Expected type: String, Actual type [[Char]] error message in Haskell

Question

I'm working on a Haskell implementation of the board game Othello and still at the beginning of the process. I'm currently trying to create a function that will display the board. At the minute I am simply trying to get the function to output the coordinate value of each cell. My idea is that I can recurse through each row and then through each cell and output the value of each. I then call this function in main and output the board as a string.

I'm having trouble with string concatenations however and run into the error "Expected type: String, Actual type: [[Char]].

I know that in Haskell, String is essentially a pseudonym for [Char] but I can't see why I am getting a [[Char]] when I use the "++" function.

showGameState :: GameState -> String
showGameState g =
      let boardStart = (1,1)
      in drawBoard boardStart (board g) (size $ board g)

//the standard board is size 8*8
//here im trying to recursively call the drawBoardRow function increasing the row each time
drawBoard :: (Int, Int) -> Board -> Int -> String
drawBoard pos b len =
      let string = ""
      in if len > 0
              then string ++ (drawBoardRow pos (size b) : [drawBoard (fst pos, snd pos +1) (b) (len - 1)])
              else return string


//callig the drawBoardCell function recursively increasing the column position each time
drawBoardRow :: (Int, Int) -> Int -> String
drawBoardRow pos len =
      let string = ""
      in if len > 0
              then string ++ (drawBoardCell pos : [drawBoardRow(fst pos + 1, snd pos) (len - 1)]) ++ "\n"
              else return string


//At this stage i simply want to add each the coordinate of the cell the string containing
//the row
drawBoardCell :: (Int, Int) -> String
drawBoardCell pos =
      let string = ""
      in return (string ++ " " ++  show(fst pos) ++ show(snd pos) ++ " ")

Ideally I want this function to output this:

11 21 31 41 51 61 71 81
12 22 32 42 52 62 72 82
13 23 33 43 53 63 73 83
14 24 34 44 54 64 74 84
15 25 35 45 55 65 75 85
16 26 36 46 56 66 76 86
17 27 37 47 57 67 77 78
18 28 38 48 58 68 78 88

Sorry if my code isn't very readable or my ideas unclear, I'm still trying to learn the basics of Haskell. Thank for your time and help in advance.

Can you give us the full error message? That way we can know which function the error is occurring in. — bradrn

molbdnilo molbdnilo · Accepted Answer · 2020-04-16T04:46:46

This is because

(drawBoardCell pos : [drawBoardRow(fst pos + 1, snd pos) (len - 1)])

is a [String], not a String, and you can't append a String to a [String].
(drawBoardCell and drawBoardRow produce Strings, so [drawBoardRow ...]is a [String], so the entire thing is also a [String].)

You need to stick to ++.

It also looks like you're using let and return as if this were some other programming language that you already know.
If you do know some other programming language, now is a good time to forget that you do.

You don't need return at all here, and most of your lets serve no purpose.

My own beginner-level take would look like this:

drawBoard :: (Int, Int) -> Board -> Int -> String
drawBoard _ _ 0 = ""
drawBoard pos@(x,y) b len =
  drawBoardRow pos (size b) ++ drawBoard (x, y + 1) b (len - 1)

drawBoardRow :: (Int, Int) -> Int -> String
drawBoardRow _ 0 = ""
drawBoardRow pos@(x, y) len =
  drawBoardCell pos ++ drawBoardRow(x + 1, y) (len - 1) ++ "\n"

drawBoardCell :: (Int, Int) -> String
drawBoardCell (x, y) = " " ++  show x ++ show y ++ " "

Expected type: String, Actual type [[Char]] error message in Haskell

2 Answers