January 2010 - Haskell-Cafe

Determining application directory
by Matveev Vladimir 01 Feb '10

01 Feb '10

Hi, I'm writing cross-platform application in Haskell which should be running under Windows and Linux. Under Linux configuration is stored in the /etc directory, and under Windows configuration is meant to be in the application directory. So, is there a way to get an application directory path under Windows? I remember that there is a way to do this using WinAPI, but how to do this Haskell?

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HWN Issue 148 -- unicode error
by Joe Fredette 31 Jan '10

31 Jan '10

Indeed, the Unicode only made it through my system, and not the mailer itself. In fact, the name should look something like "Ćwikłowski", and not the garbled mess that made it through. Apologies again, Bartek! /Joe

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Haskell Weekly News: Issue 148 - January 31, 2010
by jfredett＠gmail.com 31 Jan '10

31 Jan '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100131 Issue 148 - January 31, 2010 --------------------------------------------------------------------------- Welcome to issue 148 of HWN, a newsletter covering developments in the [1]Haskell community. Hello Haskellers, this week begins with a correction. Last week, I noted a blogpost from one 'Bartek Paczesiowa', I was rapidly informed of the complete wrongness of this citation. In fact, Bartek's last name is, and I hope the Unicode makes it through here, 'wikowsk,' and Paczesiowa is his nickname. Apologies to Bartek, as penance I've reread your [2]excellent post on your blog. That said, this week's HWN if packed full of awesome potential naming errors, so I'll let you get to it. Thus, Haskellers, Your Haskell Weekly News! Announcements haskell-src-exts-1.8.0. Niklas Broberg [3]announced a new release of haskell-src-exts. Elerea 1.2.3 with some enhancements. Patai Gergely [4]announced his addition of some new features to the experimental branch of his FRP library, Elerea. hakyll-1.3. Jasper Van der Jeugt [5]announced the release of hakyll 1.3, including several new improvments and changes. ThreadScope 0.1. Satnam Singh [6]announced the release of Threadscope 0.1, the premier graphical thread profiler. The Monad.Reader Issue 15. Brent Yorgey [7]announced the most recent issue of the Monad.Reader, a monthly publication of Haskell Related expostition and discusson articles. afv-0.1.0. Tom Hawkins [8]announced a afv-0.1.0, an infinite state model checker for verifying assertions about embedded C programs. adaptive-tuple 0.1.0. John Lato [9]announced the initial release of adaptive-tuple, his library for combining the space-efficient properties of tuples with the utility of lists. Job opportunities at Citrix Systems (Cambridge, UK). Matthias Goergens [10]announced a opportunity available at Citrix Systems in Cambridge, UK. Discussion OT: Literature on translation of lambda calculus to combinators. Dusan Kolar [11]asked about texts regarding translating the untyped lambda calculus to a combinator calculus such as SKI or BCKW. Linguistic hair-splitting. Andrew Coppin [12]asked an interesting offtopic linguistic question about what we call a number, a field, an element, and a monad. Adopting hpodder? John Goerzen [13]asked if there were any interested adoptive maintainers for his hpodder project. This is an excellent opportunity for a Haskell Neophyte to help the community and learn about project management hopefully we can find hpodder a new maintainer! Blog noise [14]Haskell news from the [15]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Holumbus: [16]HolHac at FH-Wedel. * ++Arch Haskell Team++: [17]Arch Linux updates to GHC 6.12. * Neil Brown: [18]Exploring a Communicating Sequential Processes EDSL. * The GHC Team: [19]Yielding more improvements in parallel performance. Quotes of the Week * Berengal: I'm going to write a module Hmm with a (.) operator in it, so I can go 'Hmm..' in my code * clarkb,: in CS they dont teach you to program...You learn Data Structures, Algorithms, Logic, Discrete Math, Language theory, etc and happen to pick up programming on the way * dons: hey i love core. i dream about unboxes * cale: Differential geometry is the study of manifolds under change of notation. * kmc: the irony being, the abstraction that gets the most complaining and general noise [from imperative programmers] is the one that captures imperative programming * kmc: i am Jack's monad operator * arw: ...and a basic law of haskell is, 50% of all documentation has to be monad tutorials :) * Cale: Here [#haskell], we feed trolls until they explode. * bartek: It took me 2 years of studying teachings of Oleg Kiselyov (who was raised among types, where he learned to speak their language), but finally, I have the solution. * kmc: I think 250 milliolegs is enough to kill an elephant olsner: ... to kill an elephant - in the type system! * syntaxglitch: every time I have a cool idea about something that might work in Haskell, I go check Oleg's stuff and find that 1) he already did it 2) thought it out better 3) did it incidentally while working on something way more interesting * DRMacIver:: I dread to think what category theory would look like after the software engineering world had got their grubby paws on it. Enterprise variant functors. Commutative UML diagrams. About the Haskell Weekly News New editions are posted to [20]the Haskell mailing list as well as to [21]the Haskell Sequence and [22]Planet Haskell. [23]RSS is also available, and headlines appear on [24]haskell.org. To help create new editions of this newsletter, please see the information on [25]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [26]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://paczesiowa.blogspot.com/2010/01/pure-extensible-exceptions-and-self.… 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69749 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69745 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69731 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69612 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69535 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69517 9. http://article.gmane.org/gmane.comp.lang.haskell.general/17764 10. http://article.gmane.org/gmane.comp.lang.haskell.general/17763 11. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69643 12. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69620 13. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69528 14. http://planet.haskell.org/ 15. http://haskell.org/haskellwiki/Blog_articles 16. http://holumbus.fh-wedel.de/blog/?p=32 17. http://archhaskell.wordpress.com/2010/01/31/arch-linux-updates-to-ghc-6-12/ 18. http://chplib.wordpress.com/2010/01/27/exploring-a-communicating-sequential… 19. http://ghcmutterings.wordpress.com/2010/01/25/yielding-more-improvements-in… 20. http://www.haskell.org/mailman/listinfo/haskell 21. http://sequence.complete.org/ 22. http://planet.haskell.org/ 23. http://sequence.complete.org/node/feed 24. http://haskell.org/ 25. http://haskell.org/haskellwiki/HWN 26. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Recursively traversing a data structure with HXT
by Mads Lindstrøm 31 Jan '10

