Protobuf oneOf to generate coproducts without Option-ality

[SemanticBeeng]

In proto3 there is no required constraint available.

As a consequence this

oneof value {
  A : a = 1;
  B : B = 2;
}

generates

Option[A] :+: Option[B] :+: CNil

Proposing the Option-ality is removed to generate shapeless A :+: B :+: CNil instead.

Rationale:

1. The Option-ality seems redundant in the coproduct for evident reasons: if A is present in the coproduct then it is not optional.
2. Generates more complex than necessary service apis.
3. Slows down the runtime marshalling.

Given that skeuomorph is schema transformation library the user should have more flexibility at the time use from mu (time of service api design) and not be bound by the semantics of source IDL (context #91).

Resources

1. https://stackoverflow.com/questions/42955621/require-a-oneof-in-protobuf
2. https://news.ycombinator.com/item?id=18188519

[AdrianRaFo]

The right protobuff syntax would be

oneof value {
  A a = 1;
  B B = 2;
}

and AFAIK skeuomorph will generate the Coproduct without Options. Am I wrong @rafaparadela?

[rafaparadela]

I agree that optional values in Coproducts is redundant and doesn't make sense.

We might want to keep this:

case ProtobufF.TNamedType(name) => TOption(A.algebra(TNamedType(name)))

but maybe we should just optimize the printer for Coproducts

[rafaparadela]

In the case that @pepegar and @BeniVF approve this approach, this is the PR that closes this issue:
#119