diff --git a/examples/parser-by-do.ts b/examples/parser-by-do.ts
index 549bcae..9069918 100644
--- a/examples/parser-by-do.ts
+++ b/examples/parser-by-do.ts
@@ -36,7 +36,7 @@ const between = <T, S>(parser: Parser<T, S>, pre: Parser<unknown, S>, post = pre
 const or = <T, U, S>(left: Parser<T, S>, right: Parser<U, S>) =>
     qo<T | U, S>(perform => {
         const [success, result] = perform.try(
-            [false] as const,
+            [false],
             () => [true, perform(left)] as const,
         );
         return success ? result : perform(right);
diff --git a/src/do.ts b/src/do.ts
index 640f0c2..5e20bd4 100644
--- a/src/do.ts
+++ b/src/do.ts
@@ -11,7 +11,7 @@ export type PerformOptions = {
 
 export type Perform<S> = {
     <T>(parser: Parser<T, S>, options?: PerformOptions): T;
-    try<T, U = T>(defaultValue: T, runner: () => U): T | U;
+    try<const T, U = T>(defaultValue: T, runner: () => U): T | U;
 };
 
 export const qo = <T, S>(
diff --git a/src/string.ts b/src/string.ts
index e20b139..9990312 100644
--- a/src/string.ts
+++ b/src/string.ts
@@ -134,7 +134,7 @@ export const regexGroup = (re: RegExp): Parser<RegExpExecArray, string> => {
 export const regex: {
     (re: RegExp): Parser<string, string>;
     (re: RegExp, groupId: number | string): Parser<string | undefined, string>;
-    <T>(
+    <const T>(
         re: RegExp,
         groupId: number | string,
         defaultValue: T,