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Question about TagSoup
by David Virebayre 06 Dec '10

06 Dec '10

Hello café, I have seen tutorials about extracting information from a tag soup, but I have a different use case: I want to read a xml file, find a tag, change its content, and write the xml file back. This is an example of the files <?xml version="1.0" encoding="UTF-8" standalone="yes"?> <idPkg:Story xmlns:idPkg=" http://ns.adobe.com/AdobeInDesign/idml/1.0/packaging" DOMVersion="7.0"> <Story Self="ub9fad" AppliedTOCStyle="n" TrackChanges="false" StoryTitle="$ID/" AppliedNamedGrid="n"> <StoryPreference OpticalMarginAlignment="false" OpticalMarginSize="12" FrameType="TextFrameType" StoryOrientation="Horizontal" StoryDirection="LeftToRightDirection"/> <InCopyExportOption IncludeGraphicProxies="true" IncludeAllResources="false"/> <ParagraphStyleRange AppliedParagraphStyle="ParagraphStyle/prix"> <CharacterStyleRange AppliedCharacterStyle="CharacterStyle/$ID/[No character style]"> <Content>zzznba5</Content> </CharacterStyleRange> </ParagraphStyleRange> </Story> </idPkg:Story> Assuming I want to change the content of the "Content" tag, this is what I came up with (simplified), I'm using direct recursion. Is there a better way ? ts = do soup <- parseTags `fmap` readFile "idml/h00/Stories/Story_ub9fad.xml" writeFile "test" $ renderTagsOptions renderOptions{optMinimize = const True} $ modif soup modif [] = [] modif (x@(TagOpen "Content" []):TagText _m : xs) = x : TagText "modified" : modif xs modif (x:xs) = x : modif xs David.

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Cabal, xmonad-contrib, X11-xft, pkg-config ... questioning my sanity
by Jacek Generowicz 06 Dec '10

06 Dec '10

In short: I can't cabal install xmonad-contrib. Longer version: It a appears that a program which does not exist on my computer seems to insist on a package version which does not exist in my universe, I'm starting to wonder whether I have lost my marbles. Could some kind soul please point me in some sensible direction? cabal install xmonad-contrib Resolving dependencies... Configuring X11-xft-0.3... cabal: pkg-config version >=0.9.0 is required but it could not be found. cabal: Error: some packages failed to install: X11-xft-0.3 failed during the configure step. The exception was: ExitFailure 1 xmonad-contrib-0.9.1 depends on X11-xft-0.3 which failed to install. cabal install x11-xft Resolving dependencies... Configuring X11-xft-0.3... cabal: pkg-config version >=0.9.0 is required but it could not be found. cabal: Error: some packages failed to install: X11-xft-0.3 failed during the configure step. The exception was: ExitFailure 1 X11-xft version 0.9.0 doesn't even exist, as far a I can tell: 0.3 seems to be the highest. So what is making cabal believe that such a version is needed? Is it pkg-config? I'm not at all familiar with pkg-config, and, as far as I can tell it's not even installed on the Mac on which I am trying to do this. Curiously, its man page *does* work (it's in /usr/X11/man, for some reason). Does Cabal have some sort of embedded pkg-config?

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LLVM, getElementPtr?
by Lally Singh 06 Dec '10

06 Dec '10

Hey all, I'm trying to get started generating LLVM code, but I'm getting a rather cryptic error. I wanted to create a function that takes a string and generates a function calling to "puts" with that string as an argument: buildReaderFun :: String -> CodeGenModule (Function (IO ())) buildReaderFun nm = do puts <- newNamedFunction ExternalLinkage "puts" :: TFunction (Ptr Word8 -> IO Word32) greetz <- createStringNul nm func <- createFunction ExternalLinkage $ do tmp <- getElementPtr0 greetz (0 :: Word32, ()) call puts tmp -- Throw away return value. ret () return func (based off of some code at this blog post: http://augustss.blogspot.com/2009/01/llvm-llvm-low-level-virtual-machine-is… ) But, I get this error: /research/phd/libmet/Listener.hs:16:11: Ambiguous type variable `n' in the constraint: `Data.TypeLevel.Num.Sets.NatI n' arising from a use of `getElementPtr0' at /research/phd/libmet/Listener.hs:16:11-49 Probable fix: add a type signature that fixes these type variable(s) My attempts at figuring out what type-level has to do with this, and how to satisfy it have so far proven unsuccessful. Can anyone shed a little light on what's going on? Or pointers to some documentation on any of this? Cheers, -Lally

