. This came as a surprise, because expected these two functions to be completely allocation free, and the core does not suggest any allocation to me. I would very much welcome explanations for these reported allocations. Thank you, Rohan Lean

1 0

Re: [Haskell-cafe] ANN: scion 0.1
by Thomas Schilling 27 Aug '09

27 Aug '09

On 27 Aug 2009, at 09:17, Johan Tibell wrote: > Hi Thomas, > > This is really cool stuff. I played with it this morning and found a > potential bug. Both the emacs and vim client libraries refer to the > binary "scion_server" but the binary that gets built and put in > ~/.cabal/bin is called "scion-server" (note the dash). I worked around > the problem by putting this in my .emacs: Right, I changed that before the release and apparently forgot to change a few places. Will be fixed in the next minor version. > > (setq scion-program "scion-server") > > Also, I tried to play with the scion-open-cabal-project function. I'm > not sure I understand how it's intended to be used. Does it make sure > that if you compile a file using C-c C-x C-l the right source > directories are used? Could you give a usage example? I eventually want to get rid of it and let scion-load figure out itself when the project needs to be reconfigured. At the moment you have to delete the .dist-scion directory if you changed your .cabal file. :/ > > One problem I frequently have with emacs-mode is that you can't use it > to load a file if > > * your sources don't live in the same directory as the Cabal file, or Actually, the latest version of haskell-mode has some heuristics. It sets the working directory to where the .cabal file lives. > * the file needs preprocessing (e.g. it's a .hsc) file. > > Now when we have the Cabal file loaded would it be possible to have > Cabal compile the project and then try to load the .hs file generated > from the .hsc file that is currently opened? Right, so those files should be generated, but it's quite difficult to update them automatically and recompile the necessary files. Also, some preprocessers don't add the proper {-# LINE #-} markers, so the error messages will be all wrong. So, I think it should work this way: User opens a .hsc / .y / .x file, Scion figures out that the file needs preprocessing, loads the generated file and highlights the error messages and highlights them in the original file. > > Thanks! > > -- Johan / Thomas -- Push the envelope. Watch it bend.

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Qualified operators
by Colin Paul Adams 27 Aug '09

27 Aug '09

What's the syntax for using a qualified operator (I have a clash of !)? -- Colin Adams Preston Lancashire

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ANN: jail-0.0.1 - Jailed IO monad
by Sebastiaan Visser 27 Aug '09

27 Aug '09

Hi all, I am very pleased to announce the first release of the jail[1,2] package. A jailed IO monad that can restrict filesystem access for your code. This package will soon be an integral part of the Salvia web server. (a new and improved Salvia will be released soon) Basic documentation of the jail package is included below. Any comments, suggestions, audits, etc. are welcome! Gr, -- Sebastiaan Visser [1] Source repo: http://github.com/sebastiaanvisser/jail/ [2] Hackage: http://hackage.haskell.org/package/jail -------- Like all of us know, the IO monad from the System.IO module is a wild beast allowing all forms of insecure computations that can read, or even worse, alter the real world. Writing to sockets, deleting files or even launching missiles, its possibilities are endless. This library provides a special IO module that wraps all file and handle based IO operations from the System.IO module, but provides a possibility to run them in an restricted environment. When running a jailed IO computation a file path can be specified all IO operations will be checked against. Accessing files outside this directory is not allowed and results in a runtime error. Additionally, when running a jailed IO computation a whitelist of file handles can be specified that are accessible as well. For example, running some code with the permission to access all files within (and only within) my home directory and allowing to access the standard output and standard error can be enforced like this: Jail.run (Just "/home/sebas") [stdout, stderr] yourUntrustworthyComputation Only allowing the code to access the standard input and nothing else can be enforced like this: Jail.run Nothing [stdin] yourEvenMoreUntrustworthyComputation Because the jailed IO environment keeps track of all file handles and checks that are opened by its own operations, smuggling in evil file handles from the fierce and dangerous outside world will be punished by border patrol. Only handles from the whitelist or handles securely opened by functions like openFile will be accepted. Because of the opaque IO constructor and the restricted set of exported operations this module is not easily fooled. I would almost dare to say this module is conceptually safe and code with the jailed IO type can blindly be trusted. Except, yes unfortunately except, unsafePerformIO ruins it all. I would almost suggest adding a flag to the compiler to enforce the absence ofunsafeRuinMyTypeSafety-alike functions in order to be able to create systems in which code can be trusted by its type alone. Nonetheless, this module is one step forward in trusting your own programs. Some real http://tinyurl.com/paranoidpeople prefer writing there software in one of the most insecure programming languages and perform security audits by hand, I'd rather have my compiler do the job. (Anyone who wants to audit this library is more than welcome!)

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Re: [Haskell-cafe] Mapping over multiple values of a list at once?
by Patai Gergely 27 Aug '09

27 Aug '09

> For example, starting from > > [4,3,2,6,7] > > you need to find the averages of > > 4,3,2 and 3,2,6 and 2,6,7 > > resulting in > > [3,4,5] > > What is the most elegant way to do that? It's probably less elegant than tails, but very likely more efficient to keep track of running sums instead of summing the sublists over and over again. import Data.Ratio nsums n xs = map (% n) $ scanl (+) (sum (take n xs)) $ zipWith (-) (drop n xs) xs Gergely -- http://www.fastmail.fm - The professional email service

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Re: [Haskell-cafe] Mapping over multiple values of a list at once?
by haskell＠kudling.de 27 Aug '09

27 Aug '09

"tails" seems to be the key. I haven't thought of this before. Thanks for pointing me in the right direction, guys.

