(duration: Duration.Input): <A, E, R>(
self: Stream<A, E, R>
) => Stream<A, E, R>
<A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<
A,
E,
R
>Drops earlier elements within the debounce window and emits only the latest element after the pause.
Example (Debouncing stream elements)
import { Console, Duration, Effect, Stream } from "effect"
const stream = Stream.make(1, 2, 3).pipe(
Stream.concat(Stream.fromEffect(Effect.sleep(Duration.millis(50)).pipe(Effect.as(4)))),
Stream.concat(Stream.make(5)),
Stream.debounce(Duration.millis(30))
)
const program = Effect.gen(function*() {
const values = yield* Stream.runCollect(stream)
yield* Console.log(values)
// Output: [ 3, 5 ]
})export const const debounce: {
(duration: Duration.Input): <A, E, R>(
self: Stream<A, E, R>
) => Stream<A, E, R>
<A, E, R>(
self: Stream<A, E, R>,
duration: Duration.Input
): Stream<A, E, R>
}
Drops earlier elements within the debounce window and emits only the latest element after the pause.
Example (Debouncing stream elements)
import { Console, Duration, Effect, Stream } from "effect"
const stream = Stream.make(1, 2, 3).pipe(
Stream.concat(Stream.fromEffect(Effect.sleep(Duration.millis(50)).pipe(Effect.as(4)))),
Stream.concat(Stream.make(5)),
Stream.debounce(Duration.millis(30))
)
const program = Effect.gen(function*() {
const values = yield* Stream.runCollect(stream)
yield* Console.log(values)
// Output: [ 3, 5 ]
})
debounce: {
(duration: Duration.Inputduration: import DurationDuration.type Input =
| number
| bigint
| Duration.Duration
| readonly [seconds: number, nanos: number]
| `${number} nano`
| `${number} nanos`
| `${number} micro`
| `${number} micros`
| `${number} milli`
| `${number} millis`
| `${number} second`
| `${number} seconds`
| `${number} minute`
| `${number} minutes`
| `${number} hour`
| `${number} hours`
| `${number} day`
| `${number} days`
| `${number} week`
| `${number} weeks`
| "Infinity"
| "-Infinity"
| Duration.DurationObject
Valid input types that can be converted to a Duration.
When to use
Use when an API should accept any value that Effect can convert into a
Duration, including existing durations, millisecond numbers, nanosecond
bigints, high-resolution tuples, duration strings, infinity strings, or
duration objects.
Details
String inputs accept values like "10 seconds", "500 millis",
"Infinity", and "-Infinity". Finite fractional values that are
normalized to nanoseconds are rounded to the nearest nanosecond, with ties
away from zero.
Input): <function (type parameter) A in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>R>(self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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 Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>R>) => interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>): Stream<A, E, R>R>
<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R>(self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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 Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R>, duration: Duration.Inputduration: import DurationDuration.type Input =
| number
| bigint
| Duration.Duration
| readonly [seconds: number, nanos: number]
| `${number} nano`
| `${number} nanos`
| `${number} micro`
| `${number} micros`
| `${number} milli`
| `${number} millis`
| `${number} second`
| `${number} seconds`
| `${number} minute`
| `${number} minutes`
| `${number} hour`
| `${number} hours`
| `${number} day`
| `${number} days`
| `${number} week`
| `${number} weeks`
| "Infinity"
| "-Infinity"
| Duration.DurationObject
Valid input types that can be converted to a Duration.
When to use
Use when an API should accept any value that Effect can convert into a
Duration, including existing durations, millisecond numbers, nanosecond
bigints, high-resolution tuples, duration strings, infinity strings, or
duration objects.
Details
String inputs accept values like "10 seconds", "500 millis",
"Infinity", and "-Infinity". Finite fractional values that are
normalized to nanoseconds are rounded to the nearest nanosecond, with ties
away from zero.
Input): interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R>
} = dual<(...args: Array<any>) => any, <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input) => Stream<A, E, R>>(arity: 2, body: <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input) => Stream<A, E, R>): ((...args: Array<any>) => any) & (<A, E, R>(self: Stream<A, E, R>, duration: Duration.Input) => Stream<A, E, R>) (+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(
2,
<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R>(self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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 Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R>, duration: Duration.Inputduration: import DurationDuration.type Input =
| number
| bigint
| Duration.Duration
| readonly [seconds: number, nanos: number]
| `${number} nano`
| `${number} nanos`
| `${number} micro`
| `${number} micros`
| `${number} milli`
| `${number} millis`
| `${number} second`
| `${number} seconds`
| `${number} minute`
| `${number} minutes`
| `${number} hour`
| `${number} hours`
| `${number} day`
| `${number} days`
| `${number} week`
| `${number} weeks`
| "Infinity"
| "-Infinity"
| Duration.DurationObject
Valid input types that can be converted to a Duration.
