State Management in Effect

Overview

Effect provides functional mutable state primitives:

• Ref – Basic mutable reference
• SynchronizedRef – Ref with effectful updates
• SubscriptionRef – Ref with change notifications

All are fiber-safe and work correctly with concurrent access.

Ref – Basic Mutable Reference

Creating and Using Refs

import { Effect, Ref } from "effect"

const program = Effect.gen(function* () {
  const counter = yield* Ref.make(0)

  const current = yield* Ref.get(counter)

  yield* Ref.set(counter, 10)

  yield* Ref.update(counter, (n) => n + 1)

  const old = yield* Ref.getAndSet(counter, 0)

  const newValue = yield* Ref.updateAndGet(counter, (n) => n + 5)

  const [oldVal, result] = yield* Ref.modify(counter, (n) => [
    n,
    n * 2
  ])
})

Atomic Operations

const atomicIncrement = Effect.gen(function* () {
  const counter = yield* Ref.make(0)

  yield* Effect.all([
    Ref.update(counter, (n) => n + 1),
    Ref.update(counter, (n) => n + 1),
    Ref.update(counter, (n) => n + 1)
  ], { concurrency: "unbounded" })

  return yield* Ref.get(counter)
})

Ref in Services

const CounterService = Effect.gen(function* () {
  const ref = yield* Ref.make(0)

  return {
    increment: Ref.update(ref, (n) => n + 1),
    decrement: Ref.update(ref, (n) => n - 1),
    get: Ref.get(ref),
    reset: Ref.set(ref, 0)
  }
})

const CounterLive = Layer.effect(Counter, CounterService)

SynchronizedRef – Effectful Updates

For updates that require running effects:

import { Effect, SynchronizedRef } from "effect"

const program = Effect.gen(function* () {
  const ref = yield* SynchronizedRef.make({ count: 0, lastUpdated: Date.now() })

  yield* SynchronizedRef.updateEffect(ref, (state) =>
    Effect.gen(function* () {
      yield* Effect.log("Updating state")
      return {
        count: state.count + 1,
        lastUpdated: Date.now()
      }
    })
  )

  const result = yield* SynchronizedRef.modifyEffect(ref, (state) =>
    Effect.gen(function* () {
      const newCount = state.count + 1
      yield* sendMetric("counter", newCount)
      return [
        newCount,
        { ...state, count: newCount }
      ]
    })
  )
})

When to Use SynchronizedRef

• Updates require API calls
• Updates require logging/metrics
• Updates depend on external state
• Updates need error handling

// Cache with async refresh
const cache = yield* SynchronizedRef.make<Data | null>(null)

const refreshCache = SynchronizedRef.updateEffect(cache, () =>
  Effect.tryPromise(() => fetchLatestData())
)

SubscriptionRef – Reactive State

For state that needs to notify subscribers:

import { Effect, SubscriptionRef, Stream } from "effect"

const program = Effect.gen(function* () {
  const ref = yield* SubscriptionRef.make(0)

  const changes = yield* SubscriptionRef.changes(ref)

  yield* Effect.fork(
    Stream.runForEach(changes, (value) =>
      Effect.log(`Value changed to: ${value}`)
    )
  )

  yield* SubscriptionRef.set(ref, 1)
  yield* SubscriptionRef.update(ref, (n) => n + 1)
  yield* SubscriptionRef.set(ref, 10)
})

Reactive Patterns

const configRef = yield* SubscriptionRef.make(initialConfig)

const subscriber1 = Effect.fork(
  Stream.runForEach(
    SubscriptionRef.changes(configRef),
    (config) => updateService1(config)
  )
)

const subscriber2 = Effect.fork(
  Stream.runForEach(
    SubscriptionRef.changes(configRef),
    (config) => updateService2(config)
  )
)

yield* SubscriptionRef.set(configRef, newConfig)

Comparison

Common Patterns

Counter Service

class Counter extends Context.Tag("Counter")<
  Counter,
  {
    readonly increment: Effect.Effect<number>
    readonly decrement: Effect.Effect<number>
    readonly get: Effect.Effect<number>
  }
>() {}

const CounterLive = Layer.effect(
  Counter,
  Effect.gen(function* () {
    const ref = yield* Ref.make(0)
    return {
      increment: Ref.updateAndGet(ref, (n) => n + 1),
      decrement: Ref.updateAndGet(ref, (n) => n - 1),
      get: Ref.get(ref)
    }
  })
)

State Machine

type State = "idle" | "loading" | "success" | "error"

const stateMachine = Effect.gen(function* () {
  const state = yield* Ref.make<State>("idle")

  const transition = (from: State, to: State) =>
    Ref.modify(state, (current) =>
      current === from
        ? [true, to]
        : [false, current]
    )

  return {
    state: Ref.get(state),
    startLoading: transition("idle", "loading"),
    succeed: transition("loading", "success"),
    fail: transition("loading", "error"),
    reset: Ref.set(state, "idle")
  }
})

Accumulator

const accumulator = Effect.gen(function* () {
  const items = yield* Ref.make<Array<Item>>([])

  return {
    add: (item: Item) => Ref.update(items, (arr) => [...arr, item]),
    getAll: Ref.get(items),
    clear: Ref.set(items, []),
    count: Effect.map(Ref.get(items), (arr) => arr.length)
  }
})

Best Practices

1. Use Ref for simple state – Basic counters, flags, accumulators
2. Use SynchronizedRef for async updates – When updates need effects
3. Use SubscriptionRef for reactive patterns – When others need notifications
4. Keep state minimal – Don’t store derived data
5. Prefer immutable updates – Return new objects, don’t mutate

Additional Resources

For comprehensive state management documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• “Ref” for basic mutable references
• “SynchronizedRef” for effectful updates
• “SubscriptionRef” for reactive state