diff --git a/src/errors.js b/src/errors.js
index 96675c5..3d1c252 100644
--- a/src/errors.js
+++ b/src/errors.js
@@ -1,6 +1,6 @@
 class LexerError extends Error { }
 class ParserError extends Error { }
-class SemanticError extends Error {}
+class SemanticError extends Error { }
 class StartSymbolNotFound extends ParserError { }
 
 module.exports = Object.freeze({
diff --git a/src/parser.js b/src/parser.js
index 38b18bf..53e2e8f 100644
--- a/src/parser.js
+++ b/src/parser.js
@@ -2,6 +2,7 @@ const antlr4 = require('antlr4');
 const GrammarlangLexer = require('../grammarlang/grammarlangLexer').grammarlangLexer;
 const GrammarlangParser = require('../grammarlang/grammarlangParser').grammarlangParser;
 const errors = require('./errors');
+const DuplicatedRuleWarning = require('./warnings').DuplicatedRuleWarning;
 
 const NONTERMINAL = 0;
 const TERMINAL = 1;
@@ -15,7 +16,6 @@ class Visitor {
   visitRuleList(ctx) {
     const rules = [];
     const nonTerminals = new Set();
-    const warnings = [];
     let startSymbol = undefined;
 
     ctx.children.forEach(child => {
@@ -60,12 +60,13 @@ class Visitor {
     });
 
     // checks for duplicates
+    const warnings = [];
     Object.keys(grammar).forEach(nonTerminal => {
       const tmpRules = [];
       grammar[nonTerminal].forEach((rule, index) => {
         const equalToRule = v => JSON.stringify(v) === JSON.stringify(rule);
         if (tmpRules.some(equalToRule)) {
-          warnings.push({ message: 'Duplicated rule', nonTerminal, index });
+          warnings.push(new DuplicatedRuleWarning(nonTerminal, index));
         }
         tmpRules.push(rule);
       });
diff --git a/src/warnings.js b/src/warnings.js
new file mode 100644
index 0000000..ba74fb3
--- /dev/null
+++ b/src/warnings.js
@@ -0,0 +1,9 @@
+class DuplicatedRuleWarning {
+    constructor(nonTerminal, index) {
+        this.message = 'Duplicated rule';
+        this.nonTerminal = nonTerminal;
+        this.index = index;
+    }
+}
+
+module.exports = Object.freeze({ DuplicatedRuleWarning });
\ No newline at end of file
diff --git a/test/test-parser.js b/test/test-parser.js
index 837894d..5a274b1 100644
--- a/test/test-parser.js
+++ b/test/test-parser.js
@@ -1,6 +1,7 @@
 import test from 'ava';
 const parser = require('../src/parser.js');
 const errors = require('../src/errors');
+const warnings = require('../src/warnings');
 
 test('simple case', t => {
     const input = `S -> a S;`
@@ -210,7 +211,7 @@ test('duplicated rule case 1', t => {
         rulesNumber: 2,
         terminals: ['a'],
         nonTerminals: ['S'],
-        warnings: [{ message: 'Duplicated rule', nonTerminal: 'S', index: 1 }],
+        warnings: [new warnings.DuplicatedRuleWarning('S', 1)],
     });
 });
 
@@ -248,8 +249,8 @@ test('duplicated rule case 2', t => {
         terminals: ['a', 'b'],
         nonTerminals: ['A', 'S'],
         warnings: [
-            { message: 'Duplicated rule', nonTerminal: 'S', index: 2 },
-            { message: 'Duplicated rule', nonTerminal: 'S', index: 3 },
+            new warnings.DuplicatedRuleWarning('S', 2),
+            new warnings.DuplicatedRuleWarning('S', 3),
         ],
     });
 });
@@ -276,8 +277,6 @@ test('duplicated rule case 3', t => {
         rulesNumber: 3,
         terminals: ['a'],
         nonTerminals: ['A', 'S'],
-        warnings: [
-            { message: 'Duplicated rule', nonTerminal: 'A', index: 1 },
-        ],
+        warnings: [new warnings.DuplicatedRuleWarning('A', 1)],
     });
 });
\ No newline at end of file