problem.scala

sexp.scala

    val show: Algebra[SExp, Option[String] => String] = {
      case True => _ => "t"
      case Symbol(s) => _ => s"'$s"
      case Keyword(k) => _ => s":$k"
      case Char(c) => _ => s"?$c"
      case Int(v) => _ => v.toString
      case Nil => {
        case None => "nil"
        case Some(p) => s"$p)"
      }
      case Pair(l, r) => {
        case None => r(Some(s"(${l(None)}"))
        case Some(p) => r(Some(s"$p ${l(None) }"))
      }
    }