Scala 集合 : totally unpredictable behaviours

Question

对 Scala 2.8 的收集行为感到非常沮丧。问题是:我正在创建一个数独板。我正在标记从 A1 到 I9 的单元格(字母是行，数字是列)。我想获得棋盘上的单位列表，即第 9 行、夜间列和夜间象限。

这是我的 Scala 类:

class Square(val row:Char, val column:Int) extends Pair[Char, Int](row, column) {
    override def toString() = "" + row + column 
}

object Board {
    private val rows = "ABCDEFGHI"
    private val cols = 1 to 9
    private lazy val units = unitList(rows, cols)
    private def cross(rows:Iterable[Char], columns:Iterable[Int]):Iterable[Square] = {
        for (row <- rows; col <- columns)
            yield new Square(row, col)
    }

    private def unitList(rows:Iterable[Char], cols:Iterable[Int]) = {
        val u1 = (for (col <- cols) yield cross(rows, List(col)))
        val u2 = (for (row <- rows) yield cross(List(row), cols))
        val u3 = (for (cols <- List("ABC", "DEF", "GHI"); rows <- List(1 to 3, 4 to 6, 7 to 9)) yield cross(cols, rows))

        u1 :+ u2 :+ u3  // won't compile, reason: :+ is not a member of Iterable[Iterable[sudoku.Square]]
    }

    def run() {
        val u1 = (for (col <- cols) yield cross(rows, List(col)))
        val u2 = (for (row <- rows) yield cross(List(row), cols))
        val u3 = (for (cols <- List("ABC", "DEF", "GHI"); rows <- List(1 to 3, 4 to 6, 7 to 9)) yield cross(cols, rows))
        println(u1)
        println(u2)
        println(u3)
        val u4 = u1 :+ u2 :+ u3  // compiles
        println(u1 :+ u2 :+ u3)  // compiles and output correctly
    }
}

查看代码中的注释。具体来说，为什么相同的代码不能在 unitList 中编译，但在 run() 中编译并运行良好？

此外，当我观察 run 方法的输出时，似乎 yield 关键字返回的集合在 Vector 和 List 之间随机切换:

Vector(Vector(A1, B1, C1, D1, E1, F1, G1, H1, I1), Vector(A2, B2, C2, D2, E2, F2, G2, H2, I2), Vector(A3, B3, C3, D3, E3, F3, G3, H3, I3), Vector(A4, B4, C4, D4, E4, F4, G4, H4, I4), Vector(A5, B5, C5, D5, E5, F5, G5, H5, I5), Vector(A6, B6, C6, D6, E6, F6, G6, H6, I6), Vector(A7, B7, C7, D7, E7, F7, G7, H7, I7), Vector(A8, B8, C8, D8, E8, F8, G8, H8, I8), Vector(A9, B9, C9, D9, E9, F9, G9, H9, I9))

Vector(List(A1, A2, A3, A4, A5, A6, A7, A8, A9), List(B1, B2, B3, B4, B5, B6, B7, B8, B9), List(C1, C2, C3, C4, C5, C6, C7, C8, C9), List(D1, D2, D3, D4, D5, D6, D7, D8, D9), List(E1, E2, E3, E4, E5, E6, E7, E8, E9), List(F1, F2, F3, F4, F5, F6, F7, F8, F9), List(G1, G2, G3, G4, G5, G6, G7, G8, G9), List(H1, H2, H3, H4, H5, H6, H7, H8, H9), List(I1, I2, I3, I4, I5, I6, I7, I8, I9))

List(Vector(A1, A2, A3, B1, B2, B3, C1, C2, C3), Vector(A4, A5, A6, B4, B5, B6, C4, C5, C6), Vector(A7, A8, A9, B7, B8, B9, C7, C8, C9), Vector(D1, D2, D3, E1, E2, E3, F1, F2, F3), Vector(D4, D5, D6, E4, E5, E6, F4, F5, F6), Vector(D7, D8, D9, E7, E8, E9, F7, F8, F9), Vector(G1, G2, G3, H1, H2, H3, I1, I2, I3), Vector(G4, G5, G6, H4, H5, H6, I4, I5, I6), Vector(G7, G8, G9, H7, H8, H9, I7, I8, I9))

Vector(Vector(A1, B1, C1, D1, E1, F1, G1, H1, I1), Vector(A2, B2, C2, D2, E2, F2, G2, H2, I2), Vector(A3, B3, C3, D3, E3, F3, G3, H3, I3), Vector(A4, B4, C4, D4, E4, F4, G4, H4, I4), Vector(A5, B5, C5, D5, E5, F5, G5, H5, I5), Vector(A6, B6, C6, D6, E6, F6, G6, H6, I6), Vector(A7, B7, C7, D7, E7, F7, G7, H7, I7), Vector(A8, B8, C8, D8, E8, F8, G8, H8, I8), Vector(A9, B9, C9, D9, E9, F9, G9, H9, I9), Vector(List(A1, A2, A3, A4, A5, A6, A7, A8, A9), List(B1, B2, B3, B4, B5, B6, B7, B8, B9), List(C1, C2, C3, C4, C5, C6, C7, C8, C9), List(D1, D2, D3, D4, D5, D6, D7, D8, D9), List(E1, E2, E3, E4, E5, E6, E7, E8, E9), List(F1, F2, F3, F4, F5, F6, F7, F8, F9), List(G1, G2, G3, G4, G5, G6, G7, G8, G9), List(H1, H2, H3, H4, H5, H6, H7, H8, H9), List(I1, I2, I3, I4, I5, I6, I7, I8, I9)), List(Vector(A1, A2, A3, B1, B2, B3, C1, C2, C3), Vector(A4, A5, A6, B4, B5, B6, C4, C5, C6), Vector(A7, A8, A9, B7, B8, B9, C7, C8, C9), Vector(D1, D2, D3, E1, E2, E3, F1, F2, F3), Vector(D4, D5, D6, E4, E5, E6, F4, F5, F6), Vector(D7, D8, D9, E7, E8, E9, F7, F8, F9), Vector(G1, G2, G3, H1, H2, H3, I1, I2, I3), Vector(G4, G5, G6, H4, H5, H6, I4, I5, I6), Vector(G7, G8, G9, H7, H8, H9, I7, I8, I9)))

我完全迷失在这里。

Answer 1

for 理解中 yield 的结果是从第一个生成器的类型派生的，在你的方法中，你限制了参数的类型，到目前为止你失去了你的 :+ 方法。

//The type of 1 to 9 is show below
scala> 1 to 9
res0: scala.collection.immutable.Range.Inclusive with scala.collection.immutable.Range.ByOne = Range(1, 2, 3, 4, 5, 6, 7, 8, 9)


//If you cast it to Iterable[Int] it doens't have the :+ method
scala> (res0:Iterable[Int]) :+ 1
<console>:7: error: value :+ is not a member of Iterable[Int]
       (res0:Iterable[Int]) :+ 1
       ^

//But if you don't, you have it
scala> res0 :+ 1                
res6: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9, 1)

//And to prove that for comprehensions yield derives the type of the first generator:

scala> for(a <- res0) yield a
res7: scala.collection.immutable.IndexedSeq[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9)

scala> for(a <- (res0:Iterable[Int])) yield a
res8: Iterable[Int] = Vector(1, 2, 3, 4, 5, 6, 7, 8, 9)