Scala Constructor 模式与 Extractor 模式匹配

Question

构造器模式和抽取器模式匹配有什么区别？在 Programming in Scala 中，作者区分了第 15 章中讨论的构造函数模式和第 26 章中讨论的提取器模式。

构造函数模式的例子:

abstract class Expr
case class BinOp(operator: String, left: Expr, right: Expr) extends Expr

expr match {
    case BinOp("+", e, Number(0)) => println("a deep match")
    case _ => 
}

提取器示例:

l match {
     case List(a,b,c) => a
     case _ =>
}

Answer 1

区别在规范中表示:

An extractor pattern x(p1, …, pn) where n ≥ 0 is of the same syntactic form as a constructor pattern. However, instead of a case class, the stable identifier x denotes an object which has a member method named unapply or unapplySeq that matches the pattern.

简单地说，构造函数模式讨论案例类的分解，其中提取器模式讨论任何具有 unapply 或 unapplySeq 的对象。

List[A] 有一个 unapplySeq 由抽象 SeqFactory 提供:

/** This method is called in a pattern match { case Seq(...) => }.
 *
 *  @param x the selector value
 *  @return  sequence wrapped in an option, if this is a Seq, otherwise none
 */
def unapplySeq[A](x: CC[A]): Some[CC[A]] = Some(x)