Question

I started playing around with xtext a few days ago and just went through the tutorials. Maybe the solution has been covered in the reference somewhere but I cannot get it right quickly.

My problem is this. I tried to write a simple grammar which mixed in org.eclipse.xtext.common.Terminals . Then I wanted to insert a cusotm terminal FILE_NAME like this:

terminal FILE_NAME:
( !('/' | '\\' | ':' | '*' | '?' | '"' | '<' | '>' | '|') )+
;

That’s basically what a filename is allowed to be under Windows. However, by doing that, inherited rules like ID, INT, etc. would never be matched, because they are always generated after custom terminals.

Can that kind of problem be avoided gracefully (as repeatless as possible and as general as possible)? Thanks in advance!

Answer 1

Terminal rules (aka lexer rules) are used to tokenize the input sequenze. IMHO there should be a minimum of semantics in terminal rules.

You try to express a specialized parser rule which accepts only valid file names.

Have a look at parser phases described in the Xtext Documentation [1]. My suggestion:

1. Lexing: Instead of using a specialized terminal rule go with STRING.

2. Validation: Write a validation rule for an EClass with a ‘fileName’ EAttribute.

as repeatless as possible and as general as possible

You don’t want to repeat your validation for every EClass with a ‘fileName’ EAttribute. Introduce a new super type with a ‘fileName’ EAttribute if you have a refined Ecore model.

Than you can implement one general validation rule #check_fileName_is_valid(ElementWithFile).

And if you don’t have a refined MM use meta model hints within your grammar. If you provide a generalized super type Xtext’s Ecore inferrer will pull up common features of the subtypes. Ex:

ElementWithFile: A | B;
A: ... 'file' fileName=STRING ...;
B: ... 'file' fileName=STRING ...;
// => Ecore: ElementWithFile.fileName<EString>

[1] http://www.eclipse.org/Xtext/documentation.html#DSL