<A, E, XE extends E, XA extends A>(
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?: boolean | undefined
}
): (self: FiberHandle<A, E>) => Effect.Effect<void>
<A, E, XE extends E, XA extends A>(
self: FiberHandle<A, E>,
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?: boolean | undefined
}
): Effect.Effect<void>Sets the fiber in the FiberHandle.
Details
When the fiber completes, it will be removed from the FiberHandle. If a
fiber already exists in the FiberHandle, it will be interrupted unless
options.onlyIfMissing is set.
Example (Setting a fiber safely)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
const handle = yield* FiberHandle.make()
const fiber = Effect.runFork(Effect.succeed("hello"))
// Set the fiber safely
yield* FiberHandle.set(handle, fiber)
// The fiber is now managed by the handle
const result = yield* Fiber.await(fiber)
console.log(result) // "hello"
})export const const set: {
<A, E, XE extends E, XA extends A>(
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
): (
self: FiberHandle<A, E>
) => Effect.Effect<void>
<A, E, XE extends E, XA extends A>(
self: FiberHandle<A, E>,
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
): Effect.Effect<void>
}
Sets the fiber in the FiberHandle.
Details
When the fiber completes, it will be removed from the FiberHandle. If a
fiber already exists in the FiberHandle, it will be interrupted unless
options.onlyIfMissing is set.
Example (Setting a fiber safely)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
const handle = yield* FiberHandle.make()
const fiber = Effect.runFork(Effect.succeed("hello"))
// Set the fiber safely
yield* FiberHandle.set(handle, fiber)
// The fiber is now managed by the handle
const result = yield* Fiber.await(fiber)
console.log(result) // "hello"
})
set: {
<function (type parameter) A in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) XA in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) A in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
fiber: Fiber.Fiber<XA, XE>(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; <…;
}
import FiberFiber.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<function (type parameter) XA in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
options: | {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
onlyIfMissing?: boolean | undefinedonlyIfMissing?: boolean
readonly propagateInterruption?: boolean | undefinedpropagateInterruption?: boolean | undefined
}
): (self: FiberHandle<A, E>(parameter) self: {
deferred: Deferred.Deferred<void, unknown>;
state: { readonly _tag: "Open"; fiber: Fiber.Fiber<A, E> | undefined } | { readonly _tag: "Closed" };
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; <…;
toString: () => string;
toJSON: () => unknown;
}
interface FiberHandle<out A = unknown, out E = unknown>Scoped handle that manages at most one fiber, interrupts the current fiber
when the handle's scope closes, and removes managed fibers from the handle
when they complete.
Example (Managing a single fiber)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
// Create a FiberHandle that can hold fibers producing strings
const handle = yield* FiberHandle.make<string, never>()
// The handle can store and manage a single fiber
const fiber = yield* FiberHandle.run(handle, Effect.succeed("hello"))
const result = yield* Fiber.await(fiber)
console.log(result) // "hello"
})
FiberHandle<function (type parameter) A in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): (self: FiberHandle<A, E>) => Effect.Effect<void>
import EffectEffect.interface Effect<out A, out E = never, out R = never>The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
Effect<void>
<function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XA in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
self: FiberHandle<A, E>(parameter) self: {
deferred: Deferred.Deferred<void, unknown>;
state: { readonly _tag: "Open"; fiber: Fiber.Fiber<A, E> | undefined } | { readonly _tag: "Closed" };
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; <…;
toString: () => string;
toJSON: () => unknown;
}
interface FiberHandle<out A = unknown, out E = unknown>Scoped handle that manages at most one fiber, interrupts the current fiber
when the handle's scope closes, and removes managed fibers from the handle
when they complete.
Example (Managing a single fiber)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
// Create a FiberHandle that can hold fibers producing strings
const handle = yield* FiberHandle.make<string, never>()
// The handle can store and manage a single fiber
const fiber = yield* FiberHandle.run(handle, Effect.succeed("hello"))
const result = yield* Fiber.await(fiber)
console.log(result) // "hello"
})
FiberHandle<function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
fiber: Fiber.Fiber<XA, XE>(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; <…;
}
import FiberFiber.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<function (type parameter) XA in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
options: | {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
onlyIfMissing?: boolean | undefinedonlyIfMissing?: boolean
readonly propagateInterruption?: boolean | undefinedpropagateInterruption?: boolean | undefined
}
): import EffectEffect.interface Effect<out A, out E = never, out R = never>The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
Effect<void>
} = dual<(...args: Array<any>) => any, <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}) => Effect.Effect<void>>(isDataFirst: (args: IArguments) => boolean, body: <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}) => Effect.Effect<void>): ((...args: Array<any>) => any) & (<A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}) => Effect.Effect<void>) (+1 overload)
Creates a function that can be called in data-first style or data-last
(pipe-friendly) style.
When to use
Use to expose one implementation through both direct and pipe-friendly
call styles.
