returnsResultOf

Specifies that the function returns the result of the invocation of the function parameter lambda.

This information is currently used by the Kotlin's return value checker and instructs it to look inside lambda to decide whether function with contract is ignorable.

If the functional parameter result is returned only in one of the possible execution paths (i.e., in InvocationKind.AT_MOST_ONCE situation), contract still should mention it. If the function accepts multiple functional parameters, the contract may be specified several times and should list all potential invocations from all execution paths.

For example:

inline fun <T, R> T.selector(condition: (T) -> Boolean?, a: (T) -> R, b: (T) -> R): R? {
    contract {
        callsInPlace(a, InvocationKind.AT_MOST_ONCE)
        callsInPlace(b, InvocationKind.AT_MOST_ONCE)
        returnsResultOf(a)
        returnsResultOf(b)
    }
    return when (condition(this)) {
        true -> a(this)
        false -> b(this)
        null -> null
    }
}

a and b are mentioned in the returnsResultOf contract because each of them is returned from the function in at least one of the paths. condition is not mentioned in the contract because it is not returned from the function on all the paths.

This contract is experimental, and it is allowed to use it only with the 'Return value checker' feature enabled.