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Using HaXml
by Yves Parès 26 Mar '12

26 Mar '12

Hello café, Provided what I read, HaXml seems to be the recommended library for parsing XML files (I'm trying to browse and alter spreadsheets (ODS) and possibly release a package when I'm done), is there somewhere tutorials on how to use it? I used 'xml' package by the past, but HaXml is more substantial. Thanks.

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Execute only one step in Hood-Melville Real time queue.
by Xinyu LIU 26 Mar '12

26 Mar '12

Hi, I read the Hood-Melville real time queue realization in [1]. There are 2 lists maintained in queue, front and rear. When the queue gets unbalanced due to push/pop, it amortized the f ++ reverse r incrementally to build the new front list based on the below mechanism reverse r = reverse' r [] where reverse' [] acc = acc reverse' (x:xs) = reverse' xs (x:acc) and f ++ reverse r == reverse (reverse f) ++ reverse r == reverse' (reverse f) [] ++ reverse r == reverse' (reverse' f []) (reverse' r []) And the re-balance happens once | f | + 1 = | r |. Let's denote m = |f|. incremental f ++ reverse r takes total 2*m + 2 steps. In the the realization mentioned in [1], it execute 2 steps every push and pop to make sure the incremental computation finish before next queue re-balancing. However, I found it's possible to execute only one step per push/pop. because: 1. Next re-balance happens at earliest | f' | + 1 = |r|+|f|+1+1 = 2*m + 2 times by continuously push operation; 2. If we keep a copy of f, and a counter of how many elements left in f, which need to be incrementally 'appended', by continuously m times popping operation. we can finish the f ++ reverse r (actually, only reverse r is needed, as all elements in f are popped) Based on this fact, I rewrite the program as the following: data State a = Empty | Reverse Int [a] [a] [a] [a] -- n, f', acc_f' r, acc_r | Append Int [a] [a] -- n, rev_f', acc | Done [a] -- result: f ++ reverse r deriving (Show, Eq) -- front, length of front, on-goint reverse state, rear, length of reverse data RealtimeQueue a = RTQ [a] Int (State a) [a] Int deriving (Show, Eq) -- we skip the empty error for pop and front empty = RTQ [] 0 Empty [] 0 isEmpty (RTQ _ lenf _ _ _) = lenf == 0 -- O(1) time push push (RTQ f lenf s r lenr) x = balance f lenf s (x:r) (lenr + 1) -- O(1) time pop pop (RTQ (_:f) lenf s r lenr) = balance f (lenf - 1) (abort s) r lenr front (RTQ (x:_) _ _ _ _) = x balance f lenf s r lenr | lenr <= lenf = step f lenf s r lenr | otherwise = step f (lenf + lenr) (Reverse 0 f [] r []) [] 0 -- execute f ++ reverse r step by step step f lenf s r lenr = case s' of Done f' -> RTQ f' lenf Empty r lenr s' -> RTQ f lenf s' r lenr where s' = if null f then next $ next s else next s next (Reverse n (x:f) f' (y:r) r') = Reverse (n+1) f (x:f') r (y:r') next (Reverse n [] f' [y] r') = Append n f' (y:r') next (Append 0 _ acc) = Done acc next (Append n (x:f') acc) = Append (n-1) f' (x:acc) next s = s -- Abort unnecessary appending as the element is popped abort (Append 0 _ (_:acc)) = Done acc -- Note! we rollback 1 elem abort (Append n f' acc) = Append (n-1) f' acc abort (Reverse n f f' r r') = Reverse (n-1) f f' r r' abort s = s Note the 'where' clause in step function. This is because we need an extra step to change the state from (Append 0 _ xs) to (Done xs). I tested this program with invariant testing with QuickCheck. The behavior is correct as other queue implementation. The program can be found here: https://github.com/liuxinyu95/AlgoXY/blob/algoxy/datastruct/elementary/queu… Reference: [1]. Chris Okasaki. ``Purely functional data structures''. P102. Section 8.2.1. Cambridge University Press. ISBN 0521663504 -- Larry, LIU Xinyu https://sites.google.com/site/algoxy/ https://github.com/liuxinyu95/AlgoXY

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What happened with goa package repo?
by Dmitry Malikov 26 Mar '12

26 Mar '12

Cloning into bare repository '/usr/portage/distfiles/egit-src/goa.git'... fatal: The remote end hung up unexpectedly * ERROR: dev-haskell/goa-9999 failed (unpack phase): * git-2_initial_clone: can't fetch from git://github.com/chrisdone/goa.git https://github.com/chrisdone/goa returns 404. So it seems like Chris dropped that repo. Did someone have clone of it? It will be nice to put it back on github.

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Google Summer of Code: a NUMA wishlist!
by Sajith T S 26 Mar '12

26 Mar '12

Dear Cafe, It's last minute-ish to bring this up (in my part of the world it's still March 25), but graduate students are famously a busy and lazy lot. :) I study at Indiana University Bloomington, and I wish to propose^W rush in this proposal and solicit feedback, mentors, etc while I can. Since student application deadline is April 6, I figure we can beat this into a real proposal's shape by then. This probably also falls on the naive and ambitious side of things, and I might not even know what I'm talking about, but let's see! That's the idea of proposal, yes? Broadly, idea is to improve support for NUMA systems. Specifically: -- Real physical processor affinity with forkOn [1]. Can we fire all CPUs if we want to? (Currently, the number passed to forkOn is interpreted as number modulo the value returned by getNumCapabilities [2]). -- Also kind of associated with the above: when launching processes, we might want to specify a list of CPUs rather than the number of CPUs. Say, a -N [0,1,3] flag rather than -N 3 flag. This shall enable us to gawk at real pretty htop [3] output. -- From a very recent discussion on parallel-haskell [4], we learn that RTS' NUMA support could be improved. The hypothesis is that allocating nurseries per Capability might be a better plan than using global pool. We might borrow/steal ideas from hwloc [5] for this. -- Finally, a logging/monitoring infrastructure to verify assumptions and determine if/how local work stays. (I would like to acknowledge my fellow conspirators and leave them unnamed, lest they shall be embarrassed by my... naivete.) Thanks, Sajith. [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurre… [2] http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurre… [3] http://htop.sourceforge.net/ [4] http://groups.google.com/group/parallel-haskell/browse_thread/thread/7ec1eb… [5] http://www.open-mpi.org/projects/hwloc/ -- "the lyf so short, the craft so long to lerne." -- Chaucer.

