- Haskell-Cafe - Haskell.org

ANN: LambdaCube 3D (purely functional rendering engine) is back in business
by Patai Gergely 08 Sep '12

08 Sep '12

Hello all, We recently rebooted the LambdaCube project, which aims to provide a purely functional interface for GPU programming. The details and the background are described in our new blog [1], while the code can be found on GitHub [2]. Cheers, Gergely [1] http://lambdacube3d.wordpress.com/ [2] https://github.com/csabahruska/lc-dsl -- http://www.fastmail.fm - The professional email service

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performance issues with popCount
by Harald Bögeholz 08 Sep '12

08 Sep '12

Dear Haskell Cafe, I am struggling with the performance of the popCount function from Data.Bits. To be more precise: I downloaded the Haskell Platform 2012.2.0.0 from http://hackage.haskell.org/platform/ (64 bit, Mac OS X). In this version I found the popCount function to be broken. If I look in the online documentation at http://hackage.haskell.org/packages/archive/base/4.5.1.0/doc/html/src/Data-… it is already fixed, but included with my Haskell Platform was the broken version. Anyway, I tried this version popCount :: Integer -> Int popCount = go 0 where go c 0 = c go c w = go (c+1) (w .&. (w - 1)) and profiling showed that my program spent 80 % of its time counting bits. So I thought I'm clever and implement a table-based version like this: popCount' :: Integer -> Int popCount' = go 0 where go c 0 = c go c w = go (c+1) (w .&. (w - 1)) popCountN = 10 popCountMask :: Integer popCountMask = shift 1 popCountN - 1 popCountTable :: Array Integer Int popCountTable = listArray (0, popCountMask) $ map popCount' [0 .. popCountMask] popCount :: Integer -> Int popCount 0 = 0 popCount x = popCountTable ! (x .&. popCountMask) + popCount (x `shiftR` popCountN) turns out this is even slower ... now my program spends 90 % of its time counting bits :-(. Any hints? Thanks -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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parsing ghc type errors
by Semen Trygubenko 07 Sep '12

07 Sep '12

Dear Haskell-cafe, When dealing with ghc type errors I often see messages like Couldn't match expected type `ghc-prim:GHC.Types.IO{tc 32I} MyTypeNameHere{tc r1d8}' with actual type `ghc-prim:GHC.Types.IO{tc 32I} MyOtherTypeNameHere{tc r1d8}' Could you please help me understand the meaning of the entities in {} and why are they useful, or point to a source of information about them? Messages where familiar types are referenced are easy to read, but occasionally I get messages of the sort with actual type `[( b0{tv t3p} [tau] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )]' How do I go about reading the type [( b0{tv t3p} [tau] :: ghc-prim:GHC.Prim.*{(w) tc 34d} )] ? E.g., where the names 'b0' and '[tau]' come from? What does '(w)' in '{(w) tc 34d}' mean? Thank you, Semen -- Семен Тригубенко

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ANNOUNCE: haskell-src-exts 1.13.5
by Niklas Broberg 07 Sep '12

07 Sep '12

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.13.5! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts There are two primary reasons for this release: 1) to add the requested support for UHC's 'js' FFI calling convention (in 1.13.4), and 2) to export a stand-alone token stream lexer (in 1.13.5). NOTE: If after reading the above, you think "wow, a token stream lexer, I'll use that for some kind of refactoring tool", then you're most likely wrong. Please contact me in that case and I'll tell you why. :-) A probably unnecessary warning: This release may fail to parse some files that were previously possible to parse, if they included RULES pragmas using 'forall' as a varid. I really doubt any such exist, and they would not have been accepted by GHC if they did, so I sincerely doubt that anyone will be effected by this. Changelog: 1.13.4 --> 1.13.5 =============== * Expose Language.Haskell.Exts.Lexer, which implements a standalone token stream lexer. The module is re-exported both by Language.Haskell.Exts and by Language.Haskell.Exts.Annotated. 1.13.3 --> 1.13.4 =============== * Fix bug where operators starting with # written in parentheses would not be parsed when UnboxedTuples is turned on. Now works. * Allow 'family' and 'forall' as (non-type) varid's. This adds one more shift/reduce conflict to the parser, and its resolution means that '{-# RULES "name" forall = ... #-}' is not allowed. * Complete the set of FFI calling conventions from the Haskell 2010 report (even if no compiler implements them). Also include the 'js' calling convention, supported by UHC. Cheers, /Niklas

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From monads to monoids in a small category
by Alberto G. Corona 06 Sep '12

06 Sep '12

"Monads are monoids in the category of endofunctors" This Monoid instance for the endofunctors of the set of all elements of (m a) typematch in Haskell with FlexibleInstances: instance Monad m => Monoid (a -> m a) where mappend = (>=>) -- kleisly operator mempty = return The article can be found here: http://haskell-web.blogspot.com.es/2012/07/from-monads-to-monoids-in-small.… I would appreciate some comments.

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Haskell Weekly News: Issue 234
by Daniel Santa Cruz 06 Sep '12

06 Sep '12

Welcome to issue 243 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 26 to September 01, 2012. Quotes of the Week * companion_cube: "I don't understand this article at all" edwardk: "Well, when you finish reading it, here's a follow-up article" Top Reddit Stories * Parsing with Haskell and Attoparsec Domain: newartisans.com, Score: 64, Comments: 8 On Reddit: [1] http://goo.gl/rrlQG Original: [2] http://goo.gl/aHlTJ * FRP - Release of reactive-banana-0.7. Finally features dynamic event switching! Domain: apfelmus.nfshost.com, Score: 48, Comments: 21 On Reddit: [3] http://goo.gl/5QnBl Original: [4] http://goo.gl/8dnLg * Combinatorrent - Writing Haskell Code for Fun and Profit [InfoQ] Domain: infoq.com, Score: 46, Comments: 14 On Reddit: [5] http://goo.gl/FE4OF Original: [6] http://goo.gl/feU7U * Putting it all together -- Example project that pulls together Snap, Blaze, OpenId, IxSet, AcidState, and Sessions. Domain: self.haskell, Score: 34, Comments: 3 On Reddit: [7] http://goo.gl/huFaI Original: [8] http://goo.gl/huFaI * Invert the Inversion of Control Domain: thev.net, Score: 32, Comments: 30 On Reddit: [9] http://goo.gl/1uO5a Original: [10] http://goo.gl/89UnF * A follow up on my initial experience and why there's still something immensely attractive about Haskell Domain: binarysculpting.com, Score: 27, Comments: 19 On Reddit: [11] http://goo.gl/3RLNO Original: [12] http://goo.gl/81YF2 * Haskim -- a convenient Haskell VIM distribution Domain: github.com, Score: 25, Comments: 19 On Reddit: [13] http://goo.gl/9sJ3O Original: [14] http://goo.gl/Wq0Rq * Introducing Perdure: persistence for acyclic immutable data Domain: hackage.haskell.org, Score: 23, Comments: 12 On Reddit: [15] http://goo.gl/ACWDi Original: [16] http://goo.gl/CSi57 * Existential types and data abstraction Domain: iveselov.info, Score: 22, Comments: On Reddit: [17] http://goo.gl/K6JGl Original: [18] http://goo.gl/GBYlW * I just realized that running monads is applying a left monad action Domain: self.haskell, Score: 20, Comments: 63 On Reddit: [19] http://goo.gl/Geus4 Original: [20] http://goo.gl/Geus4 Top StackOverflow Questions * How can I recover sharing in a GADT? votes: 14, answers: 2 Read on SO: [21] http://goo.gl/I3cuW * Constructing efficient monad instances on `Set` (and other containers with constraints) using the continuation monad votes: 13, answers: 2 Read on SO: [22] http://goo.gl/Ys4Uy * Numbers as multiplicative functions (weird but entertaining) votes: 12, answers: 1 Read on SO: [23] http://goo.gl/yN8KN * Game server in Haskell votes: 10, answers: 3 Read on SO: [24] http://goo.gl/QV9de * Why doesn't sortBy take (a -> a -> Bool)? votes: 10, answers: 5 Read on SO: [25] http://goo.gl/Y7EzD * Default type instances referring to each other votes: 9, answers: 1 Read on SO: [26] http://goo.gl/6cUG7 * Applying a fixed-length-vector-function to the inital part of a longer fixed-length-vector votes: 9, answers: 2 Read on SO: [27] http://goo.gl/03KcE * Haskell: example of function of type a -> a, besides the identity votes: 9, answers: 5 Read on SO: [28] http://goo.gl/q2UDW * Stack overflow when using Haskell sort function votes: 8, answers: 2 Read on SO: [29] http://goo.gl/D31Ku * How to navigate up inside a HUET Zipper votes: 7, answers: 2 Read on SO: [30] http://goo.gl/X7dE3 * Haskell : reference to previously updated elements of list within the update function votes: 7, answers: 6 Read on SO: [31] http://goo.gl/0yRIo * Is there any way to make parsec report “shift-reduce” conflicts? votes: 7, answers: 1 Read on SO: [32] http://goo.gl/LQeDG * Haskell: how to write a monadic variadic function, with parameters using the monadic context votes: 7, answers: 1 Read on SO: [33] http://goo.gl/ThniY * Haskell parMap and parallelism votes: 7, answers: 1 Read on SO: [34] http://goo.gl/3qQHu Until next time, Daniel Santa Cruz References 1. http://newartisans.com/2012/08/parsing-with-haskell-and-attoparsec/ 2. http://www.reddit.com/r/haskell/comments/z2lir/parsing_with_haskell_and_att… 3. http://apfelmus.nfshost.com/blog/2012/08/26-frp-banana-0-7.html 4. http://www.reddit.com/r/haskell/comments/yv6ku/frp_release_of_reactivebanan… 5. http://www.infoq.com/presentations/Combinatorrent-Haskell-casestudy 6. http://www.reddit.com/r/haskell/comments/yzdyr/combinatorrent_writing_haske… 7. http://www.reddit.com/r/haskell/comments/z4iay/putting_it_all_together_exam… 8. http://www.reddit.com/r/haskell/comments/z4iay/putting_it_all_together_exam… 9. http://www.thev.net/PaulLiu/invert-inversion.html 10. http://www.reddit.com/r/haskell/comments/z4inb/invert_the_inversion_of_cont… 11. http://binarysculpting.com/2012/09/01/theres-something-about-haskell/ 12. http://www.reddit.com/r/haskell/comments/z6idz/a_follow_up_on_my_initial_ex… 13. https://github.com/jejansse/haskim 14. http://www.reddit.com/r/haskell/comments/z3orc/haskim_a_convenient_haskell_… 15. http://hackage.haskell.org/package/perdure-0.1.2 16. http://www.reddit.com/r/haskell/comments/z2sc6/introducing_perdure_persiste… 17. http://iveselov.info/posts/2012-08-30-existential-types.html 18. http://www.reddit.com/r/haskell/comments/z1ojj/existential_types_and_data_a… 19. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 20. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 21. http://stackoverflow.com/questions/12230088/how-can-i-recover-sharing-in-a-… 22. http://stackoverflow.com/questions/12183656/constructing-efficient-monad-in… 23. http://stackoverflow.com/questions/12133932/numbers-as-multiplicative-funct… 24. http://stackoverflow.com/questions/12134097/game-server-in-haskell 25. http://stackoverflow.com/questions/12152561/why-doesnt-sortby-take-a-a-bool 26. http://stackoverflow.com/questions/12199324/default-type-instances-referrin… 27. http://stackoverflow.com/questions/12221080/applying-a-fixed-length-vector-… 28. http://stackoverflow.com/questions/12230820/haskell-example-of-function-of-… 29. http://stackoverflow.com/questions/12203380/stack-overflow-when-using-haske… 30. http://stackoverflow.com/questions/12129300/how-to-navigate-up-inside-a-hue… 31. http://stackoverflow.com/questions/12144973/haskell-reference-to-previously… 32. http://stackoverflow.com/questions/12170780/is-there-any-way-to-make-parsec… 33. http://stackoverflow.com/questions/12175912/haskell-how-to-write-a-monadic-… 34. http://stackoverflow.com/questions/12224842/haskell-parmap-and-parallelism

