diff --git a/packages/effect-lens/src/Lens.ts b/packages/effect-lens/src/Lens.ts
index 1468d87..5ef8119 100644
--- a/packages/effect-lens/src/Lens.ts
+++ b/packages/effect-lens/src/Lens.ts
@@ -1,4 +1,4 @@
-import { Array, Chunk, type Context, Effect, Function, identity, type ManagedRuntime, Option, Pipeable, Predicate, Readable, type Runtime, Stream, type SubscriptionRef, type SynchronizedRef } from "effect"
+import { Array, Chunk, type Context, Effect, Function, identity, type ManagedRuntime, Option, Pipeable, Predicate, PubSub, Readable, Ref, type Runtime, Stream, type SubscriptionRef, type SynchronizedRef } from "effect"
 import type { NoSuchElementException } from "effect/Cause"
 import * as Subscribable from "./Subscribable.js"
 
@@ -17,80 +17,167 @@ export interface Lens<in out A, in out ER = never, in out EW = never, in out RR
 extends Subscribable.Subscribable<A, ER, RR> {
     readonly [LensTypeId]: LensTypeId
 
-    readonly modify: <B, E1 = never, R1 = never>(
+    readonly modifyEffect: <B, E1 = never, R1 = never>(
         f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
     ) => Effect.Effect<B, ER | EW | E1, RR | RW | R1>
 }
 
-/**
- * Internal `Lens` implementation.
- */
-export class LensImpl<in out A, in out ER = never, in out EW = never, in out RR = never, in out RW = never>
-extends Pipeable.Class() implements Lens<A, ER, EW, RR, RW> {
-    readonly [Readable.TypeId]: Readable.TypeId = Readable.TypeId
-    readonly [Subscribable.TypeId]: Subscribable.TypeId = Subscribable.TypeId
-    readonly [LensTypeId]: LensTypeId = LensTypeId
-
-    constructor(
-        readonly get: Effect.Effect<A, ER, RR>,
-        readonly changes: Stream.Stream<A, ER, RR>,
-        readonly modify: <B, E1 = never, R1 = never>(
-            f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
-        ) => Effect.Effect<B, ER | EW | E1, RR | RW | R1>,
-    ) {
-        super()
-    }
-}
-
-
 /**
  * Checks whether a value is a `Lens`.
  */
 export const isLens = (u: unknown): u is Lens<unknown, unknown, unknown, unknown, unknown> => Predicate.hasProperty(u, LensTypeId)
 
+export const LensWithInternalsTypeId: unique symbol = Symbol.for("@effect-fc/Lens/LensWithInternals")
+export type LensWithInternalsTypeId = typeof LensWithInternalsTypeId
+
+export interface LensWithInternals<in out A, in out ER = never, in out EW = never, in out RR = never, in out RW = never>
+extends Lens<A, ER, EW, RR, RW> {
+    readonly [LensWithInternalsTypeId]: LensWithInternalsTypeId
+
+    readonly update: (a: A) => Effect.Effect<void, EW, RW>
+    readonly semaphore: Effect.Semaphore
+}
+
+export const isLensWithInternals = (u: unknown): u is LensWithInternals<unknown, unknown, unknown, unknown, unknown> => Predicate.hasProperty(u, LensWithInternalsTypeId)
+
+
+export declare namespace LensImpl {
+    export interface Source<in out A, in out ER = never, in out EW = never, in out RR = never, in out RW = never> {
+        readonly get: Effect.Effect<A, ER, RR>,
+        readonly changes: Stream.Stream<A, ER, RR>,
+        readonly update: (a: A) => Effect.Effect<void, EW, RW>,
+        readonly semaphore: Effect.Semaphore,
+    }
+}
+
+export class LensImpl<in out A, in out ER = never, in out EW = never, in out RR = never, in out RW = never>
+extends Pipeable.Class() implements LensWithInternals<A, ER, EW, RR, RW> {
+    readonly [Readable.TypeId]: Readable.TypeId = Readable.TypeId
+    readonly [Subscribable.TypeId]: Subscribable.TypeId = Subscribable.TypeId
+    readonly [LensTypeId]: LensTypeId = LensTypeId
+    readonly [LensWithInternalsTypeId]: LensWithInternalsTypeId = LensWithInternalsTypeId
+
+    constructor(
+        readonly get: Effect.Effect<A, ER, RR>,
+        readonly changes: Stream.Stream<A, ER, RR>,
+        readonly update: (a: A) => Effect.Effect<void, EW, RW>,
+        readonly semaphore: Effect.Semaphore,
+    ) {
+        super()
+    }
+
+    modifyEffect<B, E1 = never, R1 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
+    ) {
+        return this.semaphore.withPermits(1)(
+            Effect.flatMap(
+                this.get,
+                a => Effect.flatMap(f(a), ([b, next]) => Effect.as(this.update(next), b),
+            ))
+        )
+    }
+}
 
