August 2010 - Haskell-Cafe

Quick Question for QuickCheck2
by Sebastian Höhn 01 Sep '10

01 Sep '10

Hello, perhaps I am just blind or is it a difficult issue: I would like to generate Char values in a given Range for QuickCheck2. There is this simple example from the haskell book: instance Arbitrary Char where arbitrary = elements (['A'..'Z'] ++ ['a' .. 'z'] ++ " ~!@#$%^&*()") This does not work in QuickCheck2 since the instance is already defined. How do I achieve this behaviour in QC2? Thanks for helping. Sebastian

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Fourth (and Fifth) GhentFPG Meeting: Date and Call for Talks
by Jeroen Janssen 31 Aug '10

31 Aug '10

(apologies if you receive multiple copies) Dear all, We would like to announce that the fourth meeting of the Ghent Functional Programming Group will be held on Thursday, October 7th at 19h. The location will again be the Technicum building of Ghent University (Sint-Pietersnieuwstraat 41, 9000 Gent). Anyone is free to give a talk, so if you feel like giving one, just contact jejansse(a)gmail.com. The final program will be announced in a few weeks. Finally, we would also like to remind you that GhentFPG is organizing BelHac, a Haskell hackathon on 5-7 November 2010. More details can be found at http://www.haskell.org/haskellwiki/Ghent_Functional_Programming_Group/BelHac The fifth GhentFPG meeting will take place at this hackathon, so if you plan on going, and feel like giving a talk at this fifth meeting, just contact me. We would be more than happy to have some international speakers at our meeting! Hope to see you then, The GhentFPG Organizing Committee.

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ANNOUNCE: iteratee-compress 0.1
by Maciej Piechotka 31 Aug '10

31 Aug '10

Iteratee-compress provides compressing and decompressing enumerators. Currently only gzip is provided but at least bzip is planned. Additionally more fine-control over stream (i.e. flushing) is planned. ---- Library currently depends on zlib haskell library only for sharing parameters data. However no zlib function is called (the needed functions are not exported) and the symbols it uses from necessity are deprecated. Would it be better to copy the parameters from zlib and drop the dependency? Please note also that the library do not bundle zlib (C) as zlib (Haskell) do.

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Arrow transformers: how to make them wright?
by Permjacov Evgeniy 31 Aug '10

31 Aug '10

A Control.Arrow in base package introduces an arrow type, and ghc have good support for arrow notation. Many things, avaible in monads, are avaible in arrows as well. There is an arrows package, that introduces some arrow classes : state, reader, writer and so on. However, it does not introduce systematic lifting of arrow classes operations. Arrows are generalisation of monads. There are libraries, that introduces systematic lifting of operations from monad classes. So, the quesions are: what operations should be in arrow transformer class? Captain Obvious says, it should look at least like: class ArrowTrans t where lift :: ( Arrow a, Arrow (t a) ) => a b c -> t a b c what laws arrow transformers must obey? C.O. says, at least lift a b c >>> lift a c d == lift ( a b c >> a c d ) how to perform lifting of actions for classes like this: class Arrow a => ArrowError a e| a-> e where raise :: a e () handle :: a e c -> a b c -> a b c class Arrow a => ArrowReader a e | a -> e where look :: a () e local :: a b c -> a (b,e) c The answers lies somewhere in category theory, C.O. says. Of course, such answer is not satisfactory.

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ANNOUNCE: Darcs hacking sprint (15-17 Oct, Orleans)
by Eric Kow 31 Aug '10

31 Aug '10

Hello Haskellers! The Darcs Team will soon be hosting its fifth hacking sprint this October. We would be delighted if you could join us. http://wiki.darcs.net/Sprints/2010-10 Details ------- When: 15-17 October 2010 Where: Orleans, France Who Anybody who wants to hack on Darcs (or Camp, Focal, etc) Beginners especially welcome! Why: Darcs aims to have bi-annual hacking sprints so that we can get together on a regular basis, hold design discussions, hack up a storm and have a lot fun. What: We'll be starting work on the next release after the upcoming Darcs 2.5. Could we see a much fast darcs over networks? Friendlier darcs annotate? Exciting times ahead... We intend to set aside at least one Darcs hacker for mentoring beginners, so if you're new to Haskell or to Darcs hacking, here's a good chance to plunge in and start working on a real world project. How: Email me <kowey(a)darcs.net> to let me know you're interested! Keep in mind that we do have funds to help pay for travel expenses; just let me know if you'd like to come :-) Thanks ------ Thanks to team Florent Becker and University of Orleans for generously offering to host this event! Thanks also to donors from the fundraising drive this year. We'll be using that money to help Darcs hackers get to Orleans Supporting Darcs ---------------- Darcs is a very happy member of the Software Freedom Conservancy. Among other things, the Software Freedom Conservancy makes it easy for open source projects to raise funds and hold assets (as part of a legal entity). For more information on supporting Darcs, see http://darcs.net/donations.html See you there! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, please try +44 (0)1273 64 2905.

