From 6111f95c79912ccd8924b676a45152612c077fa6 Mon Sep 17 00:00:00 2001
From: Phil Pluckthun <phil@kitten.sh>
Date: Fri, 10 Mar 2023 19:18:50 +0000
Subject: [PATCH] fix: Add missing filter overload to allow for type narrowing
 (#149)

---
 .changeset/khaki-lemons-argue.md |  5 +++++
 src/__tests__/operators.test.ts  |  6 ++++--
 src/operators.ts                 | 10 +++++++---
 3 files changed, 16 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/khaki-lemons-argue.md

diff --git a/.changeset/khaki-lemons-argue.md b/.changeset/khaki-lemons-argue.md
new file mode 100644
index 0000000..a1cfeda
--- /dev/null
+++ b/.changeset/khaki-lemons-argue.md
@@ -0,0 +1,5 @@
+---
+'wonka': patch
+---
+
+Add missing overload definition for `filter`, which allows types to be narrowed, e.g. by specifying a type predicate return type.
diff --git a/src/__tests__/operators.test.ts b/src/__tests__/operators.test.ts
index db19097..e22f5d6 100644
--- a/src/__tests__/operators.test.ts
+++ b/src/__tests__/operators.test.ts
@@ -231,10 +231,12 @@ describe('filter', () => {
   passesAsyncSequence(noop);
 
   it('prevents emissions for which a predicate fails', () => {
-    const { source, next } = sources.makeSubject();
+    const { source, next } = sources.makeSubject<boolean>();
     const fn = vi.fn();
 
-    sinks.forEach(fn)(operators.filter(x => !!x)(source));
+    sinks.forEach((x: true) => {
+      fn(x);
+    })(operators.filter((x): x is true => !!x)(source));
 
     next(false);
     expect(fn).not.toHaveBeenCalled();
diff --git a/src/operators.ts b/src/operators.ts
index 3a09936..06f57d2 100644
--- a/src/operators.ts
+++ b/src/operators.ts
@@ -1,4 +1,4 @@
-import { Source, Sink, Operator, SignalKind, TalkbackKind, TalkbackFn } from './types';
+import { Push, Source, Sink, Operator, SignalKind, TalkbackKind, TalkbackFn } from './types';
 import { push, start, talkbackPlaceholder } from './helpers';
 import { fromArray } from './sources';
 
@@ -268,7 +268,9 @@ export function concat<T>(sources: Source<T>[]): Source<T> {
  * );
  * ```
  */
-export function filter<T>(predicate: (value: T) => boolean): Operator<T, T> {
+function filter<In, Out extends In>(predicate: (value: In) => value is Out): Operator<In, Out>;
+function filter<T>(predicate: (value: T) => boolean): Operator<T, T>;
+function filter<In, Out>(predicate: (value: In) => boolean): Operator<In, Out> {
   return source => sink => {
     let talkback = talkbackPlaceholder;
     source(signal => {
@@ -280,12 +282,14 @@ export function filter<T>(predicate: (value: T) => boolean): Operator<T, T> {
       } else if (!predicate(signal[0])) {
         talkback(TalkbackKind.Pull);
       } else {
-        sink(signal);
+        sink(signal as Push<any>);
       }
     });
   };
 }
 
+export { filter };
+
 /** Maps emitted values using the passed mapping function.
  *
  * @param map - A function returning transforming the {@link Source | Source's} values.