错误：语法错误删除OF ID RPAREN ;;;; SML

Question

datatype SquareContent = Mine | Digit of int | Blank;
datatype Square = Revealed of SquareContent | Concealed of SquareContent;
datatype GameStatus = OnGoing | Success | Failure; 
type MineSweeperState = (Square list list * GameStatus);
type matrix = Square list list;

fun createMineSweeperGrid (n:int, mines:(int*int) list)
:(Square list list)= 
let
   fun createMines (rowCounter:int, colCounter:int
    , retGame:Square list list):(Square list list) = 
    if rowCounter=n then   
      retGame         (* finished all rows, should be n lists of size n*)
    else
      if colCounter=n then   (*finished current row, move on*)
        createMines (rowCounter+1, 0, retGame)
     else   
        if (List.exists(fn y => (rowCounter,colCounter) = y) mines) then
           createMines (rowCounter+1, 0, retGame@(Concealed of Mine)))
        else
           createMines (rowCounter+1, 0, retGame@(Concealed of Blank)))
in
  createMines (0,0,[])
end

我正在尝试编写一个函数，将我的矿井插入矿井（坐标）列表中的2dm矩阵，我从用户那里收到。 n代表尺寸。 我在标题中收到错误。

请帮助。