Details
Pass either the arity of the uncurried function or a predicate that decides
whether the current call is data-first. Arity is the common case. Use a
predicate when optional arguments make arity ambiguous.
Example (Selecting data-first or data-last style by arity)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(2, (self, that) => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Defining overloads with call signatures)
import { Function, pipe } from "effect"
const sum: {
(that: number): (self: number) => number
(self: number, that: number): number
} = Function.dual(2, (self: number, that: number): number => self + that)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
Example (Selecting data-first or data-last style with a predicate)
import { Function, pipe } from "effect"
const sum = Function.dual<
(that: number) => (self: number) => number,
(self: number, that: number) => number
>(
(args) => args.length === 2,
(self, that) => self + that
)
console.log(sum(2, 3)) // 5
console.log(pipe(2, sum(3))) // 5
dual((args: IArgumentsargs) => const isFiberHandle: (
u: unknown
) => u is FiberHandle
Returns true if a value is a FiberHandle by checking for the
FiberHandle runtime marker.
Example (Checking fiber handles)
import { Effect, FiberHandle } from "effect"
Effect.gen(function*() {
const handle = yield* FiberHandle.make()
console.log(FiberHandle.isFiberHandle(handle)) // true
console.log(FiberHandle.isFiberHandle("not a handle")) // false
})
isFiberHandle(args: IArgumentsargs[0]), <function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XA in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
self: FiberHandle<A, E>(parameter) self: {
deferred: Deferred.Deferred<void, unknown>;
state: { readonly _tag: "Open"; fiber: Fiber.Fiber<A, E> | undefined } | { readonly _tag: "Closed" };
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; <…;
toString: () => string;
toJSON: () => unknown;
}
interface FiberHandle<out A = unknown, out E = unknown>Scoped handle that manages at most one fiber, interrupts the current fiber
when the handle's scope closes, and removes managed fibers from the handle
when they complete.
Example (Managing a single fiber)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
// Create a FiberHandle that can hold fibers producing strings
const handle = yield* FiberHandle.make<string, never>()
// The handle can store and manage a single fiber
const fiber = yield* FiberHandle.run(handle, Effect.succeed("hello"))
const result = yield* Fiber.await(fiber)
console.log(result) // "hello"
})
FiberHandle<function (type parameter) A in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) E in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
fiber: Fiber.Fiber<XA, XE>(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; <…;
}
import FiberFiber.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<function (type parameter) XA in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
function (type parameter) XE in <A, E, XE extends E, XA extends A>(self: FiberHandle<A, E>, fiber: Fiber.Fiber<XA, XE>, options?: {
readonly onlyIfMissing?: boolean;
readonly propagateInterruption?: boolean | undefined;
}): Effect.Effect<void>
options: | {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
onlyIfMissing?: boolean | undefinedonlyIfMissing?: boolean
readonly propagateInterruption?: boolean | undefinedpropagateInterruption?: boolean | undefined
}
): import EffectEffect.interface Effect<out A, out E = never, out R = never>The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
Effect<void> =>
import EffectEffect.const sync: <A>(
thunk: LazyArg<A>
) => Effect<A>
Creates an Effect that represents a synchronous side-effectful computation.
When to use
Use when you need to wrap a synchronous side-effectful operation that is not
expected to throw.
Details
The provided function is evaluated lazily when the effect runs.
Gotchas
The function must not throw. If it throws, the thrown value is treated as a
defect, not as a typed failure. Use try when throwing is expected.
Example (Capturing synchronous logging in an Effect)
import { Effect } from "effect"
const log = (message: string) =>
Effect.sync(() => {
console.log(message) // side effect
})
// ┌─── Effect<void, never, never>
// ▼
const program = log("Hello, World!")
sync(() =>
const setUnsafe: {
<A, E, XE extends E, XA extends A>(
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean | undefined
readonly propagateInterruption?:
| boolean
| undefined
}
): (self: FiberHandle<A, E>) => void
<A, E, XE extends E, XA extends A>(
self: FiberHandle<A, E>,
fiber: Fiber.Fiber<XA, XE>,
options?: {
readonly onlyIfMissing?: boolean | undefined
readonly propagateInterruption?:
| boolean
| undefined
}
): void
}
self: FiberHandle<A, E>(parameter) self: {
deferred: Deferred.Deferred<void, unknown>;
state: { readonly _tag: "Open"; fiber: Fiber.Fiber<A, E> | undefined } | { readonly _tag: "Closed" };
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; <…;
toString: () => string;
toJSON: () => unknown;
}
fiber: Fiber.Fiber<XA, XE>(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; <…;
}
onlyIfMissing?: boolean | undefinedonlyIfMissing: options: | {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
onlyIfMissing?: boolean | undefinedonlyIfMissing,
propagateInterruption?: boolean | undefinedpropagateInterruption: options: | {
readonly onlyIfMissing?: boolean
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
propagateInterruption?: boolean | undefinedpropagateInterruption
})
))