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Re: [Haskell-cafe] FFI link failing due to no main?
by phil＠beadling.co.uk 27 Aug '09

27 Aug '09

> I'm not really sure, but does the linking step really need to be given > the --make flag? I would try linking without that in the first > instance. I think so - it's not just a link step, it does the following: Compile: CInterface.c -> CInterface.o Link: CInterface.o HaskellFuncs_stub.o Hasnkell.Funcs.o -> libCInterface.so I'll take a look at the full -v output and see if that reveals anything. Thanks, Phil. On 27 Aug 2009, at 04:38, Bernie Pope wrote: > Hi Phil,

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Network.Socket error in MacOS 10.5?
by kenny lu 27 Aug '09

27 Aug '09

Hi, I encountered a problem with Network.Socket in MacOS 10.5 Here is the code that I am testing, ----------------------------------------- ----------------------------------------- module Main where import qualified Network.Socket as Socket main :: IO () main = do { (hostname, _) <- Socket.getNameInfo [] True False (Socket.SockAddrUnix "localhost") -- (hostname, _) <- Socket.getNameInfo [] True False (Socket.SockAddrInet 9000 (127 + 0 * 256 + 0 * 256^2 + 1 * 256^3)) ; putStrLn (show hostname) } Running the above code yields the following error ghc --make -O2 TestSocket.hs [1 of 1] Compiling Main ( TestSocket.hs, TestSocket.o ) Linking TestSocket ... $ ./TestSocket TestSocket: getNameInfo: does not exist (ai_family not supported) If I switch to SockAddrInet instead, the error is gone. I am using GHC 6.10.3 and Network 2.2.1 Regards, Kenny

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Re: [Haskell-cafe] FFI link failing due to no main?
by phil＠beadling.co.uk 27 Aug '09

27 Aug '09

Thanks for the reply! I think this might be a Mac OS X issue. I've stripped my rather longwinded example down to the simplest case (one Haskell source file to create a single shared library containing a single exported function) and this compiles (and ultimately runs) fine on Linux. So I'm either doing something wrong which shouldn't really work on Linux (and I'm getting lucky!)... or something screwy is happening on Mac version: This exports a single function which is then #included in CInterface.c to create a new pure-C wrapper to the function. ghc -O2 -c HaskellFuncs.hs ghc -O2 -no-hs-main --make -optl '-shared' CInterface.c HaskellFuncs_stub.o HaskellFuncs.o -o libCInterface.so One Mac OS X I get the following error - but it works fine on Ubuntu. I'm using 6.10.4 on both machines: Linking libCInterface.so ... Undefined symbols: "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) ld: symbol(s) not found Could anyone comment if I'm doing anything wrong, or is this a case of unsupported functionality on (PPC/Leopard) Mac OS X? Has anyone succeeded in getting a similar example to work on Mac OS X? I notice on Linux it is still very temperamental, if I play around with the arguments even slightly I get the same error there. Cheers, Phil. On 26 Aug 2009, at 06:51, Yusaku Hashimoto wrote: > Missing -c option? > > And -v option to see what's going on. > > On Wed, Aug 26, 2009 at 10:37 AM, <phil(a)beadling.co.uk> wrote: >> Hi, >> >> After creating my stub objects etc using GHC, I'm trying to create >> a library >> with a C interface to some Haskell functions. I'm explicitly >> passing in >> -no-hs-main yet the linker still fails due to missing main? >> >> I'm sure I've had this working before with a slightly simpler >> example, but >> can't work out what is wrong here. >> >> If I give it a main (to humor it - it's not a solution), then it >> links and >> produces an executable - so it looks to me like I'm not telling the >> linker >> what I want correctly? >> >> Any ideas? >> >> Cheers, >> >> Phil. >> >> >> ghc -O2 --make -no-hs-main -package mtl -package array -optl '- >> shared' >> FFI/Octave/MyInterface.c FFI/Octave/OptionInterface_stub.o >> FFI/Octave/OptionInterface.o ./FrameworkInterface.o ./Maths/Prime.o >> ./MonteCarlo/DataStructures.o ./MonteCarlo/European.o >> ./MonteCarlo/Framework.o ./MonteCarlo/Interface.o ./MonteCarlo/ >> Lookback.o >> ./Normal/Acklam.o ./Normal/BoxMuller.o ./Normal/Framework.o >> ./Normal/Interface.o ./Random/Framework.o ./Random/Halton.o >> ./Random/Interface.o ./Random/Ranq1.o -o FFI/Octave/ >> libMyInterface.so >> Linking FFI/Octave/libMyInterface.so ... >> Undefined symbols: >> "___stginit_ZCMain", referenced from: >> ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) >> "_ZCMain_main_closure", referenced from: >> _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) >> ld: symbol(s) not found >> collect2: ld returned 1 exit status >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >>

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Haskell Weekly News: Issue 128 - August 26, 2009
by Brent Yorgey 26 Aug '09

