feat(Stream): race

.changeset/stream-race.md

@@ -0,0 +1,25 @@
+---
+"effect": minor
+---
+
+
+feat(Stream): implement `race` operator, which accepts two upstreams and returns a stream that mirrors the first upstream to emit an item and interrupts the other upstream.
+
+```ts
+import { Stream, Schedule, Console, Effect } from "effect"
+
+const stream = Stream.fromSchedule(Schedule.spaced('2 millis')).pipe(
+  Stream.race(Stream.fromSchedule(Schedule.spaced('1 millis'))),
+  Stream.take(6),
+  Stream.tap(n => Console.log(n))
+)
+
+Effect.runPromise(Stream.runDrain(stream))
+// Output each millisecond from the first stream, the rest streams are interrupted
+// 0
+// 1
+// 2
+// 3
+// 4
+// 5
+```

packages/effect/src/Stream.ts

@@ -3472,6 +3472,42 @@ export const provideSomeLayer: {
   ): Stream<A, E | E2, RIn | Exclude<R, ROut>>
 } = internal.provideSomeLayer
 
+/**
+ * Returns a stream that mirrors the first upstream to emit an item.
+ * As soon as one of the upstream emits a first value, the other is interrupted.
+ * The resulting stream will forward all items from the "winning" source stream.
+ * Any upstream failures will cause the returned stream to fail.
+ *
+ * @example
+ * import { Stream, Schedule, Console, Effect } from "effect"
+ *
+ * const stream = Stream.fromSchedule(Schedule.spaced('2 millis')).pipe(
+ *   Stream.race(Stream.fromSchedule(Schedule.spaced('1 millis'))),
+ *   Stream.take(6),
+ *   Stream.tap(Console.log)
+ * )
+ *
+ * Effect.runPromise(Stream.runDrain(stream))
+ * // Output each millisecond from the first stream, the rest streams are interrupted
+ * // 0
+ * // 1
+ * // 2
+ * // 3
+ * // 4
+ * // 5
+ * @since 3.6.0
+ * @category racing
+ */
+export const race: {
+  <AR, ER, RR>(
+    right: Stream<AR, ER, RR>
+  ): <AL, EL, RL>(left: Stream<AL, EL, RL>) => Stream<AL | AR, EL | ER, RL | RR>
+  <AL, EL, RL, AR, ER, RR>(
+    left: Stream<AL, EL, RL>,
+    right: Stream<AR, ER, RR>
+  ): Stream<AL | AR, EL | ER, RL | RR>
+} = internal.race
+
 /**
  * Returns a stream that mirrors the first upstream to emit an item.
  * As soon as one of the upstream emits a first value, all the others are interrupted.

packages/effect/src/internal/stream.ts

@@ -4873,6 +4873,24 @@ export const range = (min: number, max: number, chunkSize = DefaultChunkSize): S
     return new StreamImpl(go(min, max, chunkSize))
   })
 
+/** @internal */
+export const race: {
+  <AR, ER, RR>(
+    right: Stream.Stream<AR, ER, RR>
+  ): <AL, EL, RL>(left: Stream.Stream<AL, EL, RL>) => Stream.Stream<AL | AR, EL | ER, RL | RR>
+  <AL, EL, RL, AR, ER, RR>(
+    left: Stream.Stream<AL, EL, RL>,
+    right: Stream.Stream<AR, ER, RR>
+  ): Stream.Stream<AL | AR, EL | ER, RL | RR>
+} = dual(
+  2,
+  <AL, EL, RL, AR, ER, RR>(
+    left: Stream.Stream<AL, EL, RL>,
+    right: Stream.Stream<AR, ER, RR>
+  ): Stream.Stream<AL | AR, EL | ER, RL | RR> => raceAll(left, right)
+)
+
+/** @internal */
 export const raceAll = <S extends ReadonlyArray<Stream.Stream<any, any, any>>>(
   ...streams: S
 ): Stream.Stream<