July 2012 - Beginners

Comments on Map/Reduce Code
by Thomas Bach 17 Jul '12

17 Jul '12

Hello there, I'm new to Haskell and want to learn it a bit while experimenting with Hadoop's Map/Reduce programming model. So, I wanted to implement the standard ‘word counter’ problem in Haskell. The problem is as follows: We have several texts with words separated by white-space. We want to count the occurrences of all words in all the texts (where ‘but’ and ‘but,’ can be seen as two different words). This is done in two phases. In the Map phase a program gets a part of the text from stdout and has to produce a "KEY\tVALUE" pair (that is, the key separated with a tab to the value), which has to be passed to stdin. In our case we simply produce "WordX\t1" for every word WordX. This list is sorted by the key and later on fed as stdin to the reducer (the second phase). The reducer now has to sum up all the occurrences we trivially counted in the Map phase and put it as "WordX\tNumber" to stdout. So, here is an example: vince@roku:~/tmp echo "foo foo bar bar foo bar zoo bar foo" | runhaskell mapper.hs foo 1 foo 1 bar 1 bar 1 foo 1 bar 1 zoo 1 bar 1 foo 1 vince@roku:~/tmp echo "foo foo bar bar foo bar zoo bar foo" | runhaskell mapper.hs | sort | runhaskell reducer.hs bar 4 foo 4 zoo 1 And here is the code I've come up with: vince@roku:~/tmp cat mapper.hs import qualified Data.ByteString.Lazy.Char8 as C postFix :: C.ByteString postFix = C.pack "\t1" formatter :: C.ByteString -> C.ByteString formatter x = C.append x postFix main :: IO () main = do contents <- fmap C.words C.getContents C.putStr . C.unlines $ map formatter contents vince@roku:~/tmp cat reducer.hs import qualified Data.ByteString.Lazy.Char8 as C import qualified Data.List as L tuppleize :: String -> (String, Int) tuppleize line = (\xs -> (head xs, read (last xs))) $ words line group :: Eq a => [(a, b)] -> [[(a, b)]] group = L.groupBy (\x y -> fst x == fst y) summation :: Num b => [(a, b)] -> (a, b) summation (x:[]) = x summation (x:xs) = (fst x, (snd x) + (snd (summation xs))) formatter :: (String, Int) -> String formatter = (\w -> (fst w ++ "\t" ++ show (snd w))) main = do contents <- C.getContents putStr . unlines $ map formatter $ map summation $ group $ map tuppleize $ lines $ C.unpack contents As already said, I'm a Haskell beginner. Could you provide some comments on the code? Thanks in advance, Thomas.

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I can see lists "working" for matrix algorithms if the access patterns are mostly from the start or end of rows and columns.
by KC 15 Jul '12

15 Jul '12

Are there any such matrix algorithms that operate mainly from the start or end or rows and columns? Note: If the access pattern starts from the end of a row then one stores the row in reverse order in a list. -- -- Regards, KC

1 0

Hackage down?
by Lorenzo Bolla 14 Jul '12

14 Jul '12

Is hackage.haskell.org down? cabal update times out here... How do you guys work around this? It seems to happen pretty often. L.

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Graph Coloring Library?
by Alec Story 14 Jul '12

14 Jul '12

I have a problem where I need to find the smallest k-coloring for a graph that represents conflicts between objects. Is there a Haskell library that will do this for me? I'm not particularly concerned about speed, and it's unlikely that I'll generate really bad edge cases, but I'd prefer to do something other than write the really bad try-every-case algorithm. -- Alec Story Cornell University Biological Sciences, Computer Science 2012

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Concurrent vs GHC
by Mauricio Hernandes 12 Jul '12

12 Jul '12

Hello, I'm having a problem with GHC. When I compile the code bellow it does nothing, but If I try to use ghci it works normally. it seems a simple problem, but I can't understand. Thanks for the help Mauricio import System.IO import Control.Concurrent import Data.List main = do input <- newMVar [1..30000] ia <- newEmptyMVar ib <- newEmptyMVar ic <- newEmptyMVar forkIO $ do x <- readMVar input putMVar ia x forkIO $ do a <- readMVar ia putMVar ib ( sum a ) forkIO $ do a <- readMVar ia putMVar ic ( reverse a ) forkIO $ do b <- readMVar ib c <- readMVar ic print b print c

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Trouble compiling haskell platfrom
by Jeff Lasslett 12 Jul '12

