Scala类型不匹配什么时候不应该？

Question

I have these two functions in scala:

def recursive_func(x: Position, count: Int) : Int = {
      if(getNewPositions(x).isEmpty){
        count + 1;
      } else {
        for(n <- getNewPositions(x)){
          recursive_func(n, count+1);
        }
      }
}

def getNewPositions(x: Position) : List[Position] = {
    val possible : List[Position] = List((x._1 + 5,x._2 + 6), (x._1 + 6,x._2 + 7), (x._1 + 7,x._2 + 8), (x._1 + 8, x._2 + 9));
    val answer : List[Position] = for (n <- possible if canMakeMove(n)) yield n;
    answer;
}

The second returns a List of positions which is sometimes empty. The first is a recursive function that should return 1 if the list returned by getNewPositions is empty, or iterate over it if the list is not. However I'm getting this error:

error: type mismatch;
 found   : Unit
 required: Int

which I presume is because the compiler is trying to do for(n <- getNewPositions(x)) on an empty list even though in my head it shouldnt because of the if statement it's wrapped in? I know for sure that the getNewPositions function works fine and returns the appropriate list. Any help would be appreciated.

Answer 1

There are 2 errors in your code:

1. First:

   for(n <- getNewPositions(x)){ recursive_func(n, count+1); }

   Type of this expression is unit (this is causing your compilation error). Reason: for (n <- coll) {..} expands to coll.foreach(n => {..})

2. Second: for (n <- possible canMakeMove(n)) yield {} is definitly not a valid scala syntax