validation.kt

typealias Given<Input> = (Input) -> Boolean

infix fun <Input, Violation> Given<Input>.violates(violation: Violation): (Input) -> Violation? =
    { if (invoke(it)) violation else null }

fun <Input> given(checkForFailure: Input.() -> Boolean) = checkForFailure

fun <Input, Violation> validatorOf(vararg criteria: (Input) -> Violation?): (Input) -> List<Violation> =
    { input -> criteria.mapNotNull { it(input) } }


//DEMO
enum class PasswordViolation { MustContainUpperCase, MustContainLowerCase, MustContainNumber }

fun main(vararg args: String) {
    val validator = validatorOf(
        given<String> { equals(toLowerCase()) } violates PasswordViolation.MustContainUpperCase,
        given<String> { equals(toUpperCase()) } violates PasswordViolation.MustContainLowerCase,
        given<String> { !contains("[0-9]".toRegex()) } violates PasswordViolation.MustContainNumber
    )
    println(validator("input"))
}