Validator.kt

interface Validator<in V, out E> {
  fun validate(value: V): ValidationResult<E>
}

class CompositeValidator<in V, out E>(
  private vararg val validators: Validator<V, E>
) : Validator<V, E> {

  override fun validate(value: V): ValidationResult<E> {
    for (validator in validators) {
      val result = validator.validate(value)
      if (result is Invalid) {
        return result
      }
    }
    return Valid
  }
}

sealed class ValidationResult<out E> {
  object Valid : ValidationResult<Nothing>()
  data class Invalid<E>(val error: E) : ValidationResult<E>()

  fun errorOrNull(): E? =
    when (this) {
      is Invalid -> error
      Valid      -> null
    }
}