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Searching for Haskell (was: [Haskell-cafe] Slightly humorous: Headhunters toolbox (example for Germany))
by Henk-Jan van Tuyl 31 Aug '10

31 Aug '10

On Tue, 31 Aug 2010 13:57:42 +0200, Alberto G. Corona <agocorona(a)gmail.com> wrote: > Entering "haskell language" instead of "haskell" the ambiguity > dissapears. Then, Jamaica no longer appears in the crude reality . The > weigth of USA is hides all other country here. > > I think that a more appropriate term to look for haskell interest is > "monad" although it may catch some crazy philosophers too. The result is > interesting, > You can still get things like a page titled "Ben Haskell - Language Technologies Institute - Carnegie Mellon", and of course, not all pages about the programming language contain the word "language". To get proper results you would need a good semantic search engine, but these search engines seem to be in early research stage. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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combinators in the syb lib -- or generic heterogeneous traversals that fold
by Carter Schonwald 30 Aug '10

30 Aug '10

Hello All, In the course of some code I've been working on, I found I needed generic foldl / foldr over heterogeneous data structures, where I can easily pick whether I want top down left right, botom up left right, and ___ right left traversals, and to in tandem sensibly approach if a parent node should be included altogether What i'm wondering is if i'm somehow overlooking some simpler ways of writing such or if my attached code for the foldl case (the foldr analogue is easy to see from the example code). code with example follows --- my "foldl" that is abstracted from traversal order travL :: (b -> a -> b)-> GenericQ (Maybe a) -> (Maybe a -> b ->(b -> a -> b)->(b->b)-> b) -> GenericQ (b ->b) travL f qry merge x nil = merge (qry x) nil f (\nl-> foldl (flip ($)) nl $ gmapQ (travL f qry merge) x ) --travR could be written as --- travR f qry merge x nil = foldl (flip f) nil $ travL (flip (:)) qry merge x [] -- example usage -- takes the integers in some datastructure, and puts them in a list -- example: flipList :: Data a => a -> [Integer] flipList x = travL (flip (:) ) (mkQ Nothing (Just :: Integer -> Maybe Integer) ) (\ v nl f k -> maybe (k nl) (\y -> k $! f nl y) v ) x [] I suppose that i could simplify it to travL :: GenericQ (Maybe a) -> (Maybe a -> b ->(b->b)-> b) -> GenericQ (b ->b) and have the operand *f* of the fold work within the *merge* parameter, but that doesn't address the important bit in my mind, namely that while its pretty clear to me that I can write the *synthesize*and *everything* combinators using my "travL/R" stuff, its not clear to me that the converse or something close to it is the case. Anyways, what're everyone's thoughts? thanks! -Carter

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Ed Lambda: Functional programming meetup in Edinburgh
by Ollie Saunders 30 Aug '10

30 Aug '10

Hi guys, I'm running a meetup for functional programming in Edinburgh. The first one will be on the 13th of September at Malone's Irish Bar (14 Forrest Road) and will continue every 2nd monday of each month. For the first meetup I think we'll just be having a chat and getting to know each other but I hope to be able to expand it to talks and coding dojos once we have some regular numbers. For more details: http://meetup.com/ed-lambda/ And we're on Twitter: http://twitter.com/ed_lambda

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Support for lock-free/wait-free programming?
by Gregory Collins 30 Aug '10

30 Aug '10

Hello all, Does GHC expose any primitives for things like atomic compare-and-swap? I can't seem to find anything in the docs. I'm wondering if it's possible, for example, to implement things like the wait-free concurrent queue from [1] or a lock-free wait-free hash table like the Azul people are doing [2]. In network servers, we often have a large number of clients fighting over shared resources -- even simple stuff like "which threads are scheduled to timeout?" and "which threads are currently active, so I can kill them if I need to?". Shared mutable collections seem to be a fact of life here. Under load, pure data structures behind MVars don't perform well because of lock contention, and in my experience atomicModifyIORef is marginally better but still suffers from contention issues (probably related to thunk blackholing). Recently I got a ~35% speedup on a benchmark by switching one of these tables from a Map behind an IORef to a hashmap using a striped-locking scheme (see [3] if you're interested.) I think there's a need for high-concurrency data structures like this, and I don't see much on Hackage that addresses it. As much as we like to say that we have a handle on concurrency issues, from what I can see java.util.concurrent.* has us soundly thrashed in this department, especially as it relates to exposing atomic wait-free CPU primitives like the ones in java.util.concurrent.atomic.* [4]. Speaking as a partisan, this cannot be allowed to stand, can it? Cheers, G. ------------------------------------------------------------------------------ [1] Maged M. Michael and Michael L. Scott, "Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms": http://www.cs.rochester.edu/u/michael/PODC96.html [2] http://www.stanford.edu/class/ee380/Abstracts/070221_LockFreeHash.pdf [3] http://github.com/snapframework/snap-server/blob/master/src/Data/HashMap/Co… [4] http://download.oracle.com/javase/6/docs/api/java/util/concurrent/atomic/At… -- Gregory Collins <greg(a)gregorycollins.net>

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Hackage on Linux
by Andrew Coppin 30 Aug '10

30 Aug '10

Browsing around Hackage, I notice that a seemingly random subset of packages are available for something called "arch linux". Presumably some sort of automatic conversion system is involved, but does anyone know why only certain packages appear? I've noticed that both Debian and OpenSUSE have a very tiny selection of binary Haskell packages too. I'm guessing that these packages are also auto-generated, but presumably selected by hand. (I also don't recall seeing them listed on Hackage.) Anybody know about that? In general, is there an advantage to having native packages for Haskell things? I guess it means you can have binary packages, so you don't need to build from source. And for executables, it means the native package manager can track all the dependencies and install them all for you, potentially without needing a Haskell build environment at all. Is that it, or have I missed something?

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