AsyncFiberErrorAn error that occurs when trying to run an async fiber with Effect.runSync.
When to use
Use to inspect failures produced when synchronous runners encounter an effect that cannot complete synchronously.
Details
The fiber property stores the fiber that could not be synchronously
resolved. This error implements YieldableError.
Example (Accessing the fiber)
import { Cause } from "effect"
import type { Fiber } from "effect"
declare const fiber: Fiber.Fiber<unknown, unknown>
const error = new Cause.AsyncFiberError(fiber)
console.log(error._tag) // "AsyncFiberError"
console.log(error.fiber === fiber) // trueexport interface AsyncFiberError extends YieldableError {
readonly [const AsyncFiberErrorTypeId: "~effect/Cause/AsyncFiberError"Unique brand present on AsyncFiberError values and used by
isAsyncFiberError for runtime checks.
AsyncFiberErrorTypeId]: typeof const AsyncFiberErrorTypeId: "~effect/Cause/AsyncFiberError"Unique brand present on AsyncFiberError values and used by
isAsyncFiberError for runtime checks.
AsyncFiberErrorTypeId
readonly AsyncFiberError._tag: "AsyncFiberError"_tag: "AsyncFiberError"
readonly AsyncFiberError.fiber: Fiber<unknown, unknown>(property) AsyncFiberError.fiber: {
id: number;
currentOpCount: number;
getRef: <A>(ref: Context.Reference<A>) => A;
context: Context.Context<never>;
setContext: (context: Context.Context<never>) => void;
currentScheduler: Scheduler;
currentDispatcher: SchedulerDispatcher;
currentSpan: AnySpan | undefined;
currentLogLevel: LogLevel;
minimumLogLevel: LogLevel;
currentStackFrame: StackFrame | undefined;
maxOpsBeforeYield: number;
currentPreventYield: boolean;
addObserver: (cb: (exit: Exit<A, E>) => void) => () => void;
interruptUnsafe: (fiberId?: number | undefined, annotations?: Context.Context<never> | undefined) => void;
pollUnsafe: () => Exit<A, E> | undefined;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
interface Fiber<out A, out E = never>A runtime fiber is a lightweight thread that executes Effects. Fibers are
the unit of concurrency in Effect. They provide a way to run multiple
Effects concurrently while maintaining structured concurrency and
cancellation safety.
When to use
Use to observe, join, interrupt, or coordinate work that has already been
forked.
Details
A fiber exposes both safe Effect-based operations, such as
await
,
join
, and
interrupt
, and low-level runtime fields used by
the scheduler and runtime internals.
Gotchas
Prefer the exported functions in this module over calling interruptUnsafe
or pollUnsafe directly. The unsafe methods are immediate runtime hooks and
do not provide the same Effect-based sequencing guarantees.
Example (Awaiting a forked fiber)
import { Effect, Fiber } from "effect"
const program = Effect.gen(function*() {
// Fork an effect to run in a new fiber
const fiber = yield* Effect.forkChild(Effect.succeed(42))
// Wait for the fiber to complete and get its result
const result = yield* Fiber.await(fiber)
console.log(result) // Exit.succeed(42)
return result
})
The Fiber namespace contains utility types and functions for working with fibers.
It provides type-level utilities for fiber operations and variance encoding.
When to use
Use to reference type-level helpers associated with Fiber.
Details
The namespace currently exposes type-level support used by the Fiber
interface. Runtime operations are exported as module-level functions.
Example (Working with fiber types)
import { Effect, Fiber } from "effect"
const program = Effect.gen(function*() {
// Create a fiber
const fiber = yield* Effect.forkChild(Effect.succeed(42))
// Use namespace types for variance
const typedFiber: Fiber.Fiber<number, never> = fiber
// Access fiber properties
console.log(`Fiber ID: ${fiber.id}`)
// Join the fiber
const result = yield* Fiber.join(fiber)
return result // 42
})
Fiber<unknown, unknown>
}
/**
* Constructs an `AsyncFiberError` for a fiber that could not be resolved
* synchronously.
*
* **When to use**
*
* Use to create the error value for a fiber that could not be completed by a
* synchronous runner.
