In today’s Programming Praxis exercise our task is to write a diff command line tool. Let’s get started, shall we?
First, we import a package that gives us a longest common subsequence function.
There are three possible differences between files: deletions, additions and changes.
data Change = D | A | C
We define a few helper functions for the output: one to display the line numbers in the correct format, one for printing headers and one to show the lines that are different.
linenum :: (Int, Int) -> String linenum (s, e) = if s == e then show s else show s ++ "," ++ show e header :: (Int, Int) -> String -> (Int, Int) -> IO () header l op r = putStrLn $ linenum l ++ op ++ linenum r section :: Char -> [String] -> IO () section c = mapM_ (\s -> putStrLn $ c:' ':s)
The diff function follows the same structure as that in the Scheme solution, but with some of the repetition abstracted out.
diff :: String -> String -> IO () diff xs ys = f 0 0 (lines xs) (lines ys) where f n1 n2 = g where g  b = change A  b g a  = change D a  g a b = case lcs a b of  -> change C a b (d:_) -> case (head a == d, head b == d) of (True, True) -> rec 1 1 (True, _ ) -> change A q1 q2 >> rec len1 len2 (_ , True) -> change D q1 q2 >> rec len1 len2 _ -> change C q1 q2 >> rec len1 len2 where [q1, q2] = map (takeWhile (/= d)) [a, b] [len1, len2] = map length [q1, q2] rec l r = f (n1+l) (n2+r) (drop l a) (drop r b) change D a _ = header (n1+1, n1+length a) "d" (n2, n2) >> section '<' a change A _ b = header (n1, n1) "a" (n2+1, n2 + length b) >> section '>' b change C a b = header (n1+1, n1+length a) "c" (n2+1, n2+length b) >> section '<' a >> putStrLn "---" >> section '>' b
To test our function, just load two files and diff them.
main :: IO () main = do f1 <- readFile "file1.txt" f2 <- readFile "file2.txt" diff f1 f2
Sadly, this version isn’t all that much shorter than the Scheme version (about a third). There is still a small amount of duplicated code, but I don’t see a way of elegantly factoring it out.