Pattern matching in eval()

gistfile1.scala

  def eval(expr: Expr, env: Env): Expr = expr match {
    // ...
    case "cons" :: arg1 :: arg2 :: Nil => eval(arg2, env) match {
      case l: List[Expr] => eval(arg1, env) :: l
      case arg => throw new IllegalArgumentException(s"Second argument to 'cons' should be list, was: '$arg'")
    }
    case "cons" :: args => throw new IllegalArgumentException(s"Invalid arguments to 'cons': $args")
    case "null?" :: head :: Nil => eval(head, env) == List()
    case "null?" :: _ => throw new IllegalArgumentException("Expected exactly one argument for 'null?'")
    // ...
  }