混淆在斯卡拉在List.foldRight（）參數

Question

爲什麼此代碼的工作：

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) 
    } 

哪裏，因爲這不：

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) 
    } 

我不是在做的都一樣，只是在不同的地方？

Answer 1

取而代之的是發電機，使用中流變量綁定到內分解你的元組表達：

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