<R, E = unknown, A = unknown>(): Effect.Effect<
<XE extends E, XA extends A>(
effect: Effect.Effect<XA, XE, R>,
options?:
| {
readonly signal?: AbortSignal | undefined
readonly scheduler?: Scheduler | undefined
readonly onlyIfMissing?: boolean | undefined
readonly propagateInterruption?: boolean | undefined
}
| undefined
) => Fiber.Fiber<XA, XE>,
never,
Scope.Scope | R
>Creates a scoped run function that forks effects into a new FiberHandle.
Details
Each call returns the forked fiber, stores it in the handle, and interrupts
the previous fiber unless onlyIfMissing is set. The managed fiber is
interrupted when the handle's scope closes.
Example (Running effects with a fiber handle)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
const run = yield* FiberHandle.makeRuntime<never>()
// Run effects and get fibers back
const fiberA = run(Effect.succeed("first"))
const fiberB = run(Effect.succeed("second"))
// The second fiber will interrupt the first
const resultA = yield* Fiber.await(fiberA)
const resultB = yield* Fiber.await(fiberB)
}).pipe(Effect.scoped)export const const makeRuntime: <
R,
E = unknown,
A = unknown
>() => Effect.Effect<
<XE extends E, XA extends A>(
effect: Effect.Effect<XA, XE, R>,
options?:
| {
readonly signal?:
| AbortSignal
| undefined
readonly scheduler?:
| Scheduler
| undefined
readonly onlyIfMissing?:
| boolean
| undefined
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
) => Fiber.Fiber<XA, XE>,
never,
Scope.Scope | R
>
Creates a scoped run function that forks effects into a new FiberHandle.
Details
Each call returns the forked fiber, stores it in the handle, and interrupts
the previous fiber unless onlyIfMissing is set. The managed fiber is
interrupted when the handle's scope closes.
Example (Running effects with a fiber handle)
import { Effect, Fiber, FiberHandle } from "effect"
Effect.gen(function*() {
const run = yield* FiberHandle.makeRuntime<never>()
// Run effects and get fibers back
const fiberA = run(Effect.succeed("first"))
const fiberB = run(Effect.succeed("second"))
// The second fiber will interrupt the first
const resultA = yield* Fiber.await(fiberA)
const resultB = yield* Fiber.await(fiberB)
}).pipe(Effect.scoped)
makeRuntime = <function (type parameter) R in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
function (type parameter) E in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
function (type parameter) A in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
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<
<function (type parameter) XE in <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
function (type parameter) E in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
function (type parameter) XA in <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
function (type parameter) A in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
effect: Effect.Effect<XA, XE, R>(parameter) effect: {
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;
}
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<function (type parameter) XA in <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
function (type parameter) XE in <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
function (type parameter) R in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
options: | {
readonly signal?: AbortSignal | undefined
readonly scheduler?: Scheduler | undefined
readonly onlyIfMissing?: boolean | undefined
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
signal?: AbortSignal | undefinedsignal?: AbortSignal | undefined
readonly scheduler?: Scheduler | undefinedscheduler?: Scheduler | undefined
readonly onlyIfMissing?: boolean | undefinedonlyIfMissing?: boolean | undefined
readonly propagateInterruption?: boolean | undefinedpropagateInterruption?: boolean | undefined
}
| undefined
) => 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 <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
function (type parameter) XE in <XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined): Fiber.Fiber<XA, XE>
import ScopeScope.Scope | function (type parameter) R in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
import EffectEffect.const flatMap: {
<A, B, E1, R1>(
f: (a: A) => Effect<B, E1, R1>
): <E, R>(
self: Effect<A, E, R>
) => Effect<B, E1 | E, R1 | R>
<A, E, R, B, E1, R1>(
self: Effect<A, E, R>,
f: (a: A) => Effect<B, E1, R1>
): Effect<B, E | E1, R | R1>
}
const make: <
A = unknown,
E = unknown
>() => Effect.Effect<
FiberHandle<A, E>,
never,
Scope.Scope
>
Creates a scoped FiberHandle that can store a single fiber.
Details
When the associated Scope is closed, the contained fiber will be
interrupted. You can add a fiber to the handle using FiberHandle.run, and
the fiber will be automatically removed from the FiberHandle when it
completes.
Example (Creating a scoped fiber handle)
import { Effect, FiberHandle } from "effect"
Effect.gen(function*() {
const handle = yield* FiberHandle.make()
// run some effects
yield* FiberHandle.run(handle, Effect.never)
// this will interrupt the previous fiber
yield* FiberHandle.run(handle, Effect.never)
yield* Effect.sleep(1000)
}).pipe(
Effect.scoped // The fiber will be interrupted when the scope is closed
)
make<function (type parameter) A in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
function (type parameter) E in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>
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;
}
const runtime: <A, E>(
self: FiberHandle<A, E>
) => <R = never>() => Effect.Effect<
<XE extends E, XA extends A>(
effect: Effect.Effect<XA, XE, R>,
options?:
| {
readonly signal?:
| AbortSignal
| undefined
readonly scheduler?:
| Scheduler
| undefined
readonly onlyIfMissing?:
| boolean
| undefined
readonly propagateInterruption?:
| boolean
| undefined
}
| undefined
) => Fiber.Fiber<XA, XE>,
never,
R
>
Captures the current runtime and returns a function for forking effects into
an existing FiberHandle.
Details
Each call returns the forked fiber, stores it in the handle, and interrupts
the previous fiber unless onlyIfMissing is set.
Example (Capturing a runtime for fiber handles)
import { Context, Effect, FiberHandle } from "effect"
interface Users {
readonly _: unique symbol
}
const Users = Context.Service<Users, {
getAll: Effect.Effect<Array<unknown>>
}>("Users")
Effect.gen(function*() {
const handle = yield* FiberHandle.make()
const run = yield* FiberHandle.runtime(handle)<Users>()
// run an effect and set the fiber in the handle
run(Effect.andThen(Users, (_) => _.getAll))
// this will interrupt the previous fiber
run(Effect.andThen(Users, (_) => _.getAll))
}).pipe(
Effect.scoped // The fiber will be interrupted when the scope is closed
)
runtime(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;
}
function (type parameter) R in <R, E = unknown, A = unknown>(): Effect.Effect<(<XE extends E, XA extends A>(effect: Effect.Effect<XA, XE, R>, options?: {
readonly signal?: AbortSignal | undefined;
readonly scheduler?: Scheduler | undefined;
readonly onlyIfMissing?: boolean | undefined;
readonly propagateInterruption?: boolean | undefined;
} | undefined) => Fiber.Fiber<XA, XE>), never, Scope.Scope | R>