Functions using expression statements have an implicit return type. Changing the type of the expression accidentally, changes the functions return type. This may lead to backward incompatibility. Use a block statement to make clear this function will never return a value.

Noncompliant Code

fun errorProneUnit() = println("Hello Unit")
fun errorProneUnitWithParam(param: String) = param.run { println(this) }
fun String.errorProneUnitWithReceiver() = run { println(this) }

Compliant Code

fun blockStatementUnit() {
    // code
}

// explicit Unit is compliant by default; can be configured to enforce block statement
fun safeUnitReturn(): Unit = println("Hello Unit")

Options

• allowExplicitReturnType (default: True)

  if functions with explicit 'Unit' return type should be allowed