When to use
Use when an API should accept any value that Effect can convert into a
Duration, including existing durations, millisecond numbers, nanosecond
bigints, high-resolution tuples, duration strings, infinity strings, or
duration objects.
Details
String inputs accept values like "10 seconds", "500 millis",
"Infinity", and "-Infinity". Finite fractional values that are
normalized to nanoseconds are rounded to the nearest nanosecond, with ties
away from zero.
Input): interface Stream<out A, out E = never, out R = never>A Stream<A, E, R> describes a program that can emit many A values, fail
with E, and require R.
Details
Streams are pull-based with backpressure and emit chunks to amortize effect
evaluation. They support monadic composition and error handling similar to
Effect, adapted for multiple values.
Example (Creating and consuming streams)
import { Console, Effect, Stream } from "effect"
const program = Effect.gen(function*() {
yield* Stream.make(1, 2, 3).pipe(
Stream.map((n) => n * 2),
Stream.runForEach((n) => Console.log(n))
)
})
Effect.runPromise(program)
// Output:
// 2
// 4
// 6
Stream<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R> =>
const transformPull: <
A,
E,
R,
B,
E2,
R2,
EX,
RX
>(
self: Stream<A, E, R>,
f: (
pull: Pull.Pull<
Arr.NonEmptyReadonlyArray<A>,
E,
void
>,
scope: Scope.Scope
) => Effect.Effect<
Pull.Pull<
Arr.NonEmptyReadonlyArray<B>,
E2,
void,
R2
>,
EX,
RX
>
) => Stream<
B,
EX | Pull.ExcludeDone<E2>,
R | R2 | RX
>
Derives a stream by transforming its pull effect.
Example (Transforming a pull effect)
import { Console, Effect, Stream } from "effect"
const stream = Stream.make(1, 2, 3)
const transformed = Stream.transformPull(stream, (pull) => Effect.succeed(pull))
const program = Effect.gen(function*() {
const values = yield* Stream.runCollect(transformed)
yield* Console.log(values)
})
Effect.runPromise(program)
// Output: [ 1, 2, 3 ]
transformPull(
self: Stream<A, E, R>(parameter) self: {
channel: Channel.Channel<Arr.NonEmptyReadonlyArray<A>, E, void, unknown, unknown, unknown, R>;
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 EffectEffect.const fnUntraced: <Context.Reference<Clock> | Effect.Effect<Fiber.Fiber<never, Cause.Done<void> | E>, never, never>, Effect.Effect<readonly [A, ...A[]], Cause.Done<void> | E, R>, [pull: Pull.Pull<readonly [A, ...A[]], E, void, never>, scope: Scope.Scope]>(body: (this: unassigned, pull: Pull.Pull<readonly [A, ...A[]], E, void, never>, scope: Scope.Scope) => Generator<Context.Reference<Clock> | Effect.Effect<Fiber.Fiber<never, Cause.Done<void> | E>, never, never>, Effect.Effect<...>, never>) => (pull: Pull.Pull<...>, scope: Scope.Scope) => Effect.Effect<...> (+41 overloads)fnUntraced(function*(pull: Pull.Pull<
readonly [A, ...A[]],
E,
void,
never
>
(parameter) pull: {
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;
}
scope: Scope.Scope(parameter) scope: {
strategy: "sequential" | "parallel";
state: State.Open | State.Closed | State.Empty;
}
const clock: Clockconst clock: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
}
const Clock: Context.Reference<Clock>(alias) const Clock: {
key: string;
Service: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
};
defaultValue: () => Shape;
of: (this: void, self: Clock) => Clock;
context: (self: Clock) => Context.Context<never>;
use: (f: (service: Clock) => Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
useSync: (f: (service: Clock) => A) => Effect.Effect<A, never, never>;
Identifier: Identifier;
stack: string | 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; <…;
toString: () => string;
toJSON: () => unknown;
}
Represents a time-based clock which provides functionality related to time
and scheduling.
When to use
Use to define or provide a clock service for current-time and sleep
operations.
Example (Reading current time)
import { Clock, Effect } from "effect"
const clockOperations = Effect.gen(function*() {
const currentTime = yield* Clock.currentTimeMillis
const currentTimeNanos = yield* Clock.currentTimeNanos
console.log(`Current time (ms): ${currentTime}`)
console.log(`Current time (ns): ${currentTimeNanos}`)
})
Context reference for the active time service in the environment.
When to use
Use when you need to access or provide the full time service, including sleep
operations, rather than a single timestamp accessor.
