The question may sound silly, but there is no typo in it.
fun test(): Any {
return return true
}
This is actually possible in Kotlin. Although the compiler warns about
Unreachable code
for the outer return. But this is just a warning.
I don't want to compare Java with Kotlin, but I was interested whether the same would work in Java.
public class Test {
// ...
static int test() {
return return 1;
}
}
It does not!
/Test.java:8: error: illegal start of expression
return return 1;
^
/Test.java:8: error: not a statement
return return 1;
^
2 errors
Why was Kotlin designed this way?
return is an expression in Kotlin, with a return type of Nothing, the type that acts as a subtype of all other types. This enables you to, for example, do this in a type safe way and without extra lines of null checks:
fun getInt(): Int? = ...
fun printInt() {
val int: Int = getInt() ?: return
println(int)
}
The type of getInt() ?: return can be Int here, because that's the closest common supertype of the two sides of the Elvis operator, thanks to Nothing being a subtype of Int.
The same thing applies for throw, which you can also use neatly with the Elvis operator to indicate that you want to cancel execution on a null value without having to worry about types later.
This results in an odd quirk where things like
fun x(): Int {
return return throw return throw throw return 0
}
are valid syntax, because the Nothing type makes each of the expressions valid read from right to left. What will actually happen is that return 0 will execute and the rest of the code will never be reached, as the compiler warns.