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How to handle C library dependencies on Hackage?
by . 25 Mar '12

25 Mar '12

Hello Cafe, I recently did my first upload to Hackage: <http://hackage.haskell.org/package/jalla>. The library does not build (and therefore unfortunately the documentation is not built either), because the C library that it needs to wrap is usually not installed with normal Linux distributions. It's LAPACKE (note the E at the end). I /could/ upload the source code of that library alongside my Haskell library and build it using Setup.hs (this works on my computer). However, LAPACKE is large compated to my own library (about 16MB sources) and I don't want to flood Hackage with it if it's not necessary. Can anyone here give me some advice on how to solve this on the system that builds the hackage packages? Thanks very much, Christian

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Re: [Haskell-cafe] Open-source projects for beginning Haskell students?
by John Lato 25 Mar '12

25 Mar '12

> From: Heinrich Apfelmus <apfelmus(a)quantentunnel.de> > > Tom Murphy wrote: >> If you want to do Haskell audio synthesis, you could also use >> hsc3 (good start here: http://slavepianos.org/rd/ut/hsc3-texts/) With >> hsc3 you can start on serious audio synthesis with only a few lines of >> Haskell. In my opinion it could use a much larger community. > > While Rohan's bindings to SuperCollider are great, I have found that > SuperCollider itself is quite difficult to understand for a new user. > (My tomata-rubato project aims to be much easier to learn.) > > Also, as far as I am aware, you can't do low-level audio programming in > SuperCollider, i.e. play a list of samples that you've calculated > yourself. That's cool if you're only interested in sound design, but bad > for learning how audio programming works. I think this charge is a bit unfair. If you really want to do low-level stuff, it's possible within SC. You just have to work in SuperCollider, not Haskell (AFAIK). However, it is possible to transfer audio data between Haskell and Csound, in several ways. The hCsound package comes with some examples of transferring the audio input and output streams between csound and haskell. Named channels provide for even more complicated routing if you like. John L.

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GSoC project ideas for web technology
by Jeremy Shaw 25 Mar '12

25 Mar '12

Hello, If you are looking for some ideas for a GSoC project, I have written down some web technology related projects I would like to see. So far I mostly have ideas for improvements to HSX (a templating solution) and acid-state (a pure, haskell persistent datastore). Both these technologies can be used with any Haskell web framework (though they are embraced most fully by Happstack). http://code.google.com/p/happstack/wiki/GoogleSummerOfCode I am an officially registered Haskell GSoC mentor. I am happy to help expand any of these ideas into a genuine GSoC proposal and to act as a mentor on the project. If you have other ideas you wish to pitch, I am happy to hear those as well. - jeremy

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sinkfork using conduit 3.0
by grant 24 Mar '12

24 Mar '12

I am trying to update sinkfork to use conduits 3.0. The difficulty I am having is trying to figure out how to push a value into a SinkM constructor. Any ideas? Here is the old! conduits code 2.0: https://gist.github.com/1975383 Thanks in advance. Grant

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Alex character sets
by Omari Norman 24 Mar '12

24 Mar '12

I'm wrinting (or at least, trying to write) an Alex lexer. Say I want to group character ranges together and specify that I want a character that is *not* in any of those ranges. Example --- { module Main where } %wrapper "posn-bytestring" $lowAscii = \x00-\x1f -- unprintable stuff $lowSymbol = \x21-\x2f -- exclamation point to solidus $space = \x20 $special = [^$lowAscii $lowSymbol] lex :- $special { Word } { data Token = Word AlexPosn ByteString.ByteString } When I run this through Alex, I get "Prelude.Enum.Char.pred: bad argument". What am I doing wrong? How would I accomplish this sort of thing? Thanks. --Omari

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ANNOUNCE: options, an easy-to-use command-line option parser
by John Millikin 24 Mar '12

24 Mar '12

Hackage: http://hackage.haskell.org/package/options Home page: https://john-millikin.com/software/haskell-options/ API reference: https://john-millikin.com/software/haskell-options/reference/haskell-option… The options package lets library and application developers easily work with command-line options. The following example is a full program that can accept two options, --message and --quiet: ---------------------------------------------------------- {-# LANGUAGE TemplateHaskell #-} import Options defineOptions "MainOptions" $ do stringOption "optMessage" "message" "Hello world!" "A message to show the user." boolOption "optQuiet" "quiet" False "Whether to be quiet." main :: IO () main = runCommand $ \opts args -> do if optQuiet opts then return () else putStrLn (optMessage opts) ---------------------------------------------------------- ---------------------------------------------------------- $ ./hello Hello world! $ ./hello --message='ciao mondo' ciao mondo $ ./hello --quiet $ ---------------------------------------------------------- In addition, this library will automatically create documentation options such as --help and --help-all: ---------------------------------------------------------- $ ./hello --help Help Options: -h, --help Show option summary. --help-all Show all help options. Application Options: --message A message to show the user. --quiet Whether to be quiet. ----------------------------------------------------------

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