Question

When i do e.g:

var airlines = Set("Qantas", "JetStar", "Air NZ")
airlines += "Virgin"

airlines is an immutable Set.

+= is not defined on the immutable Set trait.

So is += a built-in operator in scala? I mean how does scala know to reassign airlines with a new set("Qantas", "JetStar", "Air NZ", "Virgin") ?

Answer 1

If an operator ending with = (e.g. +=) is used but not defined on a class, the Scala compiler will desugar this into e.g.

airlines = airlines + "Virgin"

or, for ++=, we’d have

airlines ++= airlines

desugared into

airlines = airlines ++ airlines

Of course, as dmeister notes, this will only compile if that new expression makes sense. For example, if we deal with vars.

See Scala Reference §6.12.4 Assignment Operators
^{(<= , >= and != are excluded as special cases, as are patterns also starting with =.)}