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Monospace code in Haddock
by Niklas Hambüchen 06 Sep '12

06 Sep '12

Is it possible to use inlined monospaced font in Haddock that does *not* replace < and > to links and does *not* link to Haskell functions (like @ and ' and " and ` do)? Like: -- | This function gives you @<span a1="v1">str</span>@ something :: String The @ above of course makes everything unreadable (as it thinks span... is a link). The only thing I've found to work so far is: -- | This function gives you -- -- >@<span a1="v1">str</span>@ something :: String Which is not inline.

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Haskell seems setup for iterative numerics; i.e. a standard example is Newton's method where lazy evaluation ...
by KC 05 Sep '12

05 Sep '12

separates control from computation. It seems as if Haskell would be better for iterative matrix methods rather than direct calculation. -- -- Regards, KC

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[CUFP 2012] Birds of a Feather sessions
by Ashish Agarwal 05 Sep '12

05 Sep '12

========================================================= Birds of a Feather sessions (BOFs) Commercial Users of Functional Programming Workshop (CUFP 2012) http://cufp.org/bofs-2012 Copenhagen, Denmark, September 13 - 15 ========================================================= CUFP 2012 will again include Birds of a Feather sessions as in previous years. BoFs provide a place for our community to gather informally and reach consensus on matters of importance. Any CUFP attendee can propose a BoF session, and grab one of the spare rooms in the evening slots below. Attendance in the evening is open to all. BoF sessions facilitate ad-hoc discussions and provide a place to gather and start off the chat, before moving on during the evening to a local restaurant or pub. There is no deadline for proposing a session---you can even request a room on the day---but we encourage you to do so at the earliest possible time to help us with planning and to publicize your proposed discussion. Attendees are also free to show up even if you have not confirmed, but again earlier confirmation will help us with planning. Please see http://cufp.org/bofs-2012 for details on how to propose a BoF session or confirm attendance at one of them. General information on CUFP 2012 can be found at http://cufp.org/conference/2012. The conference is co-located with ICFP. You can register for CUFP at the main ICFP website http://www.icfpconference.org/icfp2012.

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Research Position, Leeds: Parallel functional visualization
by David Duke 05 Sep '12

05 Sep '12

I have an open position in the Visualization group at Leeds for a researcher to work on one of two projects, the second of which is specific to Haskell: We are seeking to appoint an ambitious researcher within the Visualization and VR group at Leeds to work on one of two projects, (i) time-varying and higher-dimensional graphs, or (ii) high-performance visualization in Haskell. The post is available immediately, and lasts until 31 July 2016. 1. Time-varying and higher-dimensional graphs. We are developing powerful tools for the topological analysis of (time-varying) volumetric datasets. These produce large graphs capturing combinatorial structures and events, and visual analysis of these graphs provides significant insight into the data. Similar structures may be found in graphs resulting from optimisation problems in multi-parameter spaces. The research will investigate (i) the link between layout aesthetics and specific tasks over time-varying data; (ii) layout techniques that respect these aesthetics, for example embedding of the graph into higher-dimensional manifolds; (iii) techniques for eliding structural detail, and linking visual abstractions to other models of the data; and (iv) efficient implementation of layout algorithms, using established toolkits (e.g. VTK), and parallelisation on cluster-based hardware and/or many-core GPU architecture. You will be working with researchers on the "Multifield Extension of Topological Analysis" project, funded recently by the EPSRC.  2. Parallel functional visualization: Scientific visualization involves the application of compute-intensive algorithms to large volumes of data, often larger than can fit into core memory. Systems are still largely based on the "pipeline" model, extended to exploit different processing technologies (multi-core CPUs, GPUs, clusters), and strategies (streaming, tiling, out-of-core processing, etc). This research concerns an innovative approach that exploits laziness and parallelism in the pure functional language Haskell. Scivis "applications" will be constructed from expressions in a declarative domain-specific language, and transformed into imagery via lower-level DSLs and strategies that coordinate use of heterogeneous resources. Both projects involve fundamental computer science and interdisciplinary work alongside experts from other disciplines providing data and contributing to its analysis. The appointee will be expected to contribute to the programme described above through all stages of the research process, including basic research, programming, testing, evaluation, publication of results at the highest level, and assistance in developing proposals for funding. Work will involve collaboration with other senior members of the Leeds visualization group (Dr. Hamish Carr and Dr. Roy Ruddle), and there will be opportunities to assist with academic supervision of undergraduate and postgraduate students. CLOSING DATE: 21st September. For further information including person specification, and/or to submit an application, please see the following URL: http://jobs.leeds.ac.uk/fe/tpl_universityofleeds01.asp?s=FoQnTYvIgXJoLlXgd&… For informal inquiries, please feel free to contact me directly. David Duke Head, School of Computing, and Reader in Visualization E: D.J.Duke(a)leeds.ac.uk W: www.comp.leeds.ac.uk/djd/

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SYB with class: Bug in Derive.hs module
by Roman Cheplyaka 05 Sep '12

05 Sep '12

There's a bug in syb-with-class reported by Alexey Rodriguez Yakushev in 2008 [1]. I can confirm that the bug is still there (syb-with-class-0.6.1.3, ghc 7.4.1). [1]: http://www.haskell.org/pipermail/haskell-cafe/2008-March/041179.html Here's an even simpler test case: {-# LANGUAGE FlexibleContexts, FlexibleInstances, MultiParamTypeClasses, UndecidableInstances, TemplateHaskell, OverlappingInstances, DeriveDataTypeable #-} import Data.Generics.SYB.WithClass.Basics import Data.Generics.SYB.WithClass.Derive data Foo = Foo Foo | Bar deriving (Typeable, Show) deriveData [''Foo] f :: (Data NoCtx ast, Typeable ast) => ast -> TypeRep f = typeOf main = print $ f $ Foo Bar The cause of this bug is a self-referencing instance created by deriveData: instance (Data ctx Foo, Sat (ctx Foo)) => Data ctx Foo where ... What's the proper way to fix it? -- Roman I. Cheplyaka :: http://ro-che.info/

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How to waitForProcess?
by Magicloud Magiclouds 05 Sep '12

05 Sep '12

Hi, I have code like this and it leaves lots of zombies of flow-export. Then I add waitForProcess.... Well I do not know where to put it. Before or after 'hGetContents' both make the program hung. exportCSV :: FilePath -> IO [String] exportCSV file = do csv_ <- withBinaryFile file ReadMode $ \i -> do (_, Just o, _, h) <- createProcess $ CreateProcess (RawCommand "/usr/bin/flow-export" ["-f2"]) Nothing Nothing (UseHandle i) CreatePipe Inherit True False hGetContents o return $ tail $ lines csv_ -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Over general types are too easy to make.
by timothyhobbs＠seznam.cz 05 Sep '12