31 Jan '10

Hi I am trying to use HXT (8.3.2) for parsing XML. I think an example will clarify what I want to do. I want to parse XML like this: <object class="ex1"> <foo/> <foo> <object class="ex2"/> <object class="ex3"/> </foo> <object class="ex4"/> </object> and want to turn it into the following Haskell data structure: data Widget = Widget { cls :: String , children :: [Widget] } The XML above should be turned into: Widget "ex1" [Widget "ex2" [], Widget "ex3" [], Widget "ex4" []] That is, I want everything but the object-tags stripped out. And I want to keep the hierarchy of the object-tags. I thus wrote the following program: {-# LANGUAGE Arrows, NoMonomorphismRestriction #-} module Main where import Text.XML.HXT.Arrow data Widget = Widget { cls :: String , children :: [Widget] } deriving Show main = (runX (readDocument [(a_validate,v_0)] "test.xrc" >>> getObject)) >>= print getObject = deep (isElem >>> hasName "object") >>> proc x -> do cls <- getAttrValue "class" -< x cs <- listA getObject <<< getChildren -< x -- recursive call here returnA -< Widget cls cs But it do not work as intended. In stead I get the following output: [Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []},Widget {cls = "ex1", children = []}] Hopefully somebody can point me in the right direction. Greetings, Mads Lindstrøm

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ANN: haskell-src-exts-1.8.0
by Niklas Broberg 31 Jan '10

31 Jan '10

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.8.0! * 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 For users of hsx, the latest version 0.6.1 works unchanged with this release. 1.7.2 --> 1.8.0 =============== * Add an instance Show Fixity (derived). * Support for the new ANN and INLINE_CONLIKE pragmas. * Remove support for CFILES and INCLUDE pragmas. The support wasn't correct anyway, as it assumed the pragmas appeared at the top of files. As CFILES/INCLUDE pragmas can (and do) appear anywhere, there's no hope to support them in the AST. Better to remove the support altogether. Files with CFILES/INCLUDE pragmas can still be parsed of course, but those pragmas will be handled as comments. * Parsing with ignoreLinePragmas = False now correctly updates the file name. * Allow the whole SPECIALISE/INLINE family of pragmas in instance declarations. The InsInline constructor is removed, and is now represented by 'InsDecl (InlineSig ...)'. * Fix a bug with line numbering and quasi quotes, and a similar one with line numbering and CDATA. * Fix a few minor bugs in the exactPrinter. * Fix the strange handling of so called strings in LINE pragmas. Cheers, /Niklas

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ANN: Elerea 1.2.3 with some enhancements
by Patai Gergely 31 Jan '10

31 Jan '10

Hello all, I added a few new features to the experimental branch of Elerea [1], my little FRP library. The previous release added random sources -- both noise signals and instantaneous values within generators -- using mersenne-random, and the current update added an extra primitive to handle external events that can occur several times between samplings, as I heard some people needed this. All the three variants of the experimental interface support these additions. Here's a simple example showing how the new externalMulti construct can be used: import Control.Monad import FRP.Elerea.Experimental.Simple main = do (gen,sink) <- externalMulti sig <- start $ do vals <- gen cnt <- stateful 10 (+1) return (liftM2 (:) cnt vals) let act = print =<< sig act sink 3 act >> act sink 8 >> sink 2 act sink 1 >> sink 7 >> sink 9 act >> act >> act sink 4 act The program prints the following: [10] [11,3] [12] [13,2,8] [14,9,7,1] [15] [16] [17,4] As you can see, all the input received since the last sampling is collected in a list in reverse order. It is up to the programmer to store these values somewhere or build further IO actions out of them. Gergely [1] http://hackage.haskell.org/package/elerea -- http://www.fastmail.fm - Does exactly what it says on the tin

