December 2010 - Haskell-Cafe

ANNOUNCE: Haddock 2.9.0
by David Waern 07 Dec '10

07 Dec '10

-------------------------------------------- -- Haddock 2.9.0 -------------------------------------------- A new version of Haddock, the Haskell documentation tool, is out! This version is compatible with .haddock files produced by the version of Haddock that comes with GHC 7.0.1. -------------------------------------------- -- Changes in version 2.9.0 -------------------------------------------- Changes in version 2.9.0 * Drop support for ghc < 7 * New flag --qual for qualification of names * Print doc coverage information to stdout when generating docs * Include an 'All' option in the A-Z subdivided index * Make TOC group header identifiers validate * Minor changes to the API -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/package/haddock-2.9.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://code.haskell.org/haddock -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Tobias Brandt Mark Lentczner Ian Lynagh Simon Marlow Simon Michael Ryan Newton David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://code.haskell.org/haddock Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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include modules in DLL? "undefined reference"
by Gerold Meisinger 07 Dec '10

07 Dec '10

Hello! I'm trying to compile a Windows DLL and need to include an additional module (specifically FRP.Yampa). I was successful with a Hello World type of DLL but when I use Yampa I get: $ ghc -shared -o hs.dll DLLTest.o DLLTest_stub.o StartEnd.o Creating library file: hs.dll.a DLLTest.o:fake:(.text+0x46): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp a_integral_closure' DLLTest.o:fake:(.text+0x4d): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp aziVectorSpace_zdfVectorSpaceDoubleDouble_closure' DLLTest.o:fake:(.text+0x194): undefined reference to `Yampazm0zi9zi2zi3_FRPziYam pa_embed_closure' DLLTest.o:fake:(.text+0x381): undefined reference to `__stginit_Yampazm0zi9zi2zi 3_FRPziYampa_' DLLTest.o:fake:(.data+0x0): undefined reference to `Yampazm0zi9zi2zi3_FRPziYampa _integral_closure' DLLTest.o:fake:(.data+0x4): undefined reference to `Yampazm0zi9zi2zi3_FRPziYampa ziVectorSpace_zdfVectorSpaceDoubleDouble_closure' DLLTest.o:fake:(.data+0x38): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp a_embed_closure' collect2: ld returned 1 exit status How can I define the references of Yampa in the DLL? I also tried to reinstall Yampa shared with: $ cabal install --reinstall --enable-shared yampa Could not find module `Data.IORef': Perhaps you haven't installed the "dyn" libraries for package `base-3.0.3. 2'? Use -v to see a list of the files searched for. $ cabal install --reinstall --enable-shared base # :) Resolving dependencies... cabal: internal error: impossible ---- (StartEnd is taken from: http://www.haskell.org/ghc//docs/latest/html/users_guide/win32-dlls.html ) DLLTest.hs: {-# LANGUAGE ForeignFunctionInterface, Arrows #-} module DLLTest embeddedSF :: Double -> Double -> Double -> IO Double embeddedSF v0 v1 v2 = return . last $ embed integral (v0, [(1.0, Just v1), (1.0, Just v2)]) foreign export ccall embeddedSF :: Double -> Double -> Double -> IO Double

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Storables and Ptrs
by Tyler Pirtle 07 Dec '10

07 Dec '10

Hi cafe, I'm just getting into Foreign.Storable and friends and I'm confused about the class storable. For GHC, there are instances of storable for all kinds of basic types (bool, int, etc) - but I can't find the actual declaration of those instances. I'm confused that it seems that all Storable instances operate on a Ptr, yet none of these types allow access to an underlying Ptr. I noticed that it's possible via Foreign.Marshal.Utils to call 'new' and get a datatype wrapped by a Ptr, but this isn't memory managed - I'd have to explicitly free it? Is that my only choice? Is there a way that given just simply an Int I could obtain a Ptr from it, and then invoke the storable functions on it? Or for that matter, if I go and create some new data type, is there some generic underlying thing (ghc-only or otherwise) that would let me have a Ptr of it? Thanks, Tyler

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Question about TagSoup
by David Virebayre 07 Dec '10

07 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 07 Dec '10

07 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 07 Dec '10

07 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 06 Dec '10

06 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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