March 2010 - Haskell-Cafe

Cabal dependency hell
by Maciej Piechotka 14 Apr '10

14 Apr '10

While I love Haskell it's packaging system have some problems - especially with parsec. Currently I'm not able to install an practically anything using cabal due to version mismatches (or at least packages linking to both version of parsec). I found the following problems in various cabal packages (the examples are only examples - please do not be offended): - The constraints are too loose. It is written that package works with '>0.6 && <1' but in 0.8 the API has been changed (example of tagsoup & hxt. Fixed hxt versions depends on ghc 6.12... and ghc 6.10 if you change 2 lines in cabal file) - The constraints are too tight. It is written that package works with 'parsec <3' but it can run with 3.0 and 3.1 (a lot of packages) Additionally: - Monomorphism restriction & autodetection of types may cause that extending interface (such as changing signature from a -> IO a to MonadIO m => a -> m a) may break some programs - You cannot change the restrictions on already submitted programs. If I figure out that due to minor change my package does not work with parsec 3.1 I can push the new version with fix but there is no way to mark package on hackage as 'this version is not compatible with parsec >=3.1' I understand that it is not problem only with haskell/cabal but also with many other packaging systems - especially those language-specific. I understand the need of ability of changing API (and haskell-specific problem what constitutes as API change). However I hope that some improvements are possible. Best regards PS. I understand that content may be flame-gen. I am sorry in advance if such circumstances happen. However I believe that possible improvements in process are worth the risk.

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Cabal update problem
by Maciej Podgurski 12 Apr '10

12 Apr '10

Hi, after updating to cabal-install-0.8.0/Cabal-1.8.0.2 with GHC 6.10.4, I always get an error when updating the package list: > cabal update Downloading the latest package list from hackage.haskell.org cabal: Codec.Compression.Zlib: premature end of compressed stream Is there a way to fix this without rebuilding from the latest darcs repo? Best wishes, Maciej

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libraries and executables in cabal
by Paul Brauner 11 Apr '10

11 Apr '10

Hi, i'm working on a project made of - lots of modules - one excutable importing these modules - another excutable importing these same modules I don't especially want to expose those modules as libraries, especially on hackage, since they are meaningless without the executables. But, if I declare two executables in my .cabal file, cabal will compile all those modules two times each time I want to compile the executables. Is there a way to tell cabal that those modules should be considered part of a "private" library ? Paul

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developing against privately patched libraries, and cabal
by Dougal Stanton 11 Apr '10

11 Apr '10

If you're making local changes against a library you don't own (with the ultimate intention of sending those changes back upstream to the library maintainer) it makes sense change the version number to avoid clashes with the canonical version of the library. Of course, it's easy to lose track and end up publishing your own program against a non-existent (outside your hard disk) version of the library. I'd like to make it very obvious, both in mypogram.cabal and library.cabal that one is a patched copy and the other has to be compiled against a patched copy. Does cabal provide any way of marking a version private? I thought initially to just mark the version field in the patched library as X.y-dougal and enforce my program to compile against that, but it doesn't seem to recognise the -dougal suffix there. Thoughts? D -- Dougal Stanton dougal(a)dougalstanton.net // http://www.dougalstanton.net

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Alternative dependencies in Cabal file
by Matthias Reisner 11 Apr '10

11 Apr '10

Hi, for a package I need to ensure the user uses a certain package configuration. So how would I rewrite the following pseudo-cabal description? Build-Depends: packageA < X, packageB < Y or packageA >= X && < X', packageB >= Y && < Y' or packageA >= X', packageB >= Y' Build-Depends: ... common dependencies ... where neither A nor B is the base package. Maybe I have to use if/else blocks, but I don't know what conditions to use then. Regards, Matthias

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passing cpp options to c2hs from cabal
by Chris Casinghino 11 Apr '10

11 Apr '10

Hi all, I have a question about cpp, c2hs and cabal. The short version: What can I put in my cabal file to get "-cppopts=-U__BLOCKS__" passed as an argument in calls to c2hs? Longer story: I need to set up my cabal file so that c2hs gets this extra option to make things build smoothly on some macs. If I run cabal from the command line, I can pass it: cabal install --c2hs-options='--cppopts=-U__BLOCKS__' And this works great. I need to make my .cabal file do this, but c2hs-options doesn't seem to be accepted there. I tried: cpp-options: "-U__BLOCKS__" But if I run cabal install -v I can see this isn't being passed on to c2hs: ... /home/ccasin/.cabal/bin/c2hs --include=dist/build --cppopts=-D__GLASGOW_HASKELL__=610 --cppopts=-Icontrib/libpuz/include --output-dir=dist/build --output=Codec/Game/Puz/Internal.hs ./Codec/Game/Puz/Internal.chs ... Thanks! --Chris Casinghino

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Multiple versions of a cabal package Or what's the right way to update?
by david fries 11 Apr '10

