packages/effect-lens/src/Lens.test.ts

@@ -208,6 +208,31 @@ describe("Lens", () => {
         expect(result[1]).toBe(25)
     })
 
+    test("modifyEffect updates are atomic under concurrency", async () => {
+        const iterations = 100
+
+        const result = await Effect.runPromise(Effect.flatMap(
+            SubscriptionRef.make({ count: 0 }),
+            parent => {
+                const countLens = Lens.focusObjectOn(Lens.fromSubscriptionRef(parent), "count")
+
+                return Effect.flatMap(
+                    Effect.forEach(
+                        Array.from({ length: iterations }),
+                        () => Lens.updateEffect(
+                            countLens,
+                            count => Effect.as(Effect.yieldNow(), count + 1),
+                        ),
+                        { concurrency: "unbounded", discard: true },
+                    ),
+                    () => parent.get,
+                )
+            },
+        ))
+
+        expect(result.count).toBe(iterations)
+    })
+
     test("focusObjectOn focuses a nested property without touching other fields", async () => {
         const [initialCount, updatedState] = await Effect.runPromise(
             Effect.flatMap(