Scala 中如何省略匹配词? [英] How is a match word omitted in Scala?

问题描述

在 Scala 中，你可以这样做

list.filter { item =>项目匹配{case Some(foo) =>foo.bar >0}}

但是你也可以通过省略match来更快地完成:

list.filter {case Some(foo) =>foo.bar >0}

Scala 如何支持此功能?这是 2.9 的新功能吗?我一直在寻找它，我可以弄清楚是什么使这成为可能.它只是 Scala 编译器的一部分吗?

解决方案

语言规范解决了第 8.5 节中的问题.相关部分:

<块引用>

匿名函数可以由一系列 case 定义

{ case p1 =>b1 ...情况pn =>十亿 }

<块引用>

如果预期的类型是 scala.Functionk[S1, ..., Sk, R] ，则表达式为等价于匿名函数:

(x1 : S1, ..., xk : Sk) =>(x1, ..., xk) 匹配 {情况p1＝＞b1 ...情况pn =>十亿}

<块引用>

如果预期类型是 scala.PartialFunction[S, R]，则表达式被取为等价于下面的实例创建表达式:

new scala.PartialFunction[S, T ] {def apply(x: S): T = x match {情况p1＝＞b1 ...情况pn =>十亿}def isDefinedAt(x: S): Boolean = {情况p1＝＞真... case pn =>真的案例_ =>错误的}}

因此将表达式键入为 PartialFunction 或 Function 会影响表达式的编译方式.

Also trait PartialFunction [-A, +B] extends (A) ⇒ B 所以一个部分函数 PartialFunction[A,B] 也是一个 Function[A,B].

In Scala, you can do

list.filter { item =>
    item match {
      case Some(foo) => foo.bar > 0
    }
}

But you can also do the quicker way by omitting match:

list.filter {
  case Some(foo) => foo.bar > 0
}

How is this supported in Scala? Is this new in 2.9? I have been looking for it, and I can figure out what makes this possible. Is it just part of the Scala compiler?

解决方案

The language specification addresses that in section 8.5. The relevant portions:

An anonymous function can be defined by a sequence of cases

{ case p1 => b1 ... case pn => bn }

If the expected type is scala.Functionk[S1, ..., Sk, R] , the expression is taken to be equivalent to the anonymous function:

(x1 : S1, ..., xk : Sk) => (x1, ..., xk) match {
  case p1 => b1 ... case pn => bn
}

If the expected type is scala.PartialFunction[S, R], the expression is taken to be equivalent to the following instance creation expression:

new scala.PartialFunction[S, T ] {
  def apply(x: S): T = x match {
    case p1 => b1 ... case pn => bn
  }
  def isDefinedAt(x: S): Boolean = {
    case p1 => true ... case pn => true
    case _ => false
  }
}  

So typing the expression as PartialFunction or a Function influences how the expression is compiled.

Also trait PartialFunction [-A, +B] extends (A) ⇒ B so a partial function PartialFunction[A,B] is also a Function[A,B].

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