diff --git a/async/filter.ts b/async/filter.ts
index 51ae8db..2937c4c 100644
--- a/async/filter.ts
+++ b/async/filter.ts
@@ -1,3 +1,11 @@
+export function filter<T>(
+  iterable: Iterable<T> | AsyncIterable<T>,
+  fn: (value: T, index: number) => boolean | Promise<boolean>,
+): AsyncIterable<T>;
+export function filter<T, U extends T>(
+  iterable: Iterable<T> | AsyncIterable<T>,
+  fn: (value: T, index: number) => value is U,
+): AsyncIterable<U>;
 /**
  * Filters an iterable based on a function.
  *
diff --git a/filter.ts b/filter.ts
index 05a4b8b..a6326a4 100644
--- a/filter.ts
+++ b/filter.ts
@@ -1,3 +1,11 @@
+export function filter<T>(
+  iterable: Iterable<T>,
+  fn: (value: T, index: number) => boolean,
+): Iterable<T>;
+export function filter<T, U extends T>(
+  iterable: Iterable<T>,
+  fn: (value: T, index: number) => value is U,
+): Iterable<U>;
 /**
  * Filters an iterable based on a function.
  *
diff --git a/pipe/async/filter.ts b/pipe/async/filter.ts
index be160af..0660d8d 100644
--- a/pipe/async/filter.ts
+++ b/pipe/async/filter.ts
@@ -1,5 +1,11 @@
 import { filter as base } from "../../async/filter.ts";
 
+export function filter<T>(
+  fn: (value: T, index: number) => boolean | Promise<boolean>,
+): (iterable: Iterable<T> | AsyncIterable<T>) => AsyncIterable<T>;
+export function filter<T, U extends T>(
+  fn: (value: T, index: number) => value is U,
+): (iterable: Iterable<T> | AsyncIterable<T>) => AsyncIterable<U>;
 /**
  * Returns an operator that filters an iterable based on a function.
  *
diff --git a/pipe/filter.ts b/pipe/filter.ts
index 065ded8..260471a 100644
--- a/pipe/filter.ts
+++ b/pipe/filter.ts
@@ -1,5 +1,11 @@
 import { filter as base } from "../filter.ts";
 
+export function filter<T>(
+  fn: (value: T, index: number) => boolean,
+): (iterable: Iterable<T>) => Iterable<T>;
+export function filter<T, U extends T>(
+  fn: (value: T, index: number) => value is U,
+): (iterable: Iterable<T>) => Iterable<U>;
 /**
  * Returns an operator that filters an iterable based on a function.
  *