# How to use `oneOf` with interfaces to define discriminated type unions.

openapi: "3.1.0"
info:
  title: Example OpenAPI definition
paths: {}
components:
  interfaces:
    # A "oneOf" interface can be used to generate a TypeScript type union or a
    # Rust enumeration, including the "Post", etc., variants.
    #
    # This is not a superclass! It's a type union, using `|` in TypeScript to
    # allow more than one type. This affects the design in several surprising
    # ways. Among other things, we might have several union types that include
    # different concrete interface types in their union. For example,
    # `ShapeOptions`, `RoundishShapeOptions`, etc.
    #
    # We can only create a `oneOf` union of multiple interfaces if the
    # individual types _all_ use the same discriminator member (see below).
    ShapeOptions:
      description: "Options for a shape."
      oneOf:
        - $interface: "SquareShapeOptions#SameAsInterface"
        - $interface: "RoundShapeOptions#SameAsInterface"

    SquareShapeOptions:
      # The discriminator is a property of `SquareShapeOptions`, even if this
      # type appears without being part of `ShapeOptions`. This is because it
      # affects how the variants of this type get generated.
      discriminatorMemberName: "type"
      members:
        type:
          # The discriminator must be required and initializable, but not mutable.
          required: true
          initializable: true
          schema:
            type: string
            # We must always use `const` here.
            const: square
        height:
          required: true
          mutable: true
          schema:
            type: number
        width:
          required: true
          mutable: true
          schema:
            type: number

    RoundShapeOptions:
      discriminatorMemberName: "type"
      members:
        type:
          required: true
          initializable: true
          schema:
            type: string
            const: round
        radius:
          required: true
          mutable: true
          schema:
            type: number