Effect Stream Patterns

Overview

Effect Streams are pull-based, lazy sequences of values. Unlike Observables (push-based), Streams are consumed on-demand and provide powerful composition primitives.

Key characteristics:

• Pull-based — Consumer drives execution
• Lazy — Only computes what's needed
• Chunked — Emits

  Chunk<A>

  for efficiency
• Effectful — Each element can be an

  Effect

• Composable — Rich operators for transformation

Use streams for:

• Progressive data loading — Display results as they arrive
• Infinite sequences — Tickers, pollers, event sources
• Resource-efficient processing — Process large datasets without loading all into memory
• Async iteration — Cleaner than manual async loops

Canonical Sources

Effect Stream Core

• Submodule:

  ../../submodules/effect/packages/effect/src/

  • Stream.ts:52-70

    — Stream constructors

  • Stream.ts:316-362

    — Stream.async

  • Stream.ts

    (various) — Operators and consumers

Effect Website Documentation

• Submodule:

  ../../submodules/website/content/src/content/docs/docs/stream/

  • creating.mdx

    — Stream constructors

  • consuming-streams.mdx

    — Running streams

  • operations.mdx

    — Transformations

  • error-handling.mdx

    — Failure management

  • resourceful-streams.mdx

    — Resource-managed streams

TMNL Battle-tested Implementations

• Progressive search —

  src/lib/data-manager/v1/atoms/index.ts:206

  (Stream with atom updates)
• Stream-to-Atom —

  src/lib/streams/atoms/streamToAtom.ts

  (Reactive integration)
• Mock streams —

  src/lib/data-grid/mocking/stream.ts

  (Test utilities)

Patterns

Decision Tree: Which Stream Pattern?

Need a stream?
│
├─ From existing data (array, iterable)?
│  └─ Use: Stream.fromIterable(data)
│
├─ From async callback (WebSocket, EventSource)?
│  └─ Use: Stream.async((emit) => { ... })
│
├─ Ticking/polling at intervals?
│  └─ Use: Stream.fromSchedule(Schedule.spaced(...))
│
├─ Single effectful value?
│  └─ Use: Stream.fromEffect(effect)
│
├─ Range of numbers?
│  └─ Use: Stream.range(start, end)
│
├─ Infinite sequence?
│  └─ Use: Stream.iterate(initial, fn)
│
└─ From another stream with transformation?
   └─ Use: stream.pipe(Stream.map(...))

Pattern 1: Stream.fromIterable — FROM ARRAYS

When to use:

• Have existing array/iterable data
• Want to process data lazily
• Need to chunk large datasets

Signature:

Stream.fromIterable<A>(iterable: Iterable<A>): Stream.Stream<A>

Full Example:

import { Stream, Effect } from 'effect'

// From array
const numbers = Stream.fromIterable([1, 2, 3, 4, 5])

// Lazy processing (only computes when consumed)
const doubled = numbers.pipe(
  Stream.map((n) => n * 2)
)

// Consume
const result = await Stream.runCollect(doubled).pipe(Effect.runPromise)
console.log(result)  // Chunk([2, 4, 6, 8, 10])

// From generator
function* fibonacci() {
  let a = 0, b = 1
  while (true) {
    yield a
    ;[a, b] = [b, a + b]
  }
}

const fibs = Stream.fromIterable(fibonacci()).pipe(
  Stream.take(10)
)

Key Features:

• Lazy evaluation — Doesn't process until consumed
• Automatic chunking — Emits

  Chunk<A>

  for efficiency
• Supports generators — Works with any

  Iterable

Pattern 2: Stream.async — FROM ASYNC CALLBACKS

When to use:

• Integrate push-based sources (WebSocket, EventSource, DOM events)
• Wrap callbacks into pull-based stream
• Need cleanup logic

Signature:

