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HXT error handling
by Mads Lindstrøm 06 Apr '10

06 Apr '10

Hi I am trying to use HXT to evaluate XPath expressions. The XPath expressions are not specified by myself, but by users of my program. However, the idea behind HXT's error handling confuses me. Maybe somebody can enlighten me. This program fragment: evalXPath :: String -> String -> [XmlTree] evalXPath xpath xml = runLA ( xread >>> propagateNamespaces >>> getXPathTrees xpath ) xml seems to work fine, except when it comes to error handling. If the xml-string is malformed, the error is simply ignored and the evalXPath-function just returns an empty list. I can understand what happens. 'getXPathTrees xpath' tries to match the xpath to a XmlTree which represents a parse error, and it therefore never matches anything. What I do not understand, is when this behavior would be desirable. Or why do the error in 'xread' not propagate throughout the arrow, similar to how: foobar :: Either String Int foobar = do _ <- fail "Some failure" return 2 the error in 'foobar' would propagate and 'foobar = Left "Some failure"'. What if the xpath-parameter contains an invalid XPath expression? Well, then the output depends upon both the xpath and the xml -parameters. That is: length (evalXPath "/...../" " <foo/> ") == 3 whereas length (evalXPath "/...../" "<foo/>") == 1 Which just seems strange to me. If we add a getText to evalXPath: evalXPath :: String -> String -> [String] evalXPath xpath xml = runLA ( xread >>> propagateNamespaces >>> getXPathTrees xpath >>> getText -- added here ) xml Then an error in the xpath-parameter will be hidden by the getText function, similar to how 'getXPathTrees xpath' ignores errors in the xread function. Again, I understand what happens, I just cannot see why it is designed as it is. What I really would like was functions like: parseStr :: (Monad m) => String -> m XmlTrees parseStr xml = case Parsec.xread xml of (x:xs) -> if (Dom.isError x) then fail "Could not parse tree" else return (x:xs) [] -> fail "No XML tree resultet from parsing XML" And a similar function for the XPath. This could also speed up my program, as I would not parse the same XPath again and again. A stand-alone function to parse an XPass expression, would also make it easy to create a Template Haskell parse-xpath-function, that gave compile-time error messages and increased performance for XPath expression know at compile-time. Regards, Mads Lindstrøm

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[OT?] Haskell-inspired functions for BASH
by Patrick LeBoutillier 06 Apr '10

06 Apr '10

Hi all, I've been studying Haskell for about a year now, and I've really come to like it. In my daily work I write a lot of BASH shell scripts and I thought I'd try add some of the haskell features and constructs to BASH to make my scripting life a bit easier. So I've been working on a small BASH function library that implements some basic functional programming building blocks. Note: There is no actual Haskell code involved here. I put up the full manpage here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=24564 Source is here: http://svn.solucorp.qc.ca/repos/solucorp/bashkell/trunk/trunk/ All this is very prototypical, but here is an example of some of the stuff I've got so far (map, filter, foldr): $ ls data 1.txt 2.txt # basic map, argument goes on the command line $ ls -d data/* | map basename 1.txt 2.txt # map with lambda expression $ ls -d data/* | map '\f -> basename $f .txt' 1 2 # simple filter, also works with lambda $ ls -d data/* | map basename | filter 'test 1.txt =' 1.txt # sum $ ls -d data/* | map '\f -> basename $f .txt' | foldr '\x acc -> echo $(($x + $acc))' 0 3 Basically I'm looking for a bit of feedback/info: - Does anyone know if there are already similar projets out there? - Does anyone find this interesting? - Any other comment/suggestion/feedback - Where's a good place to promote such a project? Thanks a lot, Patrick LeBoutillier -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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How do I use ByteString?
by Günther Schmidt 06 Apr '10

06 Apr '10

Hi all, I've never found an easy way to deal with ByteStrings. I'm using the RSA library and it en- and decodes Data.ByteString.Lazy.ByteString. I initially start with Strings, ie. [Char], but there is no function to convert the 2 back and forth. There is however a function which takes [Word8] to BytesString and back. It all wouldn't be so confusing if there weren't several versions of ByteString in several modules to choose from. And a number of libraries requiring different types of ByteString. I am sure the designers of the bytestring package had good reason for this design, is there also a webpage which explains which one to use and under what circumstances? Günther

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FRP, arrows and loops
by Maciej Piechotka 06 Apr '10

