From 4c45f49092fdde0ef5c4ffc492ce15520f59b54a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Julien=20Valverd=C3=A9?= <julien.valverde@mailo.com>
Date: Sat, 16 May 2026 15:12:29 +0200
Subject: [PATCH] Add provide

---
 packages/effect-lens/README.md        | 14 +++++++++++
 packages/effect-lens/src/Lens.test.ts | 34 ++++++++++++++++++++++++++-
 packages/effect-lens/src/Lens.ts      | 27 ++++++++++++++++++++-
 3 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/packages/effect-lens/README.md b/packages/effect-lens/README.md
index c976d80..d177c27 100644
--- a/packages/effect-lens/README.md
+++ b/packages/effect-lens/README.md
@@ -95,6 +95,20 @@ const lens = Effect.all([
 
 Note: while Lens supports asynchronous effects for the proxy logic, we would recommend keeping them synchronous to preserve atomicity.
 
+If a `Lens` depends on a service in its environment, you can provide that service directly to the lens:
+```typescript
+class Offset extends Context.Tag("Offset")<Offset, { readonly value: number }>() {}
+
+const root = Lens.fromSubscriptionRef(ref)
+const offsetLens = Lens.mapEffect(
+    root,
+    n => Effect.map(Offset, ({ value }) => n + value),
+    (_n, next) => Effect.map(Offset, ({ value }) => next - value),
+)
+
+const runnableLens = Lens.provide(offsetLens, Offset, { value: 5 })
+```
+
 
 ### Focusing
 
diff --git a/packages/effect-lens/src/Lens.test.ts b/packages/effect-lens/src/Lens.test.ts
index fabe248..e417b42 100644
--- a/packages/effect-lens/src/Lens.test.ts
+++ b/packages/effect-lens/src/Lens.test.ts
@@ -1,9 +1,11 @@
 import { describe, expect, test } from "bun:test"
-import { Chunk, Effect, Option, SubscriptionRef } from "effect"
+import { Chunk, Context, Effect, Option, SubscriptionRef } from "effect"
 import * as Lens from "./Lens.js"
 
 
 describe("Lens", () => {
+    class Offset extends Context.Tag("Offset")<Offset, { readonly value: number }>() {}
+
     test("mapOption transforms Some values and preserves None", async () => {
         const result = await Effect.runPromise(
             Effect.flatMap(
@@ -54,6 +56,36 @@ describe("Lens", () => {
         expect(result[1]).toEqual(Option.some(50)) // 100 / 2
     })
 
+    test("provide supplies a service to get and modify", async () => {
+        const result = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make(10),
+                parent => {
+                    const lens = Lens.provide(
+                        Lens.mapEffect(
+                            Lens.fromSubscriptionRef(parent),
+                            n => Effect.map(Offset, ({ value }) => n + value),
+                            (_n, next) => Effect.map(Offset, ({ value }) => next - value),
+                        ),
+                        Offset,
+                        { value: 5 },
+                    )
+
+                    return Effect.flatMap(
+                        Lens.get(lens),
+                        value => Effect.flatMap(
+                            Lens.set(lens, 30),
+                            () => Effect.map(parent.get, parentValue => [value, parentValue] as const),
+                        ),
+                    )
+                },
+            ),
+        )
+
+        expect(result[0]).toBe(15)
+        expect(result[1]).toBe(25)
+    })
+
     test("focusObjectOn focuses a nested property without touching other fields", async () => {
         const [initialCount, updatedState] = await Effect.runPromise(
             Effect.flatMap(
diff --git a/packages/effect-lens/src/Lens.ts b/packages/effect-lens/src/Lens.ts
index ca97fd6..9c6663b 100644
--- a/packages/effect-lens/src/Lens.ts
+++ b/packages/effect-lens/src/Lens.ts
@@ -1,4 +1,4 @@
-import { Array, Chunk, Effect, Function, identity, Option, Pipeable, Predicate, Readable, Stream, type SubscriptionRef, type SynchronizedRef } from "effect"
+import { Array, Chunk, type Context, Effect, Function, identity, Option, Pipeable, Predicate, Readable, Stream, type SubscriptionRef, type SynchronizedRef } from "effect"
 import type { NoSuchElementException } from "effect/Cause"
 import * as Subscribable from "./Subscribable.js"
 
@@ -269,6 +269,31 @@ export const mapStream: {
     get modify() { return self.modify },
 }))
 
+/**
+ * Provides a single service to a `Lens`, removing it from both the read and write environments.
+ */
+export const provide: {
+    <A, ER, EW, RR, RW, I, S>(
+        self: Lens<A, ER, EW, RR, RW>,
+        tag: Context.Tag<I, S>,
+        service: NoInfer<S>,
+    ): Lens<A, ER, EW, Exclude<RR, I>, Exclude<RW, I>>
+    <I, S>(
+        tag: Context.Tag<I, S>,
+        service: NoInfer<S>,
+    ): <A, ER, EW, RR, RW>(self: Lens<A, ER, EW, RR, RW>) => Lens<A, ER, EW, Exclude<RR, I>, Exclude<RW, I>>
+} = Function.dual(3, <A, ER, EW, RR, RW, I, S>(
+    self: Lens<A, ER, EW, RR, RW>,
+    tag: Context.Tag<I, S>,
+    service: NoInfer<S>,
+): Lens<A, ER, EW, Exclude<RR, I>, Exclude<RW, I>> => make({
+    get get() { return Effect.provideService(self.get, tag, service) },
+    get changes() { return Stream.provideService(self.changes, tag, service) },
+    modify: <B, E1 = never, R1 = never>(
+        f: (a: A) => Effect.Effect<readonly [B, A], E1, R1>
+    ) => Effect.provideService(self.modify(f), tag, service),
+}))
+
 
 /**
  * Narrows the focus to a field of an object. Replaces the object in an immutable fashion when written to.