diff --git a/.changeset/new-crabs-attack.md b/.changeset/new-crabs-attack.md
new file mode 100644
index 0000000..e142e4e
--- /dev/null
+++ b/.changeset/new-crabs-attack.md
@@ -0,0 +1,5 @@
+---
+"@effect-rx/rx": patch
+---
+
+fix \*Fn not running for constant return values
diff --git a/docs/rx/Rx.ts.md b/docs/rx/Rx.ts.md
index 2d4b3b5..ed5c5ba 100644
--- a/docs/rx/Rx.ts.md
+++ b/docs/rx/Rx.ts.md
@@ -25,6 +25,7 @@ Added in v1.0.0
   - [effect](#effect)
   - [effectFn](#effectfn)
   - [family](#family)
+  - [fn](#fn)
   - [readable](#readable)
   - [runtime](#runtime)
   - [scoped](#scoped)
@@ -212,6 +213,16 @@ export declare const family: <Arg, T extends Rx<any>>(f: (arg: Arg) => T) => (ar
 
 Added in v1.0.0
 
+## fn
+
+**Signature**
+
+```ts
+export declare const fn: <Arg, A>(f: Rx.ReadFn<Arg, A>) => Writable<Option.Option<A>, Arg>
+```
+
+Added in v1.0.0
+
 ## readable
 
 **Signature**
diff --git a/packages/rx/src/Rx.ts b/packages/rx/src/Rx.ts
index e7ecfb4..ea8f003 100644
--- a/packages/rx/src/Rx.ts
+++ b/packages/rx/src/Rx.ts
@@ -2,7 +2,7 @@
  * @since 1.0.0
  */
 import * as internalRegistry from "@effect-rx/rx/internal/registry"
-import { runCallbackSyncDefault } from "@effect-rx/rx/internal/runtime"
+import { runCallbackSync, runCallbackSyncDefault } from "@effect-rx/rx/internal/runtime"
 import * as Result from "@effect-rx/rx/Result"
 import * as Chunk from "@effect/data/Chunk"
 import { dual, pipe } from "@effect/data/Function"
@@ -14,7 +14,7 @@ import { NoSuchElementException } from "@effect/io/Cause"
 import * as Effect from "@effect/io/Effect"
 import * as Exit from "@effect/io/Exit"
 import * as Layer from "@effect/io/Layer"
-import * as Runtime from "@effect/io/Runtime"
+import type * as Runtime from "@effect/io/Runtime"
 import * as Scope from "@effect/io/Scope"
 import * as Stream from "@effect/stream/Stream"
 
@@ -322,7 +322,7 @@ function makeEffectRuntime<R, E, A, RE>(
     return runtimeResult as any
   }
 
-  return makeEffect(ctx, create as any, Runtime.runCallback(runtimeResult.value))
+  return makeEffect(ctx, create as any, runCallbackSync(runtimeResult.value))
 }
 
 function makeScoped<R, E, A, RE>(
@@ -344,6 +344,44 @@ function makeScoped<R, E, A, RE>(
     : makeEffect(ctx, createScoped as any)
 }
 
+/**
+ * @since 1.0.0
+ * @category constructors
+ */
+export const fn = <Arg, A>(f: Rx.ReadFn<Arg, A>): Writable<Option.Option<A>, Arg> => {
+  const argRx = state<[number, Arg]>([0, undefined as any])
+  return writable(function(get) {
+    const [counter, arg] = get(argRx)
+    if (counter === 0) {
+      return Option.none()
+    }
+    return Option.some(f(arg, get))
+  }, function(ctx, arg) {
+    ctx.set(argRx, [ctx.get(argRx)[0] + 1, arg])
+  })
+}
+
+function makeFn<Arg, A, B>(
+  f: Rx.ReadFn<Arg, A>,
+  transform: (get: Context, _: Option.Option<Rx.Read<A>>) => B
+) {
+  const argRx = state<[number, Arg]>([0, undefined as any])
+  return writable<B, Arg>(function(get) {
+    const [counter, arg] = get(argRx)
+    if (counter === 0) {
+      return transform(get, Option.none())
+    }
+    return transform(
+      get,
+      Option.some(function(ctx) {
+        return f(arg, ctx)
+      })
+    )
+  }, function(ctx, arg) {
+    ctx.set(argRx, [ctx.get(argRx)[0] + 1, arg])
+  })
+}
+
 /**
  * @since 1.0.0
  * @category constructors
@@ -401,27 +439,15 @@ export const effectFn: {
 } = <Arg, R, E, A, RE>(
   f: Rx.ReadFn<Arg, Effect.Effect<R, E, A>>,
   options?: { readonly runtime?: RxRuntime<RE, R> }
-) => {
-  const argRx = state<[number, Arg]>([0, undefined as any])
-  const effectRx = readable<Effect.Effect<R, E, A> | undefined>(function(get) {
-    const [counter, arg] = get(argRx)
-    if (counter === 0) {
-      return undefined
-    }
-    return f(arg, get)
-  })
-  return writable<Result.Result<E, A>, Arg>(function(get) {
-    const effect = get(effectRx)
-    if (effect === undefined) {
+) =>
+  makeFn(f, function(get, create) {
+    if (create._tag === "None") {
       return Result.initial()
     }
     return options?.runtime
-      ? makeEffectRuntime(get, (_) => effect, options.runtime)
-      : makeEffect(get, (_) => effect as Effect.Effect<never, E, A>)
-  }, function(ctx, arg) {
-    ctx.set(argRx, [ctx.get(argRx)[0] + 1, arg])
+      ? makeEffectRuntime(get, create.value, options.runtime)
+      : makeEffect(get, create.value as any)
   })
-}
 
