track the position of cross-references.

[maxir269]

hallo,

I have a question about cross-references.

grammar HelloWorld

entry Model:
    (persons+=Person | greetings+=Greeting)*;

Person:
    'person' name=ID;

Greeting:
    'Hello' person=[Person:ID] '!';

hidden terminal WS: /\s+/;
terminal ID: /[_a-zA-Z][\w_]*/;

hidden terminal ML_COMMENT: /\/\*[\s\S]*?\*\//;
hidden terminal SL_COMMENT: /\/\/[^\n\r]*/;

when the input is

Hello John!
Hello Jane!

person John
person Jane

greetings happens before the defination of pereson and working fine

but in the programming we have something like

var = 10

newvar = var + 20 // should work

---

newvar = var + 20 // I want to show a problem here because var is not yet defined
var = 10

is this possible with languim to track the position?

[cdietrich]

You can extract the location from the concrete syntax tree element.$cstNode

[msujew]

More specifically, you can use node.$cstNode.offset. Alternatively, you can walk through the tree to identify which element comes first. But that's a more expensive check.

[maxir269]

Hallo langium Team,

Could you please give a small example of this?

[msujew]

You can do something like this (using the HelloWorld grammar as an example):

checkPersonPosition(greeting: Greeting, acceptor: ValidationAcceptor): void {
  const greetingOffset = greeting.$cstNode?.offset ?? 0;
  const personOffset = greeting.person.ref?.$cstNode?.offset ?? 0;
  if (person > greetingOffset) {
    acceptor('error', 'Person must be declared before greeting them', {
      node: greeting,
      property: 'person'
    });
  }
}

Depending on the complexity of your language, you might need to to more AST analysis to determine whether the variable has been declared yet. However, this simple comparison should help you along quite well.

[cdietrich-everest]

what do you mean by example?

checkUseAfterDecl(gr: Greeting, accept: ValidationAcceptor): void {
        const decl = gr.person.ref?.$cstNode?.offset
        if (decl) {
            const g = gr.$cstNode?.offset;
            if (g && decl > g) {
                accept('error', 'Greeting must be declared before it is used.', { node: gr, property: 'person' });
            }
        }
    }