RFC: Type composition

[wiegell]

Background:

• Proposal: Implicitly include properties of implemented interfaces #533
• [Proposal] Interface implementation violate "Don't Repeat Yourself" #500

The two proposals above discuss the option on how to write GraphQL in a more DRY fashion. Some concerns raised in those topics include:

• The GraphQL language is intended for reading over writing
• We should preserve backwards compatibility at all costs.

Proposal

If instead of changing the way interfaces work and/or are implemented, what if we instead enabled type composition much like Go does it. This could then be an example:

type Humanoid {
  id: String
  name: String
  appearsIn: [Episode]
}

type Human {
  Humanoid
  homePlanet: String
}

type Droid {
  Humanoid
  primaryFunction: String
}

Or multiple types could be combined:

type TimelineItem {
  date: String!
}

type Identifiable {
  id: ID!
  name: string
}

type SomeTimelineEvent {
  TimelineItem
  Identifiable
  eventName: string
}

There are some details that would need to be discussed in particular how field collisions should be handled.
I don't see any real breaking changes in this or problems with backwards compatibility and it would let the typing become more DRY - let me know what you think.