scala

Question

为什么此代码有效：

  val list = List(('a', 2), ('b', 2))

  val first = list.foldRight(List[List[(Char, Int)](List())) {
    case ((char, num), acc) =>
      (for (
        elem <- acc;
        i <- 0 to num
      ) yield (char, i) :: elem)
  }

这不是：

  val second = list.foldRight(List[List[(Char, Int)]](List())) {
    (elem, acc) =>
      (for (
        elem1 <- acc;
        (char, num) <- elem; // ERROR: value filter is not a member of (Char, Int)
        i <- 0 to num
      ) yield (char, i) :: elem1)
  }

我是不是两个都做同样的事情，只是在不同的地方？

Answer 1

使用中流变量绑定代替for生成器来分解for表达式中的元组：

for (
  elem1 <- acc;
  (char, num) = elem; 
  i <- 0 to num
) yield ...