How can I let the Kotlin compiler know I already checked for null inside a function?

Question

Basically I have a function that does some stuff but specifically it checks if two values I'm passing are null. This function is called from various places to make sure those two values are not null. Kotlin is complaining later that I'm not checking for null. Is there a way I can do this so that Kotlin already knows that I am checking for null without using !!?

Here is a simple example:

private fun stuff() {
    var possibleNull: String? = "test"
    if (testNull(possibleNull)) {
        mustPassNonNull(possibleNull)
    }
}

private fun mustPassNonNull(possibleNull: String) {
    //use value that isn't null
}

private fun testNull(possibleNull: String?): Boolean {
    return possibleNull != null
}

Basically testNull is only true if possibleNull is not null and that check is on an if right before calling mustPassNonNull so can I let Kotlin know I'm already checking that? without using !! of course.

Thanks.

Answer 1

It is possible with the use of contracts. Currently in experimental in Kotlin 1.3.

It is possible to declare contracts for your own functions, but this feature is experimental, as the current syntax is in a state of early prototype and will most probably be changed. Also, please note, that currently the Kotlin compiler does not verify contracts, so it's a programmer's responsibility to write correct and sound contracts. -kotlinlang.org

@ExperimentalContracts
fun stuff() {
   var possibleNull: String? = "test"
   if (testNull(possibleNull)) {
      mustPassNonNull(possibleNull)
   }
}

fun mustPassNonNull(possibleNull: String) {
   //use value that isn't null
}

@ExperimentalContracts
fun testNull(possibleNull: String?): Boolean {
   contract{
      returns(true) implies (possibleNull is String)
   }

   return possibleNull != null
}

Articles I referenced:

• https://kotlinlang.org/docs/reference/whatsnew13.html
• https://blog.kotlin-academy.com/understanding-kotlin-contracts-f255ded41ef2