Example from Nat Pryce

example.kt

// https://groups.google.com/d/msg/growing-object-oriented-software/mCjbhcRAGDs/P7pkf62XBgAJ

interface UserLookup {
    fun findByName(name: String): User
}

interface PermissionDecision {
    fun rejected(user: User)
    fun allowed(user: User)
}

and then the check could be implemented as:

class UserAgeRestriction(val users: UserLookup, val decision: PermissionDecision) {
    fun execute(name: String) {
        val user = users.findByName(name)
        if (user.age > 18) {
            decision.rejected(user)
        } else {
            decision.allowed(user)
        }
}