*
* **Example** (Creating an AsyncFiberError)
*
* ```ts
* import { Cause } from "effect"
* import type { Fiber } from "effect"
*
* declare const fiber: Fiber.Fiber<unknown, unknown>
*
* const error = new Cause.AsyncFiberError(fiber)
* console.log(error.message) // "An asynchronous Effect was executed with Effect.runSync"
* ```
*
* @see {@link isAsyncFiberError} for checking unknown values
*
* @category constructors
* @since 4.0.0
*/
export const const AsyncFiberError: new (
fiber: Fiber<unknown, unknown>
) => AsyncFiberError
An error that occurs when trying to run an async fiber with Effect.runSync.
When to use
Use to inspect failures produced when synchronous runners encounter an effect
that cannot complete synchronously.
Details
The fiber property stores the fiber that could not be synchronously
resolved. This error implements YieldableError.
Example (Accessing the fiber)
import { Cause } from "effect"
import type { Fiber } from "effect"
declare const fiber: Fiber.Fiber<unknown, unknown>
const error = new Cause.AsyncFiberError(fiber)
console.log(error._tag) // "AsyncFiberError"
console.log(error.fiber === fiber) // true
Constructs an AsyncFiberError for a fiber that could not be resolved
synchronously.
When to use
Use to create the error value for a fiber that could not be completed by a
synchronous runner.
Example (Creating an AsyncFiberError)
import { Cause } from "effect"
import type { Fiber } from "effect"
declare const fiber: Fiber.Fiber<unknown, unknown>
const error = new Cause.AsyncFiberError(fiber)
console.log(error.message) // "An asynchronous Effect was executed with Effect.runSync"
AsyncFiberError: new(fiber: Fiber<unknown, unknown>(parameter) fiber: {
id: number;
currentOpCount: number;
getRef: <A>(ref: Context.Reference<A>) => A;
context: Context.Context<never>;
setContext: (context: Context.Context<never>) => void;
currentScheduler: Scheduler;
currentDispatcher: SchedulerDispatcher;
currentSpan: AnySpan | undefined;
currentLogLevel: LogLevel;
minimumLogLevel: LogLevel;
currentStackFrame: StackFrame | undefined;
maxOpsBeforeYield: number;
currentPreventYield: boolean;
addObserver: (cb: (exit: Exit<A, E>) => void) => () => void;
interruptUnsafe: (fiberId?: number | undefined, annotations?: Context.Context<never> | undefined) => void;
pollUnsafe: () => Exit<A, E> | undefined;
pipe: { <A>(this: A): A; <A, B = never>(this: A, ab: (_: A) => B): B; <A, B = never, C = never>(this: A, ab: (_: A) => B, bc: (_: B) => C): C; <A, B = never, C = never, D = never>(this: A, ab: (_: A) => B, bc: (_: B) => C, cd: (_: C) => D): D; <…;
}
interface Fiber<out A, out E = never>A runtime fiber is a lightweight thread that executes Effects. Fibers are
the unit of concurrency in Effect. They provide a way to run multiple
Effects concurrently while maintaining structured concurrency and
cancellation safety.
When to use
Use to observe, join, interrupt, or coordinate work that has already been
forked.
Details
A fiber exposes both safe Effect-based operations, such as
await
,
join
, and
interrupt
, and low-level runtime fields used by
the scheduler and runtime internals.
Gotchas
Prefer the exported functions in this module over calling interruptUnsafe
or pollUnsafe directly. The unsafe methods are immediate runtime hooks and
do not provide the same Effect-based sequencing guarantees.
Example (Awaiting a forked fiber)
import { Effect, Fiber } from "effect"
const program = Effect.gen(function*() {
// Fork an effect to run in a new fiber
const fiber = yield* Effect.forkChild(Effect.succeed(42))
// Wait for the fiber to complete and get its result
const result = yield* Fiber.await(fiber)
console.log(result) // Exit.succeed(42)
return result
})
The Fiber namespace contains utility types and functions for working with fibers.
It provides type-level utilities for fiber operations and variance encoding.
When to use
Use to reference type-level helpers associated with Fiber.
Details
The namespace currently exposes type-level support used by the Fiber
interface. Runtime operations are exported as module-level functions.
Example (Working with fiber types)
import { Effect, Fiber } from "effect"
const program = Effect.gen(function*() {
// Create a fiber
const fiber = yield* Effect.forkChild(Effect.succeed(42))
// Use namespace types for variance
const typedFiber: Fiber.Fiber<number, never> = fiber
// Access fiber properties
console.log(`Fiber ID: ${fiber.id}`)
// Join the fiber
const result = yield* Fiber.join(fiber)
return result // 42
})
Fiber<unknown, unknown>) => AsyncFiberError = import effecteffect.class AsyncFiberErrorAsyncFiberError