- Haskell-Cafe - Haskell.org

ANNOUNCE: freesect-0.0.5
by Ras Far 12 Mar '12

12 Mar '12

Hi, I'm pleased to announce my first package, freesect, which is a Haskell syntax extension to generalise sections. Please visit the homepage http://www.fremissant.net/freesect for more information. The package is also available on Hackage. Some previous discussion occurred on the haskell-cafe thread http://www.mail-archive.com/haskell-cafe@haskell.org/msg97643.html It's been an exhilarating experience! Kind Regards, Andrew Seniuk (rasfar on #haskell)

1 0

ANNOUNCE : Leksah 0.12
by Hamish Mackenzie 12 Mar '12

12 Mar '12

Source is in Hackage and https://github.com/leksah OS X Binary Installers http://leksah.org/packages/leksah-0.12.0.3-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.12.0.3-ghc-7.4.1.dmg Windows Binary Installers http://leksah.org/packages/leksah-0.12.0.3-ghc-6.12.3.exe http://leksah.org/packages/leksah-0.12.0.3-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.12.0.3-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.12.0.3-ghc-7.4.1.exe Changes to the last release include: * Support for GHC 7.2 and 7.4 * Better handling of navigation from text panes * Addition of a Files pane * Support for "cabal test" * Better printer for cabal files (when Leksah is built with Cabal >1.10) * Better candy support for literal Haskell * Syntax highlighting for quasi-quoted hamlet, julius and cassius (DSLs used in Yesod) * Several bug fixes and optimizations Thanks to Sanny Sanoff for improving some of the keyboard navigation and symbol searching among other things. Thanks to Harald Jagenteufel and Stephan Fortelny we have basic source control integration (git,svn) in a repo branch, but it is not part of this release. We hope to merge this in soon. Work has been progressing on a major restructuring of Leksah to make it a modular plugin based system from the ground up. It is a big task and there is still more to do. Unfortunately this has meant we have spent less time adding features.

3 5

Haskell-platform and ghc 7.4
by G 11 Mar '12

11 Mar '12

Am new to haskell, but one thing I wanted to try out was the performance of an app in haskel. Having issues getting it to compile under ghc 7.0.4. The owner recommend trying to upgrade to 7.2/7.4 I see a warning that the haskell-platform isn't compatible with with 7.4.1, or so my homebrew install says. Can it be made to compile ?? Can I remove the haskell-platform and then install the dev tools from the haskell-platform via cabal etc once I have moved to 7.4.x etc ? Thanks

2 2

network-conduit proxy
by grant 11 Mar '12

11 Mar '12

I am trying to get a proxy working using the network-conduit package on windows. So I send a request to port 5002 and that gets forwarded to another port 5000 where I have a simple echo server running. I made a stab at it, but get intermittent send errors after the first connection Here is the code: {-# OPTIONS -Wall #-} import Data.Conduit import Data.Conduit.Network import Network (withSocketsDo) import Control.Monad.IO.Class (liftIO) import Control.Monad.Trans.Resource main::IO () main = withSocketsDo $ runTCPClient (ClientSettings 5000 "localhost") $ \src1 sink1 -> do liftIO $ print "in tcpclient section" liftIO $ withSocketsDo $ runTCPServer (ServerSettings 5002 Nothing) $ \src sink -> do liftIO $ print "in tcpserver section" _ <- liftIO $ runResourceT $ resourceForkIO $ do src1 $$ sink return () src $$ sink1 Thanks for any help, Grant

3 14

haskell platform - mac lion - installation error
by Peter 11 Mar '12

11 Mar '12

Hello - I'm a new user, having some trouble installing the Haskell Platform (2011.4.0.0 64bit.pkg) on a Macbook Pro (10.7.3). I installed Xcode 4.3.1 (4E1019), then the Haskell Platform. When I double-click on the Platform package icon, I get an installation dialogue with an error message reading "Developer Tools Missing - Please install Xcode developer tools first." Are Xcode developer tools something distinct from Xcode itself? I'm mystified by this error message. Any assistance would be greatly appreciated - thank you.

6 5

Haskell Java interoperability
by bartosz＠fpcomplete.com 11 Mar '12

11 Mar '12

I looked on haskell.org for solutions to Haskell/Java interop, but there doesn't seem to be any current project. What is the status of Lambada or its progeny (I asked Erik Meijer, but he lost track of it). Which project is closest to maturity? --Bartosz

2 1

ANNOUNCE: diagrams 0.5
by Brent Yorgey 09 Mar '12

09 Mar '12

I am pleased to announce the release of version 0.5 of diagrams [1], a full-featured framework and embedded domain-specific language for declarative drawing. Check out the gallery [2] for examples of what it can do! [1] http://projects.haskell.org/diagrams [2] http://projects.haskell.org/diagrams/gallery.html Highlights of this release include: - A new diagrams-contrib [3] package of user-contributed modules, which so far contains code for tree drawing, Apollonian gaskets, planar tilings, "wrapped" layout, and turtle graphics. - Experimental support for animation, built on top of the new active [4] library. - Numerous small additions and improvements, including more general rounded rectangle shapes and better text support. - Much better performance in some common situations, such as laying out a very long list of diagrams using 'cat' and related combinators. - Added support for GHC 7.4. See the release notes [5] for complete details, and the diagrams wiki [6] for help migrating code from 0.4 to 0.5 (changes should be minimal). [3] http://hackage.haskell.org/package/diagrams%2Dcontrib [4] http://hackage.haskell.org/package/active [5] http://projects.haskell.org/diagrams/releases.html [6] http://www.haskell.org/haskellwiki/Diagrams/Migrate0.5 Try it out ---------- For the truly impatient: cabal install gtk2hs-buildtools cabal install diagrams Diagrams is supported under GHC 6.12, 7.0, 7.2, and 7.4. However, getting cairo to build can be tricky on some platforms; see the diagrams wiki [8] for more information and workarounds regarding specific platforms. (A new native SVG backend is in the works, targeted for the 0.6 release.) To get started with diagrams, read the quick tutorial [9], which will introduce you to the fundamentals of the framework. For those who are even less impatient but want to really dig in and use the power features, read the user manual [10]. [7] http://projects.haskell.org/gtk2hs/development/#development [8] http://www.haskell.org/haskellwiki/Diagrams [9] http://projects.haskell.org/diagrams/tutorial/DiagramsTutorial.html [10] http://projects.haskell.org/diagrams/manual/diagrams-manual.html Get involved ------------ Subscribe to the project mailing list [11], and/or come hang out in the #diagrams IRC channel on freenode.org for help and discussion. Make some diagrams. Fix some bugs [12]. Submit your cool examples for inclusion in the gallery or your cool code for inclusion in the diagrams-contrib package! [11] http://groups.google.com/group/diagrams-discuss [12] http://code.google.com/p/diagrams/issues/list Happy diagramming! Brought to you by the diagrams team: - Peter Hall - Ian Ross - Michael Sloan - Ryan Yates - Brent Yorgey with contributions from: - Sam Griffin - Claude Heiland-Allen - John Lato - Vilhelm Sjoberg - Luite Stegeman - Kanchalai Suveepattananont - Scott Walck

1 0

ANNOUNCE: string-conversions-0.1
by Sönke Hahn 09 Mar '12

09 Mar '12

Hi all! string-conversions is a very simple package to facilitate dealing with different string types. It provides a simple type class that allows you to convert between values of different string types. It also provides type synonyms for these string types. Supported types are: - String - Strict ByteString - Lazy ByteString - Strict Text - Lazy Text http://hackage.haskell.org/package/string-conversions Any comments welcome. Cheers, Sönke

3 8

If you'd design a Haskell-like language, what would you do different?
by Robert Clausecker 09 Mar '12

09 Mar '12

Image you would create your own language with a paradigm similar to Haskell or have to chance to change Haskell without the need to keep any compatibility. What stuff would you add to your language, what stuff would you remove and what problems would you solve completely different? Thanks in advance for all answers, yours Robert Clausecker

39 91

The (!) operation
by Christopher Done 08 Mar '12

08 Mar '12

‘Ello. Is there a generalization of this operator? It's all over the place, it's basically (!) :: (Monad m, Indexed collection index value) => index -> container -> m value We have `(!!)` on lists, `(!)` on maps, vectors, json objects, … (doesn't seem there's one for bytestring) (Though I seem to recall the monadic return value being frowned upon but I don't recall why.) Thoughts? Ciao!

4 9

GHCi fails to load C++ object files (missing symbol)
by Yves Parès 08 Mar '12

08 Mar '12

Hi, I'm trying to have GHCi load a haskell file that depends on a C++ object file, which causes GHCi to fail because of an unknown symbol (*and I did link with **libstdc++*), whereas the link into an executable with ghc works and runs perfectly. I've reduced my code to the smallest one that produces the problem: The C++ part defines a mere class and C externals that enable communication with Haskell: // Header Stuff.hpp class Base { public: Base(); ~Base(); } extern "C" { Base* mkthing(); void delthing(Base*); } ----------- // Source Stuff.cpp #include <iostream> #include "Stuff.hpp" using namespace std; Base::Base() { cout << "Base created" << endl; } Base::~Base() { cout << "Base deleted" << endl; } extern "C" { Base* mkthing() { return new Base(); } void delthing(Base* b) { delete b; } } Haskell code (just for reference, but I'm not sure it's relevant), Main.hs: {-# LANGUAGE ForeignFunctionInterface #-} import Foreign import Foreign.C foreign import ccall unsafe "mkthing" mkthing :: IO (Ptr ()) foreign import ccall unsafe "delthing" delthing :: Ptr () -> IO () main = do p <- mkthing delthing p I then compile it with: g++ -c Stuff.cpp ghci Main.hs Stuff.o -lstdc++ And then the error is: Loading object (static) Stuff.o ... done Loading object (dynamic) /usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5.2/libstdc++.so ... done final link ... ghc: Stuff.o: unknown symbol `__dso_handle' linking extra libraries/objects failed Whereas 'ghc Main.hs Stuff.o -lstdc++' works just fine. Does GHCi lacks a link directive that regular GHC has?

