scala - 在 Scala 中，a += b 何时变为 a = a + b？

Question

Scala 只是有时会脱糖

a += b

到

a = a + b

但不总是。例如，一些可变集合定义了一个 +=方法，而是变成

a.+=(b)

这是行为吗

完全取决于是否有合适的+=方法在 a ? (包括这种行为还有其他例子吗？)

与对象是 val 还是 var 无关？

相关例子

改编自 Scala 编程，在

var s = Set("a", "b")
s += "c"

In this case, the second line of code s += "c" is essentially shorthand for:

s = s + "c"

Answer 1

When does a += b become a = a + b in Scala?

当没有适用时 +=方法，有一个适用的 +方法和 a是可赋值的(也就是说，它是一个 var 或者有一个 a_= 方法)。
或作为 the spec说:

The re-interpretation occurs if the following two conditions are fulfilled.

1. The left-hand-side l does not have a member named +=, and also cannot be converted by an implicit conversion to a value with a member named +=.
2. The assignment l = l + r is type-correct. In particular this implies that l refers to a variable or object that can be assigned to, and that is convertible to a value with a member named +.

Is this behaviour

1. entirely determined by whether there is a suitable += method on a?
2. independent of whether the objects are val or var?

不完全的。如果有合适的 +=方法，它将被调用而不管任何其他因素(例如 a 可分配)。但是当没有时，其他因素决定了它是脱糖还是您收到错误消息。
请注意，您收到的错误消息与从脱糖版本中获得的错误消息不同:当脱糖标准不适用时，您会收到一条错误消息，告诉您“+= 不是 .. .”，并解释为什么不能应用脱糖(例如“接收器不可分配”或您从 a + b 得到的类型错误，如果 a + b 会产生类型错误)。