26 Aug '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090826 Issue 128 - August 26, 2009 --------------------------------------------------------------------------- Welcome to issue 128 of HWN, a newsletter covering developments in the [1]Haskell community. New releases of haddock, gitit, jhc, formlets, and lots of other libraries and tools; Edinburgh Hack Day, ICFP, and HacPDX coming up; exciting times! The Google Summer of Code has also wrapped up. See below for final progress reports from this summer's Haskell participants. PS: Just as this was going to press, Thomas Ten Cate released the Scion library from his Google Summer of Code project; hence it isn't listed below but you should check it out anyway! Announcements GLUT 2.2.1.0. Sven Panne [2]announced a new version of the [3]GLUT package. The package is now autoconf-free, with API entries are resolved dynamically at runtime; support for sRGB framebuffers has been added; and support for context profiles has been added. Potential Network SIG. Thomas DuBuisson [4]announced the formation of a SIG to hammer out a design for a new Network API, seeing as the current API, a straight-forward Berkeley binding, doesn't seem to please anyone in a Haskell context. epoll bindings 0.1.1. Toralf Wittner [5]announced the release of [6]epoll bindings 0.1.1. Epoll is an I/O event notification facility for Linux similar to poll but with good scaling characteristics. Currently the bindings are fairly low level and close to the C API, but there are plans to add some buffer or stream abstraction on top. Eventually, when GHC can make use of epoll/kqueue in addition to select, this library will not be needed anymore. Until then it might be useful for applications which monitor large numbers of file descriptors. gitit 0.6.1. John MacFarlane [7]announced the release of [8]gitit 0.6.1, a wiki program that runs on happstack, the Haskell web application server stack, and stores pages and other content in a git or darcs filestore. The whole code base has been overhauled since the last release: gitit is now faster, more memory efficient, more modular, and more secure. It also has many new features, including page metadata and categories, atom feeds (sitewide and per-page), support for literate Haskell, a better configuration system, an improved caching system, a Haskell library exporting happstack wiki handlers, and a plugin system. jhc 0.7.1. John Meacham [9]announced the 0.7.1 release of the [10]jhc optimizing Haskell compiler. There have been a lot of changes since the last public release. Some notable ones include the use of a general compiler cache by default rather than object files; reworked library support; an updated manual, with clearer build instructions; support for writing pure C libraries in Haskell; numerous library updates; smart progress meters; typechecking before compilation; and various bug fixes and cross compilation improvements. rss2irc 0.3 released. Simon Michael [11]announced the release of [12]rss2irc version 0.3, an irc bot created by Don Stewart to watch rss feeds and announce new items on irc, now maintained by Simon. This version includes reliable http networking, irc flood protection, better error handling & reporting, extensive debugging output, Atom support, more useful defaults, precise control of irc output, and is now installable on OSX. Feedback and patches welcome. formlets 0.6. Chris Eidhof [13]announced that the formlets team has released a new version of [14]formlets, a library to build type-safe, composable web forms. Most notably, Mightybyte and Chris worked on the [15]massInput functionality, which is now ready for use! graphtype -- A simple tool to illustrate dependencies between Haskell types. Max Desyatov [16]announced the release of [17]graphtype, a tool for visualising type declarations in Haskell source files. It produces [18].dot-files for subsequent processing with graphviz. OAuth library in haskell. Diego Souza [19]announced the release of [20]hoauth, a library which helps you to deal with the [21]oauth protocol. Currently it supports only consumer side applications, but there are plans to add service providers support in near future. ByteString Nums. Jason Dusek [22]announced [23]bytestring-nums, a simple package for relatively careless parsing of numbers from ByteStrings. It works to parse out integer strings, floating point strings and hex strings. haskell-src-exts-1.1.3. Niklas Broberg [24]announced the release of [25]haskell-src-exts-1.1.3, a package for Haskell source code manipulation. It handles (almost) all syntactic extensions to the Haskell 98 standard implemented by GHC, and the parsing can be parametrised on what extensions to recognise. haskell-src-exts-1.1.3 is a highly experimental release, which does not change the current stable part of haskell-src-exts. But it includes a whole new set of modules implementing a new and more accurate syntax tree where all nodes are adorned with annotations. Together with this comes a parser that retains exact source information, stored in the aforementioned annotations. Help in testing and bug reporting is welcome and appreciated! ministg-0.2, an interpreter for STG operational semantics. Bernie Pope [26]announced the first public release of [27]Ministg, an interpreter for a high-level, small-step, operational semantics for the STG machine, the abstract machine at the core of GHC. One of the main features of Ministg is the ability to record a trace of the execution steps as a sequence of HTML files; here is an [28]example trace. OpenCLRaw 1.0.1000. Jeff Heard [29]announced the release of [30]OpenCLRaw, a raw binding to the OpenCL, a platform for single-host heterogeneous, data-parallel computing. He has future plans to create higher-level bindings on top of these raw ones. compose-trans-0.0. Miguel Mitrofanov [31]announced [32]compose-trans, a small library intended to make monad transformers composable. Haddock version 2.5.0. David Waern [33]announced the release of [34]Haddock 2.5.0. This version reverts to the old multi-page index for large packages, shows GADT records in the generated documentation, adds a --use-unicode flag for displaying prettier versions of common symbols, and many other changes. Edinburgh Meetup (Sat 29 Aug) and Hack Day (Sun 30 Aug). Eric Kow [35]sent a reminder that we will be having a [36]Hack Day in Edinburgh on Sunday 30 August at the ICFP venue. There will also be a meetup the day before, 09:30 Saturday 29 August just outside the ICFP venue; we'll have a quick wander and hopefully find some nice places to sit and chat, whip out the occasional laptop and fling a lambda or not being careful not to injure the passers-by. Cleaner networking API - network-fancy. Taru Karttunen [37]announced [38]network-fancy, which offers a cleaner API to networking facilities in Haskell. It supports high-level operations on tcp, udp and