Types 此过程（elementSeparator）在ReasonML中给出了一个类型错误

此代码：

let initialRows = 5;
    let initialCols = 7;

    /* 
    Player 1 is P1, Player 2 is P2
    */

    type whichPlayer =
      | P1
      | P2;

    /* 
    Either Player 1 or Player 2 has won the game,
    it is a draw, or it is ongoing where either
    it is Player 1's turn or Player 2's turn
    */

    type status =
      | Win(whichPlayer)
      | Draw
      | Ongoing(whichPlayer);

    /*
    The current state of the game is represented by a list of lists of tuples: 
    containing the column number of the occupancy as the first element,
    and the occupant of the specified slot as the second element,
    and a status specification indicating which player's turn it is, or
    whether the game has ended in a win or a draw.
    */

    type state = 
      | State(list(list((int, whichPlayer))), status);

    /*
    A move is represented by an int, corresponding to
    the column number of the desired occupancy 
    */

    type move = 
      | Move(int);

let rec createEmptySpaces: int => string = num =>
    switch(num){
      | x when x > 1 => "( )" ++ createEmptySpaces(num-1)
      | 1 => "( )"
    };      

let rec elementSeparator: (list((int, whichPlayer)), int) => string = (lst, num) =>
switch(lst){
  | [(colNum, p), (colNum2, p2), ... tl] when num == 1 =>{
    let difference = colNum2 - colNum;
    if(colNum != 1){
      createEmptySpaces(colNum-1) ++
      if(p == P1){
        "(o)" ++ if(difference != 1){
                    createEmptySpaces(difference - 1)
                 }
      } else {
        "(x)" ++ if(difference != 1){
                    createEmptySpaces(difference - 1)
                 }
      } 
      ++ elementSeparator([(colNum2, p2)]@tl, num + 1)
    } else {
      if(p == P1){
        "(o)" ++ if(difference != 1){
                    createEmptySpaces(difference - 1)
                 }
      } else {
        "(x)" ++ if(difference != 1){
                    createEmptySpaces(difference - 1)
                 }
      }
      ++ elementSeparator([(colNum2, p2)]@tl, num + 1)
    }
  }

  | [] => ""
};

elementSeparator([(1, P1), (3, P2), (7, P1)], 1);
elementSeparator([(3, P2), (5, P2)], 1);
elementSeparator([(2, P1), (3, P2), (4, P1), (6, P2)], 1);

给了我一个错误：

Error: This expression has type string but an expression was expected of type unit

对于第三个if表达式。我正在尝试创建一行

---

“（x）”、“（o）”及“（）”等。我不知道为什么它会抛出这个错误，因为elementSeparator应该输出一个字符串。每次我尝试在第二个if表达式之后连接另一个字符串（通过函数、过程、表达式等），它都会抛出此错误。有人能帮我吗？

如果表达式没有
其他
分支，则总是需要返回
单位
类型的值（因为如果
如果
if
条件未解析为
真
，则这是表达式的类型）

可以通过添加一个返回空字符串的
else
分支来解决此问题，或者为了简洁起见，只使用三元运算符：

difference != 1 ? createEmptySpaces(difference - 1) : ""

请不要破坏你的帖子，为别人做更多的工作。通过在Stack Exchange（SE）网络上发布，您已经在a下授予SE分发内容的不可撤销权利（即，无论您未来的选择如何）。根据SE政策，分发非故意破坏版本。因此，任何故意破坏行为都将恢复原状。请参阅：。如果允许删除，则在帖子下方左侧有一个“删除”按钮，但它仅在浏览器中，而不是移动应用程序中。