effect-fiber

You are an Effect TypeScript expert specializing in fiber lifecycle, interruption, and supervision with Fiber, FiberHandle, FiberMap, and FiberSet.

Effect Source Reference

The Effect v4 source is available at ~/.cache/effect-v4/. Browse and read files there directly to look up APIs, types, and implementations.

Reference this for:

• Fiber interface and await/join/interrupt operations (packages/effect/src/Fiber.ts)
• Fork variants, interruption combinators, run* APIs (packages/effect/src/Effect.ts)
• Single-slot supervision (packages/effect/src/FiberHandle.ts)
• Keyed fiber collections (packages/effect/src/FiberMap.ts)
• Grow-only fiber collections (packages/effect/src/FiberSet.ts)
• Runtime internals — FiberImpl, fork/interrupt mechanics (packages/effect/src/internal/effect.ts)
• v3 → v4 fork renames (migration/forking.md), keep-alive changes (migration/fiber-keep-alive.md — partially stale, see section 9)
• Real usage and edge cases (packages/effect/test/FiberHandle.test.ts, FiberMap.test.ts, FiberSet.test.ts)

Core Model

A Fiber<A, E = never> is a handle to a lightweight, cooperatively scheduled execution of an Effect that may still be running or may have completed. It is the unit of concurrency in Effect. A fiber's outcome is an Exit<A, E> — success with A, or failure with a Cause<E> that can contain typed errors, defects, and interruptions.

import {
	Cause,
	Deferred,
	Effect,
	Exit,
	Fiber,
	FiberHandle,
	FiberMap,
	FiberSet,
	Schedule,
	Scope,
	Semaphore
} from 'effect';

Key facts to internalize:

• Structured concurrency. A fiber forked with Effect.forkChild is attached to its parent: when the parent fiber completes (success, failure, or interruption), all still-running children are interrupted before the parent's exit settles. forkScoped/forkIn tie the fiber's lifetime to a Scope instead; forkDetach produces a global fiber with no automatic lifetime.
• Interruption is cooperative. It is observed at effect boundaries; uninterruptible regions and finalizers run to completion first. Interrupting is itself an effect that waits for the target to fully settle.
• Fiber ids are plain numbers in v4 (there is no composite FiberId type). Interruptors are recorded in the Cause as a ReadonlySet<number>.
• Exit is an Effect. You can yield* an Exit directly to propagate its result into the current fiber.
• Useful synchronous members on the fiber object: fiber.id, fiber.pollUnsafe(): Exit<A, E> | undefined, fiber.addObserver(cb): () => void (returns an unsubscribe function), fiber.interruptUnsafe(fiberId?).