05 Sep '12

I'd have to say that there is one(and only one) issue in Haskell that bugs me to the point where I start to think it's a design flaw: It's much easier to type things over generally than it is to type things correctly. Say we have a >data BadFoo = > BadBar{ > badFoo::Int} | > BadFrog{ > badFrog::String, > badChicken::Int} This is fine, until we want to write a function that acts on Frogs but not on Bars. The best we can do is throw a runtime error when passed a Bar and not a Foo: >deBadFrog :: BadFoo -> String >deBadFrog (BadFrog s _) = s >deBadFrog BadBar{} = error "Error: This is not a frog." We cannot type our function such that it only takes Frogs and not Bars. This makes what should be a trivial compile time error into a nasty runtime one :( The only solution I have found to this is a rather ugly one: >data Foo = Bar BarT | Frog FrogT If I then create new types for each data constructor. >data FrogT = FrogT{ > frog::String, > chicken::Int} >data BarT = BarT{ > foo :: Int} Then I can type deFrog correctly. >deFrog :: FrogT -> String >deFrog (FrogT s _) = s But it costs us much more code to do it correctly. I've never seen it done correctly. It's just too ugly to do it right :/ and for no good reason. It seems to me, that it was a design mistake to make data constructors and types as different entities and this is not something I know how to fix easily with the number of lines of Haskell code in existence today :/ >main = do > frog <- return (Frog (FrogT "Frog" 42)) > print $ > case frog of > (Frog myFrog) -> deFrog myFrog > badFrog <- return (BadBar 4) > print $ > case badFrog of > (notAFrog@BadBar{}) -> deBadFrog notAFrog The ease with which we make bad design choices and write bad code(in this particular case) is tragically astounding. Any suggestions on how the right way could be written more cleanly are very welcome! Timothy Hobbs

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Re: [Haskell-cafe] From monads to monoids in a small category
by Alberto G. Corona 05 Sep '12

05 Sep '12

Not to mention the ugly formatting ;) 2012/9/5 Richard O'Keefe <ok(a)cs.otago.ac.nz>: > > On 4/09/2012, at 10:39 PM, Alberto G. Corona wrote: > >> "Monads are monoids in the category of endofunctors" >> >> This Monoid instance for the endofunctors of the set of all elements >> of (m a) typematch in Haskell with FlexibleInstances: >> >> instance Monad m => Monoid (a -> m a) where >> mappend = (>=>) -- kleisly operator >> mempty = return >> >> The article can be found here: >> >> http://haskell-web.blogspot.com.es/2012/07/from-monads-to-monoids-in-small.… >> >> I would appreciate some comments. > > s/kleisly/Kleisli/ > In the article, > s/Lets/Let's/ > /Here 'm b' as/ s/as/is/ > s/this_are/this are/ > s/first, is/first is/ > s/haskell/Haskell/ > s/polimorphic/polymorphic/ > s/x=/x =/ > s/let's/Let's/ > s/condition, associativity/condition, associativity,/ > /if not where that way, .* guess/ > I *think* you mean to say something like > (If it were not so, it would be impossible to > define the denotational semantics of imperative > languages in terms of monads, I guess.) > Generally, it's "according TO", not "according WITH", > and "associated WITH", not "associated TO". > > instance Functor a > doesn't seem to be legal Haskell. > > At this point I stopped reading. > >

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Asking for advice on programming style and also performance
by Harald Bögeholz 04 Sep '12

04 Sep '12

Dear Haskell Cafe, I am working on a solver for Slitherlink puzzles in Haskell. Today I am completely puzzled that a small change I made had a huge performance impact. I thought the change to be merely cosmetic to make a function a little more elegant and the code more readable. But this change slowed down my program by a factor of 100! I would greatly appreciate advice from experienced Haskellers why this has happened. The program is hosted on github; this is the direct link to the change with unexpected outcome: https://github.com/ctbo/slitherlink/commit/b6f58699258ef68ddee21a1346bd1844… The program itself isn't commented at all, but I have written a sketch of how it works in the wiki: https://github.com/ctbo/slitherlink/wiki/How-it-works While I am at it: I would also appreciate any advice you might have on the programming style in general. I am relatively new to Haskell and this is my first nontrivial program. So any suggestions for improvements are welcome! Thanks for your time, -- Harald Bögeholz <bo(a)ct.de> (PGP key available from servers) Redaktion c't Tel.: +49 511 5352-300 Fax: +49 511 5352-417 http://www.ct.de/ int f[9814],b,c=9814,g,i;long a=1e4,d,e,h; main(){for(;b=c,c-=14;i=printf("%04d",e+d/a),e=d%a) while(g=--b*2)d=h*b+a*(i?f[b]:a/5),h=d/--g,f[b]=d%g;} (Arndt/Haenel) Affe Apfel Vergaser /* Heise Zeitschriften Verlag GmbH & Co. KG * Karl-Wiechert-Allee 10 * 30625 Hannover * Registergericht: Amtsgericht Hannover HRA 26709 * Persönlich haftende Gesellschafterin: Heise Zeitschriften Verlag * Geschäftsführung GmbH * Registergericht: Amtsgericht Hannover, HRB 60405 * Geschäftsführer: Ansgar Heise, Dr. Alfons Schräder */

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Cabal install fails due to recent HUnit
by Erik Hesselink 04 Sep '12

04 Sep '12

Hi all, All cabal installs using cabal-install-0.10.2 are currently failing for us. This is due to the cabal file for HUnit-1.2.5.0, which was recently uploaded to hackage. The ouput I'm getting from cabal is just: Reading available packages... Resolving dependencies... cabal: Couldn't read cabal file "HUnit/1.2.5.0/HUnit.cabal" If I unpack HUnit-1.2.5.0 and call 'cabal configure', I get: cabal: HUnit.cabal:57: The 'type' field is required for test suites. The available test types are: exitcode-stdio-1.0 The relevant lines from the cabal file are: Test-Suite hunit-tests-optimize-0 Type: exitcode-stdio-1.0 These look fine to me. Does anyone have any idea how to go about fixing this (on hackage at least)? Could this package temporarily be removed, to avoid breaking everyone's cabal? Erik

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hstats median algorithm
by David Feuer 03 Sep '12

03 Sep '12

The median function in the hstats package uses a naive O(n log n) algorithm. Is there another package providing an O(n) option? If not, what would it take to get the package upgraded?

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hellno - a somewhat different approach to tackling cabal hell
by Danny B 03 Sep '12

03 Sep '12

Like many of us, I've suffered from cabal dependency hell and sought relief. I wasn't exactly happy with sandboxes - because using per-project ones meant package duplication and shared sandboxes suffer from the same issues as the GHC package database itself, i.e. reinstalls can break other projects, etc. So I wrote hellno, which is so named because that's the expression one makes when seeing cabal hell suddenly manifest itself and that's also the expression one makes upon encountering yet ANOTHER cabal wrapper. To quote the README: > Generally, with hellno you'll get the same result as for blowing away your user > package database and doing a nice clean install but without having to recompile > everything and with ability to easily revert back and change between projects. Hellno works by keeping all the compiled packages to itself in a database, so that when you ask it to bring in the dependencies of a project, it will use the precompiled packages if available or install the deps and save them for later reuse. Hellno puts symlinks in the user package database pointing to packages in its storage to make them visible. Mutating the user db is hardly elegant, although shouldn't result in much trouble. It's been working fine for me, so I figured it may be useful to others. You can get hellno from GitHub: https://github.com/db81/hellno I don't mean to say that sandboxing is inherently bad or should not be used, but I guess it's better to consider more than one way.

5 6

Installation issues with Centos
by Manish Trivedi 03 Sep '12

03 Sep '12

# uname -a Linux <<hosename>> 2.6.32-220.2.1.el6.centos.plus.x86_64 #1 SMP Thu Dec 22 23:32:31 GMT 2011 x86_64 x86_64 x86_64 GNU/Linux # cat /proc/version Linux version 2.6.32-220.2.1.el6.centos.plus.x86_64 ( mockbuild(a)c6b18n3.bsys.dev.centos.org) (gcc version 4.4.6 20110731 (Red Hat 4.4.6-3) (GCC) ) #1 SMP Thu Dec 22 23:32:31 GMT 2011 I am trying to install Haskell framework on this server. - Installation for GHC went ok. no error - Installed ghc-7.4.1 from "ghc-7.4.1-x86_64-unknown-linux.tar" then while trying to install (in configure step) Haskell platform version 2012.2.0.0. I get following error: [root@qa-engine1 haskell-platform-2012.2.0.0]# ./configure ************************************************** * * Haskell Platform Source Installer * * For the Haskell Platform 2012.2.0.0 and GHC 7.4.1 * ************************************************** checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu checking for ghc... /usr/local/lib/ghc-7.4.1/ghc checking for ghc-pkg... /usr/local/lib/ghc-7.4.1/ghc-pkg checking for hsc2hs... /usr/local/lib/ghc-7.4.1/hsc2hs checking version of ghc... 7.4.1 checking version of ghc-pkg matches... yes checking ghc actually works... no configure: error: Your installation of ghc does not appear to work. It cannot compile a simple program (see config.log for the details). If you installed ghc from a generic binary tarball then it is worth checking that you have the 'gmp' C library and header files installed. (On Debian-based systems this package is called libgmp3-dev.) ==================================== I have made sure that libc and gmp are indeed installed. while trying use yum to make sure I have gmp installed. ==================================== [root@qa-engine1 haskell-platform-2012.2.0.0]# yum install gmp Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile * base: mirror.stanford.edu * extras: centos.mirror.facebook.net * updates: mirror.stanford.edu Setting up Install Process Package gmp-4.3.1-7.el6_2.2.x86_64 already installed and latest version Nothing to do ======================================== Any help is appreciated. Btw, I could make haskell latest version work on my mac osx. -Manish