3 5

Type classes for converting to Text and String
by Simon Hengel 08 Mar '12

08 Mar '12

Hi! When writing library code that should work with both String and Text I find my self repeatedly introducing classes like: class ToString a where toString :: a -> String class ToText a where toText :: a -> Text (I use this with newtype wrapped value types backed by Text or ByteString.) So I wonder whether it would be a good idea to have a package that provides those classes. Or maybe just ToText, and provide default implementations of toString and toText, like: class ToText a where toText :: a -> Text toText = Text.pack . toString toString :: a -> String toString = Text.unpack . toText How do you guys deal with that? Any thoughts? Cheers, Simon

4 9

Monad.Reader #20 call for copy
by Edward Z. Yang 08 Mar '12

08 Mar '12

I'm pleased to announce that I, Edward Z. Yang, will be taking over Brent Yorgey's role as lead editor of the Monad Reader! Call for Copy: The Monad.Reader - Issue 20 -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 20 will be: **Monday, March 5** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

1 1

Re: [Haskell-cafe] Summer of Code idea: Haskell Web Toolkit
by Greg Weber 07 Mar '12

07 Mar '12

This is obviously a great concept. However, it may not be appropriate for a GSoC. The design space is far too open, and it is not clear if anything in that space will end up beating plain old javascript. I think my proposal for an awesome websockets library [1] shows that this is putting the cart before the horse. For some of the potential javascript solutions, websockets is a dependency anyways. Without being able to hold open a connection to the server, required server interaction will be too slow. Yesod has been successful by explicitly avoiding any new radical paradigms, but by instead just trying to get the basics of web development established correctly. Without websockets, we don't have the basics down for fluid client-side interaction. The groundwork for this ticket has been laid: it should be easy to accomplish with time left over. So we are guaranteed a clear win for the community. With the time left over, a solution to the javascript problem can be attempted. But there is no burden for it to be solved within the summer. Next year the landscape will be more mature and we can try to come up with a solid proposal for the javscript problem if the community hasn't already created a solution. [1] http://hackage.haskell.org/trac/summer-of-code/ticket/1607

2 2

Summer of Code idea: Haskell Web Toolkit
by Dimitry Golubovsky 07 Mar '12

07 Mar '12

Hi, Alejandro Serrano Mena wrote: > My idea is to make a client-side Haskell Web Toolkit, in the spirit of > Google Web Toolkit, which would allow to program in Haskell the client part > of a web application, and would complement the web frameworks already > existing for Haskell (such as Yesod and Snap). http://www.haskell.org/haskellwiki/Haskell_web_toolkit - In memoriam ;) Feel free to use these ideas. It would be nice if you could pick it up and complete - I never did... -- Dimitry Golubovsky Anywhere on the Web

1 0

Re: [Haskell-cafe] [Haskell] Higher types in contexts
by Henning Thielemann 06 Mar '12

06 Mar '12

moved to haskell-cafe On Mon, 5 Mar 2012, Barney Hilken wrote: > Is there any deep reason why I can't write a polymorphic type in a context? I think the record update problem can be (sort of) solved if you could write: > > class Has r Rev (forall a. [a] -> [a]) => HRClass r where > setHRClass :: (forall a.[a] -> [a]) -> r -> r > > but polymorphic types are not allowed in contexts. Is this one of the problems SPJ considers "Hard" or is it a feasible extension? I don't know what you want to do, but you may wrap the (forall a. [a] -> [a]) in an existential type: data ListFunc = forall a. ListFunc ([a] -> [a]) class Has r Rev ListFunc => HRClass r where setHRClass :: ListFunc -> r -> r

4 3

Re: [Haskell-cafe] Adding type annotations to an AST?
by oleg＠okmij.org 06 Mar '12

06 Mar '12

> How do I add type annotations to interior locations in an abstract > syntax tree?.... > i.e. the type it [algorithm] infers is the type of the whole > program, I would also like the types of any internal let-rec > definitions so I can label my AST. I had exactly the same problem: type reconstruction and the annotation of all sub-terms with their inferred types. Even if the overall type inference fails, the user can still see what the type checker was able to infer before the error. Here is the solution http://okmij.org/ftp/Computation/FLOLAC/TEvalNR.hs There is a bit of the explanation here: http://okmij.org/ftp/Computation/FLOLAC/lecture.pdf

2 1

Double-dispatch
by Clark Gaebel 06 Mar '12

06 Mar '12

Is there any way in Haskell to have the correct function selected based on the types of two different types? For example, let's say I'm writing intersection tests: aABBandAABB :: AABB -> AABB -> Bool oBBandOBB :: OBB -> OBB -> Bool oBBandPoint :: OBB -> Point -> Bool Is there some way (such as with Type Families) that I can write some sort of generic method for this: intersects :: Intersectable -> Intersectable -> Bool which automatically selects the right function to call based on the types of the two intersectables? Regards, - clark

3 4

Conduit Sink fork
by t helfferich 05 Mar '12

05 Mar '12

Hi! So, it turns out I have a need for a sink that forks input into two other sinks using the Conduit package. Here is what I came up with: https://gist.github.com/1975383 Is this the right way to write it? Also, what left over value(s) should I return in the Done constructor? I appreciate any help you can give me. Thanks so much,Grant

2 6

Reasons for Super-Linear Speedup
by burak ekici 05 Mar '12

05 Mar '12

Dear List; I have parallelized RSA decryption and encryption schemes by using second generation strategies of Haskell p.l. In that case, what I got in the sense of speed up was nearly 10 times of better performances (on a quad-core CPU with 8M cache) in the parallel evaluation of 125K long plain text with 180-bit of an encryption key, in comparison with the serial evaluation, abnormally. All my thoughts are in the direction of getting wrong serial-time performances. My question here is that is there any difference between serial and parallel evaluation of any arbitrary computation in terms of "cache usage methodologies" in Haskell compiler design. If no; what could be other possible reasons? I would be much more than appreciated, if somebody brings an acceptable pinpoint for the issue. Many thanks in advance. Kind regards Burak.

2 1

[haskell-cafe] Question about 64bit target on Windows platform
by C K Kashyap 05 Mar '12

05 Mar '12

Hi All, Can someone please let me know if there is a 64bit target on Windows on the horizon for GHC? I am trying to push for changing the current implementation in my organization in C++ to Haskell - Our primary targets are Windows and Mac. Not being able to generate 64bit DLL's on Windows would be a big bottleneck. Regards, Kashyap

3 4

Looking for an heap-like data structure allowing efficient update
by Arnaud Bailly 05 Mar '12

05 Mar '12

Hello Cafe, I am looking for a data structure that would have the following informal properties: - allow efficient retrieval of minimum element for some ordering relation on a computed property of the elements - allow efficient update of any element wrt to this property I have first tried to use a PQueue from fingertree package but it does not allow deletion/update of arbitrary elements. I then started to try combining this with a map but ran into the same issue. I skimmed through Okasaki's book (oldies but goldies...) and the basic finger tree structure but these does not seem to fit my needs. Thanks for your help Regards, Arnaud Bailly

2 2

Re: [Haskell-cafe] FreeSect -- generalised sections syntax extension
by Ras Far 05 Mar '12

05 Mar '12

So we can have ... chiastic freesects? This sounds like too much fun! I find chiasmus is a term from linguistics? From [1]: "The elements of simple chiasmus are often labelled in the form ABBA, where the letters correspond to grammar, words, or meaning." So we've got love, too (Aramaic word “Abba” = love of God or so...) Also coincidental, since a lot of the literary instances are apparently found in sacred texts (although Coleridge comes up a lot too, cough). So many coincidences around this. I'd written up the idea for the Haskell extension in 2003 (calling them "arbitrary sections"), but only decided to implement them about a week ago. Had just gotten started about an hour earlier, when eyebloom on #haskell asks about the same thing, right down to the _ syntax. (I offered him collaboration but he didn't get back to me, hope you're not sore eyebloom, anyhow I don't mind you were maybe too busy.) Then I get hacking Language.Haskell.Exts and -- the most recent extension already added is "TupleSections"! And there's no other extension so related to this idea. Now the serendipity with Wren's linguistic research. And even the freegeek domain. Fantastic. Wren, your slides look great, thanks for the link, I think I'll read them tonight. Do you have a paper version of the slides at all? Slides can be a bit terse on their own. Yours Chiastically, Andrew [1] http://en.wikipedia.org/wiki/Chiastic_structure

3 3

Impact of "try" on Parsec performance
by Omari Norman 04 Mar '12

04 Mar '12

The Parsec documentation says that Parsec performs best on predictive grammars, and Parsec does not backtrack by default to improve performance (though this also improves error messages). On the other hand, I notice that attoparsec and polyparse backtrack by default, and attoparsec claims to be faster than Parsec (I can't remember if polyparse makes this claim). The Parsec documentation says there is a penalty for using try and suggests left-factoring grammars when possible. Real World Haskell says that excessive use of try can degrade Parsec performance quite significantly. On the other hand, some of the combinators provided with Parsec, such as notFollowedBy, use try. So my question is: what is the practical impact of using try? I ask because as a novice I have a simple grammar that could, perhaps, be factored to avoid use of try, but it's quite a brain puzzle for me and I wonder if it would ever be worth it. Does it matter how many characters "try" would have to store, or how often it would run?

7 7

IO() and other datatypes
by Kevin Clees 04 Mar '12

04 Mar '12

Dear Haskell programmers, I'm very confused, because I really don't know how to handle with IO's and other datatypes, such as Int or String. If I want to build a haskell program can I only use IO() method outputs ? How can I "give" a Int result from a different method back to the main? Would this mean that I have to create only IO() outputs? Is this correct? For example: -- A User has to choose something, so I need a IO() datatype main :: IO() main = do [...] let listtournementTime = [8,20,10,15] -- This is wrong: Couldn't match expected type `IO t0' with actual type `Int' -- The results of the method dauer is a Int. Do I have to transform the method to an IO() Output a <- Dauer listtournementTime [...] -- the methods dauer:: [Int] -> Int dauer (x:xs) | laenge(x:xs) == 1 = 0 | mod (laenge (x:xs)) 2 == 0 = (tmp x xs) + dauer xs | mod (laenge (x:xs)) 2 /= 0 = dauer xs | otherwise = 999999 -- failure tmp:: Int -> [Int] -> Int tmp y (x:xs) = x-y laenge :: [a] -> Integer laenge [] = 0 laenge (x:xs) = 1 + laenge xs The last three methods are working correct, if I directly put some data into the methods, like: dauer [10,15] ===Result===> 5 Thank you for any help Best greetings from Namibia