Stream.async<A, E>(
  register: (emit: Emit<A, E>) => Effect.Effect<void> | void,
  bufferSize?: number
): Stream.Stream<A, E>

Full Example:

import { Stream, Effect } from 'effect'

// WebSocket stream
const wsStream = Stream.async<string, Error>((emit) => {
  const ws = new WebSocket('wss://example.com/stream')

  ws.onmessage = (event) => {
    emit.single(event.data)  // Emit single value
  }

  ws.onerror = (error) => {
    emit.fail(new Error('WebSocket error'))  // Emit error
  }

  ws.onclose = () => {
    emit.end()  // End stream
  }

  // Cleanup function
  return Effect.sync(() => {
    ws.close()
  })
})

// EventSource stream
const sseStream = Stream.async<MessageEvent>((emit) => {
  const source = new EventSource('/api/events')

  source.onmessage = (event) => {
    emit.single(event)
  }

  source.onerror = () => {
    emit.fail(new Error('SSE error'))
  }

  return Effect.sync(() => {
    source.close()
  })
})

// DOM event stream
const clickStream = Stream.async<MouseEvent>((emit) => {
  const handler = (event: MouseEvent) => {
    emit.single(event)
  }

  document.addEventListener('click', handler)

  return Effect.sync(() => {
    document.removeEventListener('click', handler)
  })
})

Emit API:

• emit.single(value)

  — Emit one value

• emit.chunk(chunk)

  — Emit multiple values at once

• emit.fail(error)

  — Emit error and end stream

• emit.end()

  — End stream successfully

• emit.fromEffect(effect)

  — Emit from Effect

• emit.fromEffectChunk(effect)

  — Emit chunk from Effect

Key Features:

• Cleanup support — Return Effect for cleanup logic
• Buffer size — Control backpressure (default: 16)
• Error handling — Emit failures via

  emit.fail

Pattern 3: Stream.fromSchedule — TICKING STREAMS

When to use:

• Poll at regular intervals
• Emit values on a schedule
• Implement retry logic with backoff

Signature:

Stream.fromSchedule<A>(schedule: Schedule.Schedule<A>): Stream.Stream<A>

Full Example:

import { Stream, Schedule, Effect } from 'effect'

// Tick every second
const ticker = Stream.fromSchedule(Schedule.spaced('1 second'))

// Emit current time every second
const clock = ticker.pipe(
  Stream.map(() => new Date())
)

// Exponential backoff ticker
const backoff = Stream.fromSchedule(
  Schedule.exponential('100 millis').pipe(
    Schedule.compose(Schedule.recurs(5))  // Max 5 retries
  )
)

// Poll API every 5 seconds
const pollApi = Stream.fromSchedule(Schedule.spaced('5 seconds')).pipe(
  Stream.mapEffect(() =>
    Effect.tryPromise(() => fetch('/api/status').then(r => r.json()))
  )
)

// Consume with counter
const counted = ticker.pipe(
  Stream.scan(0, (count) => count + 1),
  Stream.take(10)
)

Common Schedules:

• Schedule.spaced('1 second')

  — Fixed interval

• Schedule.exponential('100 millis')

  — Exponential backoff

• Schedule.fibonacci('1 second')

  — Fibonacci backoff

• Schedule.recurs(n)

  — Limit number of emissions

Key Features:

• Schedule composition — Combine schedules with

  .pipe

• Automatic timing — No manual

  setTimeout

• Configurable — Durations, retry logic, jitter

Pattern 4: Stream.fromEffect — SINGLE EFFECTFUL VALUE

When to use:

• Stream from a single Effect
• Lift async operation into stream
• Compose with other streams

Signature:

Stream.fromEffect<A, E>(effect: Effect.Effect<A, E>): Stream.Stream<A, E>

Full Example:

import { Stream, Effect } from 'effect'

// From Effect
const userStream = Stream.fromEffect(
  Effect.tryPromise(() =>
    fetch('/api/user').then(r => r.json())
  )
)

