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Managed Resources

Overview

Resources live for the entire runtime. Some stateful handles should exist only while the Model is in a particular state: a camera stream during a video call, a WebSocket while on a chat page, or a Web Worker pool during a computation. Managed Resources give those handles a Model-driven acquire and release lifecycle, using the same dependency-diffing engine as Subscriptions.

The restaurant analogy

Resources are the kitchen equipment available all night. A Managed Resource is a specialty station set up only while the menu needs it. Changing the special tears down the old station and sets up the new one, just as changing camera requirements releases one stream and acquires another. A Command that asks for an inactive station receives ResourceNotAvailable.

Define the handle’s identity with ManagedResource.tag, then wire its lifecycle with ManagedResource.make. The modelToMaybeRequirements function returns Option.some(params) while the handle should be active and Option.none() while it should be absent.

import { Effect, Option, Schema, pipe } from 'effect'
import { ManagedResource, Runtime } from 'foldkit'

// 1. Define a Managed Resource identity
const CameraStream = ManagedResource.tag<MediaStream>()('CameraStream')

// 2. Wire the lifecycle with make. The requirements schema sits inline next
//    to its config: Option.some = active, Option.none = inactive
const managedResources = ManagedResource.make<Model, Message>()(entry => ({
  camera: entry(Schema.Option(Schema.Struct({ facingMode: Schema.String })), {
    resource: CameraStream,
    modelToMaybeRequirements: model =>
      pipe(
        model.callState,
        Option.liftPredicate(
          (callState): callState is typeof InCall.Type =>
            callState._tag === 'InCall',
        ),
        Option.map(callState => ({
          facingMode: callState.facingMode,
        })),
      ),
    acquire: ({ facingMode }) =>
      Effect.tryPromise(() =>
        navigator.mediaDevices.getUserMedia({
          video: { facingMode },
        }),
      ),
    release: stream =>
      Effect.sync(() => stream.getTracks().forEach(track => track.stop())),
    onAcquired: () => AcquiredCamera(),
    onReleased: () => ReleasedCamera(),
    onAcquireError: error => FailedAcquireCamera({ error: String(error) }),
  }),
}))

// 3. Pass to makeApplication
const application = Runtime.makeApplication({
  Model,
  init,
  update,
  view,
  container: document.getElementById('root'),
  managedResources,
})

The runtime compares the requirements after every Model change and performs the corresponding transition.

Requirements transition	Runtime behavior
`Option.none()` to `Option.some(params)`	Acquire the handle, then dispatch `onAcquired`.
`Option.some(params)` to `Option.none()`	Release the handle, then dispatch `onReleased`.
`Option.some(a)` to a different `Option.some(b)`	Release and dispatch `onReleased`, then acquire and dispatch `onAcquired`.
Structurally equal requirements	Keep the current handle.

If acquisition fails, the runtime dispatches onAcquireError as a Message. The lifecycle keeps watching for the next requirements change, and the failed acquisition does not crash the application.

Accessing Managed Resources in Commands

Commands access the current handle through .get. Because the handle may be inactive, .get can fail with ResourceNotAvailable. The Command must turn that error into one of its declared result Messages.

import { Array, Effect, Option } from 'effect'
import { Command, ManagedResource } from 'foldkit'

const CameraStream = ManagedResource.tag<MediaStream>()('CameraStream')

const TakePhoto = Command.define('TakePhoto', {
  messages: [SucceededTakePhoto, CameraUnavailable],
  execute: Effect.gen(function* () {
    const stream = yield* CameraStream.get

    const maybeTrack = Array.head(stream.getVideoTracks())
    const bitmap = yield* Option.match(maybeTrack, {
      onNone: () => Effect.fail(new Error('No video track available')),
      onSome: track => {
        const imageCapture = new ImageCapture(track)
        return Effect.tryPromise(() => imageCapture.grabFrame())
      },
    })

    return SucceededTakePhoto({ width: bitmap.width, height: bitmap.height })
  }).pipe(Effect.catch(() => Effect.succeed(CameraUnavailable()))),
})

This is the usual Command error-to-Message boundary. The Model should gate the operation, for example by enabling TakePhoto only after AcquiredCamera has been received. The error handler remains a safety net if that Model logic is wrong or the handle disappears before the Command reads it.

