packages/effect-fc/src/Lens.test.ts

@@ -0,0 +1,99 @@
+import { describe, expect, test } from "bun:test"
+import { Chunk, Effect, SubscriptionRef } from "effect"
+import * as Lens from "./Lens.js"
+
+
+describe("Lens", () => {
+    test("mapField focuses a nested property without touching other fields", async () => {
+        const [initialCount, updatedState] = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make({ count: 1, label: "original" }),
+                parent => {
+                    const countLens = Lens.mapField(Lens.fromSubscriptionRef(parent), "count")
+                    return Effect.flatMap(
+                        Lens.get(countLens),
+                        count => Effect.flatMap(
+                            Lens.set(countLens, count + 5),
+                            () => Effect.map(parent.get, state => [count, state] as const),
+                        ),
+                    )
+                },
+            ),
+        )
+
+        expect(initialCount).toBe(1)
+        expect(updatedState).toEqual({ count: 6, label: "original" })
+    })
+
+    test("mapMutableField preserves the root identity when mutating in place", async () => {
+        const original = { detail: "keep" }
+        const updated = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make(original),
+                parent => {
+                    const detailLens = Lens.mapMutableField(Lens.fromSubscriptionRef(parent), "detail")
+                    return Effect.flatMap(
+                        Lens.set(detailLens, "mutated"),
+                        () => parent.get,
+                    )
+                },
+            ),
+        )
+
+        expect(updated).toBe(original)
+        expect(updated.detail).toBe("mutated")
+    })
+
+    test("mapArrayAt updates the selected index", async () => {
+        const updated = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make([10, 20, 30]),
+                parent => {
+                    const elementLens = Lens.mapArrayAt(Lens.fromSubscriptionRef(parent), 1)
+                    return Effect.flatMap(
+                        Lens.update(elementLens, value => value + 5),
+                        () => parent.get,
+                    )
+                },
+            ),
+        )
+
+        expect(updated).toEqual([10, 25, 30])
+    })
+
+    test("mapMutableArrayAt mutates the array reference in place", async () => {
+        const original = ["foo", "bar"]
+        const updated = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make(original),
+                parent => {
+                    const elementLens = Lens.mapMutableArrayAt(Lens.fromSubscriptionRef(parent), 0)
+                    return Effect.flatMap(
+                        Lens.set(elementLens, "baz"),
+                        () => parent.get,
+                    )
+                },
+            ),
+        )
+
+        expect(updated).toBe(original)
+        expect(updated).toEqual(["baz", "bar"])
+    })
+
+    test("mapChunkAt replaces the focused chunk element", async () => {
+        const updated = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make(Chunk.make(1, 2, 3) as Chunk.Chunk<number>),
+                parent => {
+                    const elementLens = Lens.mapChunkAt(Lens.fromSubscriptionRef(parent), 2)
+                    return Effect.flatMap(
+                        Lens.set(elementLens, 99),
+                        () => parent.get,
+                    )
+                },
+            ),
+        )
+
+        expect(Chunk.toReadonlyArray(updated)).toEqual([1, 2, 99])
+    })
+})