06 Apr '10

Hello. I'm trying to understand the FRP (by implementing FRP system on my own) and I think I'm slowly getting it. 1. How to interpret ArrowLoop? I have two possible implementations: type RunSF a = a Dynamic () data SF a b c = SF (a (Dynamic, b, RunSF, Set Unique) (c, Set Unique, SF a b c)) (...) instance ArrowLoop (SF a) where loop (SF f) = loop' f undefined where loop' g d = proc (dyn, b, r, s) -> do ((c, d'), s, g') <- g <- (dyn, (b, d), r, s) returnA -< (c, s, loop' g' d') instance ArrowLoop a => ArrowLoop (SF a) where loop (SF f) = SF $! proc (d, b, r, s) -> do rec ((c, d), s, f') <- f -< (d, (b, d), r, s) returnA -< (c, s, loop f') Since the first is not unlike ArrayCircuit from arrays I guess second one but I'm not quite sure. 2. Why there is no ArrowIO in arrows? I.e. class Arrow a => ArrowIO a where liftAIO :: Kleisli IO b c -> a b c (possibly class Arrow a => ArrowST a where liftAST :: Kleisli ST b c -> a b c ) 3. Why switch is needed? How to interpret switch with current continuation? I think switch is equivalent to ArrowChoice but do I miss something? Regards

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Data Structures GSoC
by Nathan Hunter 06 Apr '10

06 Apr '10

Hello. I am hoping to take on the Data Structures project proposed two years ago by Don Stewart here<http://hackage.haskell.org/trac/summer-of-code/ticket/1549>, this summer. Before I write up my proposal to Google, I wanted to gauge the reaction of the Haskell community to this project. Particularly: -What Data Structures in the current libraries are in most dire need of improvement? -How necessary do you think a Containers Library revision is? -Should I attempt to build on the work Jamie Brandon did with Map as generalised tries, or is that beyond the scope of this project I am very excited to work with functional data structures, and I would greatly appreciate any input. -Nathan Hunter

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Haskell JSON requests from your blog, roll your own Try Haskell!
by Christopher Done 05 Apr '10

05 Apr '10

For those interested, http://chrisdone.com/posts/2010-04-05-haskell-json-service-tryhaskell.html I've updated the Haskell JSON service I whipped up a month ago and made it much simpler, written some sample code for how to include this in your blog or tutorials. It supports the JSONP way of making requests, so you can use it from any domain. Thanks to that, Try Haskell can now be run by anyone who has the HTML/JavaScript code and an internet connection, which is nice. Lowers the barrier a bit for people who'd like to write interactive tutorials but can't be bothered messing around with FastCGI and a web server. Don't worry about using it too much, it's on a Linode account with 200GB of bandwidth a month. AJAX requests really won't use that up. ;) I hope someone uses it on a tutorial. *Hint hint* BONUS. Hope you guys find it useful!

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Re: Hackage accounts and real names
by Hein Hundal 05 Apr '10

05 Apr '10

I don't like using my real name on line unless I am 100% sure that I want my statements recorded for all time and available to anyone. Using a pseudonym allows me to be more honest in my opinions and it allows me to join groups without wondering how my membership in that group will be viewed in 20 years. Simply put, if you don't want much input from me or people that think like I do about anonymity, then, go ahead and require a real name.

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Where are the haskell elders?
by Günther Schmidt 05 Apr '10

05 Apr '10

Hi all, I notice that posts from the Haskell elders are pretty rare now. Only every now and then we hear from them. How come? Günther

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Haskell on Debian
by Alex Rozenshteyn 05 Apr '10

05 Apr '10

I tend to install haskell packages from apt whenever possible. One such package is "unix", which appears to come provided by the ghc6 debian package. I'm trying to cabal install lambdabot and getting the following: $ cabal install lambdabot Resolving dependencies... Configuring lambdabot-4.2.2.1... Preprocessing executables for lambdabot-4.2.2.1... Building lambdabot-4.2.2.1... Main.hs:11:7: Could not find module `System.Posix.Signals': It is a member of the hidden package `unix-2.4.0.0'. Perhaps you need to add `unix' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. cabal: Error: some packages failed to install: lambdabot-4.2.2.1 failed during the building phase. The exception was: ExitFailure 1 I have a feeling this has something to do with the interaction between cabal and apt... Does anyone have any advice for fixing this issue other than just adding "unix" to the lambdabot.cabal file? -- Alex R

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Integers v ints
by Jens Blanck 05 Apr '10

05 Apr '10

I was wondering if someone could give me some references to when and why the choice was made to default integral numerical literals to Integer rather than to Int in Haskell. Also, if you are aware of similar discussions in other languages. I'd like to use this information to make an analogous case for defaulting real numerical literals (well, the literals are likely to be in scientific notation, i.e., floating point) to some data type of computable reals rather than to floating point Double. Jens

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