Tagged union in TypeScript

tagged-union.ts

interface Tag {
  kind: string
  data: any
}

interface Union {
  match<T>(tag: Tag, handleMap: { [kind: string]: (data: any) => T }, catchAll?: () => T): T,
  tags: { [kind: string]: (data: any) => Tag }
}

export function union(kinds: string[]): Union {
  return {
    match(tag, handleMap, catchAll) {
      const handle = handleMap[tag.kind]
      return handle ? handle(tag.data) : catchAll()
    },
    tags: kinds.reduce(
      (tags, kind) => ({
        ...tags,
        [kind]: data => ({ kind, data } as Tag)
      }),
      {}
    )
  }
}

const { match, tags } = union(['Poop'])