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Hackage down?
by Ozgur Akgun 06 Dec '10

06 Dec '10

http://downforeveryoneorjustme.com/http://hackage.haskell.org -- Ozgur Akgun

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Berlin Haskell Meeting
by Sönke Hahn 06 Dec '10

06 Dec '10

The Berlin Haskell meeting is due again: Date: Tuesday, December 7th Time: from 20:00 Location: c-base, Rungestrasse 20, 10179 Berlin See you there. Sönke

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Small strings
by Antoine Latter 06 Dec '10

06 Dec '10

Hi Haskell, I've recently created the 'smallstring' package[1] and put it on Hackage. The goal was to create a string-like type with as little memory overhead as possible, and to have equality and comparison operations be competitive with the native String type. The idea is that this type would make ideal keys into data structures like Data.Map. I've taken pains to make the library portable to other compilers, but this isn't really tested, and most of my recent development has been on GHC 7. The library is young and likely has places it could be improved, so your feedback is welcome. The API provided is small - to/fromString along with Eq and Ord instances. This isn't meant to be a replacement for the text[2] or bytestring[3] packages - the reduced size comes at a cost. Both of these libraries offer substring manipulation functions which allow the sharing of the backing memory buffers. Also, I haven't implemented any sort of string-like operations on the type as it isn't the right thing for that. I've borrowed heavily from the previously mentioned text package for the underlying array implementation, so thanks goes to Bryan O'Sullivan for maintaining the package and to all of the other authors as well. Take care, Antoine [1] http://hackage.haskell.org/package/smallstring [2] http://hackage.haskell.org/package/text [3] http://hackage.haskell.org/package/bytestring

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OverloadedStrings mixed with type classes leads to boilerplate type signatures
by Johan Tibell 05 Dec '10

05 Dec '10

Hi, I'm trying to generalize my string substitution library (http://hackage.haskell.org/package/template) to allow users to provide different kinds of key/value mappings (e.g. functions and association lists) for filling in the placeholders in a template. Here are two examples I'd like to work: ghci> :set -XOverloadedStrings ghci> "$name ate a banana." % [("name", "Johan")] "Johan ate a banana." ghci> "$name ate a banana." % (\v -> if v == "name" then "Johan" else error "KeyError") "Johan ate a banana." At the moment the context (i.e. the second argument to "%") is always a function type Context = T.Text -> T.Text I would like to generalize that to class Context a where lookup :: a -> T.Text -> T.Text Different key/value mappings fulfill different use cases: * Association lists have a small syntactic overhead, great for one-off string substitution. * Functions are flexible, allowing for arbitrary complex lookup logic, but are syntactically heavy (in this case.) Here's the gist of my implementation: {-# LANGUAGE FlexibleInstances #-} module Template where import Data.Maybe (fromMaybe) import qualified Data.Text as T import qualified Data.Text.Lazy as LT import Prelude as P class Context a where lookup :: a -> T.Text -> T.Text instance Context (T.Text -> T.Text) where lookup f = f instance Context [(T.Text, T.Text)] where lookup xs k = fromMaybe (error $ "KeyError: " ++ show k) (P.lookup k xs) -- | Perform string substitution. Example: -- -- > "$foo" % [("foo" :: T.Text, "bar" :: T.Text)] (%) :: Context c => T.Text -> c -> LT.Text (%) = undefined The problem is that the compiler is not able to deduce that string literals should have type 'Text' when used in 'Context's. For example ghci> :t "$foo" % [("foo", "bar")] <interactive>:1:8: No instance for (Context [(a, a1)]) arising from a use of `%' Possible fix: add an instance declaration for (Context [(a, a1)]) In the expression: "$foo" % [("foo", "bar")] This forces the user to provide explicit type signatures, which makes the construct more heavy weight and defeats the whole purpose of introducing the 'Context' class: ghci> :t "$foo" % [("foo" :: T.Text, "bar" :: T.Text)] "$foo" % [("foo" :: T.Text, "bar" :: T.Text)] :: LT.Text Is there any way to make the syntactically short `"$foo" % [("foo", "bar")]` work but still keep the 'Context' class? Johan

