nominal production = ()
nominal homologation = ()
nominal schema_v4 = ()
nominal draft = ()
nominal signed = ()
nominal authorized = ()

nominal phase env schema state = env * schema * state

union credential env =
    | ProductionCertificate :: credential production
    | HomologationCertificate :: credential homologation

union endpoint env schema =
    | ProductionV4Endpoint :: endpoint production schema_v4
    | HomologationV4Endpoint :: endpoint homologation schema_v4

nominal draft_payload = { id : string }
nominal signed_payload = { xml : string }
nominal authorized_payload = { protocol : string }

union document index =
    | DraftDocument :: forall env schema.
        draft_payload -> document (phase env schema draft)
    | SignedDocument :: forall env schema.
        signed_payload -> document (phase env schema signed)
    | AuthorizedDocument :: forall env schema.
        authorized_payload -> document (phase env schema authorized)

union operation from to =
    | Sign :: forall env schema.
        credential env ->
        operation
            (phase env schema draft)
            (phase env schema signed)
    | Authorize :: forall env schema.
        endpoint env schema ->
        operation
            (phase env schema signed)
            (phase env schema authorized)

inl execute forall from to.
    (operation : operation from to)
    (document : document from)
    : document to =
    match operation, document with
    | Sign _, DraftDocument payload =>
        SignedDocument (signed_payload { xml = payload.id })
    | Authorize _, SignedDocument _ =>
        AuthorizedDocument (authorized_payload { protocol = "capability-authorized" })

inl main () : string =
    inl draft_document : document (phase production schema_v4 draft) =
        DraftDocument (draft_payload { id = "NFE-20" })
    inl signed_document = execute (Sign ProductionCertificate) draft_document
    inl authorized_document = execute (Authorize HomologationV4Endpoint) signed_document
    match authorized_document with
    | AuthorizedDocument payload => payload.protocol