How can I print a newline properly in Haskell?

Question

I was trying to make Tic Tac Toe game for the monthly tutorial and wrote this code to make a box first:

box :: [String]
box = ["0 | 1 | 2",
       "---------",
       "3 | 4 | 5",
       "---------",
       "6 | 7 | 8"]

I get this output in GHCi:

["0 | 1 | 2\n","---------\n","3 | 4 | 5\n","---------\n","6 | 7 | 8\n"]

I wanted to get:

 0 | 1 | 2
 ---------
 3 | 4 | 5
 ---------
 6 | 7 | 8

How can I print the grid like this?

Answer 1

Try something like:

box = unlines $ ["0 | 1 | 2",
                 "---------",
                 "3 | 4 | 5",
                 "---------",
                 "6 | 7 | 8"]

Then output the box using putStr:

main = putStr box

What unlines does is take a list of strings and join them together using newlines. It basically treats the list of strings as a list of lines and turns them into a single string.

putStr just prints the string to STDOUT. If you used print or GHCi to look at the string instead, newlines would be rendered as \n rather than actual newlines. This happens because the show instance of a String is designed to serialize the string as you would have inputted it rather than printing it. Both print and GHCi use show before outputting to STDOUT.

If you are at the GHCi prompt and want to see what the string actually looks like, you can use putStr directly:

*Main> putStr box
0 | 1 | 2
---------
3 | 4 | 5
---------
6 | 7 | 8
*Main>

Answer 2

Another approach is

main = mapM_ putStrLn box

mapM :: Monad m => (a -> m b) -> [a] -> m [b] is the analogue of map, but for monadic functions, its underscored cousin, mapM_ :: Monad m => (a -> m b) -> [a] -> m () doesn't collect the return values and can thus be more efficient. If the return values are uninteresting, as for putStrLn s, mapM_ is the better choice.