Ttykuu

Since the Esteemed Academia wants me to learn Haskell, and Haskell seems just weird for me, I thought that to train some basic Haskell skills I'd write a simple script I now need in Haskell.

This is what the script is supposed to do... I have a file that looks like this:

oo R
j
j ai I'm a comment
kd oo
stack j
a

I needed to extract duplicated elements from this file with lines they appear on. The catch is that the tab character denotes comments; so I'm a comment should not be considered here and in particular, this a from I'm a comment should not match this a that appears as the last element from these sample lines.

If fed this particular sample file, my script should report finding duplicate elements: oo on lines 1 and 4 and j on lines 2, 3 and 5. I believe I wrote a script that does this correctly.

Here it is:

import qualified Data.Map as Map
import Data.List
import Data.Function

main = do
 input <- fmap preprocess getContents
 printDupes $ findDupes input

preprocess input = let
 untabbedLines = map (takeWhile (/= 't')) $ lines input
 wordsedLines = map words untabbedLines
 numberedLines = zip wordsedLines [1..]
 numberedWords = concat $ map indexDown numberedLines where
 indexDown (words', index) = map (flip (,) $ index) words'
 in numberedWords

findDupes entries = let
 occurences = Map.fromListWith (flip (++)) $ map (fmap (:[])) entries
 in Map.filter ((>1).length) occurences

printDupes :: Map.Map String [Int] -> IO()
printDupes dupes = let
 showPositions positions = intercalate ", " $ map show positions
 printDupe (dupe, positions) = putStrLn $ "Duplicated element " ++ dupe ++ " found on positions: " ++ showPositions positions
 in let
 sortedDupes = sortBy (compare `on` snd) $ Map.assocs dupes
 in mapM_ printDupe sortedDupes

The task is simple, but I guess it's OK for learning how to code in Haskell...

So could you kindly review this self-imposed exercise? What could've been done better than I did? Simpler? Shorter? I'm sure much, but what precisely? Also how to improve readability?