cata23.scala

type Algebra[F[_], A] = F[A] => A

val algebra0: Algebra[ExpressionF, Int] = {
  case ValueF(v) => v
  case AddF(e1, e2) => e1 + e2
  case MultF(e1, e2) => e1 * e2
}

val algebra1: Algebra[ExpressionF, String] = {
  case ValueF(v) => v.toString
  case AddF(e1, e2) => "(" ++ e1 ++ " " ++ e2 ++ ")"
  case MultF(e1, e2) => e1 ++ e2
}

val algebra2: Algebra[ExpressionF, Boolean] = {
  case ValueF(_) => true
  case AddF(e1, e2) => e1 || e2
  case MultF(e1, e2) => e1 && e2
}