unix sockets. Feedback on the API is welcome! GLFW-0.4.1. Paul L [39]announced a new version of [40]GLFW, 0.4.1. Notable changes include a workaround for a FFI bug that affects GHC < 6.10 on 64-bit machines, a fix for the compilation problem on OS X for GHC > 6.10.1, a compatibility fix to work with both OpenGL 2.3.0.0 and older versions, choice of a "dynamic" flag to link with dynamic GLFW C library instead, and a number of other fixes, cleanups and improvements. HacPDX, A Hackathon in Portland. Thomas DuBuisson [41]announced [42]HacPDX, an opportunity for Portland Haskell hackers to join together in building and improving libraries and tools. If you've never been, hackathons are typically not only a good opportunity for experienced devs to work together but also a great way for newcomers to get involved in the community. HacPDX will take place Friday September 25 to Sunday September 27 at Portland State University; see the email for more specific details. Hack on the Delve core, with Delve and Haskell. spoon [43]announced [44]Delve, a new programming language intended to bring the benefits of static type checking and functional programming to object-oriented design and development, currently being implemented in Haskell. Contributors welcome! cabal-query 0.1. Max Desyatov [45]announced the release of [46]cabal-query, a package to assist in finding a set of Cabal packages which satisfy your needs. EnumMap-0.0.1. John Van Enk [47]announced the first version of [48]EnumMap, a generalization of IntMap that constrains the key to Enum rather than forcing it to be Int. Google Summer of Code Progress updates from participants in the 2008 [49]Google Summer of Code. Haddock improvements! Isaac Dupree has [50]wrapped up his project, with patches waiting to be merged back into both Haddock and GHC. His final post contains a detailed description of the work he did; looks like we'll have much better cross-package documentation support in Haddock soon! EclipseFP. Thomas Ten Cate began adding a notion of [51]build targets to EclipseFP, so that projects can be created without .cabal files. He has [52]wrapped up the project for now, and although he isn't fully happy with the results that he achieved, he was able to make useful contributions which hopefully others can continue to build on. Improving the Haskell space profiling experience. Gergely Patai's project is done: he [53]uploaded hp2any, a set of realtime space profiling tools, to Hackage. He also [54]created a [55]haskellwiki page describing it and its use. haskell-src-exts -> haskell-src. Niklas Broberg has been [56]working on a complete revamp of the AST, lexer and parser to allow for exact source info to be kept in the tree, which in turn will allow exact printing of the code as it was read. darcs. Petr Rockai posted a [57]final report where he described his accomplishments: the hashed-storage library for reading and writing filesystem trees in hash-based formats; darcs whatsnew integration with hashed-storage; progress on a new and improved version of hashed-storage, and a branch of darcs depending on it; and darcs-benchmark, a standalone package for benchmarking darcs. Discussion Unification and matching in Abelian groups. John D. Ramsdell [58]shared some code implementing unification and matching in Abelian groups. Grouping and SIMD in parallel Haskell (using Nested Data Parallel Haskell ideas in legacy code). Zefirov Sergey [59]posted some code showing how to translate Parallel Haskell programs (expressed with par and pseq) into Nested Data Parallel Haskell. Request for Comments - hscurrency 0.0.1. Max Cantor [60]requested feedback on some simple tools to do safe calculations on different currencies. DDC compiler and effects; better than Haskell? (was Re: unsafeDestructiveAssign?). Peter Verswyvelen began a long [61]discussion about the DDC compiler and its effect system, and the relationship to Haskell and monads. Jobs Credit Suisse is hiring. Ganesh Sittampalam [62]announced that the Global Modelling and Analytics Group (GMAG) at Credit Suisse is once again looking to hire functional programmers; see his email for more information. Jane Street is Hiring (as if you didn't already know). Yaron Minsky sent out a [63]reminder that [64]Jane Street is looking to hire functional programmers; see his email for more details. He also mentioned that he will be at parts of ICFP, CUFP and DEFUN this year, so if you're interested, come and talk to him there. Galois is Hiring. Don Stewart [65]announced that Galois is continuing to hire, with multiple positions for talented functional programmers (with both junior and senior positions). They will be at ICFP and related events; see Don or Lee Pike. Blog noise [66]Haskell news from the [67]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Isaac Dupree: [68]Summer of Code Wrap-Up.. * Thomas ten Cate: [69]Endgame. * Jeff Heard: [70]Followup to my earlier post on Hilbert curve timeseries plots. * Jeff Heard: [71]Plotting timeseries in space filling curves. * Magnus Therning: [72]Making a choice from a list in Haskell, Vty (part 5, the last one). * Magnus Therning: [73]Fork/exec in Haskell. * Edward Kmett: [74]Iteratees, Parsec and Monoids (Slides). * Chris Smith: [75]Flow Equivalence Code in Haskell. * Thomas Ten Cate: [76]Build targets. * Chris Smith: [77]Catching a Mathematical Error Using Haskell's Type System. * Well-Typed: [78]Industrial Haskell Group meeting at CUFP. * London HUG: [79]Next meeting: Alex McLean, Live coding music with Haskell. * David Amos: [80]Finite fields, part 1. * Greg Bacon: [81]Simple analogy for lazy evaluation . * Magnus Therning: [82]JSON in Haskell. * Notes on the LHC: [83]Status update: New Integer implementation.. * Edward Kmett: [84]Clearer Reflections. * Petr Rockai: [85]soc final report. * Gergely Patai: [86]hp2any overview online. * Brent Yorgey: [87]New 2D text layout library. * Manuel Chakravarty: [88]World's first formal machine-checked proof of a general-purpose operating system kernel.. * Bryan O'Sullivan: [89]Haskell Platform support for Fedora: we're almost there. * Gergely Patai: [90]hp2any on Hackage. * Doug Beardsley: [91]Dynamic List Formlets in Haskell . * Niklas Broberg: [92]Quick update. * Christopher Lane Hinson: [93]FactoryArrow. * Michael Feathers: [94]Imposing the Edges Later . * Brent Yorgey: [95]Species operations: differentiation. * >>> Ron Leisti: [96]A prime number sieve in Haskell. Quotes of the Week * bos: You don't get accurate answers from Perl. It just lies to you to keep you happy. * ray: haskell' will come out in 2020 and be h98 with hierarchical modules * ray: enlarge your kleisli arrow, please the category ladies * quicksilver: making the compiler writer's job painful is one of the main duties of a language designer. * gwern: as a plugin, yes, but that's like being so out of shape that a guy in a wheelchair can outrace you - yes, he