Example (Accessing the Clock service)
import { Clock, Effect } from "effect"
const program = Effect.gen(function*() {
const clock = yield* Clock.Clock
return clock.currentTimeMillisUnsafe()
})
Clock
const const durationMs: numberdurationMs = import DurationDuration.const toMillis: (self: Input) => numberConverts a Duration to milliseconds.
Example (Converting durations to milliseconds)
import { Duration } from "effect"
console.log(Duration.toMillis(Duration.seconds(5))) // 5000
console.log(Duration.toMillis(Duration.minutes(2))) // 120000
toMillis(import DurationDuration.const fromInputUnsafe: (
input: Input
) => Duration
Decodes a Duration.Input into a Duration.
When to use
Use when the input has already been validated or comes from a trusted source
and throwing is acceptable for invalid duration syntax.
Gotchas
If the input is not a valid Duration.Input, it throws an error.
Example (Decoding duration inputs)
import { Duration } from "effect"
const duration1 = Duration.fromInputUnsafe(1000) // 1000 milliseconds
const duration2 = Duration.fromInputUnsafe("5 seconds")
const duration3 = Duration.fromInputUnsafe("Infinity")
const duration4 = Duration.fromInputUnsafe([2, 500_000_000]) // 2 seconds and 500ms
fromInputUnsafe(duration: Duration.Inputduration))
let let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
import ArrArr.type NonEmptyReadonlyArray<A> = readonly [
A,
...A[]
]
A readonly array guaranteed to have at least one element.
When to use
Use when non-emptiness must be tracked at the type level while preventing mutation.
Many Array module functions accept or return this type.
Example (Typing a non-empty array)
import type { Array } from "effect"
const nonEmpty: Array.NonEmptyReadonlyArray<number> = [1, 2, 3]
const head: number = nonEmpty[0] // guaranteed to exist
NonEmptyReadonlyArray<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A> | undefined
let let cause:
| Cause.Cause<Cause.Done | E>
| undefined
import CauseCause.interface Cause<out E>A structured representation of how an Effect failed.
When to use
Use to preserve the full structured failure information for an effect instead
of collapsing it to a single error value.
Details
Access the individual failure entries through the reasons array, then
narrow each entry with
isFailReason
,
isDieReason
, or
- Use
hasFails
/
hasDies
/
hasInterrupts
to test
for the presence of specific reason kinds without iterating.
- Use
findError
/
findDefect
to extract the first value
of a given kind.
- Use
combine
to merge two causes.
Cause implements Equal — two causes with the same reasons (by value)
compare as equal.
Example (Creating and inspecting a cause)
import { Cause } from "effect"
const cause = Cause.fail("Something went wrong")
console.log(cause.reasons.length) // 1
console.log(Cause.isFailReason(cause.reasons[0])) // true
Companion namespace for the Cause interface.
Cause<import CauseCause.interface Done<A = void>A graceful completion signal for queues and streams.
When to use
Use to model normal producer completion through a stream or queue error
channel.
Details
Done indicates that a producer has finished normally — no more elements
will arrive. It is distinct from an error or interruption; it represents
successful completion. The optional value field can carry a final
leftover payload.
Example (Signaling queue completion)
import { Cause, Effect, Queue } from "effect"
const program = Effect.gen(function*() {
const queue = yield* Queue.bounded<number, Cause.Done>(10)
yield* Queue.offer(queue, 1)
yield* Queue.end(queue)
const result = yield* Effect.flip(Queue.take(queue))
console.log(Cause.isDone(result)) // true
})
Companion namespace for the Done interface.
Creates a Done signal with an optional value.
When to use
Use when you need to construct a low-level pull completion signal directly.
Done | function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E> | undefined
let let emitAtMs: numberemitAtMs = var Infinity: numberInfinity
const const pullLatch: Latch.Latchconst pullLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
import LatchLatch.const makeUnsafe: (
open?: boolean | undefined
) => Latch
Creates a Latch synchronously, outside of Effect.
When to use
Use when you need to allocate a Latch synchronously outside an Effect
workflow.
Details
The latch starts closed by default; pass true to create it open.
Example (Creating a latch unsafely)
import { Effect, Latch } from "effect"
const latch = Latch.makeUnsafe(false)
const waiter = Effect.gen(function*() {
yield* Effect.log("Waiting for latch to open...")
yield* latch.await
yield* Effect.log("Latch opened! Continuing...")
})
const opener = Effect.gen(function*() {
yield* Effect.sleep("2 seconds")
yield* Effect.log("Opening latch...")
yield* latch.open
})
const program = Effect.all([waiter, opener])
makeUnsafe()
const const emitLatch: Latch.Latchconst emitLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
import LatchLatch.const makeUnsafe: (
open?: boolean | undefined
) => Latch
Creates a Latch synchronously, outside of Effect.
