partialgeneric.scala

// Range is a product of two ints that implicitly carries a proof that start <= end. 
// We can model this in scala as a case class with a validating apply method.
// Note: requires relatively recent 2.12 release

final case class Range private (start: Int, end: Int) {
  def copy(start: Int = this.start, end: Int = this.end): Option[Range] = Range(start, end)
}
object Range {
  def apply(start: Int, end: Int): Option[Range] =
    if (start <= end) Some(new Range(start, end)) else None
}

// We lose the ability to get a Generic[Range] due to the private constructor, which is good!
// However, it's still useful to do metaprogramming with such datatypes, perhaps via something like:
trait PartialGeneric[A] {
  type Repr
  def to(a: A): Repr
  def from(r: Repr): Option[A]
}

// This would let us derive things like codecs.

// One thing that is kind of interesting is representing Range as a straight sum of products type with an additional component in the product that represents a proof of the condition enforced in the Repr => Option[A] function.