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Strange error when using Attoparsec and Enumerator
by Crutcher Dunnavant 05 Dec '10

05 Dec '10

I have spent a good chunk of the past week tracing code, trying to solve this problem. I'm seeing an error when using Enumerator and Attoparsec that I can't explain. This is a reduced form of the problem. In general, I've observed that debugging broken iterators and enumerators is very hard. We probably want some tooling around that; I'm looking at an identity enumeratee with debug.trace shoved in, or something like that, not sure yet what would help. {- Haskell 2010, ghc 6.12.3 array-0.3.0.2 attoparsec-0.8.2.0 attoparsec-enumerator-0.2.0.2 bytestring-0.9.1.7 containers-0.4.0.0 deepseq-1.1.0.2 enumerator-0.4.2 text-0.10.0.0 transformers-0.2.2.0 -} import Control.Applicative ((<|>)) import qualified Data.Attoparsec.Char8 as AP import qualified Data.Attoparsec.Combinator as APC import qualified Data.Attoparsec.Enumerator as APE import qualified Data.ByteString.Char8 as B import qualified Data.Enumerator as E import Data.Enumerator (($$)) import System.IO as IO parseLine :: AP.Parser B.ByteString parseLine = do AP.char '+' return . B.pack =<< APC.manyTill AP.anyChar endOfLineOrInput endOfLineOrInput :: AP.Parser () endOfLineOrInput = AP.endOfInput <|> AP.endOfLine pp :: Show a => AP.Parser a -> String -> IO () pp p s = do result <- E.run $ E.enumList 1 [ B.pack s ] $$ E.sequence (APE.iterParser p) $$ E.printChunks False case result of (Right _) -> return () (Left e) -> IO.hPutStrLn stderr $ show e main = pp parseLine "+OK" {- Observed output: ["OK"] *** Exception: enumEOF: divergent iteratee Problems with this: 1) I didn't write an iteratee, enumerator, or enumeratee in this code. Something's wrong. 2) If the parser is divergent, I _should_ be getting the error message: "iterParser: divergent parser" -} -- Crutcher Dunnavant <crutcher(a)gmail.com>

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New Xmobar maintainer: request for comments
by Jose A. Ortega Ruiz 05 Dec '10

05 Dec '10

Hi, I've recently accepted Andrea Rossato's kind offer to take over maintainership of xmobar, and i wanted to ask for feedback to the community. My apologies for cross-posting to the cafe and xmonad, but xmobar didn't have a mailing list until a couple days ago. My immediate plans are to fix some low-hanging bugs, incorporate a handful of patches and contributions i have in my repo and make a new release (0.12). Besides that, i've created a mailing list (xmobar(a)projects.haskell.org, http://projects.haskell.org/cgi-bin/mailman/listinfo/xmobar, also mirrored at gmane.comp.lang.haskell.xmobar) and i will move xmobar's homepage to http://projects.haskell.org/xmobar (not done yet). Perhaps more disruptingly, i'm thinking of moving source code management from darcs to git (right now, i maintain a _mirror_ at github, http://github.com/jaor/xmobar) But xmobar has a very lively user base and many contributors, and i wouldn't like to alienate or disturb anyone: would that move be frown upon by the community? I also plan to keep hacking and adding (not many) features to xmobar in the future, and would be delighted to hear from you: please, do not hesitate to send your feature requests or any other comment directly to me or to the new list. Thanks in advance for your feedback, and special thanks to Andrea for creating xmobar and giving me this great chance of being more than a mere spectator of the haskell world. Cheers, jao -- Fools ignore complexity. Pragmatists suffer it. Some can avoid it. Geniuses remove it. - Alan Perlis, Epigrams in Programing

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getErrorStatus in HXT
by Tobias Schoofs 05 Dec '10

05 Dec '10

Hi, I have some problems with the error processing in HXT. Here is a trivial example: e <- runX (transformDoc [] someRules src dst) transformDoc cfg rules src dst = configSysVars cfg >>> readDocument [] src >>> rules >>> -- some transformations writeDocument [] dst >>> getErrStatus I would expect "e" to contain an error value >= c_err for the case where any of the processing steps in transformDoc fails. But it does not, even with an io error on writeDocument. I guess I misunderstand "getErrStatus" in some way. Any idea? Thanks, Tobias

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