Scala combinator parsers - distinguish between number strings and variable strings

def factor: Parser[ExprTree] = (wholeNumber | "(" ~ expr ~ ")" | ident) ^^ {
    case a: wholeNumber  => Number(a.toInt)
    case a: String => Variable(a)
}

def factor: Parser[ExprTree] = ( wholeNumber ^^ { Number(_.toInt) }
                               | "(" ~> expr <~ ")"
                               | ident ^^ { Variable(_) } )