5 10

Compiling a haskell project as a shared library and loading it in C/C++
by Farid Neshat 03 Sep '12

03 Sep '12

So I'm trying to follow [this blog][1]. So I tried the code, couldn't compile the haskell part, probably due to the fact the guide uses version 6, but I have version 7 of ghc. By changing the paramaters I could compile it with the following: ghc --make -dynamic -shared -fPIC -no-hs-main -optl '-shared' -optc '-DMODULE=Test' -o Test.so Test.hs module_init.c But the when trying to load it, on dlopen, I get the following error: /usr/lib/ghc/ghc-prim-0.2.0.0/libHSghc-prim-0.2.0.0-ghc7.4.1.so: undefined symbol: stg_forkOnzh I suspect it's because the haskell runtime is not linked to the shared library but adding -L/usr/lib/ghc/ didn't really help. I searched a lot, but couldn't find anymore example that somebody have done this. [Here's also some docs][2] for compiling shared libraries: [1]: http://weblog.haskell.cz/pivnik/building-a-shared-library-in-haskell [2]: http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/using-shared-libs.ht…

2 1

Vector: blit'ing a source into a destination
by Nicolas Trangez 02 Sep '12

02 Sep '12

All, For some code I need to alter an (unboxed or storable) vector by blit'ing (copying) data from another (smaller) vector into it, overwriting the existing data. I couldn't find a function for this in the vector API, so I hacked something up which cuts the jobs (see below), but it feels really ugly/dirty. Any ideas about a better way to tackle this, and wouldn't this be a useful addition to the standard API? Thanks, Nicolas import Data.Vector.Unboxed (Unbox, Vector) import qualified Data.Vector.Unboxed as UV import qualified Data.Vector.Unboxed.Mutable as MUV blit :: Unbox a => Vector a -> Vector a -> Int -> Vector a blit src dest offset = UV.modify act dest where act dest' = do let slice = MUV.slice offset len dest' src' <- UV.unsafeThaw src MUV.copy slice src' len = UV.length src main :: IO () main = do let v = UV.fromList [0 :: Int .. 10] b = UV.fromList [20, 30, 40] putStrLn $ "v = " ++ show v putStrLn $ "b = " ++ show b putStrLn $ "blit b v 2 = " ++ show (blit b v 2) -- v = fromList [0,1,2,3,4,5,6,7,8,9,10] -- b = fromList [20,30,40] -- blit b v 2 = fromList [0,1,20,30,40,5,6,7,8,9,10]

1 0

Is there a tool for automatic update of other-extensions and other-modules sections in a .cabal file?
by Tsuyoshi Ito 02 Sep '12

02 Sep '12

Hello, I have a *.cabal file which contains a package description for several executables (which share some of the source files). Is there a tool which updates other-extensions section and other-modules section in the .cabal file automatically to synchronize them to the source files? If it matters, I am using Haskell Platform 2012.2.0.0 on Windows. (I am aware that neither “cabal configure” nor “cabal build” requires these sections, and therefore it is not strictly necessary to update them during development of a program. But when I use a version control system such as Git, I find it ugly to store a .cabal file which is not synchronized to the latest source files… this is the primary reason why I would like such a tool.) Best regards, Tsuyoshi

1 0

Re: [Haskell-cafe] [Haskell] ANNOUNCE: Perdure
by Felipe Almeida Lessa 01 Sep '12

01 Sep '12

[moving discussion to haskell-cafe] Congratulations and thanks for your new open source contribution! I hope you feel at home =). Your library looks really interesting but I'm completely overwhelmed by its size. Its Cabal description is huge and there's no example of how to use the library (it actually took me some time to understand what the library tries to accomplish). I took a quick look at the modules but there are so many that I couldn't really make sense of anything in a short time. So, could you perhaps write a full example of how to use the library? It looks like a nice library but I need a starting point =). Cheers! -- Felipe.

2 1

Yesod a href render issue
by Thiago Negri 01 Sep '12

01 Sep '12

I'm following Yesod tutorial that gives this as the first example for "type-safe URLs": | getHomeR = defaultLayout [whamlet|<a href=@{Page1R}>Go to page 1!|] Worked fine, the "a href" generated looks perfect. Then I tried this: | getHomeR = defaultLayout [whamlet|Hello!<a href=@{Page1R}>Go to page 1!|] And got a "a href" referecing a URL without quotes and with no end tag for "a", something like this: | <html><head><title></title></head><body>Hello<a href=/page1>Go to page 1!</body></html> I tried in many different ways, and the only way to get it working properly was to set the "a" tag in a line by itself: | getHomeR = defaultLayout [whamlet|Hello! | <a href=@{Page1R}>Go to page 1! | |] Is this a failure of Yesod quasiquotation or am I missing something? I know Yesod clearly states that the best approach is to use external files instead of quasiquotes, but as I'm making my first steps into Yesod, I would like to use the practical embedded quasiquotes.

3 2

Build regressions due to GHC 7.6
by Bryan O'Sullivan 01 Sep '12

01 Sep '12

Since the release of the GHC 7.6 RC, I've been going through my packages and fixing up build problems so that people who upgrade to 7.6 will have a smooth ride. Sad to say, my experience of 7.6 is that it has felt like a particularly rough release for backwards incompatibility. I wanted to quantify the pain, so I did some research, and here's what I found. I maintain 25 open source Haskell packages. Of these, the majority have needed updates due to the GHC 7.6 release: - base16-bytestring - blaze-textual - bloomfilter - configurator - criterion - double-conversion - filemanip - HDBC-mysql - mwc-random - pcap - pool - riak-haskell-client - snappy - text - text-format - text-icu That's 16 out of 25 packages I've had to update. I've also either reported bugs on, or had to fix, several other people's packages along the way (maybe four?). So let's say I've run into problems with 20 out of the combined 29 packages of mine and my upstreams. The reasons for these problems fall into three bins: - Prelude no longer exports catch, so a lot of "import Prelude hiding (catch)" had to change. - The FFI now requires constructors to be visible, so "CInt" has to be imported as "CInt(..)". - bytestring finally got bumped to 0.10, so many upper bounds had to be relaxed (*cf* my suggestion that the upper-bounds-by-default policy is destructive). It has been a lot of work to test 29 packages, and then modify, rebuild, and release 20 of them. It has consumed most of my limited free time for almost two weeks. Worse, this has felt like make-work, of no practical benefit to anyone beyond scrambling to restore the status quo ante. If over half of my packages needed fixing, I'm alarmed at the thought of the effects on the rest of Hackage. I'm torn over this. I understand and agree with the impetus to improve the platform by tidying things up, and yet just two seemingly innocuous changes (catch and FFI) have forced me to do a bunch of running to stand still. I don't have any suggestions about what to do; I know that it's hard to estimate the downstream effects of what look like small changes. And so I'm not exactly complaining. Call this an unhappy data point.

10 15

Function names in Haskell lib not first-class on web!
by damodar kulkarni 01 Sep '12

01 Sep '12

Hi Cafe, It seems, the function names in Haskell libs are not first-class objects, AT LEAST when it comes to searching for them of the net! I was trying to search for the following Haskell functions in the mailing list archives. Here is a summary of the responses I have had from various servers upon searching various "valid and normally used" Haskell function identifiers from well-known libraries. Responses from the http://www.mail-archive.com >>=<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=%3E%3E%3D>, It seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=*+-%3E+*>There are 0 results. and so on... Responses from the http://search.gmane.org >>=<http://search.gmane.org/search.php?group=gmane.comp.lang.haskell.cafe&query…> Here, too, it seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://search.gmane.org/?query=*+-%3E+*&author=&group=gmane.comp.lang.haske…> Here it seems the script searched for the string "&*gt*;" and so on... *Note: google badly fails to search these functions identifiers.* My current solution to this problem is: download compressed text archives and do a simple grep on it on my machine. But the archives up to year 2000 only are found at http://www.haskell.org/pipermail/haskell-cafe/. So, as an aside: Where do I find the earlier messages for downloading? Thanks, Damodar

4 4

Data.Text UTF-8 question
by jeff p 31 Aug '12

31 Aug '12

Hello, I have a sample file (attached) which I cannot read into Text: Prelude Control.Applicative> Data.Text.IO.readFile "foo" *** Exception: utf8.txt: hGetContents: invalid argument (invalid byte sequence) Prelude Control.Applicative> Data.Text.Encoding.decodeUtf8 <$> Data.ByteString.Char8.readFile "foo" "*** Exception: Cannot decode byte '\x6e': Data.Text.Encoding.decodeUtf8: Invalid UTF-8 stream So it seems that foo doesn't contain valid UTF-8. However, System.IO.UTF8 has no problem reading the data: Prelude Control.Applicative> System.IO.UTF8.readFile "foo" "3591,,,dihigma99h,1905,5,25,CUBA,,Matanzas,1971,5,20,CUBA,,Cienfuegos,Martin,Dihigo,,Mart\65533n Magdaleno Dihigo (Llanos),,190,74,R,R,,,,dihigma99,dihigma99,dihim001,dihigma99,dihigma99\r\n" Shouldn't these all have the same behavior? I am running on Mac OS X 10.8.1, with GHC 7.4.2 and text-0.11.2.3. thanks for any insight, Jeff

