LanguageService.ts

function checkSemanticRules(ast: TodoExpressionsContext): ITodoLangError[] {
    const errors: ITodoLangError[] = [];
    const definedTodos: string[] = [];
    ast.children.forEach(node => {
        if (node instanceof AddExpressionContext) {
            // if a Add expression : ADD TODO "STRING"
            const todo = node.STRING().text;
            // If a TODO is defined using ADD TODO instruction, we can re-add it.
            if (definedTodos.some(todo_ => todo_ === todo)) {
                // node has everything to know the position of this expression is in the code
                errors.push({
                    code: "2",
                    endColumn: node.stop.charPositionInLine + node.stop.stopIndex - node.stop.stopIndex,
                    endLineNumber: node.stop.line,
                    message: `Todo ${todo} already defined`,
                    startColumn: node.stop.charPositionInLine,
                    startLineNumber: node.stop.line
                });
            } else {
                definedTodos.push(todo);
            }
        }else if(node instanceof CompleteExpressionContext) {
            const todoToComplete = node.STRING().text;
            if(definedTodos.every(todo_ => todo_ !== todoToComplete)){
                // if the the todo is not yet defined, here we are only checking the predefined todo until this expression
                // which means the order is important
                errors.push({
                    code: "2",
                    endColumn: node.stop.charPositionInLine + node.stop.stopIndex - node.stop.stopIndex,
                    endLineNumber: node.stop.line,
                    message: `Todo ${todoToComplete} is not defined`,
                    startColumn: node.stop.charPositionInLine,
                    startLineNumber: node.stop.line
                });
            }
        }

    })
    return errors;
}