// Compose multiple
const combined = Stream.mergeAll(
  Stream.fromEffect(fetchUsers),
  Stream.fromEffect(fetchPosts),
  Stream.fromEffect(fetchComments)
)

// Chain effectful streams
const users = Stream.fromEffect(fetchUserIds).pipe(
  Stream.flatMap((id) =>
    Stream.fromEffect(fetchUser(id))
  )
)

Pattern 5: Stream Transformations

map — Transform each element:

const doubled = stream.pipe(
  Stream.map((n) => n * 2)
)

mapEffect — Transform with Effect:

const validated = stream.pipe(
  Stream.mapEffect((item) =>
    Effect.tryPromise(() => validateItem(item))
  )
)

filter — Keep matching elements:

const evens = stream.pipe(
  Stream.filter((n) => n % 2 === 0)
)

flatMap — Transform to stream and flatten:

const expanded = stream.pipe(
  Stream.flatMap((item) =>
    Stream.fromIterable(item.children)
  )
)

take — Limit number of elements:

const first10 = stream.pipe(
  Stream.take(10)
)

drop — Skip first N elements:

const afterFirst5 = stream.pipe(
  Stream.drop(5)
)

scan — Accumulate state:

const cumulative = stream.pipe(
  Stream.scan(0, (sum, n) => sum + n)
)

rechunk — Change chunk size:

const batched = stream.pipe(
  Stream.rechunk(50)  // Emit in chunks of 50
)

debounce — Emit only after quiet period:

const debounced = stream.pipe(
  Stream.debounce('500 millis')
)

throttle — Limit emission rate:

const throttled = stream.pipe(
  Stream.throttle({
    cost: () => 1,
    units: 10,
    duration: '1 second'
  })
)

Pattern 6: Stream Consumers

Stream.runCollect — Collect all elements:

const result = await Stream.runCollect(stream).pipe(Effect.runPromise)
console.log(result)  // Chunk([...])

Stream.runForEach — Side effect per element:

await Stream.runForEach(stream, (item) =>
  Effect.sync(() => console.log(item))
).pipe(Effect.runPromise)

Stream.runFold — Reduce to single value:

const sum = await Stream.runFold(stream, 0, (acc, n) => acc + n)
  .pipe(Effect.runPromise)

Stream.runDrain — Run and discard results:

await Stream.runDrain(stream).pipe(Effect.runPromise)

Stream.runIntoQueue — Push to Queue:

import { Queue } from 'effect'

const queue = await Queue.unbounded<number>().pipe(Effect.runPromise)
await Stream.runIntoQueue(stream, queue).pipe(Effect.runPromise)

Pattern 7: Progressive Accumulation (TMNL Pattern)

When to use:

• Display search results as they arrive
• Progressive UI updates
• Combine Stream with effect-atom

Full Example:

import { Atom } from '@effect-atom/atom-react'
import { Stream, Effect } from 'effect'

// State atoms
const resultsAtom = Atom.make<SearchResult[]>([])
const statusAtom = Atom.make<'idle' | 'streaming' | 'complete'>('idle')

// Operation atom with progressive stream
const searchOp = runtimeAtom.fn<string>()((query, ctx) =>
  Effect.gen(function* () {
    // Create stream
    const stream = yield* SearchKernel.pipe(
      Effect.flatMap((k) => k.searchStream(query))
    )

    // Initialize state
    ctx.set(statusAtom, 'streaming')
    ctx.set(resultsAtom, [])

    // Consume stream progressively
    yield* Stream.runForEach(stream, (item) =>
      Effect.sync(() => {
        const prev = ctx.get(resultsAtom)
        ctx.set(resultsAtom, [...prev, item])
      })
    )

    // Finalize
    ctx.set(statusAtom, 'complete')
  })
)

