Xtext Parse Rules With Guards

The following small example model illustrates the usage of guards in Xtext (since 2.9).

Model: greetings+=Greeting*;

Greeting:
	'Hello' address=Address<false> '!'
		|
	'Dear' address=Address<true> '!'
;

Address<isFormal>: (<isFormal> title=Title) name=ID;

Title:"Mr"|"Mrs"|"Dr";

The first alternative of Greeting calls Address with the parameter value false, the second with true. Depending on the parameter value the title is required or causes the model to be syntactically invalid. A rule may have several parameters. The guard in the rule body need not be a simple parameter, it may be a complex expression of parameters. Analogously, Calls may combine several parameters.

Rule<ruleParam1, ruleParam2>:
	...
	Call<
		callParam2=ruleParam1|ruleParam2, 
		callParam1=!(ruleParam1&ruleParam2)
	>
	...
;
Call<callParam1, callParam2>:
	... 
	(<!callParam1|callParam2> ...)
	...
;