March 2009 - Haskell-Cafe

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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MPTC inheritance
by Derek Gladding 01 Mar '09

01 Mar '09

Please forgive me if I'm still mentally contaminated by the OO way of seeing (and discussing) the universe, but I'm trying to figure out how to "inherit an interface" from a multi-parameter type class. I have a Graph class that's parameterisable by Node and Edge type: class (Node a, Edge b) => Graph a b where (lots of stuff that you can do with Graph a b) Now, I'd like to build a FooGraph on top of this that adds additional capabilities: class (Graph a b) => FooGraph a b where (lots of additional stuff) but this isn't allowed (kind mismatch). Of couse, I can do: class (Node a, Edge b) => FooGraph a b but this means that I have to manually replicate the Graph a b operations in the FooGraph a b class definition, which is (a) work that the machine should (?) be able to do for me, and (b) fragile. Any pointers / wisdom would be very much appreciated. - Derek

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missing support for NFData in uvector package
by Manlio Perillo 01 Mar '09

01 Mar '09

Today I noticed that there is no instance declaration for NFData, in the uvector package. The definition is quite simple: instance NFData a => NFData (UArr a) where -- NOTE: UArr is already strict rnf array = array `seq` () but it is important. In a my program I was using a tuple of two arrays, and plain `seq` did not worked. Regards Manlio Perillo

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