12 Jul '12

Greetings, I'm trying to recompile & reinstall the haskell platform 12.2.0.0. I have installed it successfully on a prior occasion but I decided to reinstall as I think I may have messed up my installed packages a bit while playing around with yesod. I intend to use cabal-dev or virtualenv for that in future. So I thought I'd do a clean install. I blew away my haskell platform and ghc, both of which were installed in a dir under my home dir. I also blew away my ~/.cabal/ dir. I then reinstalled by ghc 7.4.1 into ~/prg/haskell-platform-12.2.0.0. I then unpacked the sources of haskell-platform-12.2.0.0 and configured it with the prefix set to where ghc is. configure finished without error so I ran 'make'. It quits with what follows. How do I fix this?? With thanks, Jeff * Now do "make" followed by "make install" ************************************************** jeff@chunks:~/src/haskell-platform-2012.2.0.0$ make scripts/build.sh ************************************************** Scanning system for any installed Haskell Platform components... Found: HUnit-1.2.4.2 OpenGL-2.2.3.1 GLUT-2.1.2.1 html-1.0.1.2 parallel-3.2.0.2 random-1.0.1.1 QuickCheck-2.4.2 stm-2.3 syb-0.3.6.1 haskell-src-1.0.1.5 text-0.11.2.0 transformers-0.3.0.0 mtl-2.1.1 fgl-5.4.2.4 parsec-3.1.2 network-2.3.0.13 HTTP-4000.2.3 regex-base-0.93.2 regex-posix-0.95.1 regex-compat-0.95.1 xhtml-3000.2.1 cgi-3001.1.7.4 zlib-0.5.3.3 haskell-platform-2012.2.0.0 New packages to install: None! All done. ************************************************** Building alex-3.0.1 "/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc" "--make" "Setup" "-o" "Setup" "-package" "Cabal-1.14.0" Linking Setup ... "./Setup" "configure" "--package-db=../../packages/package.conf.inplace" "--prefix=/home/jeff/prg/haskell-platform-12.2.0.0" "--with-compiler=/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc" "--with-hc-pkg=/home/jeff/prg/haskell-platform-12.2.0.0/bin/ghc-pkg" "--with-hsc2hs=/home/jeff/prg/haskell-platform-12.2.0.0/bin/hsc2hs" "--enable-library-profiling" "--ghc-pkg-option=--package-conf=../../packages/package.conf.inplace" "--user" Warning: defaultUserHooks in Setup script is deprecated. Configuring alex-3.0.1... "./Setup" "build" Building alex-3.0.1... Preprocessing executable 'alex' for alex-3.0.1... src/Data/Ranged/RangedSet.hs:64:8: Could not find module `Test.QuickCheck' There are files missing in the `QuickCheck-2.4.2' package, try running 'ghc-pkg check'. Use -v to see a list of the files searched for. Error: Building the alex-3.0.1 package failed make: *** [build.stamp] Error 2

2 2

How is takeWhile implemented?
by Carlos J. G. Duarte 09 Jul '12

09 Jul '12

3 3

Is there any pydoc or perldoc like for Haskell?
by Carlos J. G. Duarte 07 Jul '12

07 Jul '12

Python has the pydoc command to get help for builtins and modules, eg. "pydoc def" or "pydoc itertools" Perl has a similar facility with the perldoc, eg. "perldoc -tf substr" Is there such a thing available for Haskell? I have been consulting http://www.haskell.org/hoogle/ but it isn't as practical as having help available at the prompt (although very useful, though). thanks in advance.

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Using INLINE/NOINLINE pragma
by felipe zapata 07 Jul '12

07 Jul '12

Hi All, I'm being exploring the Repa Module and I got myself into a big mess trying to figure out how to use the INLINE/NOINLINE pragma. In the paper “Guiding parallel Array Fusion with Indexed Types” it is said that: if the signature contains any D tags, or type class dictionaries, then you must INLINE it to get fast code, otherwise not. But looking in the provided example of matrix multiplication the* transpose2P *defined in the Repa-example package, do not contain a “D” in the signature but nevertheless use the INLINE pragma. Besides in the Numeric Haskell: A repa Tutorial in section 1.8.5 it is said that : “... The moral of the story is to attach INLINE pragmas to all your client functiond that compute array values...”. Could you please give some advices about how to use those pragmas? Thanks in advance. Felipe

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Expert systems using Haskell
by Ramesh Kumar 06 Jul '12

06 Jul '12

Hello Folks, 1) Is it practical, if at all possible, to build rule-based expert systems using Haskell in comparison with Prolog? 2) Are there any good texts which could guide one to build expert systems using Haskell? I'm not really able to get clear answers googling. Thanks very much. Ramesh

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