From c254a8abc73d7e2589acbffe432a33f985c1003d Mon Sep 17 00:00:00 2001
From: Eduardo San Martin Morote <posva13@gmail.com>
Date: Fri, 9 Apr 2021 20:35:57 +0200
Subject: [PATCH] feat(types): fail on async patch

---
 src/types.ts             | 7 +++++--
 test-dts/store.test-d.ts | 8 ++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/src/types.ts b/src/types.ts
index bc81a256..a7a5a2a4 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -76,9 +76,12 @@ export interface StoreWithState<Id extends string, S extends StateTree> {
    * Sets or arrays and applying an object patch isn't practical, e.g. appending
    * to an array.
    *
-   * @param stateMutator - function that mutates `state`
+   * @param stateMutator - function that mutates `state`, cannot be async
    */
-  $patch(stateMutator: (state: S) => void): void
+  $patch<F extends (state: S) => void>(
+    // this prevents the user from using `async` which isn't allowed
+    stateMutator: ReturnType<F> extends Promise<any> ? never : F
+  ): void
 
   /**
    * Resets the store to its initial state by building a new state object.
diff --git a/test-dts/store.test-d.ts b/test-dts/store.test-d.ts
index 8105e2f7..5350092b 100644
--- a/test-dts/store.test-d.ts
+++ b/test-dts/store.test-d.ts
@@ -21,3 +21,11 @@ store.nonExistant
 
 // @ts-expect-error
 store.nonExistant.stuff
+
+// @ts-expect-error cannot return a value
+store.$patch(async () => {})
+store.$patch(() => {})
+store.$patch(() => {
+  // return earlier
+  return
+})
-- 
2.47.3