3 2

Is it worth adding Gaussian elimination and eigenvalues to REPA?
by KC 31 Aug '12

31 Aug '12

I realize if one wants speed you probably want to use the hMatrix interface to GSL, BLAS and LAPACK. Worth it in the sense of have a purely functional implementation. -- -- Regards, KC

2 1

Code review request - using ErrorT
by C K Kashyap 31 Aug '12

31 Aug '12

Dear gentle Haskellers, I'd appreciate it very much if you could please take a look the my code here and give me some feedback - https://github.com/ckkashyap/haskell-websocket/blob/master/src/lib/Net/WebS… I am particularly looking for advice around what would be a good type for a server handler function that does IO and needs to short circuit if it fails at any point also needs to thread a state along. I am considering this - ErrorT String (StateT Int IO) Data Regards, Kashyap

1 0

why do I need class context in declaring data constructor?
by Paul Liu 31 Aug '12

31 Aug '12

I had a toy program that encodes simply typed lambda in types. It used to work fine with GHC prior to 7.2. But now it no longer compiles. Here is a minimal fragment that demonstrates this problem. > {-# LANGUAGE GADTs, > MultiParamTypeClasses, > FlexibleInstances, > FlexibleContexts #-} > > data Abs env t v where > Abs :: g (a, env) h v -> Abs env (g (a, env) h v) (a -> v) > > class Eval g env t v where > eval :: env -> g env t v -> v > > instance Eval g (a, env) h v => > Eval Abs env (g (a, env) h v) (a -> v) where > eval env (Abs e) = \x -> eval (x, env) e The type Abs has 3 parameters: its environment, sub term (encoded in types), and type. The constructor Abs has 1 parameter: its sub term. The code loads fine in GHC 7.0.3. Here is the error reported by GHC 7.2.2 (and later): test.lhs:14:30: Could not deduce (Eval g1 (a1, env) h1 v1) arising from a use of `eval' from the context (Eval g (a, env) h v) bound by the instance declaration at test.lhs:(12,12)-(13,49) or from (g (a, env) h v ~ g1 (a1, env) h1 v1, (a -> v) ~ (a1 -> v1)) bound by a pattern with constructor Abs :: forall env (g :: * -> * -> * -> *) a h v. g (a, env) h v -> Abs env (g (a, env) h v) (a -> v), in an equation for `eval' at test.lhs:14:15-19 Possible fix: add (Eval g1 (a1, env) h1 v1) to the context of the data constructor `Abs' or the instance declaration or add an instance declaration for (Eval g1 (a1, env) h1 v1) In the expression: eval (x, env) e In the expression: \ x -> eval (x, env) e In an equation for `eval': eval env (Abs e) = \ x -> eval (x, env) e However, if I move the class context to the data constructor of definition, then it compiles fine in GHC 7.2.2 (and later): > data Abs env t v where > Abs :: Eval g (a, env) h v => g (a, env) h v -> Abs env (g (a, env) h v) (a -> v) But this is very troublesome because for every new class instance I want to make Abs of, I have to make a new class context to the data constructor. It totally defeats the purpose of making class instances to extend usage of data types. Did I missed a language extension when moving code from GHC 7.0.3 to GHC 7.2.2? What can I do to fix it for newer GHCs? -- Regards, Paul Liu

2 2

Sliding Window functional data structure
by Okasaki, Christopher D CIV USA USMA 31 Aug '12

31 Aug '12

One possibility would be a random-access list (http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.55.5156) which is a kind of list of trees. Then 'add' is trivially O(1) and 'since' is trivially O(length result). The only tricky operation is purge, which naively would be O(N), where N is the length of entries. But that can be reduced using a kind of lazy deletion. Pair the random-access list with a single k, meaning "everything <= k is considered deleted, even if it is still physically present in the data structure". Purge would update the k and scan the list of trees, removing any tree with a root <= k. There are only a logarithmic number of trees, so this scan would be O(log N). In fact, it could be made O(log #remaining entries) by noting that, when one such tree is found, the list can just be chopped off there because all later trees should also be removed. The downside of lazy deletion is that you might use a bit more space than you need, because a portion of the last tree in the list might be logically deleted but still present. I would expect this effect to be minor in practice. Taking this approach, there is one situation that would require clarification, and possibly special handling: If you call purge with a k that is larger than the most recent add, thereby purging everything, are you later permitted to add a k' that is smaller than the purged k? This might be disallowed by extending the precondition of add. But if it is allowed, then should the new k' be affected by the previous purge, or not? It's easy to implement either way; it just depends on how you want to specify the desired behavior.

1 0

Sliding Window functional data structure
by Richard O'Keefe 31 Aug '12

31 Aug '12

Consider the following interface type Ord k => Sliding_Window k v entries :: Sliding_Window k v -> [(k,v)] The cost is expected to be linear in the length of the result. The pairs are listed in increasing order of k. add :: Ord k => k -> v -> Sliding_Window k v -> Sliding_Window k v precondition: all (< k) [k' | (k',_) <- entries q] The cost should be at most O((log . length . entries) q). post: entries (add k v q) = entries q ++ [(k,v)] since :: Ord k => k -> Sliding_Window k v -> [(k,v)] answers [(k',v) | (k',v) <- entries q, k' > k] The cost should be at most O((log . length . entries) q + length result) purge :: Ord k => k -> Sliding_Window k v -> Sliding_Window k v answers q' such that entries q' = [(k',v) | (k',v) <- entries q, k' > k] The cost should be at most O((log . length . entries) q + length [k' | (k',v) <- entries q, k' <= k]) Ignoring costs, this can obviously be done trivially using a list of pairs. Paying some attention to costs, it can also be done using some sort of balanced search tree. The data structure is close to a priority queue, but subtly different. I believe I can see how to do this in an imperative language using a Dijkstra-style array of pairs: add is amortised O(1) using the well known doubling strategy, thanks to the strictly increasing key requirement; since is a binary search followed by a segment copy; purge is a binary search followed by nilling out the now unwanted elements. By adapting the back-to-back pair of lists implementation of queues, we can obviously do add in O(1) and purge in O(1+#deleted items) time in a pure functional language. Thing is, there's a baffling array of data structures to examine (AVL, RB, 2-3, 2-3-4, 1-2-brother, finger ... trees) and I wondered if anyone had any idea what would be particularly good for this rather asymmetric problem.

3 2

lambda.haskell.org is on an mxtools blacklist
by jims＠stuttard.org 31 Aug '12

31 Aug '12

I had my subscription to haskell-cafe deleted because of intermittent bounces. I raised a support ticket with my ISP bluehost.com and they suggested the bounces might be because 78.46.100.80 is on the mxtoolbox blacklist as reported here: http://www.mxtoolbox.com/SuperTool.aspx?action=blacklist%3a78.46.100.180 HTH.

1 0

ANNOUCE: monad-task-0.1.0, GLFW-task-0.1.0 and a tutorial on invert the inversion of control
by Paul Liu 31 Aug '12

31 Aug '12

Graphics program often relies on system frameworks such as GLUT or GLFW to handle window and user input. But such frameworks often impose a rigid style of callback programming, or inversion of control. It is no secret that continuation monad can deal with this issue [1]. It is also known that concurrency can be naturally expressed as continuation monads [2], and a hybrid model of event and threads [3] brings the best of both worlds. But then I'm a bit surprised to find no single library on HackageDB that can deal with this issue. Further more, I wanted something that can escape the IO monad imposed by callbacks (and FFI) and be flexible. (no, monad-control is not a solution here). So I wrote a monad-task library to handle both co-operative threads (or co-routines) and event processing in a TaskT transformer. To demonstrate its usage, I also uploaded a GLFW-task library that helps to program task monad with GLFW, which contains a sample application if you unpack its source. I documented the rationales behind this work in a (rather length but runnable literal Haskell) tutorial "Invert the Inversion of Control", and welcome any suggestions and feedbacks. monad-task: http://hackage.haskell.org/package/monad-task GLFW-task: http://hackage.haskell.org/package/GLFW-task tutorial: http://www.thev.net/PaulLiu/invert-inversion.html [1]: Quick and dirty reinversion of control, Dan Piponi. http://blog.sigfpe.com/2011/10/quick-and-dirty-reinversion-of-control.html [2]: Functional Pearls: A Poor Man's Concurrency Monad, Koen Claessen, 1999 [3]: Combining Events And Threads For Scalable Network Services. Peng Li and Steve Zdancewic. PLDI, 2007. -- Regards, Paul Liu

1 0

createProcess running non-existent programs
by Niklas Hambüchen 31 Aug '12

31 Aug '12

I just came across the fact that running createProcess (proc "asdfasdf" []) with non-existing command "asdfasdf" returns perfectly fine handles. I would expect an exception. You can even hGetContents on stdout: You just get "". I find this highly counter-intuitive. Is this intended? Thanks Niklas PS: I checked how some other programming languages do this: Python: import subprocess; subprocess.call("asdfasdf", shell=False) OSError: [Errno 2] No such file or directory Ruby: exec("asdfasdf") Errno::ENOENT: No such file or directory - asdfasdf

9 14

Re: [Haskell-cafe] A first glimps on the {-# NOUPDATE #-} pragma
by Facundo Domínguez 31 Aug '12