// React component
function SearchResults() {
  const results = useAtomValue(resultsAtom)
  const status = useAtomValue(statusAtom)

  return (
    <div>
      {status === 'streaming' && <Spinner />}
      <List items={results} />
    </div>
  )
}

Key Pattern:

1. Create state atoms at module level
2. Stream emits chunks progressively

3. Stream.runForEach

   updates atoms via

   ctx.set

4. React re-renders on each atom update
5. UI shows progressive results

TMNL Example (

src/lib/data-manager/v1/atoms/index.ts:206

export const doSearch = runtimeAtom.fn<{ query: string; limit: number }>()(
  ({ query, limit }, ctx) =>
    Effect.gen(function* () {
      const dm = yield* DataManager
      const stream = yield* dm.searchStream(query, limit)

      ctx.set(statusAtom, 'streaming')
      ctx.set(resultsAtom, [])

      yield* Stream.runForEach(stream, (result) =>
        Effect.sync(() => {
          const prev = ctx.get(resultsAtom)
          ctx.set(resultsAtom, [...prev, result])
        })
      )

      ctx.set(statusAtom, 'complete')
    })
)

Pattern 8: Error Handling

catchAll — Recover from errors:

const recovered = stream.pipe(
  Stream.catchAll((error) =>
    Stream.succeed({ error: error.message })
  )
)

retry — Retry on failure:

const retried = stream.pipe(
  Stream.retry(Schedule.exponential('100 millis').pipe(
    Schedule.compose(Schedule.recurs(3))
  ))
)

orElse — Fallback stream:

const withFallback = primaryStream.pipe(
  Stream.orElse(() => fallbackStream)
)

Pattern 9: Stream Merging & Combining

mergeAll — Merge multiple streams:

const merged = Stream.mergeAll(
  stream1,
  stream2,
  stream3
)

concat — Concatenate streams:

const concatenated = stream1.pipe(
  Stream.concat(stream2)
)

zip — Combine elements pairwise:

const zipped = stream1.pipe(
  Stream.zip(stream2)
)
// Emits: [a1, b1], [a2, b2], ...

interleave — Alternate between streams:

const interleaved = stream1.pipe(
  Stream.interleave(stream2)
)
// Emits: a1, b1, a2, b2, a3, b3, ...

Pattern 10: Resource-Managed Streams

Stream.acquireRelease — Managed resources:

const fileStream = Stream.acquireRelease(
  Effect.tryPromise(() => fs.open('file.txt')),
  (handle) => Effect.sync(() => handle.close())
).pipe(
  Stream.flatMap((handle) =>
    Stream.fromIterable(handle.readLines())
  )
)

Stream.ensuring — Run effect on completion:

const logged = stream.pipe(
  Stream.ensuring(
    Effect.sync(() => console.log('Stream completed'))
  )
)

Examples

Example 1: Infinite Ticker with Scan

import { Stream, Schedule, Effect } from 'effect'

const counter = Stream.fromSchedule(Schedule.spaced('1 second')).pipe(
  Stream.scan(0, (count) => count + 1),
  Stream.take(10)
)

await Stream.runForEach(counter, (n) =>
  Effect.sync(() => console.log(`Tick ${n}`))
).pipe(Effect.runPromise)

Example 2: WebSocket with Error Handling

import { Stream, Effect } from 'effect'

const wsStream = Stream.async<string, Error>((emit) => {
  const ws = new WebSocket('wss://example.com')

  ws.onmessage = (event) => emit.single(event.data)
  ws.onerror = () => emit.fail(new Error('Connection failed'))
  ws.onclose = () => emit.end()

  return Effect.sync(() => ws.close())
}).pipe(
  Stream.retry(Schedule.exponential('1 second').pipe(
    Schedule.compose(Schedule.recurs(3))
  )),
  Stream.catchAll((error) =>
    Stream.succeed(`Error: ${error.message}`)
  )
)

