编写通用 Scala 合并函数，无法让类型为编译器排队

Question

我正在尝试使用 Scala 学习 FP。我在一个对象上有一个函数来合并两个排序的集合并返回相同的类型。它看起来像这样：

@tailrec def merge[A, Repr <: Seq[A]](merged: Repr, l1: Repr, l2: Repr)(
  implicit ordering: Ordering[A]): Repr = {
if (l1.isEmpty) merged ++ l2
else if (l2.isEmpty) merged ++ l1
else {
  val l1Head: A = l1.head
  val l2Head = l2.head
  val orderVal = ordering.compare(l1Head, l2Head)
  if (orderVal <= 0) {
    val m2: Repr = l1Head +: merged
    merge[A, Repr](m2, l1.tail, l2)
  } else {
    merge[A, Repr](l2Head +: merged, l1, l2.tail)
  }
 }
}

它给了我编译器错误“Seq[A] 类型的表达式不符合预期的 Repr 类型

Answer 1

试试

@tailrec def merge[A, Repr](merged: Repr, l1: Repr, l2: Repr)(implicit
  ev: Repr => SeqLike[A, Repr],
  cbf: CanBuildFrom[Repr, A, Repr],
  ordering: Ordering[A]
): Repr = {
  if (l1.isEmpty) merged ++ l2
  else if (l2.isEmpty) merged ++ l1
  else {
    val l1Head: A = l1.head
    val l2Head = l2.head
    val orderVal = ordering.compare(l1Head, l2Head)
    if (orderVal <= 0) {
      val m2: Repr = l1Head +: merged
      merge[A, Repr](m2, l1.tail, l2)
    } else {
      merge[A, Repr](l2Head +: merged, l1, l2.tail)
    }
  }
}