val和无参数def之间的微妙差异 [英] Subtle differences between val and parameterless def

问题描述

好吧，这不是关于vals&定义或功能和方法。我看到，虽然这编译：

  val Extractor = new AnyRef {def unapply（s：String）=>一些（s）} 
 val x =hellomatch {case Extractor（s）=> s 
  

将 val提取器更改为 def Extractor 打破了代码。为什么这样？有点令人失望，因为我希望完整的透明性能够让我从vals（生成方法）变为defs，反之亦然。我不知道还有什么可以做一个但不是另一个？

解决方案

我不知道我可以提供一个详尽的列表，但我可以在这种情况下解释发生了什么。

Extractor 必须是一个稳定的标识符。请参阅 Scala语言规范的第8.1.8节。 （A def 不稳定; val 是。）

稳定的标识符具有一些很好的属性，可以使它们更简单地计算并具有更多的优化可能性。

Ok, this is is not another question about fundamental differences between vals & defs or functions and methods. I see that while this compiles:

val Extractor = new AnyRef { def unapply(s :String) => Some(s) }
val x = "hello" match { case Extractor(s) => s }

changing val Extractor to def Extractor breaks the code. Why is it so? It's a bit disappointing, as I hoped for complete trasnparency that would let me change the implementation from vals (generating methods anyway) to defs, or vice versa. I wonder what else can be done with one but not another?

解决方案

I'm not sure I can supply an exhaustive list, but I can explain what's going on in this case.

Extractor has to be a stable identifier. See Section 8.1.8 of the Scala Language Specification. (A def is not stable; a val is.)

Stable identifiers have certain nice properties that allow them to be more simply computed and have greater optimization possibilities.

这篇关于val和无参数def之间的微妙差异的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！