Contracts API

Given the preceding code, let's put the new functionality to good use. All we need to do is annotate the validate method with @ExperimentalContracts and add the code at the beginning of the method body:

contract {
  returns() implies (command != null)
}

The new code now looks like this:

import kotlin.contracts.ExperimentalContracts
import kotlin.contracts.contract

data class Command(val timestamp: Long)

@ExperimentalContracts
fun processCommand(command: Command?) {
    validate(command)
    println(command.timestamp)
}

@ExperimentalContracts
fun validate(command: Command?) {
    contract {
        returns() implies (command != null)
    }
    if (command == null) {
        throw IllegalArgumentException("Invalid 'command' parameter. Expecting non-null parameter")
    }

    //... more validation here
}

With the changes in place, the compiler will not raise a compilation error anymore.

The general syntax for a code contract is as follows:

fun ... {
    contract {
        Effect
    }
}

Effect is an interface that encapsulates the effect of invoking the function. It goes without saying, calling the function through reflection will not benefit from the Contracts API.