[FIXED] Why my output is "()" and how can i fix it?

Issue

I do work with text rotate from a txt file, I'm stuck on the simplest thing - output back to the same file. At the end of the program, “()” appears in the test file (output.txt). I know I need to rewrite main2 somehow, but I don’t know how exactly. There is problem with side effects, I think. So, what to do? I will be happy to get your help

import System.IO

getPixel :: [[Char]] -> Int -> Int -> Char
getPixel img x y
  | x >= 0 && x < width && y >= 0 && y < height = img !! y !! x
  | otherwise = ' '
  where
    height = length img
    width = length $ head img

rotate :: Double -> (Double, Double) -> (Double, Double)
rotate a (x, y) = (x * cos a + y * sin a, -x * sin a + y * cos a)

main :: IO ()
main = do
    output <- main2
    writeFile "output.txt" (show output)

main2 :: IO ()
main2 = do
    image <- lines <$> readFile "input.txt"
    mapM_ putStrLn $ do
        y <- [0 .. 30]
        return $ do
            x <- [0 .. 40]
            let (x', y') = rotate (pi/3) (x-5, y-1)
            return $ getPixel image (floor x') (floor y')

Solution

At the end of the program, () appears in the test file (output.txt)

Well the reason it writes () is because the signature of main2 is main2 :: IO (). This means that output in output <- main2 will be the unit type [wiki], and thus show () will return the string "()".

But you actually do not need to use mapM_ here in the first place. You can make a function that will, for a given [[Char]] generate a list of chars, for example:

rotateImg :: (Int -> Int -> Char) -> [Int] -> [Int] -> [[Char]]
rotateImg getPix ys xs = [
    [ getPix (floor x) (floor y) | x' <- xs, let (x, y) = rotate (pi/3) (fromIntegral (x'-5), fromIntegral (y'-1)) ]
    | y' <- ys
  ]

So then we can define in the main a function that reads then image info, rotates the image, and finally writes the rotated image to a file (or prints it to the stdout):

main :: IO ()
main = do
    image <- lines <$> readFile "input.txt"
    let image2 = rotateImg (getPixel image) [0..30] [0..40]
    writeFile "output.txt" (unlines output)

Answered By - Willem Van Onsem

Answer Checked By - Mildred Charles (PHPFixing Admin)