Julien Valverdé
138 lines
3.9 KiB
Markdown
138 lines
3.9 KiB
Markdown
# Effect Lens Next
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An effectful Lens for Effect v4.
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`effect-lens-next` provides a bidirectional view into shared state. A Lens can read, observe, and atomically update a value while focusing on a nested part of a larger structure.
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## Install
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```sh
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npm install effect@4.0.0-beta.85 effect-lens-next
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```
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This release targets Effect `4.0.0-beta.85` exactly while Effect v4 is in beta.
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## Quickstart
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```ts
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import { Effect, Stream, SubscriptionRef } from "effect"
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import { Lens } from "effect-lens-next"
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const program = Effect.gen(function*() {
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const state = yield* SubscriptionRef.make({ count: 0, label: "Counter" })
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const count = state.pipe(
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Lens.fromSubscriptionRef,
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Lens.focusObjectOn("count")
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)
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yield* Lens.update(count, (value) => value + 1)
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const current = yield* Lens.get(count)
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yield* count.changes.pipe(
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Stream.take(1),
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Stream.runForEach((value) => Effect.log(`count: ${value}`))
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)
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return current
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})
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```
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## Lens Type
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```ts
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Lens.Lens<
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A, // focused value
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ER, // read errors
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EW, // write errors
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RR, // read requirements
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RW // write requirements
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>
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```
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## Constructors
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- `Lens.fromSubscriptionRef` creates a reactive Lens.
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- `Lens.fromSynchronizedRef` creates a non-reactive Lens whose stream emits its value when evaluated.
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- `Lens.fromRef` creates a non-reactive Lens and returns it in an Effect because it allocates a semaphore.
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- `Lens.make` creates a Lens from custom `get`, `changes`, `commit`, and `lock` operations.
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- `Lens.unwrap` flattens an effect that produces a Lens.
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## Focusing
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```ts
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import { Effect, SubscriptionRef } from "effect"
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import { Lens } from "effect-lens-next"
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const program = Effect.gen(function*() {
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const state = yield* SubscriptionRef.make({
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users: [{ name: "Ada", score: 10 }]
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})
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const score = state.pipe(
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Lens.fromSubscriptionRef,
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Lens.focusObjectOn("users"),
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Lens.focusArrayAt(0),
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Lens.focusObjectOn("score")
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)
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yield* Lens.set(score, 11)
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})
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```
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Available focus operations include object fields, mutable object fields, arrays, mutable arrays, tuples, mutable tuples, `Chunk` values, and `Option` values. Safe indexed operations fail with `Cause.NoSuchElementError` when the index is absent.
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Effect v4 no longer treats `Option` as an Effect. Custom effectful mappings should convert optional values explicitly:
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```ts
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import { Array, Effect } from "effect"
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Lens.mapEffect(
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(values: ReadonlyArray<number>) => Effect.fromOption(Array.get(values, 0)),
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(values, value) => Effect.fromOption(Array.replace(values, 0, value))
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)
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```
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## Updating
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The update API follows Effect v4's `SubscriptionRef` conventions:
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- `get`, `set`, `getAndSet`, `setAndGet`
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- `modify`, `modifyEffect`
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- `update`, `updateEffect`
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- `getAndUpdate`, `getAndUpdateEffect`
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- `updateAndGet`, `updateAndGetEffect`
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- `modifySome`, `modifySomeEffect`
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- `getAndUpdateSome`, `getAndUpdateSomeEffect`
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- `updateSome`, `updateSomeEffect`
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- `updateSomeAndGet`, `updateSomeAndGetEffect`
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Conditional updates use `Option.none()` to skip both the write and change notification:
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```ts
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import { Effect, Option } from "effect"
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yield* Lens.updateSome(lens, (value) =>
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value < 10 ? Option.some(value + 1) : Option.none()
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)
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yield* Lens.updateSomeEffect(lens, (value) =>
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Effect.succeed(value < 10 ? Option.some(value + 1) : Option.none())
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)
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```
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## Subscribable
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Effect v4 removed its `Readable` and `Subscribable` modules. This package provides a small compatible abstraction with `get` and `changes` properties:
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```ts
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import { Lens, Subscribable } from "effect-lens-next"
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const users = Lens.fromSubscriptionRef(usersRef)
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const count = Subscribable.focusArrayLength(users)
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const current = yield* count.get
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```
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The module includes `make`, `map`, `mapEffect`, `mapError`, `tapError`, `unwrap`, and the same readonly focus helpers used by Lens. A `SubscriptionRef` can first be converted with `Lens.fromSubscriptionRef`, since every Lens is already a Subscribable.
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