2 1

ANNOUNCE: Haddock 2.10.0
by David Waern 04 Mar '12

04 Mar '12

-------------------------------------------- -- Haddock 2.10.0 -------------------------------------------- Version 2.10.0, which was released with GHC 7.4.1, is now uploaded on Hackage. Here are the release notes: -------------------------------------------- -- Changes in 2.10.0 -------------------------------------------- * Require GHC >= 7.4 * Safe Haskell indications on module pages * Type declarations on identifiers no longer necessary * Add flag --interface-version * Warn when comment refers to in-scope identifier without documentation * Bug fix: links to out-of-scope things (#78) * Bug fix: module references to other packages work again -------------------------------------------- -- Links -------------------------------------------- Hackage page: http://hackage.haskell.org/package/haddock-2.10.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://darcs.haskell.org/haddock.git -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Ian Lynagh Jose Pedro Magalhaes Simon Peyton-Jones Ryan Newton David Terei David Waern Also thanks to Simon Hengel for spotting a serious bug before the release. -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://darcs.haskell.org/haddock.git Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

2 4

ANN: 11th GhentFPG Meeting Tuesday, 20th of March: Final Program
by Jeroen Janssen 04 Mar '12

04 Mar '12

(apologies if you receive multiple postings) Dear all, The program and *new location* for the 11th GhentFPG meeting has been determined. The meeting will take place on Tuesday, the 20th of March at 19:30 in auditorium V1 of building S9 at Campus De Sterre (Krijgslaan 281, 9000 Gent). We will post signs at building S9 to guide you towards the auditorium. The program is as follows: 1. Jasper Van der Jeugt: digestive-functors 0.3 Formlets are a way of building HTML forms that are type-safe, handle errors, abstract and are easy to combine into bigger forms. I present the digestive-functors library, a formlets implementation which solves a number of issues with the original formlets library, such as annotation of errors, labelling, and separation of the validation model from the actual HTML output. 2. Jeroen Janssen: the typeclassopedia. This is part of a new type of talks we are going to try with GhentFPG. Each meeting we would like to introduce you to some of the important papers in the world of FP. This meeting I am going to introduce you to the brilliant "Typeclassopedia" paper. This paper, which originally appeared in the monad reader issue 13, introduces many of the important typeclasses in the Haskell standard library and explains how they are interrelated, both in a formal fashion as in a practical fashion. Hope to see you all there! The GhentFPG Organizing Committee.

1 0

Problems installing Data.Encoding package
by Pēteris Paikens 04 Mar '12

04 Mar '12

Hello, I'm stuck at trying to get Data.Encoding package functional. Cabal gives the following error message: cabal install encoding Resolving dependencies... /var/folders/xc/jnh69_cx1n32n1qwkgfrlc100000gq/T/encoding-0.6.620435/encoding-0.6.6/dist/setup/setup: /var/folders/xc/jnh69_cx1n32n1qwkgfrlc100000gq/T/encoding-0.6.620435/encoding-0.6.6/dist/setup/setup: cannot execute binary file cabal: Error: some packages failed to install: encoding-0.6.6 failed during the configure step. The exception was: ExitFailure 126 What could be done here? I'm on MacOS Lion, ghc v. 7.0.4 Regards, Peteris

3 2

anyone know what happened to nlpwp?
by serialhex 04 Mar '12

04 Mar '12

i was going through nlpwp.org learning haskell & natural language processing at the same time, and a few days ago it just disappeared! does anybody know what happened to it? i can still access the google cache, and learn from there, but that'd be a pain for anybody new coming along who can't access this site... :( hex -- * my blog is cooler than yours: http://serialhex.github.com * The wise man said: "Never argue with an idiot. They bring you down to their level and beat you with experience." * As a programmer, it is your job to put yourself out of business. What you do today can be automated tomorrow. ~Doug McIlroy No snowflake in an avalanche ever feels responsible. --- CFO: “What happens if we train people and they leave?” CTO: “What if we don’t and they stay?”

2 1

FreeSect -- generalised sections syntax extension
by Ras Far 04 Mar '12

04 Mar '12

Hello, I bit premature perhaps but I wanted to post it on a leap day... http://fremissant.net/freesect Thanks for eyebloom on #haskell for motivating me to finally implement an old idea. Thanks to the rest on #haskell for doing their best to talk me out of it. ;) I make no claims regarding the usefulness of the extension, but some folks might find it interesting, or may just appreciate additional examples of using HSE and SYB. I regret that I am not a better Haskell coder, but it is what it is! Kind Reg'ds, Andrew Seniuk (rasfar)

4 6

External Interfaces
by Victor Miller 03 Mar '12

03 Mar '12

I'd like to write reasonable Haskell interfaces to a few external libraries. To be specific -- all of them do limited calculations (no I/O) but each has an internal state. I know that I can have results dump everything into the IO monad, but I'd like to avoid the "sin bin" (I think it was Phil Wadler who called it that). To be specific, I have in mind the library pari ( http://en.wikipedia.org/wiki/PARI/GP ) which does symbolic algebra calculations. The way that the pari state works is that state of works is that we can add definitions of new quantities, and bind them to a calculated value. As part of the state there might be certain cached calculations. If I avoid deleting bound objects, it seems to me that it would be perfectly valid to make the interface into a monad have a complicated state which is being managed by the pari library. As long as any queries from pari give the same value (which should be the case if I never delete anything) then it should be valid without dumping things into the IO monad. Does anyone see anything wrong with my reasoning? Victor

2 1

ghc: could not execute: opt
by Graham Berks 03 Mar '12

03 Mar '12

Hi, trying out haskell. Have osx 10.7 and ghc 7.0.4 via homebrew When attempting to compile a program via cabal i get ghc: could not execute: opt I assume I have something missing / not configured with my env. Can someone point me in the direction ? Thanks

5 4

Records in Haskell: Type-Indexed Records (another proposal)
by Matthew Farkas-Dyck 03 Mar '12

03 Mar '12

Hello all. I wrote a new proposal for the Haskell record system. It can be found at http://hackage.haskell.org/trac/ghc/wiki/Records/TypeIndexedRecords Records are indexed by arbitrary Haskell types. Scope is controlled as scope of key types. No fieldLabel declarations needed (as in DORF). Cheers, strake

1 0

Does somebody know about these functions?
by Johan Holmquist 03 Mar '12

03 Mar '12

Two functions that I see useful are described here and I would like to know if they are defined in some more or less standard Haskell library. Hoogle (http://www.haskell.org/hoogle) did not reveal anything about that. Function 'inter' applies given function to each succeeding pair of elements of a list. inter :: (a -> a -> b) -> [a] -> [b] inter f [] = [] inter f l = map (uncurry f) $ zip l (tail l) Example usage: and $ inter (<=) l -- checks if 'l' is ordered inter (,) l -- gives succeeding pairs Function 'withPair' takes a pair and applies a function to it's first element, another function to it's second element and finally combines the results with yet another function. withPair :: (a' -> b' -> c) -> (a -> a') -> (b -> b') -> (a,b) -> c withPair f fa fb (a,b) = fa a `f` fb b Example usage: words [] = [] words s = withPair (:) id words (break isSpace $ dropWhile isSpace s) lines [] = [] lines s = withPair (:) id lines (break (== '\n') s) mapPair = withPair (,) This function can abstract away the (in my opinion ugly) pattern (as seen in the examples): foo list = let (a,b) = <somefun> list in <somefun2> a `<combinedWith>` foo b Anyone knows about these two functions or variants thereof? Cheers /Johan

