Add type guards

src/util.ts

@@ -1,4 +1,7 @@
-import type {Ok, Err, Result} from ".";
+import {ResultImpl, type Ok, type Err, type Result} from "./result";
+import {OptionImpl, type Option} from "./option";
+import {AsyncResult} from "./async_result";
+import {AsyncOption} from "./async_option";
 
 export type InferOk<T> = T extends Ok<infer O, any> ? O : never;
 
@@ -15,3 +18,19 @@ export type ExtractErr<T> = T extends Err<infer E, any>
 	: T extends Result<infer _, infer E>
 	? E
 	: never;
+
+export function isResult<T, E>(value: unknown): value is Result<T, E> {
+	return value instanceof ResultImpl;
+}
+
+export function isAsyncResult<T, E>(value: unknown): value is AsyncResult<T, E> {
+	return value instanceof AsyncResult;
+}
+
+export function isOption<T>(value: unknown): value is Option<T> {
+	return value instanceof OptionImpl;
+}
+
+export function isAsyncOption<T>(value: unknown): value is AsyncOption<T> {
+	return value instanceof AsyncOption;
+}