x = y =1 in Scala?

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

I strongly suspect that the author was *not* thinking of x as of Unit type - that by "valid", they meant to imply "and have some purpose". — Ed Staub, Apr 10 '12 at 17:20

score 13 · Accepted Answer · answered Apr 09 '12 at 17:49

13

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 = ()

answered Apr 09 '12 at 17:49

Tomasz Nurkiewicz

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So the whole point of this question is to show that var x = y = 1 is valid? – sc_ray Apr 09 '12 at 18:01

score 6 · Answer 2 · answered Apr 09 '12 at 17:49

6

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

answered Apr 09 '12 at 17:49

timday

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2

That's what I had in mind, except that I would have written `var x = ()` – cayhorstmann Jun 12 '12 at 15:32

score 3 · Answer 3 · edited May 23 '17 at 11:53

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.)

edited May 23 '17 at 11:53

Community

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answered Apr 09 '12 at 21:28

Jean-Philippe Pellet

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Clearly, this is not what I had in mind :-) But if you want weird solutions, how about `implicit def unit2int(u: Unit) = 42; var x = 1; var y = 2; x = y = 1` – cayhorstmann Jun 12 '12 at 15:35

score 1 · Answer 4 · answered Apr 11 '12 at 16:03

1

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

answered Apr 11 '12 at 16:03

Andriy Plokhotnyuk

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score 0 · Answer 5 · answered Jun 15 '18 at 10:16

0

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

answered Jun 15 '18 at 10:16

G.J Zhao

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x = y =1 in Scala?

5 Answers5