Building a Layer in acquire

When setup and teardown are already packaged as an Effect Layer, keep that lifecycle intact. acquire runs with the Managed Resource’s Scope in its context. Layer.build registers the Layer’s finalizers on that Scope, and the runtime closes it on release or reacquisition. Map the built Context down to the bare service value that Commands need.

import { Context, Effect, Layer, Option, Schema } from 'effect'
import { ManagedResource } from 'foldkit'

// A FEN string is a text description of a chess position.
interface ChessEngine {
  readonly bestMove: (fen: string) => Effect.Effect<string>
}

class ChessEngineService extends Context.Service<
  ChessEngineService,
  ChessEngine
>()('ChessEngineService') {}

// A heavy engine whose init and teardown are packaged as an Effect Layer.
// Building the Layer spawns the worker, and the finalizer registered by
// acquireRelease terminates it.
const engineLayer: Layer.Layer<ChessEngineService> = Layer.effect(
  ChessEngineService,
  Effect.gen(function* () {
    const worker = yield* Effect.acquireRelease(
      Effect.sync(() => new Worker('/chess-engine-worker.js')),
      worker => Effect.sync(() => worker.terminate()),
    )

    return {
      bestMove: (fen: string): Effect.Effect<string> => {
        // Your engine protocol goes here: post the FEN to the worker and
        // resolve with its best-move reply.
      },
    }
  }),
)

// 1. The Managed Resource holds the bare service value, with no wrapper.
const Engine = ManagedResource.tag<ChessEngine>()('ChessEngine')

// 2. acquire runs with the resource-lifetime Scope in its context, so
//    Layer.build registers the Layer's finalizers on it. They tear down when
//    the resource is released or re-acquired.
const managedResources = ManagedResource.make<Model, Message>()(entry => ({
  engine: entry(Schema.Option(Schema.Null), {
    resource: Engine,
    modelToMaybeRequirements: model => Option.as(model.maybeAnalysisSlug, null),
    acquire: () =>
      Layer.build(engineLayer).pipe(
        Effect.map(context => Context.get(context, ChessEngineService)),
      ),
    // The scope closes on release, so the Layer finalizers run automatically.
    release: () => Effect.void,
    onAcquired: () => StartedEngine(),
    onReleased: () => StoppedEngine(),
    onAcquireError: error => FailedStartEngine({ error: String(error) }),
  }),
}))

The resource tag holds that bare value, so Commands read it through .get with no wrapper to destructure. Any finalizer registered during acquire, through either Layer.build or Effect.addFinalizer, runs when the handle is released. In that case the explicit release can be () => Effect.void. The explicit callback runs first, followed by the Scope finalizers in Effect’s last-in-first-out order.

Composing Child Submodels

A child Submodel defines its Managed Resources in its own Model and Message terms, with no knowledge of its parent. ManagedResource.lift translates the child record through a Model accessor and a Message wrapper, matching the shape of update delegation and Subscription.lift. ManagedResource.aggregate combines root and lifted child records into the single record the runtime config expects. Duplicate keys throw at startup instead of silently replacing an entry.

Unlike Subscription.lift, toChildModel returns an Option. A Managed Resource already uses Option.none() to mean “release”, so an optional child that is not mounted naturally follows the same path. Removing the child releases its handle.

// page/call/managedResource.ts
import { Effect, Option, Schema } from 'effect'
import { ManagedResource } from 'foldkit'

import {
  ClosedSignaling,
  FailedSignaling,
  GotVideoCallMessage,
  type Message,
  OpenedSignaling,
} from './message'
import type { Model } from './model'
import * as VideoCall from './videoCall'

const SIGNALING_URL = 'wss://example.com/call/signaling'

const SignalingSocket = ManagedResource.tag<WebSocket>()('SignalingSocket')

const videoCallManagedResources = ManagedResource.lift(
  VideoCall.managedResources,
)<Model, Message>({
  toChildModel: model => model.videoCall,
  toParentMessage: message => GotVideoCallMessage({ message }),
})

const localManagedResources = ManagedResource.make<Model, Message>()(entry => ({
  signalingSocket: entry(Schema.Option(Schema.Null), {
    resource: SignalingSocket,
    modelToMaybeRequirements: model => Option.as(model.videoCall, null),
    acquire: () => Effect.try(() => new WebSocket(SIGNALING_URL)),
    release: socket => Effect.sync(() => socket.close()),
    onAcquired: () => OpenedSignaling(),
    onReleased: () => ClosedSignaling(),
    onAcquireError: error => FailedSignaling({ error: String(error) }),
  }),
}))

export const managedResources = ManagedResource.aggregate<Model, Message>()(
  videoCallManagedResources,
  localManagedResources,
)

The same operations compose across every Submodel level: make at the owner, lift through each parent, and aggregate at the root. Subscription Organization traces that leaf-to-root shape with Subscriptions; the Managed Resource structure is identical.

Resources vs Managed Resources

Use resources for services that live with the runtime, such as an RpcClient or analytics client. Use managedResources for handles whose lifetime follows the Model, such as camera streams, an AudioContext, or WebSocket connections.

Resources and Managed Resources cover long-lived services and Model-scoped handles. Unrecoverable errors in update, view, or a Command follow a different runtime path. The next page covers crash views.