31 Aug '12

Hi, > upd_noupd n = > let l = myenum' 0 n > in last l + length l This could be rewritten as > upd_noupd n = > let l n = myenum' 0 n > in last (l n) + length (l n) Or a special form of let could be introduced to define locally-scoped macros: > upd_noupd n = > let# l = myenum' 0 n > in last l + length l What's the strength of the {-# NOUPDATE #-} approach? Regards, Facundo > Date: Wed, 29 Aug 2012 10:20:21 +0200 > From: Joachim Breitner <breitner(a)kit.edu> > Subject: Re: [Haskell-cafe] A first glimps on the {-# NOUPDATE #-} > pragma > To: haskell-cafe(a)haskell.org > Message-ID: <1346228421.4271.9.camel@kirk> > Content-Type: text/plain; charset="iso-8859-15" > > Hi, > > Am Dienstag, den 28.08.2012, 18:16 -0400 schrieb Carter Schonwald: >> On Tuesday, August 28, 2012, Yves Par?s wrote: >> Monad? Simple strictness anotation is enough in that case: >> >> upd_noupd n = >> let l = myenum' 0 n >> h = head l >> in h `seq` last l + h > > darn, I though I changed my examples before posting to: > > upd_noupd n = > let l = myenum' 0 n > in last l + length l > > ("head" was a bad example because actually due to strictness analysis > and worker/wrapper transformation, GHC would yield good code even > without "`seq`"). > > In this case (which is actually the benchmarked case), reordering or > strictness annotations do not help. > > Greetings and sorry for the confusion, > Joachim > > > -- > Dipl.-Math. Dipl.-Inform. Joachim Breitner > Wissenschaftlicher Mitarbeiter > http://pp.info.uni-karlsruhe.de/~breitner

4 5

happstack simpleHTTP & state monad
by Corentin Dupont 30 Aug '12

30 Aug '12

Hi all, I'm trying to make a web server that manages its own state. The user can issue commands that modifies the state. I did like below but unfortunatly the state is not keep after a command is issued... What is the right way to do it? Is there any example sites with an internal state with happstack? *data Game = (the state of my game) type NomicServer = ServerPartT (StateT Game IO) launchWebServer :: Game -> IO () launchWebServer initialState = do putStrLn "Starting web server...\nTo connect, drive your browser to \" http://localhost:8000/Login\"" d <- getDataDir simpleHTTP' unpackStateT nullConf $ server d server :: FilePath -> ServerPartT (StateT Game IO) Response server d sh = mconcat [fileServe [] d, do decodeBody (defaultBodyPolicy "/tmp/" 4096 4096 4096) html <- implSite "http://localhost:8000/" "" nomicSite return $ toResponse html] unpackStateT:: Game -> UnWebT (StateT Game IO) Response -> UnWebT IO Response unpackStateT g w = evalStateT w g --handler for web routes nomicSite :: Site PlayerCommand (NomicServer Html) nomicSite = setDefault (Noop 0) Site { handleSite = \f url -> unRouteT (routedNomicCommands url) f , formatPathSegments = \u -> (toPathSegments u, []) , parsePathSegments = parseSegments fromPathSegments }* Thanks a lot, Corentin

2 2

paths in .cabal/config
by Semen Trygubenko 30 Aug '12

30 Aug '12

Dear Haskell-Cafe, I am wondering if there is a way to use variables in paths that appear in .cabal/config file? I have written a custom config file that I would like to share. However, my config file contains full paths s.a. world-file: /home/semen/.cabal/world etc. Is there any way I could make it less personal, e.g. world-file: /home/${HOME}/.cabal/world Many thanks, Sem -- Семен Тригубенко

1 0

Problem with searching mailing-lists archives
by damodar kulkarni 30 Aug '12

30 Aug '12

Hi Cafe, I was trying to search for the following Haskell functions in the mailing list archives. Here is a summary of the responses I have had from various servers upon searching various "valid and normally used" Haskell function identifiers. Responses from the http://www.mail-archive.com >>=<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=%3E%3E%3D>, It seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=*+-%3E+*>There are 0 results. and so on... Responses from the http://search.gmane.org >>=<http://search.gmane.org/search.php?group=gmane.comp.lang.haskell.cafe&query…> Here, too, it seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://search.gmane.org/?query=*+-%3E+*&author=&group=gmane.comp.lang.haske…> Here it seems the script searched for the string "&*gt*;" and so on... *Note: google badly fails to search these functions identifiers.* My current solution to this problem is: download compressed text archives and do a simple grep on it on my machine. But the archives up to year 2000 only are found at http://www.haskell.org/pipermail/haskell-cafe/. So, as an aside: Where do I find the earlier messages for downloading? Thanks, Damodar

1 0

Darcs fetches too little files
by Ramana Kumar 30 Aug '12

30 Aug '12

On Aug 29, 2012 10:56 PM, "Henk-Jan van Tuyl" <hjgtuyl(a)chello.nl> wrote: > > On Wed, 29 Aug 2012 23:10:24 +0200, Stefan Monnier < monnier(a)iro.umontreal.ca> wrote: > >>> Albert Einstein said: >>> Insanity: doing the same thing over and over again and expecting >>> different results. >>> I repeated the command today and it worked! >> >> >> So, did you expect the result to be different, or did you re-try just to >> confirm that it doesn't work? > > > The day after I tried the command the first time, I thought the result might be different; I have the experience that my browser crashes, and starting it again results in an immediate crash. Logging off and on again solves the problem. This might be because of a corrupted DLL or a corrupted data area of a DLL. > > The command failure could have also been caused by a problem at the server side. > > In conclusion: repeating the same thing could give different results. This depends on what counts as "the same". (There is a nice book called "The Subtlety of Sameness".) I'm surprised that as users of a language that makes a big deal out of referential transparency none of you have said that neither invoking the same command at a different time nor invoking a browser after changing context (logging in again) is doing the same thing. > > > Regards, > Henk-Jan van Tuyl > > > -- > http://Van.Tuyl.eu/ > http://members.chello.nl/hjgtuyl/tourdemonad.html > Haskell programming > -- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Weekly News: Issue 242
by Daniel Santa Cruz 30 Aug '12

