add withFallback api

tim-smart · Oct 25, 2023 · aa3e481 · aa3e481
1 parent 47c7af7
commit aa3e481
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diff --git a/.changeset/odd-donkeys-pull.md b/.changeset/odd-donkeys-pull.md
@@ -0,0 +1,5 @@
+---
+"@effect-rx/rx": patch
+---
+
+add withFallback api
diff --git a/packages/rx/src/Rx.ts b/packages/rx/src/Rx.ts
@@ -854,6 +854,62 @@ export const family = typeof WeakRef === "undefined" || typeof FinalizationRegis
     }
   }
 
+/**
+ * @since 1.0.0
+ * @category combinators
+ */
+export const withFallback: {
+  <E2, A2>(
+    fallback: Rx<Result.Result<E2, A2>>
+  ): <R extends Rx<Result.Result<any, any>>>(
+    self: R
+  ) => [R] extends [Writable<infer _, infer RW>]
+    ? Writable<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>, RW>
+    : Rx<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>>
+  <R extends Rx<Result.Result<any, any>>, E2, A2>(
+    self: R,
+    fallback: Rx<Result.Result<E2, A2>>
+  ): [R] extends [Writable<infer _, infer RW>]
+    ? Writable<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>, RW>
+    : Rx<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>>
+} = dual<
+  <E2, A2>(
+    fallback: Rx<Result.Result<E2, A2>>
+  ) => <R extends Rx<Result.Result<any, any>>>(
+    self: R
+  ) => [R] extends [Writable<infer _, infer RW>]
+    ? Writable<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>, RW>
+    : Rx<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>>,
+  <R extends Rx<Result.Result<any, any>>, E2, A2>(
+    self: R,
+    fallback: Rx<Result.Result<E2, A2>>
+  ) => [R] extends [Writable<infer _, infer RW>]
+    ? Writable<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>, RW>
+    : Rx<Result.Result<Result.Result.Failure<Rx.Infer<R>> | E2, Result.Result.Success<Rx.Infer<R>> | A2>>
+>(2, (self, fallback) => {
+  function withFallback(get: Context) {
+    const result = get(self)
+    if (result._tag === "Initial" || (result._tag === "Waiting" && result.previous._tag === "Initial")) {
+      return Result.waiting(Result.noWaiting(get(fallback)))
+    }
+    return result
+  }
+  return isWritable(self)
+    ? writable(
+      withFallback,
+      self.write,
+      self.refresh ?? function(refresh) {
+        refresh(self)
+      }
+    )
+    : readable(
+      withFallback,
+      self.refresh ?? function(refresh) {
+        refresh(self)
+      }
+    ) as any
+})
+
 /**
  * @since 1.0.0
  * @category combinators

diff --git a/packages/rx/test/Rx.test.ts b/packages/rx/test/Rx.test.ts
@@ -601,6 +601,22 @@ describe("Rx", () => {
     r.set(count, 1)
     assert.deepEqual(r.get(count), 1)
   })
+
+  it("withFallback", async () => {
+    const count = Rx.effect(() =>
+      Effect.succeed(1).pipe(
+        Effect.delay(100)
+      )
+    ).pipe(
+      Rx.withFallback(Rx.effect(() => Effect.succeed(0))),
+      Rx.keepAlive
+    )
+    const r = Registry.make()
+    assert.deepEqual(r.get(count), Result.waiting(Result.success(0)))
+
+    await vitest.advanceTimersByTimeAsync(100)
+    assert.deepEqual(r.get(count), Result.success(1))
+  })
 })
 
 interface Counter {