6 10

Parallel Haskell Digest 8
by Eric Kow 02 Mar '12

02 Mar '12

Parallel Haskell Digest 8 ========================= 2012-03-01 Hello Haskellers! It's time for our next catch-up with the Parallel Haskell community. Did you have a chance to see Simon Peyton Jones's talk [The Future is Parallel, and the Future of Parallel is Declarative][v2]? It's a good survey of some of the directions that parallel Haskell has been taking, and if you're somewhat new to this stuff, a great feel for the breadth of the community. You'll get a better idea why you see people in this digest muttering about arrays, others trying to make channels and transactions work together, and yet others talking up the 0MQ protocol. So check it out! We at Well-Typed are writing the digest as part of our mission to push Haskell parallelism and concurrency out into the real world. We are excited about these technologies and we want to do whatever it takes to make it more accessible to everybody. More news below on how we're doing in this Parallel GHC project. HTML version of this digest at http://www.well-typed.com/blog/64 News ---------------------------------------------------------------------- * [GHC 7.4.1 released][n1] Of particular interest to parallel Haskellers: better profiling flags, multicore profiling, vastly improved DPH, event logging, more convenient RTS flags (no more `-rtsopts` needed for common options like `-N`). See that new event logging stuff? Could be a good chance to play with the new spark profiling features from [ThreadScope][ts-tour]. * [Summer School on Functional Programming for Parallel and Concurrent Applications][n2] (2 weeks) Imagine this: two summer weeks in the Southern France on the Côte d'Azur, learning about Parallel Haskell. This would be the CEA-EDF-INRIA Summer School on Functional Programming for Parallel and Concurrent Applications. It takes place on 11-22 June 2012, Castle of Cadarache, Saint Paul Lez Durance, France. No functional programming experience needed. The aim of the summer school is to give a thorough and application-oriented introduction to functional programming using the programming language Haskell. A special focus is on parallel and concurrent programming, highlighting the ways in which features such as strong typing and purity make it dramatically easier to write reliable parallel or concurrent code. The school is split into three different courses that highlight different aspects of functional programming. All courses consist of lectures and hands-on sessions where everyone can try out the language on several exercises. Well-Typed's Andres Löh will be there, as well as Ralf Hinze, and Simon Marlow. The registration deadline is on 30 May, but I wouldn't procrastinate if I were you; the course sounds really tempting! Jobs ---- * [PhD program at Portland State accepting applications for Fall 2012][n3] (18 Jan) There's quite a bit of functional programming research at PSU, and one of the professors (Andrew Black) is a co-author on the Cloud Haskell paper! * [6 Funded Research Studentships at St Andrews][n4] The School of Computer Science at the University of St Andrews has six prize PhD studentships available. These studentships are funded through the Scottish Informatics and Computer Science Alliance and provide both fees and maintenance (£13,500) per annum. They can be taken up by students of any nationality (including non-EU students). The deadline is *today* (1 March), so if you're interesting, it might not be too late to get in touch with [Kevin Hammond](mailto:kh@cs.st-andrews.ac.uk). * [Computer Science Lecturer Job at Kent][n5] The School of Computing at the University of Kent is seeking to appoint a high flying academic who will spend the first 3 years in post developing as an independent researcher of internationally excellent quality in an area relevant to the School of Computing. This does not look Parallel Haskell specific, or even FP specific, but Simon Thompson did tweet that “the functional programmers at Kent would be particularly keen to recruit someone who would work with them!“ Parallel GHC Project Update ---------------------------------------------------------------------- ThreadScope 0.2.1 has been released! This version contains the features we had demonstrated at the Haskell Implementor's Workshop in September 2011. Since our workshop prototype, we have greatly refined the spark histogram feature, tuning the visualisations so that they are easier to understand. We've also written a small tutorial to go with the new release. The [ThreadScope Tour][ts-tour] works through concrete examples on using ThreadScope to debug the performance of parallel programs. We'd love feedback you have about the tutorial, especially things you feel like you need a little more help with. Along with the new ThreadScope and tutorial, we also have a new version of the ghc-events package which now provides an encoding of the meanings of events in state machines. This makes it possible to validate eventlogs, and doubles as an always up-to-date source of code as documentation. We've made some progress on our work in developing a swappable transport layer for Cloud Haskell. We now have a prototype implementation “distributed-process” (intended to be the sucessor to “remote”, the current Clound Haskell implementation). For more details, see the [distributed-process GitHub page][dp-github], particularly the examples and the [design document][dp-design], which incorporates feedback on our initial proposal. Finally a bit of partner news to wrap things up: * Tim (LANL) has been fine-tuning and profiling their physics simulation tool. One change in particular has been to switch to a “counter-based” RNG scheme which lends itself nicely to use in parallel or distributed settings. They are also currently having a look at Haskell-MPI and Cloud Haskell. * Finlay (Dragonfly) is working on a tricky performance problem, trying to speed up a program with multiple relatively small computations and relatively many synchronization points in between. * Toni (Telefonica) is benchmarking the maximal clique enumeration algorithm on a large number of randomly generated graphs. He is getting good speedups for up to 6 cores, with some stagnation around 8 cores. Current research is either finding a graph traversal strategy that works reasonably well with all graphs, or have one which somehow adapts to the properties of the graph. * Kazu (IIJ) has made several new releases of his libraries, and bug-fixes for a lot of third party libraries. He has also been participating in the discussion that lead to the development of Michael Snoyman's conduit library, and switched Mighttpd over to good effect. * Mark (VETT) is learning how to use Cloud Haskell. He is currently writing some example programs to test his understanding. Hopefully more from Mark when he's had more time to play with this Word of the month ---------------------------------------------------------------------- Over the next few digests, we'll be switching our focus from parallelism to concurrency. We tend to stress the distinction because Haskell offers ways to write parallel programs without making explicit use of concurrency. Parallelism done right gets us faster programs. Concurrency on the other hand buys us… concurrency. It's not going away. If every multicore computer in existence were to vanish, we would want to solve concurrent problems. Whether the simultaneity is real or simulated, we would still want to do more than one thing at the same time – accept user input, display progress messages, serve multiple clients. So let's dig in! We first got a taste of concurrency in second Parallel Haskell digest, where we introduced the notion of [threads][ph2]. As an abstraction, threads give us a way to express the separation of concerns between different jobs. But this isn't enough. Sometimes we need to undo the separation just enough to pass information from one thread to another. This brings us to our word of the month: `MVar`. The humble `MVar` (pronounced “em-var”) is one of many solutions for this communication problem, a fairly low-level one by Haskell standards, but one that is still useful enough that you'll see it used very frequently. An `MVar` is like a burri… wait, wrong tutorial. Actually, it is helpful to think of an `MVar` as a box in the sense that it holds values and can either be full or empty. The `MVar` type takes a type variable, so an `MVar Int` might hold an integer , an `MVar String` a String, an `MVar [String]` a list of strings and so on. -- Control.Concurrent.MVar data MVar a newEmptyMVar :: IO (MVar a) takeMVar :: MVar a -> IO a putMVar :: MVar a -> a -> IO () To give an idea how this might be used, below is a small program that fetches some URL in one thread while doing something else in the other. We fork off a Haskell thread that does the fetching and write to `MVar` to indicate what we've retrieved. In the main thread, we do our other work and then just wait until the page has been fetched. main = do m <- newEmptyMVar forkIO $ do r <- getURL "http://en.wikipedia.org/wiki/Shovel" putMVar m r doSomethingElse r <- takeMVar m putStr r These `MVar`'s may look a little familiar if you've used `IORef`s in Haskell. Here is a mini API for comparison: -- Data.IORef data IORef a newIORef :: IO (IORef a) readIORef :: IORef a -> IO a writeIORef :: IORef a -> a -> IO () So what exactly do `MVar`'s buy us? Why not just use `IORef`s to share mutable variable across threads? The reason is that coordination between threads can get messy: we want to make sure that any value we pass from one thread to another is accounted for (and not accidentally overwritten before being consumed), that we don't try to consume values that are somehow out of date with respect to other threads (that updated values are received instead of an old value being read twice). Suppose we wanted to fetch a URL while doing something else at the same time. How do we know when we have successfully retrieved it? -- don't write this at home! inadvisableMain = do m <- newIORef "" -- default value? :-( forkIO $ do r <- getURL "http://en.wikipedia.org/wiki/Shovel" writeIORef m r -- are we overwriting something? :-( doSomethingElse r <- readIORef m -- is there something to read? :-( putStr r In the example above, we have no idea if the page at URL would have been fetched by the time we try to display its contents. What we are looking for is a *synchronisation* mechanism. We need a way to indicate that our shared values are ready to be used. For example, we might hit upon the idea of combining `IORef` with `Maybe`. Now we have a some extra bureaucracy to handle. If we read a value and get `Nothing` we would then know that there isn't yet a value ready to be read. One way or another we would have to account for this case, for example busy waiting until we get a `Just`. On the flip side, we want to make sure that when somebody has written a value, the intended recipient has read it before we write over it. This sort of bureaucracy would be best packaged up into helper functions, functions that look awfully like `takeMVar` and `putMVar` might. Notice the change in name even. Now we're not just reading, but *taking*, emptying the box to signal that it's been read; and we're not just writing, but *putting*, only writing when the box is empty. Throw in a little help from the runtime system so that we're doing something smarter than busy waiting and we'll have worked our way back to the `MVar`. So the `MVar` combines references with locking to provide for synchronisation between threads. If you're coming from other languages, this should sound rather familiar. C programmers may have used mutexes (`MVar ()`) or semaphores (`MVar Int`) to protect shared data. Java programmers may have used `synchronized` methods and statements to prevent thread interference and memory inconsistency problems. The `MVar` is just a slightly nicer Haskell packaging to the same sort of idea. This means it suffers the same problems as its locky sisters. Sure, having the locks be implicit and putting them where they count (the data being shared) makes life a bit simpler, but at the end of the day locks are still locks. Take them in the wrong order, and you can still get deadlocks, races and all those subtle hard-to-reproduce bugs that keep programmers up at night. So what is the hapless foot-shooting programmer to do? The good news is that `MVar`'s are only one of several mechanisms for dealing with concurrency. Safer mechanisms exist, albeit at a cost. `MVar`'s present a compromise between performance and safety. If you are extra extra careful, you can on the one hand squeeze some serious performance out of `atomicallyModifyIORef` for concurrent data structures. If on the other hand, if you're willing to take a potential performance penalty in exchange for never worrying about a deadlock again, stay tuned for our next word of the month, “[transaction][stm]“. For more about `MVar`'s in the meantime, have a look at Edward Z. Yang's blog for an [MVar overview][mvar-ezyang] as well as the updated [API documentation][mvar-api], and finally the [concurrency chapter][mvar-rwh] from Real World Haskell. Videos ---------------------------------------------------------------------- * [It's Raining Haskell][v1] (30 minutes) Charles from MSDN's Channel 9 had a chance to catch Simon Peyton Jones and John Hughes for a chat at the YOW conference in Sydney. As John observed during the interview, “Haskell