30 Aug '12

Welcome to issue 242 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 19 to 25, 2012. Quotes of the Week * srhb: I think that's going into space rather than diving into the deep end. * edwardk: normally i don't bother with little things like "documentation", "benchmarks" and "tests", but I recently discovered that when I do, I get these curious things called "users". * shachaf: "Number of comonads released by edwardk will exceed the number of words in the English language by 2015, experts report." Top Reddit Stories * Hackage2 server ready for testing Domain: haskell.org, Score: 75, Comments: 16 On Reddit: [1] http://goo.gl/WTebL Original: [2] http://goo.gl/dHkGo * Monads In Pictures Domain: newartisans.com, Score: 60, Comments: 24 On Reddit: [3] http://goo.gl/QMfGv Original: [4] http://goo.gl/ycmvx * A new fast and easy to use CSV library Domain: blog.johantibell.com, Score: 53, Comments: 26 On Reddit: [5] http://goo.gl/eRKSm Original: [6] http://goo.gl/uRmfz * duckduckgo.com just became my favorite search engine due to haskell support Domain: self.haskell, Score: 50, Comments: 14 On Reddit: [7] http://goo.gl/1tBCg Original: [8] http://goo.gl/1tBCg * An example on how ridiculously powerful haskells type system is Domain: chrisdone.com, Score: 44, Comments: 61 On Reddit: [9] http://goo.gl/iwJT2 Original: [10] http://goo.gl/NovM7 * Galois Internship Available: Come build high-assurance autonomous vehicles! Domain: corp.galois.com, Score: 43, Comments: 13 On Reddit: [11] http://goo.gl/iJEih Original: [12] http://goo.gl/8ckL4 * Issue 20 « The Monad.Reader Domain: themonadreader.wordpress.com, Score: 34, Comments: 8 On Reddit: [13] http://goo.gl/K5Cc7 Original: [14] http://goo.gl/dBSdK * Functional Game Design: Making Pong in Elm Domain: elm-lang.org, Score: 29, Comments: 11 On Reddit: [15] http://goo.gl/sCdfE Original: [16] http://goo.gl/RL2i1 * Making HaskellDB slightly more type-safe Domain: chrisdone.com, Score: 28, Comments: 3 On Reddit: [17] http://goo.gl/b16v5 Original: [18] http://goo.gl/7v38l * Identifying outdated packages in cabal install plans Domain: byorgey.wordpress.com, Score: 24, Comments: 5 On Reddit: [19] http://goo.gl/4yTrR Original: [20] http://goo.gl/Zjw7b * ghcLiVE project escapes! Domain: ghclive.wordpress.com, Score: 23, Comments: 16 On Reddit: [21] http://goo.gl/IiXaA Original: [22] http://goo.gl/QzUHW * My view on why relational algebra matters Domain: users.utu.fi, Score: 22, Comments: 16 On Reddit: [23] http://goo.gl/byYZW Original: [24] http://goo.gl/3NSmI * ANN basic-prelude 0.3 (now comes with a fully-featured Prelude replacement) Domain: haskell.org, Score: 21, Comments: On Reddit: [25] http://goo.gl/klUAO Original: [26] http://goo.gl/4Ccyv * I just realized that running monads is applying a left monad action Domain: self.haskell, Score: 21, Comments: 53 On Reddit: [27] http://goo.gl/Geus4 Original: [28] http://goo.gl/Geus4 * You can soon play in the cabal sandbox Domain: blog.johantibell.com, Score: 20, Comments: 10 On Reddit: [29] http://goo.gl/QuVo0 Original: [30] http://goo.gl/6Q9a9 * Unordered tuples and type algebra Domain: byorgey.wordpress.com, Score: 18, Comments: 3 On Reddit: [31] http://goo.gl/N8iuz Original: [32] http://goo.gl/8l11g * Haskell-Type-Exts Passed the Final Evaluation Domain: cleantypecheck.wordpress.com, Score: 16, Comments: 3 On Reddit: [33] http://goo.gl/ok7CL Original: [34] http://goo.gl/ttsFb * LambdaJam Conference (from the creator of Strange Loop) -- Coming to Chicago in Summer 2013! Domain: lambdajam.com, Score: 15, Comments: On Reddit: [35] http://goo.gl/ytWNE Original: [36] http://goo.gl/ziyYc * Hello world in SKI combinator calculus Domain: self.haskell, Score: 14, Comments: 16 On Reddit: [37] http://goo.gl/qVDMw Original: [38] http://goo.gl/qVDMw * Is this Monoid instance in the standard library somewhere? Domain: self.haskell, Score: 14, Comments: 7 On Reddit: [39] http://goo.gl/cFHG7 Original: [40] http://goo.gl/cFHG7 * Applicative functors: definition and syntax (a reply) Domain: tomasp.net, Score: 13, Comments: 7 On Reddit: [41] http://goo.gl/4VPKR Original: [42] http://goo.gl/saU2j Top StackOverflow Questions * Lazy Evaluation and Time Complexity votes: 40, answers: 7 Read on SO: [43] http://goo.gl/f9QTO * What is the purpose of Rank2Types? votes: 21, answers: 3 Read on SO: [44] http://goo.gl/XrPHP * “Modern” HList? votes: 16, answers: 1 Read on SO: [45] http://goo.gl/X0LJ4 * Overloading of operators votes: 14, answers: 1 Read on SO: [46] http://goo.gl/1rB0O * When is a composition of catamorphisms a catamorphism? votes: 11, answers: 4 Read on SO: [47] http://goo.gl/x7eBk * Tacit function composition in Haskell votes: 10, answers: 2 Read on SO: [48] http://goo.gl/TIvMN * Haskell pattern matching on GADTs with Data Kinds votes: 9, answers: 1 Read on SO: [49] http://goo.gl/5fgAR * Store polymorphic callbacks in Haskell votes: 8, answers: 1 Read on SO: [50] http://goo.gl/QMOlq * Having trouble understanding Haskell's type system votes: 8, answers: 2 Read on SO: [51] http://goo.gl/n8FfC * Function composition hint votes: 7, answers: 2 Read on SO: [52] http://goo.gl/kvgZj * Comparing speed of Haskell and C for the computation of primes votes: 6, answers: 4 Read on SO: [53] http://goo.gl/gPh02 Until next time, Daniel Santa Cruz References 1. http://www.haskell.org/pipermail/cabal-devel/2012-August/008977.html 2. http://www.reddit.com/r/haskell/comments/yr9t1/hackage2_server_ready_for_te… 3. http://newartisans.com/2012/08/monads-in-pictures/ 4. http://www.reddit.com/r/haskell/comments/ykmgg/monads_in_pictures/ 5. http://blog.johantibell.com/2012/08/a-new-fast-and-easy-to-use-csv-library.… 6. http://www.reddit.com/r/haskell/comments/yqh7z/a_new_fast_and_easy_to_use_c… 7. http://www.reddit.com/r/haskell/comments/yjdkh/duckduckgocom_just_became_my… 8. http://www.reddit.com/r/haskell/comments/yjdkh/duckduckgocom_just_became_my… 9. http://chrisdone.com/posts/2010-10-07-haskelldb-and-typeoperator-madness.ht… 10. http://www.reddit.com/r/haskell/comments/yosoe/an_example_on_how_ridiculous… 11. http://corp.galois.com/blog/2012/8/21/galois-internship-available.html 12. http://www.reddit.com/r/haskell/comments/yl84x/galois_internship_available_… 13. http://themonadreader.wordpress.com/2012/08/25/issue-20/ 14. http://www.reddit.com/r/haskell/comments/yu9bu/issue_20_the_monadreader/ 15. http://elm-lang.org/blog/games-in-elm/part-0/Making-Pong.html 16. http://www.reddit.com/r/haskell/comments/yp27x/functional_game_design_makin… 17. http://chrisdone.com/posts/2012-08-25-haskelldb-more-type-safe.html 18. http://www.reddit.com/r/haskell/comments/ysuvr/making_haskelldb_slightly_mo… 19. http://byorgey.wordpress.com/2012/08/25/identifying-outdated-packages-in-ca… 20. http://www.reddit.com/r/haskell/comments/ytckc/identifying_outdated_package… 21. http://ghclive.wordpress.com/2012/08/20/ghclive-project-escapes/ 22. http://www.reddit.com/r/haskell/comments/yjtql/ghclive_project_escapes/ 23. http://users.utu.fi/machra/posts/2012-08-23-relalgebra.html 24. http://www.reddit.com/r/haskell/comments/ypdox/my_view_on_why_relational_al… 25. http://www.haskell.org/pipermail/haskell/2012-August/023477.html 26. http://www.reddit.com/r/haskell/comments/yralu/ann_basicprelude_03_now_come… 27. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 28. http://www.reddit.com/r/haskell/comments/ytovd/i_just_realized_that_running… 29. http://blog.johantibell.com/2012/08/you-can-soon-play-in-cabal-sandbox.html 30. http://www.reddit.com/r/haskell/comments/ys359/you_can_soon_play_in_the_cab… 31. http://byorgey.wordpress.com/2012/08/24/unordered-tuples-and-type-algebra/ 32. http://www.reddit.com/r/haskell/comments/ysuyr/unordered_tuples_and_type_al… 33. http://cleantypecheck.wordpress.com/2012/08/24/haskell-type-exts-passed-the… 34. http://www.reddit.com/r/haskell/comments/yrzcq/haskelltypeexts_passed_the_f… 35. http://lambdajam.com/ 36. http://www.reddit.com/r/haskell/comments/yprlk/lambdajam_conference_from_th… 37. http://www.reddit.com/r/haskell/comments/ypcu7/hello_world_in_ski_combinato… 38. http://www.reddit.com/r/haskell/comments/ypcu7/hello_world_in_ski_combinato… 39. http://www.reddit.com/r/haskell/comments/yqt89/is_this_monoid_instance_in_t… 40. http://www.reddit.com/r/haskell/comments/yqt89/is_this_monoid_instance_in_t… 41. http://tomasp.net/blog/applicative-functors.aspx 42. http://www.reddit.com/r/haskell/comments/ykto4/applicative_functors_definit… 43. http://stackoverflow.com/questions/12057658/lazy-evaluation-and-time-comple… 44. http://stackoverflow.com/questions/12031878/what-is-the-purpose-of-rank2typ… 45. http://stackoverflow.com/questions/12078792/modern-hlist 46. http://stackoverflow.com/questions/12076475/overloading-of-operators 47. http://stackoverflow.com/questions/12103309/when-is-a-composition-of-catamo… 48. http://stackoverflow.com/questions/12061176/tacit-function-composition-in-h… 49. http://stackoverflow.com/questions/12067503/haskell-pattern-matching-on-gad… 50. http://stackoverflow.com/questions/12044490/store-polymorphic-callbacks-in-… 51. http://stackoverflow.com/questions/12047481/having-trouble-understanding-ha… 52. http://stackoverflow.com/questions/12025230/function-composition-hint 53. http://stackoverflow.com/questions/12026668/comparing-speed-of-haskell-and-…

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Darcs fetches too little files
by Henk-Jan van Tuyl 30 Aug '12

30 Aug '12

I am trying to fetch wxHaskell with the command darcs get --lazy http://code.haskell.org/wxhaskell/ but there are much too little files downloaded; what could be the problem? I'm working on Windows XP, both in an MS-DOS shell and an MSYS shell. Installed Darcs version: 2.8.1 Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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ANNOUNCE: persistent-vector-0.1.0.1
by Tristan Ravitch 29 Aug '12

29 Aug '12

I uploaded a package implementing persistent vectors using array mapped tries (based on the implementation in clojure). Version 0.1.0.0 was broken, so I am starting off with 0.1.0.1. http://hackage.haskell.org/package/persistent-vector Persistent vectors are a sequence container offering efficient and purely functional append (snoc), indexing, and updates. This is similar to Data.Sequence from containers. The array mapped trie is closely related to the data structure used in the unordered-containers package. Comparison to Sequence: * Faster indexing and append * Slightly slower updates to existing elements * O(1) slicing * Sequence offers efficient prepend and concatenate (persistent-vector does not implement prepend, while concatenate is O(n)). I tried to model the API after Sequence as much as was reasonable, but a few functions are still missing. Some are reasonable to implement and some would be difficult to make efficient. The results from criterion (mostly comparing against Sequence and IntMap) are posted here: http://pages.cs.wisc.edu/~travitch/pvec.html The *Vec* runs are the vectors from this package and IM and Seq are IntMap and Sequence from containers. Hopefully someone will find this useful. Comments and suggestions are definitely welcome for anything: API, implementation, test suite, or benchmarks.

