Scala: tuples as var

Question

Tuples are immutable in Scala, so why is it even allowed to declare a tuple as a var and not always val?

var pair = (99, "Luftballons") 
println(pair._1) // Ok
pair._1 = 89 // <console>:14: error: reassignment to val

Using ScalaIDE Eclipse, Windows 10

Thanks,

• smac89: Having a mutable reference does not make the object mutable. Using var in this case just means you can do pair = some_other_tuple
• shanlodh: thanks, and some_other_tuple has to be same type as pair

Answer 1

There's a difference between mutable data structures and mutable references - see this answer for details.

In this particular case, you're using mutable reference to immutable data structure, which means you can only replace it with completely different one. This would work:

var pair = (99, "Luftballons") 
println(pair._1) // Ok
pair = (100, "Luftballons") // OK

As others already pointed out there's a convenience method copy defined for Tuple, which allows creating a copy of an object (potentially replacing some fields).

pair = pair.copy(5, "Kittens") // OK

Answer 2

You have to update your pair like this:

pair = (89, pair._2)
pair: (Int, String) = (89,Luftballons)

by a new assignment to the pair, not to the underlying tuple. Or you use pair.copy, like suggested by chengpohi.

scala> pair = pair.copy(_1=101)
pair: (Int, String) = (101,Luftballons)

• chengpohi: how about pair = pair.copy(_1=89), :)