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Strange random choice algorithm
by michael rice 31 Jan '10

31 Jan '10

I'm not sure where I got this PICK function from, and don't understand why it's written as it is, so I wanted to test it for randomness. It seems random enough. But if I understand the algorithm correctly, instead of selecting one of the elements from the list, it eliminates all the elements but one and that's the value it returns. Seems like a roundabout way of doing it. Comments? Also, is there a more direct way of printing an array? Output below. Michael ================= import System.Random import Data.Array.IO pick :: [a] -> IO a pick [] = undefined pick [x] = do return x pick (x:xs) = pick' x xs (2 :: Int) pick' :: (Num p, Random p) => t -> [t] -> p -> IO t pick' curr [] _ = do return curr pick' curr (next:rest) prob = do r <- getStdRandom (randomR (1,prob)) let curr' = if r == 1 then next else curr pick' curr' rest (prob+1) main = do arr <- newArray (1,9) 0 :: IO (IOArray Int Int) doLoop arr [1,2,3,4,5,6,7,8,9] 0 doLoop arr z k = do p <- pick z a <- readArray arr p writeArray arr p (a+1) if k > 10000 then do v <- readArray arr 1 print v v <- readArray arr 2 print v v <- readArray arr 3 print v v <- readArray arr 4 print v v <- readArray arr 5 print v v <- readArray arr 6 print v v <- readArray arr 7 print v v <- readArray arr 8 print v v <- readArray arr 9 print v else do doLoop arr z (k+1) =============== [michael@localhost ~]$ runhaskell array1.hs 1110 1117 1080 1169 1112 1119 1137 1084 1074 [michael@localhost ~]$

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C functional programming techniques?
by Maurício CA 31 Jan '10

31 Jan '10

Hi, all, Sorry if this looks weird, but do you know of experiences with functional programming, or "type programming", with C? Using macro tricks or just good specifications? I know this is not absurd to a small extent. I've heard of proof tool certificated C code on the net (although I don't understand that theory), and I was also able to use what I learned from Haskell in a few C-like tasks. [*] So, I imagine if someone has done such kind of experiment seriously, like others have done with object-oriented programming (gobject etc.). Google told me nothing. Thanks, Maurício [*] This may actually be interesting. I had to use a proprietary script language mixing the worst of FORTRAN and C to drive an application, and I tried to use what I learned from Haskell by, say, prohibiting side effects out of some isolated code areas that got well specified input and gave well specified output -- even if the inside of the block itself was a huge mess. The result was actually fun to use, and after two years without looking at that code I was asked to adapt the application to be used inside another, which I could do in half a day! Maybe I should write a "Why functional programming matters - 2" on that :) It's easy to show FP is cool with abstract problems, but it's far more cool when we can show it can save even programmers with no CS theory background from huge headaches.

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ANN: hakyll-1.3
by Jasper Van der Jeugt 30 Jan '10

30 Jan '10

Hello, Today I'd like to announce the release of hakyll-1.3. Changes since 1.2: - Categories were added (as opposed to tags). You can find some information in this tutorial[1]. - createListing and createListingWith functions were added, as a way of having a more high-level way to create listings. All tutorials have been updated to work with listings now, a short explanation is available here[2]. In short, this basically replaces the old and verbose renderAndConcat/createCustomPage workflow. Changes since 1.1: - Rewrote caching system. - Rewrote templating system. - 4x speed improvement, mostly because of the Data.Binary library. You can get the latest version from hackage[3]. All feedback/criticism/comments are welcome. Kind regards, Jasper Van der Jeugt [1]: http://jaspervdj.be/hakyll/tutorial6.html [2]: http://jaspervdj.be/hakyll/tutorial3.html#custom-pages [3]: http://hackage.haskell.org/package/hakyll

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Why cannot i add the `let` declaration ?
by zaxis 30 Jan '10

30 Jan '10

find_the_day sDay 0 = sDay find_the_day sDay nDay = if (is_trade_day $ nextDay sDay) then find_the_day (nextDay sDay) (nDay - 1) else find_the_day (nextDay sDay) nDay nextDay sDay = addDays 1 sDay The above code works fine. But the following always reports `compiling error`: find_the_day sDay 0 = sDay find_the_day sDay nDay = let nextDay = addDays 1 sDay if (is_trade_day $ nextDay) then find_the_day nextDay (nDay - 1) else find_the_day nextDay nDay Any suggestion is appreciated! ----- fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/Why-cannot-i-add-the-%60let%60-declaration---tp273818… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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