diff --git a/packages/effect-lens/src/Lens.test.ts b/packages/effect-lens/src/Lens.test.ts
index 6d5613a..57eee62 100644
--- a/packages/effect-lens/src/Lens.test.ts
+++ b/packages/effect-lens/src/Lens.test.ts
@@ -83,7 +83,7 @@ describe("Lens", () => {
     test("focusTupleAt updates the selected tuple index immutably", async () => {
         const updated = await Effect.runPromise(
             Effect.flatMap(
-                SubscriptionRef.make(["a", "b", "c"] as const),
+                SubscriptionRef.make<readonly [string, string, string]>(["a", "b", "c"]),
                 parent => {
                     const elementLens = Lens.focusTupleAt(Lens.fromSubscriptionRef(parent), 1)
                     return Effect.flatMap(
@@ -98,7 +98,7 @@ describe("Lens", () => {
     })
 
     test("focusMutableTupleAt mutates the tuple reference in place", async () => {
-        const original = ["foo", "bar"] as ["foo", "bar"]
+        const original: [string, string] = ["foo", "bar"]
         const updated = await Effect.runPromise(
             Effect.flatMap(
                 SubscriptionRef.make(original),
diff --git a/packages/effect-lens/src/Lens.ts b/packages/effect-lens/src/Lens.ts
index d7f7c6f..679e0dc 100644
--- a/packages/effect-lens/src/Lens.ts
+++ b/packages/effect-lens/src/Lens.ts
@@ -300,43 +300,40 @@ export const focusMutableArrayAt: {
  * Narrows the focus to an indexed element of a readonly tuple. Replaces the tuple in an immutable fashion when written to.
  */
 export const focusTupleAt: {
-    <T extends readonly [any, ...any[]], ER, EW, RR, RW>(
+    <T extends readonly [any, ...any[]], I extends number, ER, EW, RR, RW>(
         self: Lens<T, ER, EW, RR, RW>,
-        index: number,
-    ): Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW>
-    <T extends readonly [any, ...any[]], ER, EW, RR, RW>(
-        index: number
-    ): (self: Lens<T, ER, EW, RR, RW>) => Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW>
-} = Function.dual(2, <T extends readonly [any, ...any[]], ER, EW, RR, RW>(
+        index: I,
+    ): Lens<T[I], ER, EW, RR, RW>
+    <T extends readonly [any, ...any[]], I extends number, ER, EW, RR, RW>(
+        index: I
+    ): (self: Lens<T, ER, EW, RR, RW>) => Lens<T[I], ER, EW, RR, RW>
+} = Function.dual(2, <T extends readonly [any, ...any[]], I extends number, ER, EW, RR, RW>(
     self: Lens<T, ER, EW, RR, RW>,
-    index: number,
-): Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW> => mapEffect(
+    index: I,
+): Lens<T[I], ER, EW, RR, RW> => map(
     self,
-    Array.get(index),
-    (a, b) => Array.replaceOption(a, index, b) as any,
+    Array.unsafeGet(index),
+    (a, b) => Array.replace(a, index, b) as any,
 ))
 
 /**
  * Narrows the focus to an indexed element of a mutable tuple. Mutates the tuple in place when written to.
  */
 export const focusMutableTupleAt: {
-    <T extends [any, ...any[]], ER, EW, RR, RW>(
+    <T extends [any, ...any[]], I extends number, ER, EW, RR, RW>(
         self: Lens<T, ER, EW, RR, RW>,
-        index: number,
-    ): Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW>
-    <T extends [any, ...any[]], ER, EW, RR, RW>(
-        index: number
-    ): (self: Lens<T, ER, EW, RR, RW>) => Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW>
-} = Function.dual(2, <T extends [any, ...any[]], ER, EW, RR, RW>(
+        index: I,
+    ): Lens<T[I], ER, EW, RR, RW>
+    <T extends [any, ...any[]], I extends number, ER, EW, RR, RW>(
+        index: I
+    ): (self: Lens<T, ER, EW, RR, RW>) => Lens<T[I], ER, EW, RR, RW>
+} = Function.dual(2, <T extends [any, ...any[]], I extends number, ER, EW, RR, RW>(
     self: Lens<T, ER, EW, RR, RW>,
-    index: number,
-): Lens<T[number], ER | NoSuchElementException, EW | NoSuchElementException, RR, RW> => mapEffect(
+    index: I,
+): Lens<T[I], ER, EW, RR, RW> => map(
     self,
-    Array.get(index),
-    (a, b) => Effect.flatMap(
-        Array.get(a, index),
-        () => Effect.as(Effect.sync(() => { a[index] = b }), a),
-    ),
+    Array.unsafeGet(index),
+    (a, b) => { a[index] = b; return a },
 ))
 
 /**