What does “with” mean in Kotlin?

with is used to access an object’s members and methods without having to refer to the object once per access. It is (mostly) for abbreviating your code. It is frequently used when constructing an object:

// Verbose way, 204 characters:
var thing = Thingummy()
thing.component1 = something()
thing.component2 = somethingElse()
thing.component3 = constantValue
thing.component4 = foo()
thing.component5 = bar()
parent.children.add(thing)
thing.refcount = 1

// Terse way, 182 characters:
var thing = Thingummy()
with(thing) {
  component1 = something()
  component2 = somethingElse()
  component3 = constantValue
  component4 = foo()
  component5 = bar()
  parent.children.add(this)
  refcount = 1
}