How can I solve this error when I´m writing a model based on a previous metamodel using xText?

Question

I have written this metamodel in a xText project:

Screen:
"Screen name: " name = ID
    "Level: " level = STRING
    "End: " end = INT
    background = [Background]
    character = [Character]
    (enemies+=Enemy)*
    (elements+=Element)*

Background:
"Background{
    Id: " id = ID
    "Image: " image = [Image]
"}";
...

An all the metaclasses you see there (Background, Character, Enemy, etc) I haven´t had any problem creating the metamodel in ecore but when I try to define a specific model like this (using Ctrl+Space):

Screen name: s1
Level: "easy"
End: 1000

I get an error: mismatched INPUT expecting RULE_ID and I can't keep on writing with Ctrl+Space. Any ideas why this is not working? Thank you.

Answer 1

The problem with the grammar snippet is a misconception of cross references and containment references. You did not define a background somewhere in your sample file but the grammar expects a reference to a background. If you change the definition to something along these lines, it'll work:

Screen:
"Screen name: " name = ID
    "Level: " level = STRING
    "End: " end = INT
    background = Background
    character = Character
    (enemies+=Enemy)*
    (elements+=Element)*

Background:
"Background{
    Id: " id = ID
    "Image: " image = [Image]
"}";

However, I expect that you actually wanted to use a cross reference instead of a contained background in a given screen. So you have to provide either your complete grammar or define a background prior to using it in a screen.