 /**
  * Creates a `Lens` by supplying how to read the current value, observe changes, and apply transformations.
- *
- * Either `modify` or `set` needs to be supplied.
  */
 export const make = <A, ER, EW, RR, RW>(
-    options: {
-        readonly get: Effect.Effect<A, ER, RR>
-        readonly changes: Stream.Stream<A, ER, RR>
-    } & (
-        | {
-            readonly modify: <B, E1 = never, R1 = never>(
-                f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
-            ) => Effect.Effect<B, ER | EW | E1, RR | RW | R1>
-        }
-        | { readonly set: (a: A) => Effect.Effect<void, EW, RW> }
-    )
-): Lens<A, ER, EW, RR, RW> => new LensImpl<A, ER, EW, RR, RW>(
-    options.get,
-    options.changes,
-    Predicate.hasProperty(options, "modify")
-        ? options.modify
-        : <B, E1 = never, R1 = never>(
-            f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
-        ) => Effect.flatMap(
-            options.get,
-            a => Effect.flatMap(f(a), ([b, next]) => Effect.as(options.set(next), b)
-        )),
-)
+    source: LensImpl.Source<A, ER, EW, RR, RW>
+): Lens<A, ER, EW, RR, RW> => new LensImpl(source.get, source.changes, source.update, source.semaphore)
+
+
+export class LensLazyImpl<in out A, in out ER = never, in out EW = never, in out RR = never, in out RW = never>
+extends Pipeable.Class() implements LensWithInternals<A, ER, EW, RR, RW> {
+    readonly [Readable.TypeId]: Readable.TypeId = Readable.TypeId
+    readonly [Subscribable.TypeId]: Subscribable.TypeId = Subscribable.TypeId
+    readonly [LensTypeId]: LensTypeId = LensTypeId
+    readonly [LensWithInternalsTypeId]: LensWithInternalsTypeId = LensWithInternalsTypeId
+
+    constructor(
+        readonly source: LensImpl.Source<A, ER, EW, RR, RW>
+    ) {
+        super()
+    }
+
+    get get() { return this.source.get }
+    get changes() { return this.source.changes }
+    get update() { return this.source.update }
+    get semaphore() { return this.source.semaphore }
+
+    modifyEffect<B, E1 = never, R1 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
+    ) {
+        return this.semaphore.withPermits(1)(
+            Effect.flatMap(
+                this.get,
+                a => Effect.flatMap(f(a), ([b, next]) => Effect.as(this.update(next), b),
+            ))
+        )
+    }
+}
+
+/**
+ * Creates a `Lens` by supplying how to read the current value, observe changes, and apply transformations.
+ */
+export const makeLazy = <A, ER, EW, RR, RW>(
+    source: LensImpl.Source<A, ER, EW, RR, RW>
+): Lens<A, ER, EW, RR, RW> => new LensLazyImpl(source)
+
+
+export declare namespace SubscriptionRefLensImpl {
+    export interface SubscriptionRefWithInternals<in out A>
+    extends SubscriptionRef.SubscriptionRef<A> {
+        readonly ref: Ref.Ref<A>
+        readonly pubsub: PubSub.PubSub<A>
+        readonly semaphore: Effect.Semaphore
+    }
+}
+
+export class SubscriptionRefLensImpl<in out A>
+extends Pipeable.Class() implements LensWithInternals<A, never, never, never, never> {
+    readonly [Readable.TypeId]: Readable.TypeId = Readable.TypeId
+    readonly [Subscribable.TypeId]: Subscribable.TypeId = Subscribable.TypeId
+    readonly [LensTypeId]: LensTypeId = LensTypeId
+    readonly [LensWithInternalsTypeId]: LensWithInternalsTypeId = LensWithInternalsTypeId
+
+    readonly ref: SubscriptionRefLensImpl.SubscriptionRefWithInternals<A>
+
+    constructor(
+        ref: SubscriptionRef.SubscriptionRef<A>
+    ) {
+        super()
+        this.ref = ref as SubscriptionRefLensImpl.SubscriptionRefWithInternals<A>
+    }
+
+    get get() { return this.ref.get }
+    get changes() { return this.ref.changes }
+    update(a: A) {
+        return Effect.zipLeft(
+            Ref.set(this.ref.ref, a),
+            PubSub.publish(this.ref.pubsub, a),
+        )
+    }
+    get semaphore() { return this.ref.semaphore }
+
+    modifyEffect<B, E1 = never, R1 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
+    ) {
+        return this.semaphore.withPermits(1)(
+            Effect.flatMap(
+                this.get,
+                a => Effect.flatMap(f(a), ([b, next]) => Effect.as(this.update(next), b),
+            ))
+        )
+    }
+}
 
 /**
  * Creates a `Lens` that proxies a `SubscriptionRef`.
  */
 export const fromSubscriptionRef = <A>(
     ref: SubscriptionRef.SubscriptionRef<A>
-): Lens<A, never, never, never, never> => make({
-    get get() { return ref.get },
-    get changes() { return ref.changes },
-    modify: <B, E1 = never, R1 = never>(
-        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
-    ) => ref.modifyEffect(f),
-})
+): Lens<A, never, never, never, never> => new SubscriptionRefLensImpl(ref)
+
 
 /**
  * Creates a `Lens` that proxies a `SynchronizedRef`.
@@ -108,6 +195,7 @@ export const fromSynchronizedRef = <A>(
     ) => ref.modifyEffect(f),
 })
 
+
 /**
  * Flattens an effectful `Lens`.
  */
@@ -116,11 +204,9 @@ export const unwrap = <A, ER, EW, RR, RW, E1, R1>(
 ): Lens<A, ER | E1, EW | E1, RR | R1, RW | R1> => make({
     get: Effect.flatMap(effect, l => l.get),
     changes: Stream.unwrap(Effect.map(effect, l => l.changes)),
-    modify: <B, E2 = never, R2 = never>(
-        f: (a: A) => Effect.Effect<readonly [B, A], E2, R2>
-    ) => Effect.flatMap(effect, l => l.modify(f)),
-})
+    update: a => Effect.flatMap(effect, l => (l as LensWithInternals<A, ER, EW, RR, RW>).update(a))
 
+})
 
 /**
  * Derives a new `Lens` by applying synchronous getters and setters over the focused value.