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Hint & Cabal Install weirdness.
by Joseph Fredette 02 Mar '09

02 Mar '09

I'm attempting to install hint, I started by doing: cabal install hint and it gives me the following error: cabal: dependencies conflict: ghc-6.10.1 requires process ==1.0.1.1 however process-1.0.1.1 was excluded because ghc-6.10.1 requires process ==1.0.1.0 So I decided to try installing it via darcs, expecting to either have it work, or boil it down to one or two dependencies that were causing the problem, my hunch was right, it boils down to the ghc-mtl package, leading to the same error as above. My question is, how can I fix this? Doing a ghc-pkg hide on either process package breaks Cabal and Haddock (among others) according to the error message, and even after that happens, cabal complains about needing ghc >= 6.6 (or 6.8, depending on which package I disable), even though I have ghc 6.10. Detailed system info follows, help me haskell-cafe-kenobi(s), you're (all) my only hope(s)! Sysinfo: Arch Linux: uname -a gives: Linux Erdos 2.6.28-ARCH #1 SMP PREEMPT Mon Feb 2 22:22:59 UTC 2009 i686 Intel(R) Celeron(R) CPU 3.06GHz GenuineIntel GNU/Linux ghc --version gives: The Glorious Glasgow Haskell Compilation System, version 6.10.1 cabal --version: cabal-install version 0.6.0 using version 1.6.0.1 of the Cabal library Thanks again, /Joe

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Supplying a default implementation for a typeclass based on another class
by Svein Ove Aas 02 Mar '09

02 Mar '09

I'm in the process of writing a distributed filesystem (in haskell, yay), which of course means I'm using Binary for de/serialization. Now, that's fine enough, but for simplicity (and for wireshark), I'd like to be able to have Binary fall back on an instance based on Show/Read for any type that lacks any other Binary instance.. Well, I understand why this would be somewhere between extremely hard and impossible in haskell '98, but I'm not entirely up on all the extensions, so I thought I'd ask - given every extension implemented in ghc 6.10.1, is there any reasonable way to do this?

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Re: a newbies confusion with repositories - darcs or git
by trentbuck＠gmail.com 02 Mar '09

02 Mar '09

Max Battcher <me(a)worldmaker.net> writes: > +1. For the most part this was a great email. It probably belongs > immortalized on the darcs blog and elsewhere. It could even go in the manual! I think the first two chapters of the manual should be 1. Why Darcs? 2. Quick Start ...and the stuff about compiling darcs and migrating from other VCSs moved either to the end (as appendices) or simply onto the wiki.

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nubBy changed?
by Tim Newsham 02 Mar '09

02 Mar '09

Did nubBy change recently? In 6.8.2 I could generate primes as: nubBy (\a b -> b `mod` a == 0) [2..] but in 6.10.1 I have to use nubBy (\a b -> a `mod` b == 0) [2..] Is this change intentional? If so, what is the reason? Tim Newsham http://www.thenewsh.com/~newsham/

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Re: [Haskell-cafe] Re: memory issues
by Martin Huschenbett 01 Mar '09

01 Mar '09

ChrisK schrieb: > Bulat is right about making Block's fields strict. > >> >> -- | Get the offsets between entries in a list >> getSizes :: [Integer] -> [Integer] >> getSizes (x:y:[]) = [y - x] >> getSizes (x:y:ys) = (y - x):(getSizes (y:ys)) > > You should change the first part to add maxSize: > > > getSizes :: [Integer] -> [Integer] > > getSizes (x:y:[]) = [y - x,maxSize] > > getSizes (x:y:ys) = (y - x):(getSizes (y:ys)) > > This avoids the ugly use of (++) below. Note that appending to a singly > linked list is a bad "code smell": But Chris' version changed semantics. It should be > getSizes :: [Integer] -> [Integer] > getSizes (x:y:[]) = [y-x,maxSize-y] > getSizes (x:y:ys) = (y-x):getSizes (y:ys) instead. But > getSizes :: [Integer] -> [Integer] > getSizes [x] = [maxSize-x] > getSizes (x:y:ys) = (y-x):getSizes (y:ys) is even better as it doesn't repeat the y-x term. Regards, Martin.