is picking up more and more real users now, and that's very pleasing, but one of the reasons is because it offers a way of getting to grips with parallelism without suffering all the difficulties that you do with imperative programming languages, so I think that's a big opportunity for Haskell. I'd hope it won't just be a source of ideas for other languages, but that it will actually see a lot of real use itself.” * [The Future is Parallel, and the Future of Parallel is Declarative][v2] (1 hour) Simon wasn't at YOW just to give interviews on park benches. During his presentation, Simon gives us a sense of what a parallel future might look like. He skips past the usual FP-for-multicore propaganda, jumping straight to the prediction that “in the end, *something* declarative will win”. The bad news is that “there is no parallelism without tears”. If we want parallelism, we need to develop parallel algorithms, and understand the cost models behind the approaches we use. But one cost model does not fit all problems, so “no silver bullet; embrace diversity”. Simon then presents some of the different paradigms on offer, using Haskell as a single linguistic framework, but not specifically pushing the language. He covers task parallelism (threads, STM, Cloud Haskell), semi-implicit algorithm (`par` and friends) and data parallelism (Repa, DPH). It's worth knowing that the YOW conference is “for developers by developers” and Simon has given the talk for a non-Haskell audience. So if you really just want an idea what all the parallel fuss is about, give his talk a try! * [Real-time ray tracing demo with Repa ][v3] As posted by Ben Lippmeier: Real-time ray tracing demo written in Haskell with a pre-release of the Gloss-Field and Repa 3.0.0 parallel array libraries. Demo is running in real time in parallel on a four core machine with two hardware threads per core. * [Barnes-Hut Gravity Simulation][v4] More demos? OK! How about this gravity simulation using Gloss? A naive n-body simulation has quadratic runtime complexity, O(n^2); but with the Barnes-Hut algorithm, we can approximate one with only an O(n log n) runtime. These simulations lend themselves well to parallelism; however, at present (2012-02), the sequential version of this program (using Data.Vector) still runs faster than its parallel equivalent (using DPH). The good news is after working on DPH vectorisation for 6 months, the asymptotic complexity of the DPH version now matches that of the sequential one. Hooray! Well OK, so there's still pesky constant factor involved. The sequential version is still 10 times faster, but if Ben can improve DPH enough to make the parallel version faster by August, he's buying everybody beer. See [his blog post][b4] for more details, as well as [Manuel Chakravarty's slides][b5] on vectorisation and the shared data problem. Blogs and packages ---------------------------------------------------------------------- * [Making OpenCL™ Simple with Haskell][b1] (Jan 2011) Benedict R. Gaster published a set of slides (53 slides) presenting the results of his collaboration with Garrett Morris (AMD intenern and PSU PhD student). “It's a heterogeneous world” out there, with a modern platform including some CPUs, GPUs, and more. OpenCL is a framework for writing parallel programs on this mixture of devices. Benedict's slides introduce OpenCL, and talk about the low-level Haskell FFI to OpenCL C, and the use of monads and Template Haskell quasiquotation so that we can write something higher-level and more Haskelly. See the slides also for some comments about Haskell at AMD. * [Propane: Functional synthesis of images and animations in Haskell][b2] Following up on his quasicrystal code, Keegan McAllister has released Propane, a library for functional synthesis of images and animations. > Propane uses Repa for data-parallel array computations. That means > it automatically uses multiple CPU cores for rendering, provided > the program is compiled and run with threads enabled. That said, > it's not yet been optimized for speed in other ways. * [Black-Box Reverse Engineering ZMQ][b3] [ØMQ (ZeroMQ)][zmq] and the ZMTP protocol provide a high performance messaging layer for concurrent/distributed applications. Unfortunately, for Xavier Lange, “the ZMQ spec is not fully cooked”. This he discovered when trying to use libzmq and find tha the data sent by it doesn't quite match the specification. So what's a ZMQ implementor to do? In this blog post, Xavier inserts a packet sniffer between his Haskell code and a pure C libzmq test app and teases apart the “real” protocol as implemented by the library. He'll be studying the (LGPL) code next to see if he got the right idea from his black box debugging efforts. * [stm-channelize 0.1.1][stm-channelize] (20 Jan) This package arose from Joey Adam's [frustrations with asynchronous IO in Haskell][m5] for a networking layer he was writing. The package turn I/O operations into STM transactions on channels. * [stm-conduit-0.2.1][m14] (9 Feb) Clark Gaebel invites us to check out his stm-conduit package, which “introduces conduits to the wonderful world of concurrency“: > My package solves the common problem of constant bottleneck > switching loaders have. This is when, for example, we stream XML > from the disk and then parse the XML in one conduit pipeline. While > it streams a file from the disk, the process is clearly IO bound, > and while it parses the XML, the process is CPU bound. By putting > each task on its own thread, the disk IO doesn't need to wait for > the CPU to parse a document before loading the next file. By using > stm-based conduits, we have full resource utilization. Mailing lists ---------------------------------------------------------------------- ### Parallelism * *Help wanted!* [Optimizations and parallel execution in the IO for a small spellchecker][m17] (15 Feb) Paul Sujkov wrote a simple [spellchecker][suj-spell] that checks for words that aren't in a dictionary file. He's used it to benchmark the various Haskell hash table implementations on hackage, and now wants to see if he can take things a step further with parallelisation. He's tried using monad-par and the parallel packages both, but seems to have performance trouble due to the IO part of the problem. Any tips for Paul? * [SMP parallelism increasing GC time dramatically][m1] (9 Jan) Mikolaj Konarski followed up on a discussion started by Tom Thorne a while ago. Tom was nagged (in stereo!) to ThreadScope his performance problems, which he would have done had ThreadScope not crashed. Ben Gamari thinks he may have experienced something similar. What about you? Has ThreadScope ever crashed on you? * Data Parallel Haskell on MacOS X (and GHC 7.4) (15-21 Jan, 9 Feb) Mukesh Tiwari has been a series of problems getting dph to work on MacOS X Lion. He has gotten messages like “Couldn't figure out LLVM version” and “Error running clang!” and more recently, the fact that `-fdph-par` is no longer recognised by GHC. Some solutions (Brandon Allbery and Carter Schonwald) seem to be to learn that the LLVM that comes with your Mac is not a full distribution, and that you would need to install one (eg. `brew install llvm` for Homebrew users). As for GHC 7.4, the folks at UNSW have not had a chance to update the DPH wiki. In the meantime, try Ben Lippmeier's suggestion to use `-package dph-lifted-vseg` to select the backend, and also look at the dph-examples cabal file to see what flags to use when compiling. - [Installing dph-examples in Mac OS X Version 10.7.2][m6] - [Error in installing dph-examples on Mac OS X 10.7.3][m15] (9 Feb) - [Compiling dph package with ghc-7.4.0.20111219][m8] (21 Jan) * [Pruned Sparks in Original Strategies][m7] (17 Jan) Burak Ekici is trying to parallelise a sampling of programs to see how first and second generation Strategies compare. In his understanding, the garbage collection mechanism in this older version had an issue which prevented sparks from being pruned. Yet he finds sparks being pruned anyway with the original version. Any reasons why this might be the case? * [Speculative Parallelism][m10] (30 Jan) Burak was also thinking about the idea that second generation strategies can exploit speculative parallelism where the original could not. How can he convince himself of this? Does it indicate speculative parallelism when all and the same number of created sparks are converted by both strategies? * [Evaluating parallel computations in order of finishing (approximately)][m12] (6 Feb) Victor Miller wants a way to evaluate a list of computations in parallel, returning them in (roughly) the order of completion. Does something of that sort exist? It does! Ryan Newton added that this is a common non-deterministic merge operation used in stream-processing frameworks. You might have an IO action like `parCompletionOrder :: [a] -> IO [a]`, but one which returns immediately allowing the list to be lazily consumed. The function [Control.Concurrent.Chan.getChanContents][chan] should be helpful in implementing such a function. * [Merging two sorted list in parallel ( dph )][p2] (29 Jan) Mukesh Tiwari has two sorted lists of triples that he wants to merge in parallel. The triples have the form `(i,j,val)`, and are sorted by their `j` and `i` elements. If both lists have an element with the same `j`/`i`, we take the sum of their `val`. Mukesh has implemented half of a merging algorithm involving binary search, but he's stuck on updating his DPH arrays. Is there a function that does something like the below? updateWithFun :: (a -> a -> a) -> [:(Int,a):] -> [:a:] -> [:a:] * [repa nested sequential warning][p3] (27 Jan) Rick got some warnings in Repa about “performing nested parallel computation sequentially“ with the recommendation to use the `force` function. See the thread for some specific questions you might have about Repa and forcing arrays. The overall answer from Ben Lippmeier is to use Repa 3 if possible, which should resolve the problems Rick was facing and also provide some less confusing terminology (eg. renaming `force` to `computeP`) * [how to write this with Control.Concurrent.Strategies ?][p4] (24 Jan) Johannes Waldmann would like to be able to say to his students “mergesort already *is* an inherently parallel algorithm” and "you can execute it as such - you just need to annotate the source text". He has an implementation written in the `par/pseq` style but wants to know the “right” way to express it with Strategies. Patrick Maier posted an attempt which gets similar speedups to Johannes' original implementation. For comparison, Simon Marlow added a version using monad-par. Same problem, three techniques! This posting could be useful if you're just getting started learning about parallelism. Also, Chris Brown observed that merge sort is heavily sequential at the merge point and does not parallelise very well. But Johannes is only really looking for mild parallelism, enough to keep 4 to 8 cores busy. For people wanting to get a lot of paralellism out of merge sort, Chris Brown suggests a technique using balanced binary trees, and Ryan Newton a parallel divide-and-conquer (during merge phase), as used in the `cilkmerge` function in the Cilk sorting code. ### Concurrency * [STM: "nested atomically" error][m3] (12 Jan) Johan Brinch is getting a “Control.Concurrent.STM.atomically was nested” error and can't understand where it's coming from. There is some unsafe IO in there, but only in the form of debug messages. These innocuous-looking debugs may be the culprit, according to Andrew Coppin and Brandon Allberry. Their respective guesses are that (1) something to be evaluated that wouldn't be otherwise or (2) that something is being lazily evaluated as usual, but perhaps being forced within a transaction. Brandon adds in particular that the GHC IO system uses STM internally, which might explain nested transactions in the presence of unsafePerformIO. These sorts of shenanigans are normally prevented by the type system, but all bets are off once you go unsafe. * [STM atomic blocks in IO functions][m4] (13 Jan) Rob Stewart would like to know if he would still be guaranteed thread safety if he had some separate `atomically` transactions in an IO action, as opposed to one big block. Bryan O'Sullivan replied that “[i]f you want successive operations to see a consistent state, they must occur in the same atomically block.” This confused Ketil Malde. Isn't the idea for state to be consistent on entry/exit of each block, that breaking a program into multiple transactions is fine so long as each transaction is semantically complete unit? Steffen Schuldenzucker offered a clarification, that the “consistent state” Bryan was talking about is being sure that no other thread has modified a value during the span of the block. Rob had a follow-up question: is it possible to perform IO within an STM block? In a word, no. Daniel Waterworth explains that it would not make sense to allow this, because an STM transaction may be retried, which is typically not what you want to happen to your IO actions. If it's not possible to do with out, [TwilightSTM][twilight] or some sort of locking solution with a TMVar might be options. But usually you can factor the IO out so that it is not necessary to do within transactions. * [How to make asynchronous I/O composable and safe?][m5] (14 Jan) Joey Adams is “not happy with asynchronous I/O in Haskell. It's hard to reason about, and doesn't compose well”. He was grappling with several issues simultaneously (hopefully fodder for the next revision of Simon Marlow's [Concurrent Haskell tutorial][pcp-tutorial]). The setup is that Joey is writing a networking layer an application. Writing a simple client/server to test this layer turned out to be horrible on the server side (“an abomination”). The main problem seems to be that he is forced to use two threads per client (sending and receiving), because GHC does not provide a way to simultaneously wait for input and wait for an STM transaction to succeed. Issues that arise from this are * synchronizing the threads (e.g. when one dies, kill the other one) * multiple places where an exception can arise (which can lead to problems like exceptions interrupting atomic operations) * sharing a connection handle between multiple threads (you may recognise this problem if you've seen garbled/interleaved output from your concurrent code, eg. “AnOonteh esre nsteenntceen.c e.