Question

I can define a variable (by var) that is immutable:

var x = scala.collection.immutable.Set("aaaaaa","bbbbbb")
println(x.isInstanceOf[scala.collection.immutable.Set[String]])
x += "cccc"
println(x.isInstanceOf[scala.collection.immutable.Set[String]])

This results in:

true
true

+= method is not a member of scala.collection.immutable.Set, so what is happening?

Answer 1

The compiler looks for x.+= ..., and if it can’t find it, then it tries to transform the statement into x = x + ... (which only succeeds if x is a var, or x desugars into a call to some update method). Since immutable.Set implements a + operator, and x is a var, this succeeds.