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differences between Data.Array and Data.Vector
by Manlio Perillo 01 Mar '09

01 Mar '09

Hi. In Hackage there are some packages named "*array*", and others named "*vector*". What are the differences? Is available a guide to the various data structures available in Haskell? Thanks Manlio Perillo

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odd problem with export list of System.IO
by Manlio Perillo 01 Mar '09

01 Mar '09

Hi. Maybe I'm missing something, but I don't understand why the following program: module Main where import System.IO (Handle, withFile, ReadMode) main = do return fails to compile, with a Module `System.IO' does not export `ReadMode' Thanks Manlio Perillo

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memory-efficient data type for Netflix data - UArray Int Int vs UArray Int Word8
by Kenneth Hoste 01 Mar '09

01 Mar '09

Hello, I'm having a go at the Netflix Prize using Haskell. Yes, I'm brave. I kind of have an algorithm in mind that I want to implement using Haskell, but up until now, the main issue has been to find a way to efficiently represent the data... For people who are not familiar with the Netflix data, in short, it consist of roughly 100M (1e8) user ratings (1-5, integer) for 17,770 different movies, coming from 480,109 different users. The idea I have in mind requires fast access to all the ratings for a particular user, so I was thinking about an IntMap in terms of user ids, which maps to movie id/rating pairs somehow. To see if I could efficiently represent the data set in this way, I wrote a small Haskell program (attached) which uses the following data type: type data = IntMap (UArray Int Word8) -- map of user IDs to ratings (lacks movie IDs) For convenience, and because I've been discussing this with various people in #haskell @ IRC, the code is also available here: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1671#a1671 . I'm aware that the way in which the UArray's are constructed (i.e. by adding a single element each time), is far from efficient performance-wise, but that's not the point here... By reformatting the raw data, I can easily read in the data more efficiently. The issue I ran into is that changing the data type to the following, doesn't yield any significant different in memory usage. type data = IntMap (UArray Int Int) -- use Int instead of Word8 for a user rating Someone (dolio) @ #haskell suggested that maybe UArray is not byte packed for Word8, which would cause little difference with a UArray containing Int's, but someone else (dons @ #ghc) was able to tell me it _is_ byte packed. Does anyone know why the Word8 version is not significantly better in terms of memory usage? greetings, Kenneth PS: My adventures on trying to tackle the Netflix Prize problem with Haskell can be followed at http://boegel.kejo.be. -- Kenneth Hoste ELIS - Ghent University email: kenneth.hoste(a)elis.ugent.be blog: http://www.elis.ugent.be/~kehoste/blog website: http://www.elis.ugent.be/~kehoste

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Problem with haddock - it dislikes my comments
by Colin Paul Adams 01 Mar '09

01 Mar '09

Having just read the Haddock manual , I am haddocking (may I copyleft that verb?) my code. But half-way through this data type, I discover that haddock rejects my non-haddock comments, although GHC is fine: -- | Non-interactive representation of a Chu Shogi move data Move = Pass Piece Coordinate -- ^ Double move to empty square and back to origin | Igui Piece Coordinate -- ^Capture without moving Piece Coordinate -- Captured piece | Double_move Piece -- ^ Other double moves Coordinate -- Initial location Coordinate -- First destination (Maybe Piece) -- Capture? Coordinate -- Second destination (Maybe Piece) -- Capture? | Capture Piece Coordinate Piece Coordinate Bool -- Promote? Bool -- Declined to promote | Move Piece Coordinate Coordinate Bool Bool How can I document the components of each constructor? -- Colin Adams Preston Lancashire

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Can't install mkcabal because of pcre-lite
by Colin Paul Adams 01 Mar '09

01 Mar '09

cabal: Error: some packages failed to install: mkcabal-0.4.2 depends on pcre-light-0.3.1 which failed to install. pcre-light-0.3.1 failed during the configure step. The exception was: exit: ExitFailure 1 Downloading pcre-light-0.3.1... Configuring pcre-light-0.3.1... cabal: Missing dependency on a foreign library: * Missing C library: pcre This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. I'm on fedora 10. I have package pcre installed. But yum install pcre-dev so no such package. Anyone know the anser? -- Colin Adams Preston Lancashire

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