为什么包scala.collections.immutable中的VectorBuilder是？ [英] Why is VectorBuilder in the package scala.collections.immutable?

问题描述

VectorBuilder 在与 Vector 相同的源文件中定义。 Vector 是不可变的，并且在 scala.collections.immutable 包中，因此生成器位于同一包中。

VectorBuilder is defined in the same source file as Vector. Vector is immutable and in the scala.collections.immutable package, so as a consequence the builder is in the same package.

据我所知， CanBuildFrom 使用的是 VectorBuilder 。

• 是否有没有在单独文件中包含构建器的原因


• 构建器不是要直接使用吗？如果是这样，将使用哪个生成器或缓冲区来创建 Seq ？

• Is there a reason for not having the builder in a separate file in the mutable package?
• Is the builder not meant to be used directly? If so, which builder or buffer is to be used to create a Seq?

推荐答案

VectorBuilder 不能直接使用。如果要获取 Vector 的生成器，则只需调用 Vector.newBuilder [T] ，返回 Builder [T，Vector [T]] （基础实例为 VectorBuilder ）。

VectorBuilder is not meant to be used directly. If you want to get a builder for a Vector, you only need to call Vector.newBuilder[T], which returns a Builder[T, Vector[T]] (with the underlying instance being a VectorBuilder).

因此，如果您想要用于创建 Seq 的默认构建器，则只需调用 Seq.newBuilder ：

So if you want the default builder that would be used to create a Seq, you only need to call Seq.newBuilder:

scala> Seq(1,2,3)
res0: Seq[Int] = List(1, 2, 3)

scala> Seq.newBuilder[Int]
res1: scala.collection.mutable.Builder[Int,Seq[Int]] = ListBuffer()

scala> Seq.newBuilder[Int].result
res2: Seq[Int] = List()

上面的代码显示了 Seq 的默认实现是列表，并且在逻辑上， Seq 的默认构建器是实际上是 mutable.ListBuffer 。

The above shows that the default implementation of Seq is list, and, logically, the default builder for a Seq is actually a mutable.ListBuffer.

ListBuffer 更多不仅仅是 List 生成器，这就是为什么它位于 collection.mutable 而 VectorBuilder 不是 Buffer ，除构建 Vector 之外，不能用于其他任何用途。这可能就是为什么在 Vector 中本地定义它的原因。我不确定为什么它不是 private ，我无法在 Vector 本身的公共API中看到它的引用。 。也许应该是（私人）。

ListBuffer is more than just a List builder, that's why it is in collection.mutable whereas VectorBuilder is not a Buffer, it cannot be used for anything else other than build a Vector. That's probably why it is defined locally in Vector. I am not sure why it isn't private, I cannot see it referenced anywhere in the public API of Vector itself. Maybe it should be (private).

仅供参考， CanBuildFrom ，几乎总是通过上面的 newBuilder ：

Just for reference, there is no hidden magic happening with CanBuildFrom, it almost always just goes through the newBuilder above:

1. 当您未指定期望的集合类型时，例如 Seq（1,2）.map（_ + 1），则唯一可用 CanBuildFrom 来自伴侣对象一个 Seq ，类型为 CanBuildFrom [Seq [_] ，T，Seq [T]] 。这意味着结果也将是 Seq 。

1. When you do not specify the expected collection type, as in Seq(1,2).map(_+1), the only available CanBuildFrom comes from the companion object a Seq, and is of type CanBuildFrom[Seq[_], T, Seq[T]]. That means the result will be a Seq too.

与集合的大多数随播对象一样， CanBuildFrom 实例 Seq 仅提供一件事：调用 Seq.newBuilder （在中定义GenTraversableFactory ...）

Like most companion objects of collections, the CanBuildFrom instance Seq provides only does one thing: call Seq.newBuilder (that's defined in GenTraversableFactory ...)

这就是为什么 CanBuildFrom Vector 的c>使用 VectorBuilder 。例如，在此：

That's why CanBuildFrom for Vector uses a VectorBuilder. For example, in this:

scala> Vector(1,2,3).map(_+1)
res12: scala.collection.immutable.Vector[Int] = Vector(2, 3, 4)

使用的构建器是：

scala> implicitly[CanBuildFrom[Vector[Int], Int, Vector[Int]]].apply()
res13: scala.collection.mutable.Builder[Int,Vector[Int]] = 
  scala.collection.immutable.VectorBuilder@43efdf93

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