Scala RegexParser calculator example right-associativity - scala

Question

Javadoc for the RegexParsers trait contains the following example:
object Calculator extends RegexParsers {
def number: Parser[Double] = """\d+(\.\d*)?""".r ^^ { _.toDouble }
def factor: Parser[Double] = number | "(" ~> expr <~ ")"
def term : Parser[Double] = factor ~ rep( "*" ~ factor | "/" ~ factor) ^^ {
case number ~ list => (number /: list) {
case (x, "*" ~ y) => x * y
case (x, "/" ~ y) => x / y
}
}
def expr : Parser[Double] = term ~ rep("+" ~ log(term)("Plus term") | "-" ~ log(term)("Minus term")) ^^ {
case number ~ list => list.foldLeft(number) { // same as before, using alternate name for /:
case (x, "+" ~ y) => x + y
case (x, "-" ~ y) => x - y
}
}
def apply(input: String): Double = parseAll(expr, input) match {
case Success(result, _) => result
case failure : NoSuccess => scala.sys.error(failure.msg)
}
}
When parsing expression 5 - 4 - 2 it treats it like (5 - 4) - 2 and returns -1. How can I change this parser to be right-associative so it will actually evaluate 5 - (4 - 2) and return 3?

Answer 1

I have managed to make the grammar to be right associative:
trait Tree
case class Node(op: String, left: Tree, right: Tree) extends Tree
case class Leaf(value: BigInt) extends Tree
class ExpressionParser extends RegexParsers {
var result : BigInt = 0
def number: Parser[Tree] = """-?\d+""".r ^^ { s => Leaf(BigInt(s))}
def expr : Parser[Tree] = (term ~ ("+" | "-") ~ expr ^^ {
case ((x ~ op) ~ y) => Node(op, x, y)
}) | term
def term : Parser[Tree] = (factor ~ ("*" | "/") ~ term ^^ {
case ((x ~ op) ~ y) => Node(op, x, y)
}) | factor
def factor : Parser[Tree] = number | "(" ~> expr <~ ")"
}