x = y = 1 在 Scala 中？

Question

While going through the book Scala for the Impatient , I came across this question:

Come up with one situation where the assignment x = y = 1 is valid in Scala. (Hint: Pick a suitable type for x.)

I am not sure what exactly the author means by this question. The assignment doesn't return a value, so something like var x = y = 1 should return Unit() as the value of x. Can somebody point out what might I be missing here?

Thanks

Answer 1

In fact, x is Unit in this case:

var y = 2
var x = y = 1

can be read as:

var y = 2
var x = (y = 1)

and finally:

var x: Unit = ()

Answer 2

You can get to the point of being able to type x=y=1 in the REPL shell with no error thus:

var x:Unit = {}
var y = 0
x = y = 1

Answer 3

Here's another less known case where the setter method returns its argument. Note that the type of x is actually Int here:

object AssignY {
  private var _y: Int = _
  def y = _y
  def y_=(i: Int) = { _y = i; i }
}

import AssignY._

var x = y = 1

(This feature is used in the XScalaWT library, and was discussed in that question .)

Answer 4

BTW if assigning of the same value to both variables still required then use:

scala> var x@y = 1
x: Int = 1
y: Int = 1

Answer 5

It's valid, but not sensible, this make confusion.

scala> var x=y=1
x: Unit = ()

scala> y
res60: Int = 1

scala> var x@y = 1
x: Int = 1
y: Int = 1