Parsing parentheses on search expression - Scala parser combinator - scala

Question

I am writing a parser for a search expression.
Ex.
a = "zyx" and ( b < 5 or c > 9)
I wrote this parser but it's not able to match the parentheses, getting this error:
failure: identifier expected
a = "zyx" and ( b < 5 or c > 9)
^
What can I do to be able to match the paretheses
class SearchQueryParser extends StandardTokenParsers {
def expr: Parser[Expression] = orExp | "(" ~> orExp ~ ")"
def orExp: Parser[Expression] = {
andExp *("or" ^^^ {(a: Expression, b: Expression) => BoolExp("OR", (a, b))})
}
def andExp: Parser[Expression] = {
compareExp *("and" ^^^ {(a: Expression, b: Expression) => BoolExp("AND", (a, b))})
}
def compareExp: Parser[Expression] = {
identifier ~ rep(
("=" | "!=" | "<" | ">") ~ literal ^^ {
case op ~ rhs => (op, rhs)
}
) ^^ {
case (acc, ("=", rhs: Expression)) => Binomial("=", acc, rhs)
case (acc, ("!=", rhs: Expression)) => Binomial("!=", acc, rhs)
case (acc, ("<", rhs: Expression)) => Binomial("<", acc, rhs)
case (acc, (">", rhs: Expression)) => Binomial(">", acc, rhs)
}
}
}

Answer 1

Your current grammar only allows parentheses in the expr rule, which I assume is your main rule, and the expr rule is never used by any other rule. So parentheses are only allowed around the entire expression.
What you want to do is to put "(" ~ expr ~ ")" in the lowest place where parenthesized expressions would be allowed. As I understand your grammar, that would probably meaning allowing it as an alternative to compareExp in andExp (assuming you don't want to allow parentheses inside compareExps).

Answer 2

As #sepp2k mentioned you need to put your parentheses on the lowest place where parentheses is allowed. In your case it should be in the compareExp:
def compareExp: Parser[Expression] = {
"(" ~> expr <~ ")" | identifier ~ rep(
("=" | "!=" | "<" | ">") ~ literal ^^ {
case op ~ rhs => (op, rhs)
}
) ^^ {
case (acc, ("=", rhs: Expression)) => Binomial("=", acc, rhs)
case (acc, ("!=", rhs: Expression)) => Binomial("!=", acc, rhs)
case (acc, ("<", rhs: Expression)) => Binomial("<", acc, rhs)
case (acc, (">", rhs: Expression)) => Binomial(">", acc, rhs)
}
}
and exp method should not handle the parentheses
def expr: Parser[Expression] = orExp