ExpressionParser.scala

lazy val expression: PackratParser[Expression] =
    expression ~ ("add" ~> termExpression) ^^ {case l ~ r => PlusExp(l, r)} |
    expression ~ ("sub" ~> termExpression) ^^ {case l ~ r => MinusExp(l, r)} |
    termExpression

  lazy val termExpression: PackratParser[Expression] =
    termExpression ~ ("mul" ~> factor) ^^ {case a ~ b => MultiplyExp(a, b)} |
    termExpression ~ ("div" ~> factor) ^^ {case a ~ b => DivideExp(a, b)} |
    termExpression ~ ("mod" ~> factor) ^^ {case a ~ b => ModExp(a, b)} |
    factor

  lazy val factor: PackratParser[Expression] =
    ("not" ~> factor) ^^ NotExp |
    factor ~ ("(" ~> expressionList <~ ")") ^^ {case id ~ param => CallExp(id, param)} |
    number |
    boolean |
    string |
    idn |
    "(" ~> predicate <~ ")"


  lazy val expressionList: PackratParser[Seq[Expression]] = repsep(predicate, ",")

  lazy val propertyList: PackratParser[Seq[Property]] = repsep(idn, ",")

  lazy val idn: PackratParser[Property] = ident ^^ Property

  lazy val number: PackratParser[Number] = floatingPointNumber ^^ Number

  lazy val string: PackratParser[StringLiteral] = ("\'" + """([^"\p{Cntrl}\\]|\\[\\/bfnrt]|\\u[a-fA-F0-9]{4})*""" + "\'").r ^^ StringLiteral | stringLiteral ^^ StringLiteral

  lazy val boolean: PackratParser[Expression] = "true" ^^^ TrueExpr() | "false" ^^^ FalseExpr()