Generic derivation of enumeration schemas

[julienrf]

#183 added support for enumerations in the algebra. However, generic derivation of schemas only produces oneOf schemas instead of enumerations. We could improve our derivation mechanism to return an enumeration schema for sealed traits having case objects only.

One approach could be to test whether a Coproduct is an enumeration in an implicit rule with a higher priority than the current rule for coproducts (genericTagged):

implicit def genericEnumeration[A, R](implicit
  gen: LabelledGeneric.Aux[A, R],
  enum: GenericEnum[R],
  ct: ClassTag[A]
): GenericEnum[R] = …

Where GenericEnum is defined as follows:

trait GenericEnum[A] extends GenericJsonSchema[A] {
  def jsonSchema: Enum[A]
}

And its derivation rule checks that all the elements of the coproduct are singleton objects.