packages/effect-lens/README.md

@@ -241,8 +241,11 @@ Currently available:
 | - | - |
 | `focusObjectOn` | Focuses to the field of an object |
 | `focusArrayAt` | Focuses to an indexed entry of an array |
+| `focusArrayLength` | Focuses to the length of an array |
 | `focusTupleAt` | Focuses to an indexed entry of a tuple |
 | `focusChunkAt` | Focuses to an indexed entry of a `Chunk` |
+| `focusChunkSize` | Focuses to the size of a `Chunk` |
+| `focusIterableSize` | Focuses to the size of an iterable |
 
 
 ## Todo

packages/effect-lens/src/Subscribable.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, test } from "bun:test"
+import { Chunk, Effect, SubscriptionRef } from "effect"
+import * as Subscribable from "./Subscribable.js"
+
+
+describe("Subscribable", () => {
+    test("focusArrayLength reads the current array length and reflects updates", async () => {
+        const result = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make([1, 2, 3]),
+                parent => {
+                    const sizeSub = Subscribable.focusArrayLength(parent)
+                    return Effect.flatMap(
+                        sizeSub.get,
+                        initial => Effect.flatMap(
+                            SubscriptionRef.set(parent, [1, 2, 3, 4, 5]),
+                            () => Effect.map(sizeSub.get, next => [initial, next] as const),
+                        ),
+                    )
+                },
+            ),
+        )
+
+        expect(result).toEqual([3, 5])
+    })
+
+    test("focusChunkSize reads the current chunk size and reflects updates", async () => {
+        const result = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make(Chunk.make(1, 2) as Chunk.Chunk<number>),
+                parent => {
+                    const sizeSub = Subscribable.focusChunkSize(parent)
+                    return Effect.flatMap(
+                        sizeSub.get,
+                        initial => Effect.flatMap(
+                            SubscriptionRef.set(parent, Chunk.make(1, 2, 3, 4)),
+                            () => Effect.map(sizeSub.get, next => [initial, next] as const),
+                        ),
+                    )
+                },
+            ),
+        )
+
+        expect(result).toEqual([2, 4])
+    })
+
+    test("focusIterableSize also works for array values", async () => {
+        const result = await Effect.runPromise(
+            Effect.flatMap(
+                SubscriptionRef.make([1, 2, 3]),
+                parent => {
+                    const sizeSub = Subscribable.focusIterableSize(parent)
+                    return Effect.flatMap(
+                        sizeSub.get,
+                        initial => Effect.flatMap(
+                            SubscriptionRef.set(parent, [1, 2, 3, 4, 5]),
+                            () => Effect.map(sizeSub.get, next => [initial, next] as const),
+                        ),
+                    )
+                },
+            ),
+        )
+
+        expect(result).toEqual([3, 5])
+    })
+})

packages/effect-lens/src/Subscribable.ts

@@ -1,4 +1,4 @@
-import { Array, Chunk, Effect, Function, Option, Subscribable } from "effect"
+import { Array, Chunk, Effect, Function, Iterable, Option, Subscribable } from "effect"
 import type { NoSuchElementException } from "effect/Cause"
 
 
@@ -71,6 +71,13 @@ export const focusArrayAt: {
     index: number,
 ): Subscribable.Subscribable<A[number], E | NoSuchElementException, R> => Subscribable.mapEffect(self, Array.get(index)))
 
+/**
+ * Narrows the focus to the length of an array.
+ */
+export const focusArrayLength = <A extends readonly any[], E, R>(
+    self: Subscribable.Subscribable<A, E, R>,
+): Subscribable.Subscribable<number, E, R> => Subscribable.map(self, a => a.length)
+
 /**
  * Narrows the focus to an indexed element of a readonly tuple.
  */
@@ -102,3 +109,17 @@ export const focusChunkAt: {
     self: Subscribable.Subscribable<Chunk.Chunk<A>, E, R>,
     index: number,
 ): Subscribable.Subscribable<A, E | NoSuchElementException, R> => Subscribable.mapEffect(self, Chunk.get(index)))
+
+/**
+ * Narrows the focus to the size of a `Chunk`.
+ */
+export const focusChunkSize = <A, E, R>(
+    self: Subscribable.Subscribable<Chunk.Chunk<A>, E, R>,
+): Subscribable.Subscribable<number, E, R> => Subscribable.map(self, Chunk.size)
+
+/**
+ * Narrows the focus to the size of a `Iterable`.
+ */
+export const focusIterableSize = <A, E, R>(
+    self: Subscribable.Subscribable<Iterable<A>, E, R>,
+): Subscribable.Subscribable<number, E, R> => Subscribable.map(self, Iterable.size)