What is the use of `*.this` in Scala? - scala

Question

class S {
case class A(a: Int)
}
abstract class R(val s: S) {
type T1 = R.this.s.A
type T2 = s.A
implicitly[T1 =:= T2] // compiles
type T3 = R.this.type
type T4 = this.type
implicitly[T3 =:= T4] // compiles
val v1 = R.this // v1 == `this`
val v2 = R.this.s // v2 == `s`
}
Looks like the .this part has no effect whatsoever. To make a concrete question:
When do you use the .this ?

Answer 1

It matters for inner classes. E.g.
class Outer(val x: Int) {
class Inner(val x: Int) {
def outer_this = Outer.this.x
def inner_this = this.x // or just x
}
}
val outer = new Outer(0)
val inner = new outer.Inner(1)
println(inner.outer_this) // 0
println(inner.inner_this) // 1
Each Outer.Inner instances "belongs to" a specific Outer instance, and can refer to that instance as Outer.this. By itself x in Inner refers to its own property, so if you need the enclosing instance's x property, you write Outer.this.x.

Answer 2

One rule is Scala never infers singleton type this.type. For example first consider the mechanics of it
scala> trait Foo {
| type T
| def f() = this // we left out the return type to see what Scala will infer
| }
// defined trait Foo
scala> new Foo { type T = String }
val res0: Foo{T = String} = anon$1#6d3ad37a
scala> res0.f()
val res1: Foo = anon$1#6d3ad37a
Note how res1 has return type of Foo and not Foo { type T = String }, so we have lost some type information
scala> val x: res1.T = ""
1 |val x: res1.T = ""
| ^^
| Found: ("" : String)
| Required: res1.T
Note compiler does not know res1.T is actually a String. So compiler did not infer the singleton type this.type which would have all the type information including what type member T was instantiated to
scala> trait Foo {
| type T
| def f(): this.type = this
| }
// defined trait Foo
scala> new Foo { type T = String }
val res2: Foo{T = String} = anon$1#7d381eae
scala> res2.f()
val res3: Foo{T = String} = anon$1#7d381eae
scala> val x: res3.T = ""
val x: res3.T = ""
Note how after we explicitly declared singleton return type this.type compiler knows T is a String.
Here is another mechanical example of what happens because compiler does not infer singleton type this.type
scala> trait Foo {
| def f() = this // let inference do its thing
| }
// defined trait Foo
scala> trait Bar {
| def g() = 42
| }
// defined trait Bar
scala> trait Bar extends Foo {
| def g(): Int = 42
| }
// defined trait Bar
scala> new Bar {}
val res5: Bar = anon$1#6a9a6a0c
scala> res5.f()
val res6: Foo = anon$1#6a9a6a0c
scala> res6.g()
1 |res6.g()
|^^^^^^
|value g is not a member of Foo
Note how f() call typed to Foo and not perhaps expected Bar. On the other hand if we provide explicit singleton return type this.type then
scala> trait Foo {
| def f(): this.type = this
| }
// defined trait Foo
scala> trait Bar extends Foo {
| def g(): Int = 42
| }
// defined trait Bar
scala> new Bar {}
val res7: Bar = anon$1#4707d60a
scala> res7.f()
val res8: Bar = anon$1#4707d60a
scala> res8.g()
val res9: Int = 42
we see f() call typed to Bar.
Those were the mechanics but what about practical applications? Two uses I am aware of are:
supporting fluent style of programming
supporting summoners of type class instances (see also Scala 3 summon)