“) Some of the discussion that came up went over using channels to ensure the use of whole messages only (avoiding the issue with exceptions) and help “separate [the] application thread from the complexities of connection management“ (Bardur Arantsson, Daniel Waterworth); the observation that a pair of input/output channels is essential a stream processor arrow (Peter Simons); the possibility of using 0MQ to handle all the nasty details and get a simple message-based interface; some general thoughts on dealing with IO (Peter Simons thinks that one central IO loop plus callbacks is better than spreading IO all over programs); and the prospect of a wait-free concurrency model based on vats (David Barbour); and the avoidance of asynchronous exceptions except as a last resort (David, Joey). Joey eventually followed up by writing an [stm-channelize package][stm-channelize] that creates an STM layer over a network connection. * [Exceeding OS limits for simultaneous socket connections][m9] (30 Jan) Rob Stewart has a dozen machines sending messages to each other in a master/slave setup. The machines are running a Haskell program that sends thousands of messages (lazy bytestrings) to a master node in short succession. He sometimes gets an “accept: resource exhausted (Too many open files)” message because the master node exceeds its limit for simultaneous socket connections. Is there a good way to deal with this problem? Surely, it's something the Haskell web frameworks have had to contend with? Marc Weber suggests (A) to limit the number of simultaneous connections accepted by controlling the number of threads that can run simultaneously (see his mail for a sketch how), and (B) to use strict IO on the handle and close it himself, which Donn Cave points out will only reduce the size of the problem but not make it go away entirely. Best to stick with option (A). * [Memory usage with TVar?][m11] (1 Feb) Johan Brinch gets a surprising amount of memory usage from a tiny program that just cons'es a bunch of numbers onto a `TVar [(Int,Int)]`. Where is it all coming from? Jake MacArthur points us to a StackOverflow posting on the [memory footprint of Haskell data types][haskell-mem], apparently nothing STM specific so much as the result of having lots of cons cells, boxed Ints, and tuples. * [Concurrency strategy for 2 threads and rare events][m13] (8 Feb) JP Moresmau is looking for a good strategy to deal with a setup where he has one worker thread, and another user input thread ocassionally telling things to the worker thread. JP supposes he could just try a `tryTakeMVar`, but the use of `MVar`'s seems a bit heavyweight to him, as the second thread is just blocking for user input most of the time. What's a good setup? Erik Hesselink suggests maybe a `throwTo`. Yves Parès wondered what JP means by overkill; `MVar`s are both “teeny tiny” and convenient (they are!), perfect for the job. Edward Amsden suggested an `IORef` modified by `atomicModifyIORef`, cheaper than an `MVar` and usable when you don't have to lock multiple threads. * [How to increase performance using concurrency for sequential producer-consumer problem][m16] (13 Feb) Roel van Dijk sketches out a program for us: producer produces values and feeds them to a callback function (blocking until there are values to produce), converter does CPU-only work (and is the bottleneck of the program), consumer consumes values (important: in the order they were produced). A non-concurrent driver for these may look like `producer (consumer . converter)`. OK now what if Roel wants to make use of concurrency, ideally spawing off a worker thread for each core in his system? The important thing here is that the consumer continues to get values in the order they come out of the producer. Some suggestions: conduits and the newly released stm-conduit package (Clark Gamari), monad-coroutine (Mario Blažević), bounded STM channels (John Lato). John [points out][m16b] that while something like iteratee/conduit-stm would give you concurrency between the producer/converter/consumer, but would not directly help with the concurrency between converters. * [New distributed-process (Cloud Haskell) backend/transport design and prototype][p1] Duncan Coutts followed up on his design proposal for the Cloud Haskell transport design. As mentioned in the Parallel GHC news above, we've been working on doing some prototyping, and now have code for people to look at and play with. Check out the [GitHub page][dp-github] and [design document][dp-design] for distributed-process. * [Round-robin scheduling and forkIO][p5] (2 Feb) Andreas Voellmy wanted to check what happens when you `forkIO` a new thread, if it it executed in the same HEC as the original or in some new thread. He points to [Runtime support for multicore Haskell][rts-multicore], which says that threads within the run queue of a single HEC are scheduled in round-robin order; but does not talk about the threads as a whole. This can lead, Andreas suggests, to unfair scheduling of threads if you have the situation where an HEC has a single always-runnable thread, and another HEC has lots of threads (see his follow-up for example code illustrating the problem). Simon Marlow both that threads are executed in the current HEC and that the described situation can happen (“It wouldn't be too hard to do something more sensible here”). Simon suggests looking in rts/Schedule.c, particularly the function `schedulePushWork ()`. * [Synchronizations in memory allocation?][p6] (11 Feb) Andreas has a program with several Haskell threads each reading a different TCP sockets. He's investigating a problem where the performance of his program starts to degrade after 20 cores. Andreas suspects a scaling problem in allocating memory and has written a micro-benchmark to explore the issue. The program forks some worker threads that just allocated some memory without trying to synchronise or share any data. He is surprised to find that the benchmark takes longer using `mallocPlainForeignPtrBytes` the more threads he forks. The benchmark makes for a good stress test — it allocates over 10 Gb/s — and as result of Andreas' investigations, Simon Marlow has modified the pinned memory allocator to “steal complete blocks from the nursery rather than allocating them from the global block allocator.” The changes will show up in GHC 7.4.2. 20 cores, eh? [There's no pleasing some people.][no-pleasing] StackOverflow and Reddit ---------------------------------------------------------------------- * [Installing dph-examples in Mac OS X 10.7.2][s1] * [Programming language for functional parallelism: F# vs Haskell][s2] * [Parallel IO Causes Random Text Output in Terminal][s3] * [Repa Without Parallelization][s4] * [what exactly is dynamic parallelism?][s5] * [Writing “fib” to run in parallel: -N2 is slower?][s6] * [How to use Parallel Strategies in Haskell][s7] * [Parallel graphics processing in Haskell][s8] * [Why is concurrent haskell non deterministic while parallel haskell primitives (par and pseq) deterministic?][s9] * [Runtime performance degradation for C FFI Callback when pthreads are enabled][s10] * [Memory footprint in threaded mode with forkIO when there is lot of contention for single MVar][s11] Help and Feedback ---------------------------------------------------------------------- If you'd like to make an announcement in the next Haskell Parallel Digest, then get in touch with me, Eric Kow, at <parallel(a)well-typed.com>. Please feel free to leave any comments and feedback! [chan]: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurre… [dp-design]: https://github.com/haskell-distributed/distributed-process/wiki/New-backend… [dp-github]: https://github.com/haskell-distributed/distributed-process [dph-ghci]: http://warmfuzzything.posterous.com/loading-dph-codes-in-ghci [haskell-mem]: http://stackoverflow.com/questions/3254758/memory-footprint-of-haskell-data… [mvar-api]: http://www.haskell.org/ghc/docs/latest/html/libraries/base-4.5.0.0/Control-… [mvar-ezyang]: http://blog.ezyang.com/2011/02/all-about-mvars/ [mvar-rwh]: http://book.realworldhaskell.org/read/concurrent-and-multicore-programming.… [no-pleasing]: https://plus.google.com/107890464054636586545/posts/VFaU1CNj1z4 [pcp-tutorial]: http://community.haskell.org/~simonmar/par-tutorial.pdf [ph2]: http://www.well-typed.com/blog/53 [pp-frp]: http://conal.net/papers/push-pull-frp/ [rts-multicore]: http://community.haskell.org/~simonmar/papers/multicore-ghc.pdf [stm-channelize]: http://hackage.haskell.org/package/stm-channelize [stm]: http://www.haskell.org/haskellwiki/STM [suj-spell]: http://hpaste.org/63732 [ts-tour]: http://haskell.org/haskellwiki/ThreadScope_Tour [ts-tour]: http://www.haskell.org/haskellwiki/ThreadScope_Tour [twilight]: http://proglang.informatik.uni-freiburg.de/projects/twilight/ [zmq]: http://www.zeromq.org/ [n1]: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/release-7-4-1.html [n2]: http://www.haskell.org/pipermail/haskell/2012-February/023181.html [n3]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098604.html [n4]: http://blogs.cs.st-andrews.ac.uk/csblog/2011/11/17/funded-research-students… [n5]: http://www11.i-grasp.com/fe/tpl_kent01.asp?newms=jj&id=36123&newlang=1 [v1]: http://channel9.msdn.com/Blogs/Charles/YOW-2011-Simon-Peyton-Jones-and-John… [v2]: http://yow.eventer.com/events/1004/talks/1055 [v3]: https://www.youtube.com/watch?v=jBd9c1gAqWs [v4]: http://youtu.be/tDgOBM29ny4 [b1]: http://developer.amd.com/zones/OpenCLZone/publications/assets/MakingOpenCLS… [b2]: http://mainisusuallyafunction.blogspot.com/2011/12/propane-functional-synth… [b3]: http://xrl.tureus.com/black-box-reverse-engineering-zmq [b4]: http://disciple-devel.blogspot.com/2012/02/vectorisation-without-replicatio… [b5]: http://tumblr.justtesting.org/post/16916036670/the-n-body-problem-and-vecto… [m14]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099298.html [m1]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098390.html [m2]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098428.html [m3]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098453.html [m4]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098482.html [m5]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098495.html [m6]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098524.html [m7]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098570.html [m8]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098705.html [m9]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098969.html [m10]: http://www.haskell.org/pipermail/haskell-cafe/2012-January/098986.html [m11]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099035.html [m12]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099185.html [m13]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099264.html [m15]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099302.html [m16]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099378.html [m16b]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099440.html [m17]: http://www.haskell.org/pipermail/haskell-cafe/2012-February/099472.html [p1]: https://groups.google.com/d/msg/parallel-haskell/TJ4PeVoaato/th5pP4sUyjEJ [p2]: https://groups.google.com/d/msg/parallel-haskell/WkmOAgUAJIE/Wsr7fu7Sm7IJ [p3]: https://groups.google.com/d/msg/parallel-haskell/Na5-B-9XgZE/bdDVryd7R1YJ [p4]: https://groups.google.com/d/msg/parallel-haskell/0Ql1wBNMbI8/5h_sQCc1AA4J [p5]: https://groups.google.com/d/msg/parallel-haskell/rXiu-4E6_AA/BBPOvnej3ywJ [p6]: https://groups.google.com/d/msg/parallel-haskell/fsHrxz3ei70/NUpu21JQolgJ [s1]: http://stackoverflow.com/questions/8864696/installing-dph-examples-in-mac-o… [s2]: http://stackoverflow.com/questions/5492930/programming-language-for-functio… [s3]: http://stackoverflow.com/questions/8647692/parallel-io-causes-random-text-o… [s4]: http://stackoverflow.com/questions/8652363/repa-without-parallelization [s5]: http://stackoverflow.com/questions/9215284/what-exactly-is-dynamic-parallel… [s6]: http://stackoverflow.com/questions/9024672/writing-fib-to-run-in-parallel-n… [s7]: http://stackoverflow.com/questions/8852220/how-to-use-parallel-strategies-i… [s8]: http://stackoverflow.com/questions/8630659/parallel-graphics-processing-in-… [s9]: http://stackoverflow.com/questions/8582580/why-is-concurrent-haskell-non-de… [s10]: http://stackoverflow.com/questions/8902568/runtime-performance-degradation-… [s11]: http://stackoverflow.com/questions/8870648/memory-footprint-in-threaded-mod… -- Eric Kow <http://erickow.com>