 /**
  * @since 1.0.0
@@ -436,25 +462,13 @@ export const scopedFn: {
 } = <Arg, R, E, A, RE>(
   f: Rx.ReadFn<Arg, Effect.Effect<R, E, A>>,
   options?: { readonly runtime?: RxRuntime<RE, R> }
-) => {
-  const argRx = state<[number, Arg]>([0, undefined as any])
-  const effectRx = readable<Effect.Effect<R, E, A> | undefined>(function(get) {
-    const [counter, arg] = get(argRx)
-    if (counter === 0) {
-      return undefined
-    }
-    return f(arg, get)
-  })
-  return writable<Result.Result<E, A>, Arg>(function(get) {
-    const effect = get(effectRx)
-    if (effect === undefined) {
+) =>
+  makeFn(f, function(get, create) {
+    if (create._tag === "None") {
       return Result.initial()
     }
-    return makeScoped(get, (_) => effect, options)
-  }, function(ctx, arg) {
-    ctx.set(argRx, [ctx.get(argRx)[0] + 1, arg])
+    return makeScoped(get, create.value, options)
   })
-}
 
 /**
  * @since 1.0.0
@@ -492,7 +506,7 @@ export const runtime: {
 function makeStream<E, A>(
   ctx: Context,
   create: Rx.Read<Stream.Stream<never, E, A>>,
-  runCallback = Effect.runCallback
+  runCallback = runCallbackSyncDefault
 ): Result.Result<E | NoSuchElementException, A> {
   const previous = ctx.self<Result.Result<E | NoSuchElementException, A>>()
 
@@ -518,7 +532,9 @@ function makeStream<E, A>(
       }
     }
   )
-  ctx.addFinalizer(cancel)
+  if (cancel !== undefined) {
+    ctx.addFinalizer(cancel)
+  }
 
   if (previous._tag === "Some") {
     return Result.waitingFrom(previous)
@@ -541,7 +557,7 @@ function makeStreamRuntime<R, E, A, RE>(
     return runtimeResult as any
   }
 
-  return makeStream(ctx, create as any, Runtime.runCallback(runtimeResult.value))
+  return makeStream(ctx, create as any, runCallbackSync(runtimeResult.value))
 }
 
 /**
@@ -579,27 +595,16 @@ export const streamFn: {
 } = <Arg, R, E, A, RE>(
   f: Rx.ReadFn<Arg, Stream.Stream<R, E, A>>,
   options?: { readonly runtime?: RxRuntime<RE, R> }
-) => {
-  const argRx = state<[number, Arg]>([0, undefined as any])
-  const streamRx = readable<Stream.Stream<R, E, A> | undefined>(function(get) {
-    const [counter, arg] = get(argRx)
-    if (counter === 0) {
-      return undefined
-    }
-    return f(arg, get)
-  })
-  return writable<Result.Result<E | NoSuchElementException, A>, Arg>(function(get) {
-    const stream = get(streamRx)
-    if (stream === undefined) {
+) =>
+  makeFn(f, function(get, create) {
+    if (create._tag === "None") {
       return Result.initial()
     }
     return options?.runtime
-      ? makeStreamRuntime(get, (_) => stream, options.runtime)
-      : makeStream(get, (_) => stream as Stream.Stream<never, E, A>)
-  }, function(ctx, arg) {
-    ctx.set(argRx, [ctx.get(argRx)[0] + 1, arg])
+      ? makeStreamRuntime(get, create.value, options.runtime)
+      : makeStream(get, create.value as any)
   })
-}
+
 /**
  * @since 1.0.0
  * @category models