Any way to tell Kotlin compiler that prior callable function already did a check for null

Question

If I have a code fragment like

val myNullableString : String?
val resultingString = if (myNullableString.isNullOrBlank()) 
myNullDefaultString else String.format(myNullableString!!, someOtherString)

I have to use myNullableString!! in String.format() since the compiler would not be able to figure out that isNullOrBlank() includes a null-check. Is this correct or is there any way to tell the compiler that a function will infer that the instance is not null?

Answer 1

Kotlin 1.3 introduced a concept called contracts. From now on you can tell the compiler that a null check has been done. It has already been implemented for all necessary functions in the standard library (for isNullOrBlank() too).

So, your example would compile now without !!

val myNullableString: String? = null

val result = if (myNullableString.isNullOrBlank()) {
    "default"
} else {
    String.format(myNullableString, "hello") // myNullableString is smart-casted to String
}