1 0

Researcher / Lecturer position at the Univ. of Kent
by Simon Thompson 02 Mar '12

02 Mar '12

We're advertising a lectureship at Kent, with a special twist: the candidate will spend their first three years "developing as an independent researcher of internationally excellent quality in an area relevant to the School of Computing." In other words it's a research position that morphs into a traditional lectureship, and is designed to be particularly attractive to people at the start of their careers. The functional programmers at Kent would be particularly keen to recruit someone who would work with them! Online application: http://www11.i-grasp.com/fe/tpl_kent01.asp?newms=jj&id=36123&newlang=1 Closing date: 1/4/12, interviews: 30/4/12 If you have any questions do get in touch. Regards. Simon Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

1 0

Conduits and large ConduitResult chunks
by Nathan Howell 02 Mar '12

02 Mar '12

I'm porting lzma-enumerator over to conduits and I've run into a snag. The output chunks from lzma can be quite large, so I'd like to stream the results out in smaller chunks instead of tens (or hundreds) of megabytes at a time. It seems as though this should be possible, as it is with enumeratees, but I can't see how. Is there a way to get this working without unsafeInterleaveIO? https://github.com/alphaHeavy/lzma-conduit/blob/2b1451f5596b2db5a23df9a0fb1… thanks, -n

2 1

need help with monad transformers
by Alexander V Vershilov 02 Mar '12

02 Mar '12

Hello. I'm trying to add monad stack into network-conduit, and everything works except some details [1]. I've run runReaderT $ runTCPServer (wrapper around runResourceT) and inside conduit I want to run writer to gather results of inner computation. In inner computation I want to use IO, data from outter stack (ReaderT) so I'm running {-1-}: (k,t) <- lift $ runWriterT $ ask >>= \x -> tell [x] {- 1 -} and {-2-} (k,t) <- lift $ runWriterT $ do {- 2 -} x <- ask liftIO $ print $x+1 tell [x] and that will work (except I've thought I should not lift runWriterT, but calling functions inside. And finally in computation that will run once I want to register cleaning function (for example register $ putStrLn "cleaned") ({-3-}) (k,t) <- lift $ runWriterT $ do {- 3 -} x <- ask liftIO $ print $x+1 register $ print "freed2" tell [x] but I've got type error. If I'm running register outside runWriterT everything will work. I would apperated if there will be any suggestions how to make this code better or use register in internal computation (runWriterT) [1] https://gist.github.com/1941151 -- Best regards, Alexander V Vershilov

2 4

Re: [Haskell-cafe] Records in Haskell
by Evan Laforge 01 Mar '12

01 Mar '12

[ ccing the list because the wiki page was flawed and I made a bunch of changes, hope you don't mind ] > Thanks Evan, but I think that wiki page isn't doing your > proposal justice. There seem to be several typos in critical > places that make it hard to follow (for me at least). Sorry about the sloppy editing. I updated it and added more detailed examples. I also realized that as stated it didn't quite work for lens updates, so I extended it a little. > I think it would really help to include a record decl. to > show where `a` comes from, especially since you say that > record syntax doesn't change. Good point, added. > Could you explain what the TH does. Perhaps give an example > of what gets generated from a record decl? Sure, I added an example of that too. > And perhaps you could explain what you mean by a "type > directed function"? Aren't all overloaded functions type > directed? Can I have both a `#f` and a `f` in scope? What's > the difference? The idea is they're not overloaded functions. #f is not a symbol that can be in scope, it's special syntax to go *find* a `f` in some module. So `x = #f` is not in conflict with `x = f` provided #f is desugared to SomeModule.f. Unless of course the argument is defined in the current module, in which case it will desugar to plain `f` and then they will be the same. Since it's syntax for resolving a name, assigning it like `#f = xyz` doesn't make sense since you can't put a qualified name on the left of an `=` sign.

2 2

Haskell showcase in 5 minutes - follow-up
by Arnaud Bailly 01 Mar '12

01 Mar '12

Hello Cafe, Thanks to all of you who provided ideas and suggestions for introducing Haskell. I finally went the simple way, presenting the "knapsack" problem slightly reframed using tapas (spanish dishes). This problem is small enough to fit in the 5 minutes time-frame and it actually illustrates some of the salient points of the langauge: laziness of course, list-comprehensions, monadic computations (replicateM), first-class and higher-order functions, the REPL, integration with emacs, command-line compiling. Here is the code https://gist.github.com/1948301 I got a lot of positive feedback during the evening as I did some more live coding on simple problems (fizz-buzz, fibonacci) and I ended up showing some code I wrote doing "real stuff", notably the simple music synthesizer I wrote (https://github.com/abailly/haskell-synthesizer) for presenting Haskell to Paris Scala user Group and inspired of course by Paul Hudak's book and Joao Pizani's blog ( http://joaopizani.hopto.org/en/2012/01/haskell-synth/) The meeting ended with an impromptu panel on the disadvantages of the languages presented by each speaker which was really hard for me given the sheer perfection of Haskell. One advantage I mentionned somewhat indirectly which I should stress here is the Haskell community: reactive, vibrant and kind are a few adjectives that spring to my mind. Thanks again for your support, Arnaud Bailly

1 0

ANN: stm-chans 1.3.1
by wren ng thornton 01 Mar '12

01 Mar '12

-------------------------------------------- -- stm-chans 1.3.1 -------------------------------------------- The stm-chans package offers a collection of channel types, similar to Control.Concurrent.STM.TChan but with additional features. -------------------------------------------- -- Changes (since 1.2.0) -------------------------------------------- * (Version 1.3.1) The new stm-2.3 has finally been released with all the new optimized operations on TVars, TMVars, and TChans. I've updated the CPP macros in the Compat modules so that now they will actually switch over to the optimized implementations (instead of waiting for stm-9.0 :) It's highly recommended that all users bump their minimum stm-chans requirement to version 1.3.1. * (Version 1.3.0) Added Control.Concurrent.STM.TMVar.Compat for the stm-2.3 function tryReadTMVar. -------------------------------------------- -- Long description -------------------------------------------- In particular stm-chans offers these types: * Control.Concurrent.STM.TBChan: Bounded FIFO channels. When the channel is full, writers will block/retry. This ensures that the writers do not get too far ahead of the readers, which helps to make sure that memory and cpu resources are used responsibly. * Control.Concurrent.STM.TMChan: Closeable FIFO channels. This is like TChan (Maybe a) but with a monotonicity guarantee that once Nothing is returned all future reads will be Nothing as well. * Control.Concurrent.STM.TBMChan: Bounded Closeable FIFO channels. This combines the capabilities of TBChan and TMChan. In addition, the stm-chans package offers a (partial) compatibility layer for the recent API improvements to the stm package (see ticket #5104 [1]). Among others, these new functions include: * tryReadTChan :: TChan a -> STM (Maybe a) A version of readTChan which does not retry. Instead it returns Nothing if no value is available. * peekTChan :: TChan a -> STM a Get the next value from the TChan without removing it, retrying if the channel is empty. * tryPeekTChan :: TChan a -> STM (Maybe a) A version of peekTChan which does not retry. Instead it returns Nothing if no value is available. [1] http://hackage.haskell.org/trac/ghc/ticket/5104 -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/stm-chans Darcs: http://community.haskell.org/~wren/stm-chans Haddock (Darcs version): http://community.haskell.org/~wren/stm-chans/dist/doc/html/stm-chans -- Live well, ~wren