11 Apr '10

Dear Caféistas I desperately need your collective knowledge I have been working on porting the haskell-platform to the FreeBSD operating system for a while now and some versioning issues have come up. FreeBSD is still running on GHC-6.10.4 (6.12 is in the works). The ghc package contains a couple of libraries that ghc is built against and requires to run. For instance, network-2.2.1.2 is one of them. The problem is that the Haskell Platform requires version 2.2.1.4. Under normal circumstances we would port network-2.2.1.4 to FreeBSD and then specify it as a dependency of the Platform port. But then we end up with two (exposed) versions of network installed on the system. For example, let's assume that I have the following Haskell source (Networking.hs): module Networking where import Data.ByteString.Lazy import Data.Maybe import Network.HTTP import Network.Stream import Network.URI getBinary :: String -> IO ByteString getBinary url = do uri <- return $ fromJust $ parseURI url rsp <- simpleHTTP $ request uri getResponseBody rsp where request uri = Request { rqURI = uri , rqMethod = GET , rqHeaders = [] , rqBody = empty } When I try to compile it (`ghc --make Networking.hs`) I get the following: Networking.hs:12:30: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the first argument of `request', namely `uri' In the second argument of `($)', namely `request uri' In a stmt of a 'do' expression: rsp <- simpleHTTP $ request uri However, the following thing could help: $ ghc --make Networking.hs -package network-2.2.1.2 The workaround ain't acceptable to the FreeBSD committers, so I have to come up with something better... In a perfect world, I'd be able to install the required HP dependencies without tripping ghc up. Then we could switch over to the platform as a foundation of GHC-6.12. I have lots of questions. Is it smart to have multiple installed versions of the same library? Up until now, we could avoid such situations. What would be the correct approach to updating network? I guess it would involve rebuilding all installed packages that had previously been compiled with ghc? Though I hope not - our package management system could pull that off if necessary. If it is perhaps a bad idea to update certain "core" libraries, is there a way to declare them as such in the cabal file? regards and TIA, dave

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Cabal-install
by Andrew Coppin 11 Apr '10

11 Apr '10

OK, so apparently my Google skills are lacking. I can't find anything but the most terse documentation of the cabal-install tool online. Somewhere there surely must be something which explains how to control this thing. Anyway, can anybody tell me how I can change the default settings so that I get profiling libraries built by default, and Haddock documentation built by default? (I'm on Windows, in case that makes a difference...)

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Performance question
by Arnoldo Muller 09 Apr '10

09 Apr '10

Hello! I am trying to implement a binary search function that returns the index of an exact or the (index + 1) where the item should be inserted in an array if the item to be searched is not found (I am not trying to insert data in the array) . Right now, the bottleneck of my program is in binarySearch', the function must be called a few billion times. Do you have any ideas on how to improve the performance of this function? import Data.Array.IArray type IntArray a = Array Int a -- The array must be 0 indexed. binarySearch :: Ord a => a -> IntArray a -> Int binarySearch query array = let (low, high) = bounds array in binarySearch' query array low high binarySearch' :: Ord a => a -> IntArray a -> Int -> Int -> Int binarySearch' query array !low !high | low <= high = let ! mid = low + ((high - low) `div` 2) ! midVal = array ! mid in next mid midVal | otherwise = -(low + 1) where next mid midVal | midVal < query = binarySearch' query array (mid + 1) high | midVal > query = binarySearch' query array low (mid - 1) | otherwise = mid Thank you! Arnoldo Muller

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Haskell.org re-design
by Christopher Done 08 Apr '10

08 Apr '10

This is a post about re-designing the whole Haskell web site. We got a new logo but didn't really take it any further. For a while there's been talk about a new design for the Haskell web site, and there are loads of web pages about Haskell that don't follow a theme consistent with Haskell.org's, probably because it doesn't really have a proper theme. I'm not a designer so take my suggestion with a grain of salt, but something that showed pictures of the latest events and the feeds we currently have would be nice. The feeds let you know that the community is busy, and pictures tell you that we are human and friendly. Anyway, I came up with something to kick off a discussion: http://haskell.org/haskellwiki/Image:Haskell-homepage-idea.png It answers the basic questions: - What's Haskell? - Where am I on the site? (Answered by a universally recognised tab menu) - What's it like? - How do I learn it? - Does it have an active community? - What's going on in the community? What are they making? - This language is weird. Are they human? -- Yes. The picture of a recent event can fade from one to another with jQuery. The colours aren't the most exciting, but someone who's a professional designer could do a proper design. But I like the idea of the site being like this; really busy but not scarily busy. Subsections of the site could use the header and footer and heading theme, but have a completely different primary-content layout. Probably sub-sections would need a left-nav. Keeping the design simple like this also makes it easy to theme the current Wiki to fit in with it seamlessly. Personally I don't have a problem with the existing site, functionally. It has all the stuff I want to look at. The only stuff that I had issue with as a newbie was finding The One Book I Should Read and The One Download I Should Get. The current site is starting to address this with a "Download Haskell" button. However, looking at it as a marketing site, it does look pretty lame and messy, and it gives you that impression of Haskell. So if people who own the site are going to redesign it, I thought I'd contribute a bit. Anyway, please contribute your ideas. (Again, I'm not a designer, so you don't need to pick at the aesthetics, a real designer can sort that out.) Cheers!

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