May 2011 - Haskell-Cafe

Haskell Weekly News - Issue 180
by Daniel Santa Cruz 04 May '11

04 May '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of April 24 to 30, 2011. Announcements Eric Kow suggested (http://goo.gl/bc4JM) the creation of a Haskell User Group starter kit. If you have ideas in this department, please see what Eric has started! Christopher Done wrote (http://goo.gl/2m7AJ) a response to a blog from Cedric entitled "Why Scala's Option and Haskell's Maybe types won't save you from Null." A lively conversation followed. Henning Thielemann started an very interesting thread (http://goo.gl/FWyDE) with the title "Python is lazier than Haskell." (See the quotes of the week for the full quote) Quotes of the Week These quotes are taken from the #haskell channel, and from the haskell/haskell-cafe mailing lists. * Henning Thielemann: If Haskell is great because of its laziness, then Python must be even greater, since it is lazy at the type level. * Ben Lippmeier: Laziness at the value level causes space leaks, and laziness at the type level causes mind leaks. Neither are much fun. When people start wanting laziness at the kind level we'll have to quarantine them before the virus spreads... * Gregg Reynolds: If the designers could find a way to support laziness at the programmer level, I for one would be very grateful. * Felipe Almeida Lessa: But I am sure that john-millikin-is-great.txt must be increasing like a memory leak ;) * James Cook: I think Haskell questions on SO tend to the opposite extreme; no matter how poorly thought-out the question, the Haskell community will descend on it like a swarm of helpful piranhas. * Philippa: C++ seeing a renaissance as a functional language, is this a bit like rusty nails seeing a renaissance as sex toys for masochists? * #haskell: <aristid> preflex: seen FunctorSalad * preflex_ is now known as preflex. * copumpkin is now known as postflex. * xplat is now known as circumflex. <circumflex> FunctorSalad was never seen on #haskell-world-conquest -4 years, 3 days, 7i minutes and 0 seconds ago, saying: RELEASE THE KRAKEN! * tolkad: Without a standard you are lost, adrift in a sea of unspecified semantics. Hold fast to the rules of the specification lest you be consumed by ambiguity. * Saizan: it's much easier via type theory, 1) prove bottom 2) ???? 3) quodlibet * dons: Think of a monad as a spacesuit full of nuclear waste in the ocean next to a container of apples. Now, you can't put oranges in the space suite or the nuclear waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need. * tolkad: Perhaps by using it we could modernise haskell by applying the principles of the new kind of science Top Reddit Stories * The Irish Times: "Functional programming is more than just esoteric; it's becoming somewhat cool." Domain: irishtimes.com, Score: 44, Comments: 6 On Reddit: http://goo.gl/cfUbb Original: http://goo.gl/1nHEY * Why Silkapp uses Haskell Domain: blog.silkapp.com, Score: 43, Comments: 0 On Reddit: http://goo.gl/IFREi Original: http://goo.gl/1SwNa * Harper again: The real point of laziness Domain: existentialtype.wordpress.com, Score: 38, Comments: 89 On Reddit: http://goo.gl/vA8oD Original: http://goo.gl/hqcpy * Patent 5,893,120 reduced to mathematical formulae (via Haskell) Domain: paulspontifications.blogspot.com, Score: 34, Comments: 13 On Reddit: http://goo.gl/PmB4K Original: http://goo.gl/QknmV * IO evaluates the Haskell heap Domain: blog.ezyang.com, Score: 33, Comments: 3 On Reddit: http://goo.gl/FzaGV Original: http://goo.gl/KNHku * Why Haskell? Domain: blog.mired.org, Score: 33, Comments: 19 On Reddit: http://goo.gl/t1r8I Original: http://goo.gl/cV6KL * Filter and Visualize Data in Seconds with Silk (Haskell-based startup) Domain: thenextweb.com, Score: 33, Comments: 18 On Reddit: http://goo.gl/hA9HF Original: http://goo.gl/NIR3U * Haskell libraries you should use Domain: blog.johantibell.com, Score: 29, Comments: 3 On Reddit: http://goo.gl/PPTWG Original: http://goo.gl/LmbT6 * Tying the Knot - a really mind bending Haskell technique Domain: haskell.org, Score: 27, Comments: 8 On Reddit: http://goo.gl/iEdSM Original: http://goo.gl/5Zg6f * haskell.org GSoC accepted projects are up. Domain: self.haskell, Score: 26, Comments: 32 On Reddit: http://goo.gl/mnrl9 Original: /r/haskell/comments/gxj1h/haskellorg_gsoc_accepted_projects_are_up/ Top StackOverflow Questions * What are the best Haskell libraries to operationalize a program? votes: 57, answers: 3 Read on SO: http://goo.gl/rYQiL * How do you design programs in Haskell or other functional programming languages? [migrated] votes: 24, answers: 0 Read on SO: http://goo.gl/IW7Z5 * Haskell: why the convention to name a helper function "go"? votes: 22, answers: 3 Read on SO: http://goo.gl/CE1O7 * What haskell topics need to be addressed in a Real-World-Haskell style? votes: 21, answers: 5 Read on SO: http://goo.gl/3tgE4 * What are some interesting uses of high order functions? votes: 14, answers: 14 Read on SO: http://goo.gl/WdGeF * How are mutable arrays implemented in Haskell? votes: 10, answers: 3 Read on SO: http://goo.gl/LMK8u * Performance problem with Euler problem and recursion on Int64 types votes: 10, answers: 6 Read on SO: http://goo.gl/CLXD2 About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Can't install haskellnet with ghc7
by Charles-Pierre Astolfi 04 May '11