1 0

Haskell Weekly News: Issue 217
by Daniel Santa Cruz 01 Mar '12

01 Mar '12

Welcome to the 217th issue of the HWN, a newsletter covering developments in the Haskell community during the week of February 19 to 25, 2012. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2012/02/haskell-weekly-news-issue-217.h… Quotes of the Week * nand`: oh, wait, ANSI is the physical layout * cmccann: some programmers have a problem to solve and think "I know, I need an existential type." now they have another problem, but can't solve it because all they know about it is that the problem exists. * Philippa: The only data type is Nat! (Which succs). * hpc: peano arithmetic SSSSZ life * cmccann: unicode snowman is a valid identifier in Haskell, BTW * Jafet: Haskell is an imperatively functional abstract enterprise programming language template * Cale: Array is immutable boxed Cale: UArray is immutable unboxed Cale: IOArray is mutable boxed elliott: IOUArray is an array of debts. * c_wraith: It [C] frees you to use whatever data structure you want, by providing you none! * Brandon Allbery: If you absolutely insist on shooting yourself in the foot this way in Haskell, it can be done; but there is no reason Haskell should make it *easy* to do so. That's C's superpower. :) Top Reddit Stories * ANNOUNCE: Sodium, a new Haskell FRP (Reactive Programming) system Domain: hackage.haskell.org, Score: 41, Comments: 38 On Reddit: [1] http://goo.gl/IY2a1 Original: [2] http://goo.gl/OOpl1 * Adventures in Parsec - Parsing logs in Haskell Domain: variadic.me, Score: 31, Comments: 24 On Reddit: [3] http://goo.gl/pkHdy Original: [4] http://goo.gl/fDRdE * Wanted: an efficient union algorithm for hash-array mapped tries Domain: blog.johantibell.com, Score: 30, Comments: 13 On Reddit: [5] http://goo.gl/p7n4c Original: [6] http://goo.gl/4XCui * How reliable is Hackage supposed to be? Domain: self.haskell, Score: 29, Comments: 20 On Reddit: [7] http://goo.gl/qfqJ3 Original: [8] http://goo.gl/qfqJ3 * Mathematics in the Age of the Turing Machine -- I recommend starting in section 2 Domain: math.pitt.edu, Score: 28, Comments: 1 On Reddit: [9] http://goo.gl/T4jGx Original: [10] http://goo.gl/9SmzX * Conduits: Simplifying ResourceT Domain: yesodweb.com, Score: 28, Comments: 6 On Reddit: [11] http://goo.gl/PW0C2 Original: [12] http://goo.gl/9a5Dz * Clarifying a mis-understanding about regions (and iteratees) Domain: haskell.org, Score: 27, Comments: 4 On Reddit: [13] http://goo.gl/P3sVE Original: [14] http://goo.gl/WWjUb * Concatenative Haskell Part II: Custom DSL Syntax Using QuasiQuotes Domain: github.com, Score: 25, Comments: 6 On Reddit: [15] http://goo.gl/DP0bY Original: [16] http://goo.gl/Lvlnc * Designing a serialization API Domain: blog.johantibell.com, Score: 24, Comments: 1 On Reddit: [17] http://goo.gl/SFMTx Original: [18] http://goo.gl/vcdsS * Getting Started with Disciple Core Domain: disciple.ouroborus.net, Score: 19, Comments: 1 On Reddit: [19] http://goo.gl/FCPla Original: [20] http://goo.gl/7J8oN * hs-logo: Logo turtle graphics interpreter written in Haskell Domain: deepakjois.github.com, Score: 18, Comments: 5 On Reddit: [21] http://goo.gl/wRIqZ Original: [22] http://goo.gl/JE37a * The Simply-typed Lambda Calculus with Constraints Domain: flippac.wordpress.com, Score: 16, Comments: 3 On Reddit: [23] http://goo.gl/ZNNB1 Original: [24] http://goo.gl/QbT6a * Data Structure Agnostic JSON Serialization Domain: blog.melding-monads.com, Score: 14, Comments: 1 On Reddit: [25] http://goo.gl/9QD9o Original: [26] http://goo.gl/WsPtb Top StackOverflow Questions * Evaluation strategy votes: 19, answers: 4 Read on SO: [27] http://goo.gl/rcJyz * Guards vs. if-then-else vs. cases in Haskell votes: 18, answers: 2 Read on SO: [28] http://goo.gl/BmH0X * Division by zero in Haskell votes: 15, answers: 4 Read on SO: [29] http://goo.gl/a0cER * Memory efficient strings in Haskell votes: 15, answers: 2 Read on SO: [30] http://goo.gl/4kUdm * What prevents an efficient Haskell virtual machine (like JVM)? votes: 12, answers: 4 Read on SO: [31] http://goo.gl/RphqH * Is indexing of Data.Vector.Unboxed.Mutable.MVector really this slow? votes: 11, answers: 2 Read on SO: [32] http://goo.gl/72fhd * Haskell: filtering a heterogenous list by type votes: 10, answers: 2 Read on SO: [33] http://goo.gl/uFm1I * ghci tab-completion in haskell-mode votes: 10, answers: 1 Read on SO: [34] http://goo.gl/QaAo5 * Non-maintainer uploads to Hackage votes: 10, answers: 2 Read on SO: [35] http://goo.gl/lVFHN * withFile vs. openFile votes: 9, answers: 4 Read on SO: [36] http://goo.gl/KQ6Fu Until next time, Daniel Santa Cruz References 1. http://hackage.haskell.org/package/sodium 2. http://www.reddit.com/r/haskell/comments/pyxpx/announce_sodium_a_new_haskel… 3. http://variadic.me/posts/2012-02-24-adventures-in-parsec.html 4. http://www.reddit.com/r/haskell/comments/q41rr/adventures_in_parsec_parsing… 5. http://blog.johantibell.com/2012/02/wanted-efficient-union-algorithm-for.ht… 6. http://www.reddit.com/r/haskell/comments/q0a02/wanted_an_efficient_union_al… 7. http://www.reddit.com/r/haskell/comments/pwjc9/how_reliable_is_hackage_supp… 8. http://www.reddit.com/r/haskell/comments/pwjc9/how_reliable_is_hackage_supp… 9. http://www.math.pitt.edu/~thales/papers/turing.pdf 10. http://www.reddit.com/r/haskell/comments/pwx2b/mathematics_in_the_age_of_th… 11. http://www.yesodweb.com/blog/2012/02/simplifying-resourcet 12. http://www.reddit.com/r/haskell/comments/q37rf/conduits_simplifying_resourc… 13. http://www.haskell.org/pipermail/haskell-cafe/2012-February/099689.html 14. http://www.reddit.com/r/haskell/comments/q293l/clarifying_a_misunderstandin… 15. https://github.com/leonidas/codeblog/blob/master/2012/2012-02-21-concatenat… 16. http://www.reddit.com/r/haskell/comments/pzh53/concatenative_haskell_part_i… 17. http://blog.johantibell.com/2012/02/designing-serialization-api.html 18. http://www.reddit.com/r/haskell/comments/q0kcv/designing_a_serialization_ap… 19. http://disciple.ouroborus.net/wiki/Tutorial/Core 20. http://www.reddit.com/r/haskell/comments/q1url/getting_started_with_discipl… 21. http://deepakjois.github.com/hs-logo/ 22. http://www.reddit.com/r/haskell/comments/pzf9k/hslogo_logo_turtle_graphics_… 23. http://flippac.wordpress.com/2012/02/23/the-simply-typed-lambda-calculus-wi… 24. http://www.reddit.com/r/haskell/comments/q2ga5/the_simplytyped_lambda_calcu… 25. http://blog.melding-monads.com/2012/02/24/data-structure-agnostic-json-seri… 26. http://www.reddit.com/r/haskell/comments/q3t70/data_structure_agnostic_json… 27. http://stackoverflow.com/questions/9439470/evaluation-strategy 28. http://stackoverflow.com/questions/9345589/guards-vs-if-then-else-vs-cases-… 29. http://stackoverflow.com/questions/9354016/division-by-zero-in-haskell 30. http://stackoverflow.com/questions/9398572/memory-efficient-strings-in-hask… 31. http://stackoverflow.com/questions/9352744/what-prevents-an-efficient-haske… 32. http://stackoverflow.com/questions/9409634/is-indexing-of-data-vector-unbox… 33. http://stackoverflow.com/questions/9396023/haskell-filtering-a-heterogenous… 34. http://stackoverflow.com/questions/9431779/ghci-tab-completion-in-haskell-m… 35. http://stackoverflow.com/questions/9435513/non-maintainer-uploads-to-hackage 36. http://stackoverflow.com/questions/9406463/withfile-vs-openfile

1 0

Haskell showcase in 5 minutes
by Arnaud Bailly 29 Feb '12

29 Feb '12

Hello Cafe, I will be (re)presenting Haskell in a "Batlle Language" event Wednesday evening: A fun and interactive contest where various programming language champions try to attract as much followers as possible in 5 minutes. Having successfully experimented the power of live coding in a recent Haskell introduction for the Paris Scala User Group, I would like to do the same but given the time frame I need a simpler example than the music synthesizer program. So I would like to tap in the collective wisdom looking for some concise, eye-opening, mind-shaking and if possible fun example of what one can achieve in Haskell. Things that sprung to my mind are rather dull: prime factors, fibonacci numbers. Thanks in advance, Arnaud

11 17

question about conduit source
by Alexander V Vershilov 29 Feb '12

29 Feb '12

Hello, cafe. Is it possible to read data from different concurrent sources, i.e. read data from source as soon as it become avaliable, e.g. runResourceT $ (source1 stdin $= CL.map Left) >=> (source2 handle $= CL.map Right) $= application $$ sink where >=> - stands for concurrent combining of sources It would be good if it can be sources of different types (handle or STM channel, etc..). Currently I've found no good way to handle with this situation, except of using STM Channels for collecting data source1 ---+ | | sink | output sink +---] Channel [-------> application----->] | source source2 ---+ | From this point of view application takes concurent data, but this implementation requires additional thread per data processing. Also in many cases it will require run additional runResourceT (see later example). So if there any possible simplifications? Or ideas how to make (>=>) operator. Example: So I've got next code in my network-conduit based application: main :: IO () main = do pool <- createDBPool "..." 10 let r = ServerInit pool forkIO $ forever clientConsole --read channel list and send "Left" flip runReaderT r $ runTCPServer (ServerSettings 3500 Nothing) (protoServer) myServer src sink = do ch <- liftIO $ atomically $ newTBMChan 16 initState <- lift $ ask _ <- liftIO $ fork . (flip runReaderT initState) $ runResourceT $ src $= C.sequence decode $= CL.map Right $$ sinkTBMChan ch sourceTBMChan ch $= process $= C.sequence encode $$ sinkHandle stdout But in this situation I don't know if freeing of all resources are guaranteed, because I'm running additional resourceT in main resourceT scope. So can you advice is it possible to make concurrent sources now with currenly implemented library? If it's not possible but worth of implementing, so I can make that functions? Is it correct to runResourceT inside another resourceT? -- Best regards, Alexander V Vershilov

3 4

Use Template Haskell recover inside the IO monad
by Federico Mastellone 28 Feb '12

28 Feb '12

I'm new to Template Haskell and just started playing with it. I have Haskell platform 2011.4.0 and I get this error when I try to run this simple code. main = do code <- runQ $ recover (return []) th putStrLn (pprint code) putStrLn (show code) th :: Q [Dec] th = [d| sum a b = a + b |] Template Haskell error: Can't do `recover' in the IO monad POC.hs: user error (Template Haskell failure) How can I get declarations with the "recover" function so I can print a custom error message using "location"? If I can't do it inside the IO monad, how can I do a program to pretty print Haskell code? Thanks!

1 0

Re: [Haskell-cafe] GSoc-2012: Lock-free Data Structures
by Sergiu Ivanov 28 Feb '12

28 Feb '12

Hello, On Tue, Feb 21, 2012 at 8:11 AM, Ryan Newton <rrnewton(a)gmail.com> wrote: > > Oops, I've attached the PDF. It's a good overview that includes a bunch of > the key ideas that appear in recent work in the area. Very cool article, thank you! I genuinely enjoyed reading it :-) > Absolutely! Feel free to make a pull request. I'm going to play with the existing structures first. However, I can't see a list of issues anywhere. Could you suggest a place where I could look for indications of some issues which I could fix for starters? Sergiu P.S. Sorry for my sluggishness :-(

1 0

Vim plugin for ghc-mod
by Kazu Yamamoto 28 Feb '12

28 Feb '12

Hello, eagletmt implemented a Vim plugin for ghc-mod: https://github.com/eagletmt/ghcmod-vim Happy Haskell programming on Vim! --Kazu

5 4

Haskell showcase in 5 minutes
by Doug McIlroy 28 Feb '12

28 Feb '12

Sorry, a typo in the url for the power-series example. It should have been http://www.cs.dartmouth.edu/~doug/powser.html

1 0

Haskell showcase in 5 minutes
by Doug McIlroy 28 Feb '12

28 Feb '12

Here's an example that fits comfortably in 5 minutes--if your audience knows elementary calculus: http://www.cs.dartmouth.edu/~doug/powswer.html It depends critically on lazy evaluation, which knocks out a lot of competing languages right from the start. The five-minute version would begin with power-series addition--trivial. Then comes multiplication--an eye-opener. No subscripts! No worry about how many terms to carry in intermediate results. That's about all you have time to really derive. Go on to mention that division is about equally easy. Then allude to substitution (also called composition) and its inverse, reversion. Lots of finicky papers have been written about reversion over more than two centuries. Throw that one-liner on the screen side-by-side with Algorithm S (which is only pseudocode!) from Knuth section 4.7. That should convince the most skeptical observer. Doug

3 2

Re: [Haskell-cafe] Theorems for free!
by Patrick Browne 27 Feb '12

27 Feb '12

1 0