HackerRank.com - Functional Programming - Prefix Compression - Haskell

a_prefix_compression.hs

slv n "" _ = n
slv n _ "" = n
slv n a b
    | fa == fb = slv (n + 1) (tail a) (tail b)
    | otherwise = n
    where fa = head a
          fb = head b

main = do
    raw <- readFile "input.txt"
    let lns = lines raw
        str1 = head lns
        str2 = last lns
        plen = slv 0 str1 str2
        pfx = take plen str1
        rstr1 = drop plen str1
        rstr2 = drop plen str2
    putStrLn $ (show plen) ++ " " ++ pfx
    putStrLn $ (show $ length rstr1) ++ " " ++ rstr1
    putStrLn $ (show $ length rstr2) ++ " " ++ rstr2

input.txt

abcdefpr
abcpqr

output.txt

3 abc
5 defpr
3 pqr