needs a tool, but you should still be ashamed of yourself * Cale's Lemma: Any sufficiently long string of operator symbols looks like a fish. * randomwords: How "complete" does an application before it's OK to upload to hackage? <ray> there are no standards <randomwords> lawless wasteland. Got it. * ndm: I was browsing through the Yhc standard libraries, as one does on the weekend, and was drawn to Yhc's sort function. * michaelfeathers: I did a parody post to Haskell Cafe last year where I had some code that was calling (nub . nub) zip12 and asked if there was a zip13 and no one called it out as a joke. About the Haskell Weekly News New editions are posted to [97]the Haskell mailing list as well as to [98]the Haskell Sequence and [99]Planet Haskell. [100]RSS is also available, and headlines appear on [101]haskell.org. To help create new editions of this newsletter, please see the information on [102]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [103]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17442 3. http://hackage.haskell.org/package/GLUT 4. http://article.gmane.org/gmane.comp.lang.haskell.libraries/11813 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62782 6. http://hackage.haskell.org/package/epoll 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62776 8. http://gitit.johnmacfarlane.net/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62759 10. http://repetae.net/computer/jhc 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62743 12. http://hackage.haskell.org/package/rss2irc 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62738 14. http://hackage.haskell.org/package/formlets 15. http://softwaresimply.blogspot.com/2009/08/dynamic-list-formlets-in-haskell… 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62736 17. http://hackage.haskell.org/package/graphtype 18. http://i.piccy.info/i4/00/90/bfa07290012c2d3b455696bdaa86.png 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62735 20. http://hackage.haskell.org/package/hoauth 21. http://oauth.net/ 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62695 23. http://hackage.haskell.org/package/bytestring-nums 24. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62682 25. http://hackage.haskell.org/package/haskell-src-exts 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62559 27. http://www.haskell.org/haskellwiki/Ministg 28. http://www.cs.mu.oz.au/~bjpop/trace/step0.html 29. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62414 30. http://hackage.haskell.org/package/OpenCLRaw 31. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62390 32. http://hackage.haskell.org/package/compose%2Dtrans 33. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62341 34. http://www.haskell.org/haddock 35. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62324 36. http://www.haskell.org/haskellwiki/Hac7 37. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62277 38. http://hackage.haskell.org/package/network%2Dfancy 39. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62259 40. http://hackage.haskell.org/package/GLFW 41. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62195 42. http://haskell.org/haskellwiki/HacPDX 43. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62173 44. http://killersmurf.blogspot.com/ 45. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62139 46. http://hackage.haskell.org/package/cabal%2Dquery 47. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62119 48. http://hackage.haskell.org/package/EnumMap 49. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 50. http://haddock2009.wordpress.com/2009/08/26/summer-of-code-wrap-up/ 51. http://eclipsefp.wordpress.com/2009/08/20/build-targets/ 52. http://eclipsefp.wordpress.com/2009/08/25/endgame/ 53. http://just-bottom.blogspot.com/2009/08/hp2any-on-hackage.html 54. http://just-bottom.blogspot.com/2009/08/hp2any-overview-online.html 55. http://www.haskell.org/haskellwiki/Hp2any 56. http://nibrofun.blogspot.com/2009/08/just-quick-note-since-i-realise-i.html 57. http://web.mornfall.net/blog/soc_final_report.html 58. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62486 59. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62403 60. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62342 61. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/62205 62. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62406 63. http://article.gmane.org/gmane.comp.lang.haskell.general/17436 64. http://www.janestreet.com/ 65. http://article.gmane.org/gmane.comp.lang.haskell.cafe/62191 66. http://planet.haskell.org/ 67. http://haskell.org/haskellwiki/Blog_articles 68. http://haddock2009.wordpress.com/2009/08/26/summer-of-code-wrap-up/ 69. http://eclipsefp.wordpress.com/ 70. http://vis.renci.org/jeff/2009/08/24/followup-to-my-earlier-post-on-hilbert… 71. http://vis.renci.org/jeff/2009/08/24/plotting-timeseries-in-space-filling-c… 72. http://therning.org/magnus/archives/732 73. http://therning.org/magnus/archives/727 74. http://comonad.com/reader/2009/iteratees-parsec-and-monoid/ 75. http://cdsmith.wordpress.com/2009/08/20/flow-equivalence-code-in-haskell/ 76. http://eclipsefp.wordpress.com/2009/08/20/build-targets/ 77. http://cdsmith.wordpress.com/2009/08/19/catching-a-mathematical-error-using… 78. http://blog.well-typed.com/2009/08/industrial-haskell-group-meeting-at-cufp/ 79. http://www.londonhug.net/2009/08/18/next-meeting-alex-mclean-live-coding-mu… 80. http://haskellformaths.blogspot.com/2009/08/finite-fields-part-1.html 81. http://gbacon.blogspot.com/2009/08/simple-analogy-for-lazy-evaluation.html 82. http://therning.org/magnus/archives/719 83. http://lhc-compiler.blogspot.com/2009/08/status-update-new-integer.html 84. http://comonad.com/reader/2009/clearer-reflection/ 85. http://web.mornfall.net/blog/soc_final_report.html 86. http://just-bottom.blogspot.com/2009/08/hp2any-overview-online.html 87. http://byorgey.wordpress.com/2009/08/14/new-2d-text-layout-library/ 88. http://justtesting.org/post/162408446/worlds-first-formal-machine-checked-p… 89. http://www.serpentine.com/blog/2009/08/12/haskell-platform-support-for-fedo… 90. http://just-bottom.blogspot.com/2009/08/hp2any-on-hackage.html 91. http://softwaresimply.blogspot.com/2009/08/dynamic-list-formlets-in-haskell… 92. http://nibrofun.blogspot.com/2009/08/just-quick-note-since-i-realise-i.html 93. http://blog.downstairspeople.org/2009/08/09/factoryarrow/ 94. http://blog.objectmentor.com/articles/2009/08/08/imposing-the-edges-later 95. http://byorgey.wordpress.com/2009/08/05/species-operations-differentiation/ 96. http://www.ronleisti.com/wp/2009/07/26/a-prime-number-sieve-in-haskell/ 97. http://www.haskell.org/mailman/listinfo/haskell 98. http://sequence.complete.org/ 99. http://planet.haskell.org/ 100. http://sequence.complete.org/node/feed 101. http://haskell.org/ 102. http://haskell.org/haskellwiki/HWN 103. http://code.haskell.org/~byorgey/code/hwn/

