Schedule: fix unsafe tapOutput signature (#4439)

Author: gcanti

.changeset/gold-jobs-love.md

@@ -0,0 +1,33 @@
+---
+"effect": patch
+---
+
+Schedule: fix unsafe `tapOutput` signature.
+
+Previously, `tapOutput` allowed using an output type that wasn't properly inferred, leading to potential runtime errors. Now, TypeScript correctly detects mismatches at compile time, preventing unexpected crashes.
+
+**Before (Unsafe, Causes Runtime Error)**
+
+```ts
+import { Effect, Schedule, Console } from "effect"
+
+const schedule = Schedule.once.pipe(
+  Schedule.as<number | string>(1),
+  Schedule.tapOutput((s: string) => Console.log(s.trim())) // ❌ Runtime error
+)
+
+Effect.runPromise(Effect.void.pipe(Effect.schedule(schedule)))
+// throws: TypeError: s.trim is not a function
+```
+
+**After (Safe, Catches Type Error at Compile Time)**
+
+```ts
+import { Console, Schedule } from "effect"
+
+const schedule = Schedule.once.pipe(
+  Schedule.as<number | string>(1),
+  // ✅ Type Error: Type 'number' is not assignable to type 'string'
+  Schedule.tapOutput((s: string) => Console.log(s.trim()))
+)
+```

packages/effect/dtslint/Schedule.ts

@@ -0,0 +1,33 @@
+import { Console, Schedule } from "effect"
+
+// -------------------------------------------------------------------------------------
+// tapOutput
+// -------------------------------------------------------------------------------------
+
+// $ExpectType Schedule<string | number, unknown, never>
+Schedule.once.pipe(
+  Schedule.as<number | string>(1),
+  Schedule.tapOutput((
+    x // $ExpectType string | number
+  ) => Console.log(x))
+)
+
+// The callback should not affect the type of the output (`number`)
+// $ExpectType Schedule<number, unknown, never>
+Schedule.once.pipe(
+  Schedule.as(1),
+  Schedule.tapOutput((x: string | number) => Console.log(x))
+)
+// $ExpectType Schedule<number, unknown, never>
+Schedule.tapOutput(
+  Schedule.once.pipe(
+    Schedule.as(1)
+  ),
+  (x: string | number) => Console.log(x)
+)
+
+Schedule.once.pipe(
+  Schedule.as<number | string>(1),
+  // @ts-expect-error
+  Schedule.tapOutput((s: string) => Console.log(s.trim()))
+)

packages/effect/src/Schedule.ts

@@ -1763,16 +1763,17 @@ export const spaced: (duration: Duration.DurationInput) => Schedule<number> = in
 export const stop: Schedule<void> = internal.stop
 
 /**
- * Returns a schedule that runs once and produces the specified constant value.
+ * Returns a schedule that recurs indefinitely, always producing the specified
+ * constant value.
  *
  * @since 2.0.0
  * @category Constructors
  */
 export const succeed: <A>(value: A) => Schedule<A> = internal.succeed
 
 /**
- * Returns a schedule that runs once, evaluating the given function to produce a
- * constant value.
+ * Returns a schedule that recurs indefinitely, evaluating the given function to
+ * produce a constant value.
  *
  * @category Constructors
  * @since 2.0.0
@@ -1816,12 +1817,12 @@ export const tapInput: {
  * @category Tapping
  */
 export const tapOutput: {
-  <XO extends Out, X, R2, Out>(
-    f: (out: XO) => Effect.Effect<X, never, R2>
+  <X, R2, Out>(
+    f: (out: Types.NoInfer<Out>) => Effect.Effect<X, never, R2>
   ): <In, R>(self: Schedule<Out, In, R>) => Schedule<Out, In, R2 | R>
-  <Out, In, R, XO extends Out, X, R2>(
+  <Out, In, R, X, R2>(
     self: Schedule<Out, In, R>,
-    f: (out: XO) => Effect.Effect<X, never, R2>
+    f: (out: Out) => Effect.Effect<X, never, R2>
   ): Schedule<Out, In, R | R2>
 } = internal.tapOutput
 

packages/effect/src/internal/schedule.ts

@@ -1362,19 +1362,25 @@ export const tapInput = dual<
 
 /** @internal */
 export const tapOutput = dual<
-  <XO extends Out, X, R2, Out>(
-    f: (out: XO) => Effect.Effect<X, never, R2>
-  ) => <In, R>(self: Schedule.Schedule<Out, In, R>) => Schedule.Schedule<Out, In, R | R2>,
-  <Out, In, R, XO extends Out, X, R2>(
+  <X, R2, Out>(
+    f: (out: Types.NoInfer<Out>) => Effect.Effect<X, never, R2>
+  ) => <In, R>(self: Schedule.Schedule<Out, In, R>) => Schedule.Schedule<Out, In, R2 | R>,
+  <Out, In, R, X, R2>(
     self: Schedule.Schedule<Out, In, R>,
-    f: (out: XO) => Effect.Effect<X, never, R2>
+    f: (out: Out) => Effect.Effect<X, never, R2>
   ) => Schedule.Schedule<Out, In, R | R2>
->(2, (self, f) =>
-  makeWithState(self.initial, (now, input, state) =>
-    core.tap(
-      self.step(now, input, state),
-      ([, out]) => f(out as any)
-    )))
+>(
+  2,
+  <Out, In, R, X, R2>(
+    self: Schedule.Schedule<Out, In, R>,
+    f: (out: Out) => Effect.Effect<X, never, R2>
+  ): Schedule.Schedule<Out, In, R | R2> =>
+    makeWithState(self.initial, (now, input, state) =>
+      core.tap(
+        self.step(now, input, state),
+        ([, out]) => f(out)
+      ))
+)
 
 /** @internal */
 export const unfold = <A>(initial: A, f: (a: A) => A): Schedule.Schedule<A> =>

packages/effect/test/Schedule.test.ts

@@ -814,6 +814,20 @@ describe("Schedule", () => {
         )
         deepStrictEqual(exit, Exit.die(exception))
       }))
+    it.effect("tapOutput", () =>
+      Effect.gen(function*() {
+        const log: Array<number | string> = []
+        const schedule = Schedule.once.pipe(
+          Schedule.as<number | string>(1),
+          Schedule.tapOutput((x) =>
+            Effect.sync(() => {
+              log.push(x)
+            })
+          )
+        )
+        yield* Effect.void.pipe(Effect.schedule(schedule))
+        deepStrictEqual(log, [1, 1])
+      }))
   })
 })