A simple Haskell program to count the number of lines in a given file.

CountLines.hs

import System.Environment (getArgs)

countLines :: String -> IO Int
countLines file = do
  contents <- readFile file
  return (length $ lines contents)
  
main :: IO ()
main = do
  args <- getArgs
  case (length args) of
    0 -> putStrLn "You need to tell me which file you want to count the lines of"
    _ -> let file = head args in do
            putStr $ "The number of lines in the file" ++ file ++ "is: "
            lines <- countLines file
            putStrLn $ show lines