04 May '11

Hi -cafe, Did anybody managed to install haskellnet from hackage with ghc7? I tried on windows and mac and I get the following type error: [ 4 of 11] Compiling Network.HaskellNet.SMTP ( Network/HaskellNet/SMTP.hs, dist/build/Network/HaskellNet/SMTP.o ) Network/HaskellNet/SMTP.hs:269:25: Couldn't match expected type `Data.Text.Internal.Text' with actual type `[Char]' Expected type: Data.Text.Internal.Text Actual type: String In the first argument of `simpleMail', namely `to' In a stmt of a 'do' expression: myMail <- simpleMail to from subject plainBody htmlBody attachments Am I missing something? -- Cp

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warning - Euler problem spoiler enclosed
by Barbara Shirtcliff 04 May '11

04 May '11

Hi, In the following solution to problem 24, why is nub ignored? I.e. if you do lexOrder of "0012," you get twice as many permutations as with "012," even though I have used nub. puzzled, Bar -- file Euler.hs module Euler where import Data.List {- problem 24 A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are: 012 021 102 120 201 210 What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9? -} lexI :: Char -> String -> Int lexI c s = maybe 1 (id) $ elemIndex c s lexOrder :: [Char] -> [[Char]] lexOrder s | length s == 1 = [s] | length s == 2 = z : [reverse z] | otherwise = concat $ map (\n -> h n) [0..((length s) - 1)] where z = sort $ nub s -- why is the nub ignored here? h :: Int -> [String] h n = map (z!!n :) $ lexOrder $ filter (\c -> lexI c z /= n) z p24 = (lexOrder "1234567890")!!999999 main :: IO() main = do putStrLn $ show $ p24

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Advertisement: the Haskell Stack Overflow Q & A site
by Don Stewart 04 May '11

04 May '11

Hey all, I thought I'd just make a quick advertisement for the Haskell Stack Overflow community: http://stackoverflow.com/questions/tagged/haskell as a forum for questions and answers on beginner to advanced Haskell problems. The site is very active, with roughly as many questions being asked on SO as on haskell-cafe these days. One of the benefits of a site like SO as a forum is the ability to record and link to prior work, edit for technical errors, and easily search and categorize past answers. It is also less prone to noise, for those suffering from cafe overload. Cheers, Don

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Another good video tutorial on monads
by C K Kashyap 04 May '11

04 May '11

http://vimeo.com/20717301 I really liked how he starts off with the let statement. Regards, Kashyap

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Re: [Haskell-cafe] Please add a method for optimized concat to the Semigroup class
by John Lato 04 May '11

04 May '11

> > From: Edward Kmett <ekmett(a)gmail.com> > > On Tue, May 3, 2011 at 3:43 PM, Yitzchak Gale <gale(a)sefer.org> wrote: > > > I'm sure there are countless other natural examples of semigroups > > in the wild, and that the typical non-trivial ones will benefit > > from an optimized sconcat. > > > > Sold! (modulo the semantic considerations above) > Somewhat academic, but would there be a case for implementing sconcat in terms of Foldable (or similar)? John L

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Fwd: Re: Binary and Serialize classes
by Alberto G. Corona 04 May '11

04 May '11

---------- Forwarded message ---------- De: "Alberto G. Corona" <agocorona(a)gmail.com> Fecha: 03/05/2011 11:24 Asunto: Re: [Haskell-cafe] Binary and Serialize classes Para: "Antoine Latter" <aslatter(a)gmail.com> E1610With the exception of heavy serialization usage, for example, in very optimized RPC applications (and even there SOAP shows that this is not ever the case), text serialization is better. The unwritten rules of good design says that data representation and compression must be orthogonal. The binary formats were designed for performing both functionalities in the times when memory were measured in Kbytes. 2011/4/28 Antoine Latter <aslatter(a)gmail.com> > > On Thu, Apr 28, 2011 at 10:00 AM, Evan Laforge <...

