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