Send Beginners mailing list submissions to
beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
beginners-requ...@haskell.org
You can reach the person managing the list at
beginners-ow...@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re: Comments on Map/Reduce Code (Brent Yorgey)
2. Trouble compiling haskell platfrom (Jeff Lasslett)
----------------------------------------------------------------------
Message: 1
Date: Tue, 10 Jul 2012 10:19:48 -0400
From: Brent Yorgey <byor...@seas.upenn.edu>
Subject: Re: [Haskell-beginners] Comments on Map/Reduce Code
To: beginners@haskell.org
Message-ID: <20120710141948.ga16...@seas.upenn.edu>
Content-Type: text/plain; charset=us-ascii
On Thu, Jul 05, 2012 at 03:50:32PM +0200, Thomas Bach wrote:
> Hello there,
Hi Thomas,
Looks pretty good. I've interspersed a few comments below.
> 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
The above looks fine, except that generally the recommendation is to
use the text package [1] for dealing with text, whereas ByteString is
for binary data that you wish to manipulate as a sequence of bytes.
You can get away with the above only when the text consists entirely
of ASCII characters.
[1] http://hackage.haskell.org/package/text
> 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
What happens when the line is empty?
> group :: Eq a => [(a, b)] -> [[(a, b)]]
> group = L.groupBy (\x y -> fst x == fst y)
The above lambda can also be written as ((==) `on` fst). 'on' can be
imported from Data.Function.
>
> summation :: Num b => [(a, b)] -> (a, b)
> summation (x:[]) = x
> summation (x:xs) = (fst x, (snd x) + (snd (summation xs)))
Instead of using (fst x) and (snd x), you should pattern-match on x,
like
summation ((x,y):xs) = (x, y + ...)
> formatter :: (String, Int) -> String
> formatter = (\w -> (fst w ++ "\t" ++ show (snd w)))
The same goes here. I would also put the w argument on the left-hand
side of the =, like
formatter (s,i) = s ++ "\t" ++ show i
>
> main = do
> contents <- C.getContents
> putStr . unlines $ map formatter $ map summation $ group $ map tuppleize $
> lines $ C.unpack contents
Instead of using a chain of ($), it's generally considered better
style to use a chain of (.) with a single $ at the end, like
putStr . unlines . map formatter ... lines . C.unpack $ contents
-Brent
------------------------------
Message: 2
Date: Wed, 11 Jul 2012 10:07:38 +1000
From: Jeff Lasslett <jeff.lassl...@gmail.com>
Subject: [Haskell-beginners] Trouble compiling haskell platfrom
To: Beginners@haskell.org
Message-ID:
<CAK6+hbxU5k=p0-3myvxf6j5xvao9lta7fkrg2kotvv6wtrp...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
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
------------------------------
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners
End of Beginners Digest, Vol 49, Issue 11
*****************************************