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ANN: scion 0.1
by Thomas Schilling 26 Aug '09

26 Aug '09

Hello, I am pleased to announce the first release of Scion. Scion [1] is a Haskell library that aims to implement those parts of a Haskell IDE which are independent of a particular front-end. Scion is based on the GHC API and Cabal. It provides both a Haskell API and a server for non-Haskell clients such as Emacs and Vim. Scion is bundled with two front-ends, Emacs and Vim, and is used by the Haskell Eclipse plugin EclipseFP. Work on Yi support is underway. Scion's current features include: - Opening (that is, typechecking) a component of a Cabal project or a single file [Emacs, Vim]. Error messages are highlighted in the source [Emacs]. - Saving a file automatically typechecks the file [Emacs, Vim]. This feature can be turned off. - Look up the type of any identifier (even local variables) [Emacs, Vim]. Requires that the file typechecks. - Jump to definition of an identifier defined in the current project [Emacs]. For more details on the provided features and instructions of how to set up a particular front-end see the README [2]. Patches are always welcome. The main source repository is hosted on Github [3]. If you find a problem please file a bug [4] and/or join the developer's mailing list [5]. [1]: http://code.google.com/p/scion-lib/ [2]: http://github.com/nominolo/scion/blob/master/README.markdown [3]: http://github.com/nominolo/scion [4]: http://code.google.com/p/scion-lib/issues/list [5]: http://groups.google.com/group/scion-lib-devel Thanks ------ The following people contributed to patches to this release: Marc Weber Thomas ten Cate Sudish Joseph

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haddock: parse error in doc string
by Peter Verswyvelen 26 Aug '09

26 Aug '09

I'm getting the error mentioned in the subject, but without any indication where in my file this error occurs. What does this mean? Thanks, Peter

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Edinburgh Meetup (Sat 29 Aug) and Hack Day (Sun 30 Aug)
by Eric Kow 26 Aug '09

26 Aug '09

Dear Haskellers, Just a quick reminder that we will be having a Hack Day in Edinburgh on Sunday 30 August (ICFP venue). That's in two weeks! For the interested, we will also be meeting up the day before 09:30 Saturday 29 August just outside the RCPE (ie. the ICFP venue again). We'll have a quick wander and hopefully find some nice places to sit and chat, whip out the occasional laptop and fling a lambda or not being careful not to injure the passers-by. For more details, see http://www.haskell.org/haskellwiki/Hac7 And do send me an email if you're interested in attending. It'll help us to get an idea how many people will be there. See you soon! :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Suggestion: define a standard keyboard codes library?
by Peter Verswyvelen 26 Aug '09

26 Aug '09

Several libraries define their own codes for they keyboard (GLFW, GTK, GLUT, etc) Maybe it would be nice to agree on a standard datatype for keys? This might also include digital buttons on a joystick, etc... The Windows API has virtual key codes for this.

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Is it possible to prove type *non*-equality in Haskell?
by Ryan Ingram 26 Aug '09

26 Aug '09

Short version: How can I get from (Z ~ S n) to a useful contradiction? Type equality coercions[1] let us write proofs in Haskell that two types are equal: > {-# LANGUAGE GADTs, RankNTypes, TypeFamilies #-} > {-# OPTIONS_GHC -Wall #-} > module TEq where > data TEq a b = (a ~ b) => TEq This provides all the expected properties for equality: > refl :: TEq a a > refl = TEq > symm :: TEq a b -> TEq b a > symm TEq = TEq > trans :: TEq a b -> TEq b c -> TEq a c > trans TEq TEq = TEq Here's an example use: > data Z = Z > newtype S n = S n > data Nat a where > Nz :: Nat Z > Ns :: Nat x -> Nat (S x) > proveEq :: Nat a -> Nat b -> Maybe (TEq a b) > proveEq Nz Nz = return TEq > proveEq (Ns a) (Ns b) = do > TEq <- proveEq a b > return TEq > proveEq _ _ = Nothing But if you get "Nothing" back, there's no proof that the two types are in fact non-equal. You can use "_|_ as negation": > newtype Not p = Contr { contradiction :: forall a. p -> a } > > nsymm :: Not (TEq a b) -> Not (TEq b a) > nsymm pf = Contr (contradiction pf . symm) We know by parametricity that "contradiction n p" isn't inhabited as its type is (forall a. a) But I can't figure out a way to write this without "error": > notZeqS :: forall n. Not (TEq Z (S n)) > notZeqS = Contr (\x -> x `seq` error "impossible") As a first step, I'd like to write this: > -- notZeqS = Contr (\TEq -> error "impossible") but the compiler complains immediately about the pattern match being unsound: TEq.lhs:39:20: Couldn't match expected type `S n' against inferred type `Z' In the pattern: TEq In the first argument of `Contr', namely `(\ TEq -> error "impossible")' In the expression: Contr (\ TEq -> error "impossible") Is there any way to use the obvious unsoundness we get from (Z ~ S n) to generate a contradiction? Ideally I'd like to be able to implement ] natEqDec :: Nat a -> Nat b -> Either (TEq a b) (Not (TEq a b)) as follows: > predEq :: TEq (f a) (f b) -> TEq a b > predEq TEq = TEq > natEqDec :: Nat a -> Nat b -> Either (TEq a b) (Not (TEq a b)) > natEqDec Nz Nz = Left TEq > natEqDec (Ns a) (Ns b) = case natEqDec a b of > Left TEq -> Left TEq > Right pf -> Right $ Contr $ \eq -> contradiction pf (predEq eq) > natEqDec Nz (Ns _) = Right notZeqS > natEqDec (Ns _) Nz = Right (nsymm notZeqS) Which compiles successfully, but the "error" call in "notZeqS" is a big wart. Is there a better implementation of "Not" that allows us to avoid this wart? -- ryan [1] http://research.microsoft.com/en-us/um/people/simonpj/papers/ext-f/

