structural_subtyping.ml

let ident_parser stream =
  let ident = stream#get_ident () in
  print_endline ("ident_parser got ident `" ^ ident ^ "'");
  ident

let const_parser stream =
  let const = stream#get_const () in
  print_endline ("const_parser got const `" ^ const ^ "'");
  const

let def_parser stream =
  let ident = ident_parser stream in
  stream#get_equal_sign ();
  let const = const_parser stream in
  print_endline ("def_parser got definition `" ^ ident ^ " = " ^ const ^ "'");
  (ident, const)

let () =
  ignore (
    def_parser (object
      method get_ident () = "great_answer"
      method get_const () = "42"
      method get_equal_sign () = ()
    end))

StructuralSubtyping.scala

object IdentParser {
  def parse[S <: { def getIdent(): String }](stream: S) = {
    val ident = stream.getIdent()
    println("IdentParser got ident `" + ident + "'")
    ident
  }
}

object ConstParser {
  def parse[S <: { def getConst(): String }](stream: S) = {
    val const = stream.getConst()
    println("ConstParser got const `" + const + "'")
    const
  }
}

object DefParser {
  def parse[S <: {
    def getIdent(): String
    def getConst(): String
    def getEqualSign(): Unit }](stream: S) = {
    val ident = IdentParser.parse(stream)
    stream.getEqualSign()
    val const = ConstParser.parse(stream)
    println("DefParser got definition `" + ident + " = " + const + "'")
    (ident, const)
  }
}

object StructuralSubtyping {
  def main(args: Array[String]) {
    DefParser.parse(new {
      def getIdent() = "greatAnswer"
      def getConst() = "42"
      def getEqualSign() = ()
    })
  }
}