When moving on to more complex expressions, such as addition, we need to write a recursive rule since the left and right parts of an addition are expressions themselves. It would be natural to express such a rule as follows:
Expression:
... as above
{Plus} left=Expression '+' right=Expression;However, this results in an error from the Xtext editor as shown in the following screenshot:
Xtext uses a parser algorithm that is suitable for interactive editing due to its better handling of error recovery. Unfortunately, this parser algorithm does not deal with left recursive rules. A rule is left recursive when the first symbol of the rule is non-terminal and refers to the rule itself. The preceding rule for addition is indeed left recursive and is rejected by Xtext. Note that a rule can also be left recursive via multiple rule calls without any token consumption.