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Re: [Haskell-cafe] A first glimps on the {-# NOUPDATE #-} pragma
by Carter Schonwald 29 Aug '12

29 Aug '12

You got me there. Excellent point On Tuesday, August 28, 2012, Yves Parès wrote: > Monad? Simple strictness anotation is enough in that case: > > upd_noupd n = > let l = myenum' 0 n > h = head l > in h `seq` last l + h > > Le mardi 28 août 2012 22:39:09 UTC+2, Carter Schonwald a écrit : >> >> Hey Joachim, >> isn't this an example where the exact same issue could be solved via some >> suitable use of a monad for ordering those two computations on l? >> >> cheers >> -Carter >> >> On Tue, Aug 28, 2012 at 12:44 PM, Joachim Breitner <brei...(a)kit.edu>wrote: >> >>> Dear GHC users, >>> >>> I am experimenting with ways to /prevent/ sharing in Haskell, e.g. to >>> avoid space leaks or to speed up evaluation. A first attempt was to >>> duplicate closures on the heap to preserve the original one, see >>> http://arxiv.org/abs/1207.2017 for a detailed description and >>> information on the prototype implementation; no GHC patching required >>> for that. >>> >>> Currently I am trying a different angle: Simply avoid generating the >>> code that will update a closure after its evaluation; hence the closure >>> stays a thunk and will happily re-evaluate the next time it is used. >>> >>> Here is a classical example. Take this function (it is basically [f..t] >>> but with a fixed type and no risk of existing rules firing): >>> >>> myenum :: Int -> Int -> [Int] >>> myenum f t = if f <= t >>> then f : myenum (f + 1) t >>> else [] >>> >>> and this example where sharing hurts performance badly: >>> >>> upd_upd n = >>> let l = myenum 0 n >>> in last l + head l >>> >>> The problem is that during the evaluation of "last l", the list is live >>> and needs to be kept in memory, although in this case, re-evaluating l >>> for "head l" would be cheaper. If n is 50000000, then this takes 3845ms >>> on my machine, measured with criterion, and a considerable amount of >>> memory (3000MB). >>> >>> So here is what you can do now: You can mark the value as >>> non-updateable. We change myenum to >>> >>> myenum' :: Int -> Int -> [Int] >>> myenum' f t = if f <= t then f : ({-# NOUPDATE #-} myenum' (f + >>> 1) t) else [] >>> >>> and use that: >>> >>> upd_noupd n = >>> let l = myenum' 0 n >>> in last l + head l >>> >>> The improvement is considerable: 531ms and not much memory used (18MB) >>> >>> >>> Actually, it should suffice to put the pragma in the definition of l >>> without touching myenum: >>> >>> noupd_noupd n = >>> let l = {-# NOUPDATE #-} myenum 0 n >>> in last l + head l >>> >>> but this does not work with -O due to other optimizations in GHC. (It >>> does work without optimization.) >>> >>> >>> The next step would be to think of conditions under which the compiler >>> could automatically add the pragma, e.g. when it sees that evaluation a >>> thunk is very cheap but will increase memory consumption considerable. >>> >>> >>> Also this does not have to be a pragma; it could just as well be a >>> function "noupdate :: a -> a" that is treated specially by the compiler, >>> similar to the "inline" function. >>> >>> >>> If you want to play around this, feel free to fetch it from the unshare >>> branch of my ghc repository at http://git.nomeata.de/?p=ghc.**git<http://git.nomeata.de/?p=ghc.git>or >>> https://github.com/nomeata/ghc for the GitHub-lovers. Note that the >>> branch is repeatedly rebased against ghc master. >>> >>> >>> Greetings, >>> Joachim >>> >>> >>> -- >>> Dipl.-Math. Dipl.-Inform. Joachim Breitner >>> Wissenschaftlicher Mitarbeiter >>> http://pp.info.uni-karlsruhe.**de/~breitner<http://pp.info.uni-karlsruhe.de/%7Ebreitner> >>> >>> ______________________________**_________________ >>> Haskell-Cafe mailing list >>> Haskel...(a)haskell.org >>> http://www.haskell.org/**mailman/listinfo/haskell-cafe<http://www.haskell.org/mailman/listinfo/haskell-cafe> >>> >>> >>

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A first glimps on the {-# NOUPDATE #-} pragma
by Joachim Breitner 28 Aug '12

28 Aug '12

Dear GHC users, I am experimenting with ways to /prevent/ sharing in Haskell, e.g. to avoid space leaks or to speed up evaluation. A first attempt was to duplicate closures on the heap to preserve the original one, see http://arxiv.org/abs/1207.2017 for a detailed description and information on the prototype implementation; no GHC patching required for that. Currently I am trying a different angle: Simply avoid generating the code that will update a closure after its evaluation; hence the closure stays a thunk and will happily re-evaluate the next time it is used. Here is a classical example. Take this function (it is basically [f..t] but with a fixed type and no risk of existing rules firing): myenum :: Int -> Int -> [Int] myenum f t = if f <= t then f : myenum (f + 1) t else [] and this example where sharing hurts performance badly: upd_upd n = let l = myenum 0 n in last l + head l The problem is that during the evaluation of "last l", the list is live and needs to be kept in memory, although in this case, re-evaluating l for "head l" would be cheaper. If n is 50000000, then this takes 3845ms on my machine, measured with criterion, and a considerable amount of memory (3000MB). So here is what you can do now: You can mark the value as non-updateable. We change myenum to myenum' :: Int -> Int -> [Int] myenum' f t = if f <= t then f : ({-# NOUPDATE #-} myenum' (f + 1) t) else [] and use that: upd_noupd n = let l = myenum' 0 n in last l + head l The improvement is considerable: 531ms and not much memory used (18MB) Actually, it should suffice to put the pragma in the definition of l without touching myenum: noupd_noupd n = let l = {-# NOUPDATE #-} myenum 0 n in last l + head l but this does not work with -O due to other optimizations in GHC. (It does work without optimization.) The next step would be to think of conditions under which the compiler could automatically add the pragma, e.g. when it sees that evaluation a thunk is very cheap but will increase memory consumption considerable. Also this does not have to be a pragma; it could just as well be a function "noupdate :: a -> a" that is treated specially by the compiler, similar to the "inline" function. If you want to play around this, feel free to fetch it from the unshare branch of my ghc repository at http://git.nomeata.de/?p=ghc.git or https://github.com/nomeata/ghc for the GitHub-lovers. Note that the branch is repeatedly rebased against ghc master. Greetings, Joachim -- Dipl.-Math. Dipl.-Inform. Joachim Breitner Wissenschaftlicher Mitarbeiter http://pp.info.uni-karlsruhe.de/~breitner

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Conduit: Where to run monad stacks?
by Niklas Hambüchen 28 Aug '12

28 Aug '12

Today I was surprised that transPipe is called for every chunk of data going through my pipe, rendering the StateT I put in useless, because it was always restarted with the initial value. It would be nice to have some explanation about this, as it makes it easy to write compiling code that has completely unexpected behaviour. I wrote this function (also on http://hpaste.org/73538) conduitWithState :: (MonadIO m) => Conduit Int (StateT Int m) String conduitWithState = do liftIO $ putStrLn $ "Counting Int->String converter ready!" awaitForever $ \x -> do i <- lift get lift $ modify (+1) liftIO $ putStrLn $ "Converting " ++ show x ++ " to a string! " ++ "Processed so far: " ++ show i yield (show x) and ran it like this: countingConverterConduit :: (MonadIO m) => Conduit Int m String countingConverterConduit = transPipe (\stateTint -> evalStateT stateTint 1) conduitWithState main :: IO () main = do stringList <- CL.sourceList [4,1,9,7,3] $= countingConverterConduit $$ CL.consume print stringList However, the output is not what I expected, but only: Processed so far:1 Processed so far:1 ... Dan Burton proposed a fix, making the whole sink-conduit-source construction run on the StateT: main = do stringList <- flip evalStateT 1 $ ... So the question is: What is the rationale for this? I was expecting that if I have an IO pipe in my main conduit, I could easily run stuff on top of that in parts of the pipe. Thanks Niklas

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OS-independent auto-monitoring of a program to do things depending on resource usage at runtime
by Alberto G. Corona 28 Aug '12

28 Aug '12

For a caching library, I need to know the runtime usage of memory of the program and the total amount of memory, the total memory used by all the programs etc. I need not do profiling or monitoring but to do different things inside my program depending on memory usage. The search is difficult because all searches go to profiling utilities which I don´t need. Are there some portable way to to this? . The various monitoring libraries indicates that there are ways to do it, but they seem not to allow runtime "internal automonitoring"

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When is a composition of catamorphisms a catamorphism?
by Sebastien Zany 28 Aug '12

28 Aug '12

>From page 3 of http://research.microsoft.com/en-us/um/people/emeijer/Papers/meijer94more.p… : it is not true in general that catamorphisms are closed under composition When is this true?

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Simple shell scripts
by Eric Tanter 27 Aug '12

27 Aug '12

Hi, Here is a simple shell script (upper.hs): import Data.Char main = interact $ map toUpper which composes fine with other scripts: bash-3.2$ yes | head -n 3 | runghc upper.hs Y Y Y but not always: bash-3.2$ yes | runghc upper.hs | head -n 3 Y Y Y <stdout>: hFlush: resource vanished (Broken pipe) Any idea why this error occurs/how to avoid it? (running just: yes | runghc upper.hs gives the expected infinite stream of Ys) Thanks! -- Éric

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map over Bijections
by Sergey Mironov 27 Aug '12

27 Aug '12

Hi. I need map equivalent for Bijection type which is defined in fclabels: data Bijection (~>) a b = Bij { fw :: a ~> b, bw :: b ~> a } instance Category (~>) => Category (Bijection (~>)) where ... I can define this function as follows: mapBij :: Bijection (->) a c -> Bijection (->) [a] [b] -> Bijection (->) [a] [c] mapBij b1 b = (map (fw b1)) `Bij` (map (bw b1)) but do I really need to do it explicitly? Can I obtain same result using some Category combinators or other common stuff? Sergey

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