Example 3: Batched API Polling

import { Stream, Schedule, Effect } from 'effect'

const pollUsers = Stream.fromSchedule(Schedule.spaced('5 seconds')).pipe(
  Stream.mapEffect(() =>
    Effect.tryPromise(() =>
      fetch('/api/users').then(r => r.json())
    )
  ),
  Stream.take(10),
  Stream.rechunk(3)  // Batch 3 responses together
)

await Stream.runForEach(pollUsers, (batch) =>
  Effect.sync(() => console.log(`Batch:`, batch))
).pipe(Effect.runPromise)

Example 4: Stream-to-Atom (TMNL Testbed)

import { Atom } from '@effect-atom/atom-react'
import { Stream, Schedule, Effect } from 'effect'

// Create stream atom
const tickerAtom = Atom.make(
  Stream.fromSchedule(Schedule.spaced('1 second')).pipe(
    Stream.scan(0, (n) => n + 1),
    Stream.take(10)
  )
)

// React component
function Ticker() {
  const result = useAtomValue(tickerAtom)

  if (Result.isInitial(result)) return <div>Starting...</div>
  if (Result.isSuccess(result)) return <div>Count: {result.value}</div>
  return <div>Error: {result.error.message}</div>
}

Anti-Patterns

1. Not Consuming Streams

// WRONG — Stream is lazy, nothing happens
const stream = Stream.fromIterable([1, 2, 3]).pipe(
  Stream.map((n) => n * 2)
)

// CORRECT — Must consume
const result = await Stream.runCollect(stream).pipe(Effect.runPromise)

2. Ignoring Errors

// WRONG — Errors crash stream
const stream = Stream.fromIterable(urls).pipe(
  Stream.mapEffect((url) => Effect.tryPromise(() => fetch(url)))
)

// CORRECT — Handle errors
const stream = Stream.fromIterable(urls).pipe(
  Stream.mapEffect((url) =>
    Effect.tryPromise(() => fetch(url))
  ),
  Stream.catchAll((error) =>
    Stream.succeed({ error: error.message })
  )
)

3. Blocking Operations in map

// WRONG — Blocking sync operation
const stream = Stream.fromIterable(items).pipe(
  Stream.map((item) => {
    const result = await fetchData(item)  // ❌ Can't await in map
    return result
  })
)

// CORRECT — Use mapEffect
const stream = Stream.fromIterable(items).pipe(
  Stream.mapEffect((item) =>
    Effect.tryPromise(() => fetchData(item))
  )
)

4. Not Cleaning Up Resources

// WRONG — No cleanup
Stream.async((emit) => {
  const ws = new WebSocket('wss://example.com')
  ws.onmessage = (e) => emit.single(e.data)
  // No cleanup!
})

// CORRECT — Return cleanup Effect
Stream.async((emit) => {
  const ws = new WebSocket('wss://example.com')
  ws.onmessage = (e) => emit.single(e.data)
  return Effect.sync(() => ws.close())
})

Quick Reference

Stream.fromIterable

Stream.fromIterable([1, 2, 3])

Stream.async

Stream.async((emit) => { ... })

Stream.fromSchedule

Stream.fromSchedule(Schedule.spaced('1 second'))

Stream.fromEffect

Stream.fromEffect(fetchUser)

Stream.map

stream.pipe(Stream.map((n) => n * 2))

Stream.filter

stream.pipe(Stream.filter((n) => n > 0))

Stream.take

stream.pipe(Stream.take(10))

Stream.scan

stream.pipe(Stream.scan(0, (s, n) => s + n))

Stream.runCollect

Stream.runCollect(stream)

Stream.runForEach

Stream.runForEach(stream, (x) => Effect.log(x))

Stream.runFold

Stream.runFold(stream, 0, (s, n) => s + n)

Related Skills

• effect-atom-integration — Integrate streams with React atoms
• effect-service-authoring — Use streams in service methods
• effect-testing-patterns — Test stream-based code