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Re: [Haskell-cafe] How to calculate de number of digits of an integer? (was: Is logBase right?)
by George Pollard 26 Aug '09

26 Aug '09

2009/8/26 Lennart Augustsson <lennart.augustsson(a)gmail.com>: > Try 10^1000 Cheat :)

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oauth in haskell - reviewers?
by Diego Souza 26 Aug '09

26 Aug '09

Hi all, I wrote a small library in haskell do deal with oauth authentication. It turns out it is my first library in haskell as well. As I'm beginner in haskell, I'm asking for a review of someone more experienced/proficient before even daring to create a cabal pkg and dist it to hackage. :-) Any help/comments on this will be highly welcome. http://projects.bitforest.org/hoauth/ $ darcs get http://projects.bitforest.org/hoauth/ # should do the trick Thanks in advance, -- ~dsouza yahoo!im: paravinicius gpg key fingerprint: 71B8 CE21 3A6E F894 5B1B 9ECE F88E 067F E891 651E

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FFI link failing due to no main?
by phil＠beadling.co.uk 26 Aug '09

26 Aug '09

Hi, After creating my stub objects etc using GHC, I'm trying to create a library with a C interface to some Haskell functions. I'm explicitly passing in -no-hs-main yet the linker still fails due to missing main? I'm sure I've had this working before with a slightly simpler example, but can't work out what is wrong here. If I give it a main (to humor it - it's not a solution), then it links and produces an executable - so it looks to me like I'm not telling the linker what I want correctly? Any ideas? Cheers, Phil. ghc -O2 --make -no-hs-main -package mtl -package array -optl '- shared' FFI/Octave/MyInterface.c FFI/Octave/OptionInterface_stub.o FFI/ Octave/OptionInterface.o ./FrameworkInterface.o ./Maths/Prime.o ./ MonteCarlo/DataStructures.o ./MonteCarlo/European.o ./MonteCarlo/ Framework.o ./MonteCarlo/Interface.o ./MonteCarlo/Lookback.o ./Normal/ Acklam.o ./Normal/BoxMuller.o ./Normal/Framework.o ./Normal/ Interface.o ./Random/Framework.o ./Random/Halton.o ./Random/ Interface.o ./Random/Ranq1.o -o FFI/Octave/libMyInterface.so Linking FFI/Octave/libMyInterface.so ... Undefined symbols: "___stginit_ZCMain", referenced from: ___stginit_ZCMain$non_lazy_ptr in libHSrts.a(Main.o) "_ZCMain_main_closure", referenced from: _ZCMain_main_closure$non_lazy_ptr in libHSrts.a(Main.o) ld: symbol(s) not found collect2: ld returned 1 exit status

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How to convert a list to a vector encoding its length in its type?
by Bas van Dijk 25 Aug '09

25 Aug '09

Hello, We're working on a Haskell binding to levmar[1] a C implementation of the Levenberg-Marquardt optimization algorithm. The Levenberg-Marquardt algorithm is an iterative technique that finds a local minimum of a function that is expressed as the sum of squares of nonlinear functions. It has become a standard technique for nonlinear least-squares problems. It's for example ideal for curve fitting. The binding is nearly finished and consists of two layers: a low-level layer that directly exports the C bindings and a medium-level layer that wraps the lower-level functions and provides a more Haskell friendly interface. The Medium-Level (ML) layer has a function which is similar to: > levmarML :: (a -> [Double] -> Double) > -> [Double] > -> [(a, Double)] > -> [Double] > levmarML model initParams samples = ... So you give it a model function, an initial guess of the parameters and some input samples. levmarML than tries to find the parameters that best describe the input samples. Of course, the real function has more arguments and return values but this simple version will do for this discussion. Al-dough this medium-level layer is more Haskell friendly than the low-level layer it still has some issues. For example a model function is represented as a function that takes a list of parameters. For example: > \x [p1, p2, p3] -> p1*x^2 + p2*x + p3 You have to ensure that the length of the initial guess of parameters is exactly 3. Otherwise you get run-time pattern-match failures. So I would like to create a new High-Level (HL) layer that overcomes this problem. First of all I need the following language extensions: > {-# LANGUAGE EmptyDataDecls #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE GADTs #-} I want to represent the model function as a normal Haskell function. So instead of passing the parameters as a list I want to pass them as arguments: > \x p1 p2 p3 -> p1*x^2 + p2*x + p3 Because we would like to be able to use model functions of different arity it means the levmarHL function needs to be polymorphic in the model function: > levmarHL :: (ModelFunc f, Arity f ~ n, Arg f ~ Double) > => (a -> f) > -> Vector Double n > -> [(a, Double)] > -> Vector Double n > levmarHL model initParams samples = > fromList $ levmarML (\x params -> model x $* fromList params) > (toList initParams) > samples You can see that the initial parameters and the result are passed as Vectors that encode their length 'n' in their type. This length must equal the arity of the model function: 'Arity f ~ n'. The arity of the model function and the length of the vectors are represented as type level naturals: > data Z > data S n Lets look at the implementation of ModelFunc: > class ModelFunc f where > type Arg f :: * > type Arity f :: * > > ($*) :: f -> Vector (Arg f) (Arity f) -> Arg f $* is similar to $ but instead of applying a function to a single argument it applies it to a vector of multiple arguments. > instance (ModelFunc f, Arg f ~ b) => ModelFunc (b -> f) where > type Arg (b -> f) = b > type Arity (b -> f) = S (Arity f) > > f $* (x :*: xs) = f x $* xs You can see that we restrict the arguments of module functions to have the same type: 'Arg f ~ b'. This is the base case: > instance ModelFunc Double where > type Arg Double = Double > type Arity Double = Z > > d $* Nil = d We represent Vectors using a GADT: > data Vector a n where > Nil :: Vector a Z > (:*:) :: a -> Vector a n -> Vector a (S n) > infixr :*: Converting a Vector to a list is easy: > toList :: Vector b n -> [b] > toList Nil = [] > toList (x :*: xs) = x : toList xs My single question is: how can I convert a list to a Vector??? > fromList :: [b] -> Vector b n > fromList = ? regards, Roel and Bas van Dijk [1] http://www.ics.forth.gr/~lourakis/levmar/