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runST readSTRef type error
by Ken Takusagawa II 04 May '11

04 May '11

I run into the following type error: foo :: ST s (STRef s Int) -> Int foo p = (runST (p >>= readSTRef)) with ghc 6.12.1 st.hs:8:16: Couldn't match expected type `s1' against inferred type `s' `s1' is a rigid type variable bound by the polymorphic type `forall s1. ST s1 a' at st.hs:8:9 `s' is a rigid type variable bound by the type signature for `foo' at st.hs:7:10 Expected type: ST s1 (STRef s Int) Inferred type: ST s (STRef s Int) In the first argument of `(>>=)', namely `p' In the first argument of `runST', namely `(p >>= readSTRef)' However, if I add {-# LANGUAGE RankNTypes #-} and change the type signature to foo :: (forall s.ST s (STRef s Int)) -> Int it works. I don't fully understand what's going on here. Is this the "right" way to fix the problem? Are there other options? My gut feeling is, for such a simple use case of the ST monad, I shouldn't need such a big hammer as RankNTypes. --ken

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ANN: Leksah 0.10.0
by Hamish Mackenzie 03 May '11

03 May '11

Yesterday we uploaded our official 0.10.0 release (0.10.0.4) to Hackage and we have Windows and OS X installers at http://leksah.org/download.html Anyone using an earlier version should upgrade. This release is mainly small improvements and bug fixes. Some of the improvements include: * Better messages for some common problems. * Improved grep (now searches all source directories in your workspace) * Updated build system for more reliable rebuilding of dependent packages in your workspace We have automated the building of the Leksah installers and this should allow us to do more frequent releases in the future. There is a lot more to do and we would love to hear from anyone who has time to help out. Worth mentioning because they can be hard to find: * <Ctrl> R adds missing imports * <Ctrl> <Enter> evaluates current line or selection in GHCi * Right click menu in the editor has other debug functions * Comment and uncomment are now <Ctrl> D and <Ctrl> <Shift> D To build Leksah yourself: * Install Gtk and GtkSourceView2 (C libraries and headers) * Install The Haskell Platform (or GHC >= 6.12.3) * cabal update * cabal install gtk2hs-buildtools * cabal install leksah * leksah Making a Gtk application look nice on Windows and OS X is not easy so we recommend using one of these prebuilt binaries: Windows users using GHC 7.0.3 http://leksah.org/packages/leksah-0.10.0.4-ghc-7.0.3.exe Windows users using GHC 6.12.3 http://leksah.org/packages/leksah-0.10.0.4-ghc-6.12.3.exe OS X 10.6 users using GHC 7.0.3 http://leksah.org/packages/leksah-0.8.0.6.dmg (These installers do not install Gtk2Hs packages on your system) Please consult http://leksah.org/download.html for more details about installation! Background information can be found in the Leksah manual http://leksah.org/leksah_manual.pdf. Known bugs and problems: * Leksah does not run with GHC 6.12.2 (http://http://hackage.haskell.org/trac/ghc/ticket/4038) Have fun Jürgen & Hamish See you on #leksah IRC

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Filtering / branching enumeratee
by Skirmantas Kligys 03 May '11

03 May '11

Hello, I am using enumerator-0.4.10, and I need to distribute processing of different parts of the incoming stream to different iteratees (I am parsing a huge XML file, and different sub-trees have different processing logic). Only a single iteratee will be active at a time since the sub-trees don't intersect. I wrote a simple example that filters the stream and passes the result to one iteratee; please see below. However, with multiple nested iteratees it seems to me that I can no longer use an enumeratee. Do I need to write my own multi-enumeratee that holds multiple inner iteratees? Any better ideas? Here is my (beginner's) code for a single nested iteratee: module Main ( main ) where import qualified Data.Enumerator as E ( Enumeratee, Step(..), Stream(..), checkDone, checkDoneEx, continue, enumList, joinI, run_, yield ) import Data.Enumerator ( ($$), (>>==) ) import qualified Data.Enumerator.List as EL ( consume ) -- cribbed from EL.concatMap concatMapAccum :: Monad m => (s -> ao -> (s, [ai])) -> s -> E.Enumeratee ao ai m b concatMapAccum f s0 = E.checkDone (E.continue . step s0) where step _ k E.EOF = E.yield (E.Continue k) E.EOF step s k (E.Chunks xs) = loop s k xs loop s k [] = E.continue (step s k) loop s k (x:xs) = case f s x of (s', ais) -> k (E.Chunks $ ais) >>== E.checkDoneEx (E.Chunks xs) (\k' -> loop s' k' xs) passFromTo :: Monad m => ((a -> Bool), (a -> Bool)) -> Bool -> E.Enumeratee a a m b passFromTo (from, to) pass0 = concatMapAccum updatePass pass0 where updatePass pass el = case (pass, from el, to el) of (True, _, to_el) -> (not to_el, [el]) (False, True, _) -> (True, [el]) (False, False, _) -> (False, []) main :: IO() main = do E.run_ (E.enumList 3 [1..20] $$ E.joinI $ passFromTo ((\e -> e == 3 || e == 13), (\e -> e == 7 || e == 17)) False $$ EL.consume) >>= print $ ./dist/build/StatefulEnumeratee/StatefulEnumeratee [3,4,5,6,7,13,14,15,16,17]

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