Mixin tagged variants with unboxed

The trick I’ve found is to define the component parts as single-element variants, then use variant spreads. It does mean that the shared Block module doesn’t really work, but it’s at least an option for a tradeoff.

module LevelId = {
  type t = string
}

module PageBreak = {
  @tag("block_type")
  type t = 
    | @as("page-break-1.0") PageBreak({uuid: string, break_type: [#soft | #hard]})
}

module CustomSection = {
  @tag("block_type")
  type t = 
    | @as("custom-section-1.0") CustomSection({uuid: string, name: string, level_id: LevelId.t})
}

module CustomSectionTemplate = {
  @tag("block_type")
  type t = 
    | @as("custom-section-template-1.0") CustomSectionTemplate({uuid: string, name: string})
}

module ReportBlock = {
  @tag("block_type")
  type t =
    | ...PageBreak.t
    | ...CustomSection.t
}

module ReportTemplateBlock = {
  @tag("block_type")
  type t =
    | ...PageBreak.t
    | ...CustomSectionTemplate.t
}

module Examples = {
  let x: ReportBlock.t = PageBreak({uuid: "1", break_type: #soft})
  let y: ReportBlock.t = CustomSection({
    uuid: "2",
    name: "Some section",
    level_id: "0000-0000",
  })

  let x1: ReportTemplateBlock.t = PageBreak({uuid: "3", break_type: #soft})
  let y1: ReportTemplateBlock.t = CustomSectionTemplate({
    uuid: "4",
    name: "Some section",
  })
}

I used this pretty liberally in my graphql bindings for codegen.