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ANN: gitit 0.6.1
by John MacFarlane 25 Aug '09

25 Aug '09

I'm pleased to announce the release of gitit 0.6.1. Gitit is a wiki program that runs on happstack, the Haskell web application server stack, and stores pages and other content in a git or darcs filestore. The wiki can be updated either directly through the VCS or through gitit's web interface. Pages can be written in (extended) markdown, reStructuredText, HTML, or LaTeX, and exported in ten different formats. TeX math is rendered using MathML by default, and syntax highlighting is provided for over fifty languages. demo: http://gitit.johnmacfarlane.net manual: http://gitit.johnmacfarlane.net/README api: http://hackage.haskell.org/package/gitit-0.6.1 code: http://github.com/jgm/gitit bugs: http://code.google.com/p/gitit/issues/list group: http://groups.google.com/group/gitit-discuss Here is how you can install and run gitit. You'll need GHC and cabal-install. If you don't have these, install the Haskell Platform <http://hackage.haskell.org/platform/>. Then: cabal update cabal install gitit mkdir mywiki cd mywiki gitit # now browse to http://localhost:5001 Or, if you want to change the defaults (say, reStructuredText instead of markdown, or darcs instead of git): gitit --print-default-config > gitit.conf # edit gitit.conf, which is self-documenting gitit -f gitit.conf The whole code base has been overhauled since the last release. Gitit is now faster, more memory efficient, more modular, and more secure. It also has many new features, including - page metadata and categories - atom feeds (sitewide and per-page) - support for literate Haskell - a better configuration system - an improved caching system - a Haskell library exporting happstack wiki handlers - a plugin system The last two items are the most exciting and deserve special comment. First, in addition to providing an executable, gitit now provides a library, Network.Gitit, which makes it easy to include a gitit wiki (or many of them) in any happstack application. It is even possible to use the containing application's authentication system for the wiki. Second, gitit can now be extended through plugins, short Haskell programs that are loaded dynamically when the server starts. For examples of the things that can be done with plugins, see the plugins directory, which contains (among other things) a plugin for adding graphviz diagrams to pages and a plugin for adding interwiki links. For a full description of the plugin system, see the haddock documentation for Network.Gitit.Interface. Full changes from version 0.5.3, as well as upgrade instructions, are available in the file CHANGES. Thanks are due to - the happstack team, for big improvements in happstack-server that make it much easier to work with, - the darcs team, for using gitit/darcsit for <http://wiki.darcs.net>, giving gitit a real-world test, - Gwern Branwen, who helped to optimize gitit, wrote the InterwikiPlugin, and wrote the guts of the Feed module, - Simon Michael, who contributed several patches, - Henry Laxen, who added support for password resets and helped with the apache proxy instructions, - Anton van Straaten, who made the process of page generation more modular by adding Gitit.ContentTransformer, - Robin Green, who helped improve the plugin API and interface, fixed a security problem with the reset password code, and made saving of the user's file more robust, - Thomas Hartman, who helped improve the index page, making directory browsing persistent, - Kohei Ozaki, who contributed the ImgTexPlugin, - mightybyte, who suggested making gitit available as a library, and contributed a patch to the authentication system, - and everyone else who contributed suggestions and bug reports.

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ANN: epoll bindings 0.1.1
by Toralf Wittner 25 Aug '09

25 Aug '09

Hi, I am pleased to announce the release of epoll bindings 0.1.1 available from: http://hackage.haskell.org/package/epoll Epoll is an I/O event notification facility for Linux similar to poll but with good scaling characteristics. Currently the bindings are fairly low level and close to the C API. In the future I hope to add some buffer or stream abstraction on top. Eventually, when GHC can make use of epoll/kqueue etc. in addition to select, this library will not be needed anymore. Until then it might be useful for applications which monitor large numbers of file descriptors. -Toralf

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Is logBase right?
by Roberto 25 Aug '09

25 Aug '09

Hi, There is a mistake is logBase: $ ghci GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> logBase 10 10 1.0 Prelude> logBase 10 100 2.0 Prelude> logBase 10 1000 2.9999999999999996 <--- eeeerrgghhhh! Prelude> logBase 10 10000 4.0 My host is a Debian GNU/Linux 5.0.2 (lenny) with the following GHC packages: ii ghc6 6.10.4-1 ii ghc6-doc 6.10.4-1 ii libghc6-mtl-dev 1.1.0.2-7+b1 ii libghc6-utf8-string-dev 0.3.5-1+b1 ii libghc6-x11-dev 1.4.5-6 rc libghc6-x11-doc 1.4.2-1 ii libghc6-x11-xft-dev 0.3-3+b3 ii libghc6-xmonad-contrib-dev 0.8.1-3+b3 rc libghc6-xmonad-contrib-doc 0.8-2 ii libghc6-xmonad-dev 0.8.1-5 Regards!

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