-- | Input/Output in Haskell
-- Examples to introduce and illustrate how I/O works in Haskell
-- Functional Programming course 2016.
-- Thomas Hallgren

{-
This is just a skeleton, the definitions will be filled in
during the lecture.
-}

import Data.List(sort)

main = showTheDifference2

showTheDifference1 :: IO ()
showTheDifference1 = do putStrLn "Enter two numbers"
                        x <- readLn
                        y <- readLn
                        putStr "The difference is: "
                        print (x-y)



-- | Using 'return' to return results from IO actions
getTheDifference :: IO Integer
getTheDifference = do putStrLn "Enter two numbers"
                      x <- readLn
                      y <- readLn
                      return (x-y)


-- | Reimplementation of showTheDifference1, using getTheDifference
showTheDifference2 :: IO ()
showTheDifference2 = do diff <- getTheDifference
                        putStr "The difference is: "
                        print diff


copyFile :: FilePath -> FilePath -> IO ()
copyFile from to = do s <- readFile from
                      writeFile to s


sortFile :: FilePath -> FilePath -> IO ()
sortFile from to = do s <- readFile from
                      writeFile to (sortLines s)

sortLines = unlines . sort . lines


doTwice :: IO a -> IO (a,a)
doTwice io = do x <- io
                y <- io
                return (x,y)

don't :: IO a -> IO ()
don't io = do print 42
              return ()


-- | Reimplementation of getTheDifference, using doTwice
getTheDifference2 :: IO Integer
getTheDifference2 = do putStrLn "Enter two numbers"
                       (x,y) <- doTwice readLn
                       return (x-y)


-- | An example of combining do blocks and recursion
numbers 0 = return ()
numbers n = do print n
               numbers (n-1)