diff --git a/src/parser/common/basic-parser-types.ts b/src/parser/common/basic-parser-types.ts
index 8a9c0b6..728fdab 100644
--- a/src/parser/common/basic-parser-types.ts
+++ b/src/parser/common/basic-parser-types.ts
@@ -4,9 +4,9 @@
  */
 export interface CaretPosition {
     /** start at 1 */
-    lineNumber: number;
+    readonly lineNumber: number;
     /** start at 1 */
-    column: number;
+    readonly column: number;
 }
 
 /**
@@ -45,23 +45,23 @@ export enum SyntaxContextType {
 
 export interface WordRange {
     /** content of word */
-    text: string;
+    readonly text: string;
     /** start at 0 */
-    startIndex: number;
-    stopIndex: number;
+    readonly startIndex: number;
+    readonly stopIndex: number;
     /** start at 1 */
-    line: number;
+    readonly line: number;
     /** start at 1 */
-    startColumn: number;
-    stopColumn: number;
+    readonly startColumn: number;
+    readonly stopColumn: number;
 }
 
 /**
  * Suggested information analyzed from the input
  */
 export interface SyntaxSuggestion<T = WordRange> {
-    syntaxContextType: SyntaxContextType;
-    wordRanges: T[];
+    readonly syntaxContextType: SyntaxContextType;
+    readonly wordRanges: T[];
 }
 
 /**
@@ -71,22 +71,22 @@ export interface Suggestions<T = WordRange> {
     /**
      * Suggestions about syntax
      */
-    syntax: SyntaxSuggestion<T>[];
+    readonly syntax: SyntaxSuggestion<T>[];
     /**
      * Suggestions about keywords
      */
-    keywords: string[];
+    readonly keywords: string[];
 }
 
 export interface TextSlice {
     /** start at 0 */
-    startIndex: number;
-    endIndex: number;
+    readonly startIndex: number;
+    readonly endIndex: number;
     /** start at 1 */
-    startLine: number;
-    endLine: number;
+    readonly startLine: number;
+    readonly endLine: number;
     /** start at 1 */
-    startColumn: number;
-    endColumn: number;
-    text: string;
+    readonly startColumn: number;
+    readonly endColumn: number;
+    readonly text: string;
 }
diff --git a/src/parser/common/parseErrorListener.ts b/src/parser/common/parseErrorListener.ts
index d7deff1..3876767 100644
--- a/src/parser/common/parseErrorListener.ts
+++ b/src/parser/common/parseErrorListener.ts
@@ -5,23 +5,23 @@ import { ATNSimulator } from 'antlr4ts/atn/ATNSimulator';
  * Converted from {@link SyntaxError}.
  */
 export interface ParseError {
-    startLine: number;
-    endLine: number;
-    startCol: number;
-    endCol: number;
-    message: string;
+    readonly startLine: number;
+    readonly endLine: number;
+    readonly startCol: number;
+    readonly endCol: number;
+    readonly message: string;
 }
 
 /**
  * The type of error resulting from lexical parsing and parsing.
  */
 export interface SyntaxError<T> {
-    recognizer: Recognizer<T, ATNSimulator>;
-    offendingSymbol: Token;
-    line: number;
-    charPositionInLine: number;
-    msg: string;
-    e: RecognitionException;
+    readonly recognizer: Recognizer<T, ATNSimulator>;
+    readonly offendingSymbol: Token;
+    readonly line: number;
+    readonly charPositionInLine: number;
+    readonly msg: string;
+    readonly e: RecognitionException;
 }
 
 /**