diff --git a/packages/effect-fc/src/Lens.ts b/packages/effect-fc/src/Lens.ts
index cd49464..1d00bfa 100644
--- a/packages/effect-fc/src/Lens.ts
+++ b/packages/effect-fc/src/Lens.ts
@@ -5,7 +5,7 @@ import * as Writable from "./Writable.js"
 export const LensTypeId: unique symbol = Symbol.for("@effect-fc/Lens/Lens")
 export type LensTypeId = typeof LensTypeId
 
-export interface Lens<in out A, ER = unknown, RR = unknown, EW = unknown, RW = unknown>
+export interface Lens<in out A, ER = never, RR = never, EW = never, RW = never>
 extends LensPrototype, Subscribable.Subscribable<A, ER, RR>, Writable.Writable<A, EW, RW> {}
 
 
@@ -29,7 +29,7 @@ export const make = <A, ER, RR, EW, RW>(
         readonly get: Effect.Effect<A, ER, RR>
         readonly changes: Stream.Stream<A, ER, RR>
     } & (
-        | { readonly modify: <B>(f: (a: A) => readonly [B, A]) => Effect.Effect<B, EW, RW> }
+        | { readonly modify: <B, E1 = never, R1 = never>(f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>) => Effect.Effect<B, EW | E1, RW | R1> }
         | { readonly set: (value: A) => Effect.Effect<void, EW, RW> }
     )
 ): Lens<A, ER, RR, EW, RW> => Object.setPrototypeOf({
@@ -37,10 +37,11 @@ export const make = <A, ER, RR, EW, RW>(
     changes: options.changes,
     modify: Predicate.hasProperty(options, "modify")
         ? options.modify
-        : <B>(f: (a: A) => readonly [B, A]) => Effect.flatMap(options.get, a => {
-            const [b, next] = f(a)
-            return Effect.map(options.set(next), () => b)
-        }),
+        : <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.map(options.set(next), () => b)
+            )
+        ),
 }, LensPrototype)
 
 /**
@@ -51,13 +52,15 @@ export const fromSubscriptionRef = <A>(
 ): Lens<A, never, never, never, never> => make({
     get: ref.get,
     changes: ref.changes,
-    modify: ref.modify,
+    modify: ref.modifyEffect,
 })
 
 export const unwrap = <A, ER, RR, EW, RW, E1, R1>(
     effect: Effect.Effect<Lens<A, ER, RR, EW, RW>, E1, R1>
 ): Lens<A, ER | E1, RR | R1, EW | E1, RW | R1> => make({
-    get: Effect.flatMap(effect, p => p.get),
-    changes: Stream.unwrap(Effect.map(effect, p => p.changes)),
-    set: (v: A) => Effect.flatMap(effect, p => p.modify(() => [undefined as void as any, v]) as any),
+    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)),
 })
diff --git a/packages/effect-fc/src/Writable.ts b/packages/effect-fc/src/Writable.ts
index b415702..9883fd0 100644
--- a/packages/effect-fc/src/Writable.ts
+++ b/packages/effect-fc/src/Writable.ts
@@ -5,7 +5,7 @@ export const WritableTypeId: unique symbol = Symbol.for("@effect-fc/Writable/Wri
 export type WritableTypeId = typeof WritableTypeId
 
 export interface Writable<in out A, E = never, R = never> extends WritablePrototype {
-    readonly modify: <B>(f: (a: A) => readonly [B, A]) => Effect.Effect<B, E, R>
+    readonly modify: <B, E1 = never, R1 = never>(f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>) => Effect.Effect<B, E | E1, R | R1>
 }
 
 
@@ -25,7 +25,7 @@ export const isWritable = (u: unknown): u is Writable<unknown, unknown, unknown>
  * Construct a `Writable` from a `modify` function.
  */
 export const make = <A, E, R>(
-    modify: <B>(f: (a: A) => readonly [B, A]) => Effect.Effect<B, E, R>
+    modify: <B, E1 = never, R1 = never>(f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>) => Effect.Effect<B, E | E1, R | R1>,
 ): Writable<A, E, R> => Object.setPrototypeOf({ modify }, WritablePrototype)
 
 /**
@@ -33,6 +33,8 @@ export const make = <A, E, R>(
  */
 export const unwrap = <A, E, R, E1, R1>(
     effect: Effect.Effect<Writable<A, E, R>, E1, R1>
-): Writable<A, E | E1, R | R1> => make(<B>(
-    f: (a: A) => readonly [B, A]
-) => Effect.flatMap(effect, w => w.modify(f)))
+): Writable<A, E | E1, R | R1> => make(
+    <B, E2 = never, R2 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E2, R2>
+    ) => Effect.flatMap(effect, w => w.modify(f))
+)