“val"或“var"，可变还是不可变? [英] 'val' or 'var', mutable or immutable?

问题描述

我可以定义一个不可变的变量(通过var):

I can define a variable (by var) that is immutable:

var x = scala.collection.immutable.Set("aaaaaa","bbbbbb")
println(x.isInstanceOf[scala.collection.immutable.Set[String]])
x += "cccc"
println(x.isInstanceOf[scala.collection.immutable.Set[String]])

结果:

true
true

+= 方法不是 scala.collection.immutable.Set 的成员，所以发生了什么?

+= method is not a member of scala.collection.immutable.Set, so what is happening?

推荐答案

编译器寻找 x.+= ... ，如果找不到，则尝试转换将语句转化为 x = x + ...(仅当 x 是 var 或 x 时才成功desugars 调用一些 update 方法).由于 immutable.Set 实现了一个 + 操作符，而 x 是一个 var，这就成功了.

The compiler looks for x.+= ..., and if it can't find it, then it tries to transform the statement into x = x + ... (which only succeeds if x is a var, or x desugars into a call to some update method). Since immutable.Set implements a + operator, and x is a var, this succeeds.

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