Concurrency & Fibers

When to use

• Parallelizing independent work safely with limits

• Coordinating background tasks and lifecycle

• Racing operations for latency control

Parallel Patterns

const results = yield* Effect.all(tasks, { concurrency: 10 })

const processed = yield* Effect.forEach(items, processItem, { concurrency: 5 })

Fiber Lifecycle

const fiber = yield* Effect.fork(work) const value = yield* Fiber.join(fiber) yield* Fiber.interrupt(fiber)

Racing / Timeouts

const fastest = yield* Effect.race(slow, fast) const withTimeout = yield* Effect.timeout(operation, "5 seconds")

Guidance

• Limit concurrency to protect resources

• Use fork for background loops; always manage interruption

• Prefer Effect.all for independent operations

• Use Effect.forEach with concurrency for pools

• Combine with retries and timeouts for resilient parallelism

Pitfalls

• Unbounded concurrency can exhaust CPU/IO or hit rate limits

• Always interrupt background fibers on shutdown

• Don’t block inside fibers; keep work asynchronous/effectful

Cross-links

• Errors & Retries: backoff + jitter for transient failures

• Streams & Pipelines: concurrent map over streams

• 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

• Effect: docs/effect-source/effect/src/Effect.ts

• Fiber: docs/effect-source/effect/src/Fiber.ts

• Duration: docs/effect-source/effect/src/Duration.ts

Example Searches

Find Effect.all and concurrency patterns

grep -F "Effect.all" docs/effect-source/effect/src/Effect.ts

Find forEach with concurrency

grep -rF "forEach" docs/effect-source/effect/src/ | grep -F "concurrency"

Study Fiber lifecycle operations

grep -F "export" docs/effect-source/effect/src/Fiber.ts | grep -E "fork|join|interrupt"

Find race and timeout implementations

grep -F "race" docs/effect-source/effect/src/Effect.ts grep -F "timeout" docs/effect-source/effect/src/Effect.ts

Workflow

• Identify the concurrency API you need (e.g., Effect.all, fork)

• Search docs/effect-source/effect/src/Effect.ts for the implementation

• Study the types and concurrency options

• Look at test files for usage examples

• 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