Kotlin contract infer return value instead of argument value

Question

I have a function that looks something like:

fun MyInput?.toOutput() : Output? {
  if (this == null) return null
  return Output(this.someValue)
}

In places where I know that my MyInput is non-null (for example, inside a method that takes a input: MyInput as an arg), I'd like to be able to use input.toOutput as Output instead of Output?

I've tried using

contract {
  returnsNotNull() implies (this@toOutput != null)
}

But that has the implication backwards. That tells me that if toOutput returns a non-null type, that my input was non-null. I want to tell the analyzer things about the return value based on the arguments. In Java, I could use org.jetbrains.annotations.@Contract("null -> null ; !null -> !null") to accomplish this.

Is there a way to do this in Kotlin?

Note that for this, you might as well not offer `MyInput?.toOutput()` at all. Just provide `MyInput.toOutput()`, and then let callers write `myInput?.toOutput()` with the question mark. That'll make the nullability behavior much more obvious to users as well as letting you avoid this entire issue. — Louis Wasserman, Apr 25 '19 at 17:27
@LouisWasserman That's a great idea, but would it work to call `toOutput` from Java? — karl, Apr 26 '19 at 19:42
It would work in the sense that you'd have to check for null on the Java end. — Louis Wasserman, Apr 26 '19 at 23:37

score 8 · Accepted Answer · answered Apr 25 '19 at 16:34

You don't need contracts for this. You just need to make a non-nullable overload. Like this:

fun MyInput?.toOutput(): Output? {
  if (this == null) return null
  return Output(this.someValue)
}

fun MyInput.toOutput(): Output = Output(this.someValue)

However, this will not work out of the box on the JVM, because the function signatures will clash. To make it work, you have to give one of the functions a new name with the @JvmName annotation. For example:

@JvmName("toOutputNonNull")
fun MyInput.toOutput(): Output = Output(this.someValue)

You will still be able to call it like input.toOutput() from Kotlin, but it will become something like FileNameKt.toOutputNonNull(input) if you call it from Java.

Kotlin contract infer return value instead of argument value

1 Answers1