Streams & Pipelines

When to use

• You’re building data pipelines with batching/backpressure
• You need controlled concurrency per element
• You must process large inputs with constant memory

Create

const s = Stream.fromIterable(items)

Transform

const out = s.pipe(
  Stream.mapEffect(processItem, { concurrency: 4 }),
  Stream.filter((a) => a.valid),
  Stream.grouped(100)
)

Consume

yield* Stream.runDrain(out)
// or
const all = yield* Stream.runCollect(out)

Resource-Safe

const fileLines = Stream.acquireRelease(open(), close).pipe(
  Stream.flatMap(readLines)
)

Resilience

const resilient = s.pipe(
  Stream.mapEffect((x) => op(x).pipe(Effect.retry(retry)))
)

Real-world snippet: Stream to S3 with progress and scoped background ticker

let downloadedBytes = 0

yield* Effect.gen(function* () {
  // background progress ticker
  yield* Effect.repeat(
    Effect.gen(function* () {
      const bytes = yield* Effect.succeed(downloadedBytes)
      yield* Effect.log(`Downloaded ${bytes}/${contentLength} bytes`)
    }),
    Schedule.forever.pipe(Schedule.delayed(() => "2 seconds"))
  ).pipe(Effect.delay("100 millis"), Effect.forkScoped)

  yield* s3.putObject(key,
    resp.stream.pipe(
      Stream.tap((chunk) => { downloadedBytes += chunk.length; return Effect.void })
    ),
    { contentLength }
  )
}).pipe(Effect.scoped)

Guidance

• Prefer

  Stream.mapEffect

  with

  concurrency

  to control parallel work
• Use

  grouped(n)

  for batching network/DB operations
• Always model resource acquisition with

  acquireRelease

Pitfalls

• Collecting massive streams into memory → prefer

  runDrain

  or chunked writes
• Doing blocking IO in transformations → keep operations effectful and non-blocking

Cross-links

• Concurrency: pools and timeouts for per-item work
• Resources: scope/finalizers for pipeline resources
• EffectPatterns inspiration: https://github.com/PaulJPhilp/EffectPatterns

Local Source Reference

CRITICAL: Search local Effect source before implementing

The full Effect source code is available at

docs/effect-source/

. Always search the actual implementation before writing Effect code.

Key Source Files

• Stream:

  docs/effect-source/effect/src/Stream.ts

• Sink:

  docs/effect-source/effect/src/Sink.ts

• Channel:

  docs/effect-source/effect/src/Channel.ts

Example Searches

# Find Stream creation patterns
grep -F "fromIterable" docs/effect-source/effect/src/Stream.ts
grep -F "make" docs/effect-source/effect/src/Stream.ts
grep -F "fromEffect" docs/effect-source/effect/src/Stream.ts

# Study Stream transformations
grep -F "mapEffect" docs/effect-source/effect/src/Stream.ts
grep -F "filter" docs/effect-source/effect/src/Stream.ts
grep -F "grouped" docs/effect-source/effect/src/Stream.ts

# Find Stream consumption
grep -F "runDrain" docs/effect-source/effect/src/Stream.ts
grep -F "runCollect" docs/effect-source/effect/src/Stream.ts

# Look at Stream test examples
grep -F "Stream." docs/effect-source/effect/test/Stream.test.ts

Workflow

1. Identify the Stream API you need (e.g., mapEffect, grouped)
2. Search

   docs/effect-source/effect/src/Stream.ts

   for the implementation
3. Study the types and pipeline patterns
4. Look at test files for usage examples
5. Write your code based on real implementations

Real source code > documentation > assumptions

References

• Agent Skills overview: https://www.anthropic.com/news/skills
• Skills guide: https://docs.claude.com/en/docs/claude-code/skills