Example.kt

enum class EducationLevel {
    HIGH_SCHOOL, BACHELORS, MASTERS, PHD
}

data class Person(val age: Int, val smoker: Boolean, val income: Int, val education: EducationLevel)

val people = listOf(
        Person(19, false, 10000, EducationLevel.HIGH_SCHOOL),
        Person(49, true, 120000, EducationLevel.BACHELORS),
        Person(55, false, 400000, EducationLevel.MASTERS),
        Person(23, true, 10000, EducationLevel.BACHELORS),
        Person(70, false, 70000, EducationLevel.PHD),
        Person(34, false, 90000, EducationLevel.MASTERS),
        Person(90, true, 0, EducationLevel.HIGH_SCHOOL)
)

fun income_by_smoking(people: List<Person>) =
        people
                .exclude_incomes_under(10000).
                separate_people_by { it.smoker }.
                average_income()

fun List<Person>.exclude_incomes_under(amount: Int): List<Person> =
        this.filter { it.income > amount }

fun <K> List<Person>.separate_people_by(selector: (Person) -> K): Map<K, List<Person>> =
        this.groupBy { selector(it) }

fun Map<out Any, List<Person>>.average_income(): Map<out Any, Int> =
        this.mapValues { it.value.sumBy { it.income } }


fun main(args: Array<String>) {
    val message = """
Result: ${income_by_smoking(people)}
"""

    println(message)
}

ExampleWithoutExplicitReturnTypes.kt

enum class EducationLevel {
    HIGH_SCHOOL, BACHELORS, MASTERS, PHD
}

data class Person(val age: Int, val smoker: Boolean, val income: Int, val education: EducationLevel)

val people = listOf(
        Person(19, false, 10000, EducationLevel.HIGH_SCHOOL),
        Person(49, true, 120000, EducationLevel.BACHELORS),
        Person(55, false, 400000, EducationLevel.MASTERS),
        Person(23, true, 10000, EducationLevel.BACHELORS),
        Person(70, false, 70000, EducationLevel.PHD),
        Person(34, false, 90000, EducationLevel.MASTERS),
        Person(90, true, 0, EducationLevel.HIGH_SCHOOL)
)

fun income_by_smoking(people: List<Person>) =
        people
                .exclude_incomes_under(10000).
                separate_people_by { it.smoker }.
                average_income()

fun List<Person>.exclude_incomes_under(amount: Int) =
        this.filter { it.income > amount }

fun <K> List<Person>.separate_people_by(selector: (Person) -> K) =
        this.groupBy { selector(it) }

fun Map<out Any, List<Person>>.average_income() =
        this.mapValues { it.value.sumBy { it.income } }


fun main(args: Array<String>) {
    val message = """
Result: ${income_by_smoking(people)}
"""

    println(message)
}