When to use
Use when you need to allocate a Latch synchronously outside an Effect
workflow.
Details
The latch starts closed by default; pass true to create it open.
Example (Creating a latch unsafely)
import { Effect, Latch } from "effect"
const latch = Latch.makeUnsafe(false)
const waiter = Effect.gen(function*() {
yield* Effect.log("Waiting for latch to open...")
yield* latch.await
yield* Effect.log("Latch opened! Continuing...")
})
const opener = Effect.gen(function*() {
yield* Effect.sleep("2 seconds")
yield* Effect.log("Opening latch...")
yield* latch.open
})
const program = Effect.all([waiter, opener])
makeUnsafe()
const const endLatch: Latch.Latchconst endLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
import LatchLatch.const makeUnsafe: (
open?: boolean | undefined
) => Latch
Creates a Latch synchronously, outside of Effect.
When to use
Use when you need to allocate a Latch synchronously outside an Effect
workflow.
Details
The latch starts closed by default; pass true to create it open.
Example (Creating a latch unsafely)
import { Effect, Latch } from "effect"
const latch = Latch.makeUnsafe(false)
const waiter = Effect.gen(function*() {
yield* Effect.log("Waiting for latch to open...")
yield* latch.await
yield* Effect.log("Latch opened! Continuing...")
})
const opener = Effect.gen(function*() {
yield* Effect.sleep("2 seconds")
yield* Effect.log("Opening latch...")
yield* latch.open
})
const program = Effect.all([waiter, opener])
makeUnsafe()
yield* pull: Pull.Pull<
readonly [A, ...A[]],
E,
void,
never
>
(parameter) pull: {
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;
}
Pipeable.pipe<Pull.Pull<readonly [A, ...A[]], E, void, never>, Effect.Effect<readonly [A, ...A[]], Cause.Done<void> | E, never>, Effect.Effect<void, Cause.Done<void> | E, never>, Effect.Effect<never, Cause.Done<void> | E, never>, Effect.Effect<never, Cause.Done<void> | E, never>, Effect.Effect<Fiber.Fiber<never, Cause.Done<void> | E>, never, never>>(this: Pull.Pull<...>, ab: (_: Pull.Pull<readonly [A, ...A[]], E, void, never>) => Effect.Effect<readonly [A, ...A[]], Cause.Done<void> | E, never>, bc: (_: Effect.Effect<...>) => Effect.Effect<...>, cd: (_: Effect.Effect<...>) => Effect.Effect<...>, de: (_: Effect.Effect<...>) => Effect.Effect<...>, ef: (_: Effect.Effect<...>) => Effect.Effect<...>): Effect.Effect<...> (+21 overloads)pipe(
const pullLatch: Latch.Latchconst pullLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.whenOpen<A, E, R>(self: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>Runs the given effect only after the latch allows waiting fibers to
continue.
When to use
Use to gate an effect behind the latch signal.
whenOpen,
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>
}
arr: readonly [A, ...A[]](parameter) arr: {
0: A;
length: number;
toString: () => string;
toLocaleString: { (): string; (locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string };
concat: { (...items: Array<ConcatArray<A>>): Array<A>; (...items: Array<A | ConcatArray<A>>): Array<A> };
join: (separator?: string) => string;
slice: (start?: number, end?: number) => Array<A>;
indexOf: (searchElement: A, fromIndex?: number) => number;
lastIndexOf: (searchElement: A, fromIndex?: number) => number;
every: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): this is readonly S[]; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): boolean };
some: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => boolean;
forEach: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => void, thisArg?: any) => void;
map: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => U, thisArg?: any) => Array<U>;
filter: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): Array<S>; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): Array<A> };
reduce: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
reduceRight: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
find: { (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findIndex: (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
entries: () => ArrayIterator<[number, A]>;
keys: () => ArrayIterator<number>;
values: () => ArrayIterator<A>;
includes: (searchElement: A, fromIndex?: number) => boolean;
flatMap: (callback: (this: This, value: A, index: number, array: Array<A>) => U | ReadonlyArray<U>, thisArg?: This | undefined) => Array<U>;
flat: (this: A, depth?: D | undefined) => Array<FlatArray<A, D>>;
at: (index: number) => A | undefined;
findLast: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findLastIndex: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
toReversed: () => Array<A>;
toSorted: (compareFn?: ((a: A, b: A) => number) | undefined) => Array<A>;
toSpliced: { (start: number, deleteCount: number, ...items: Array<A>): Array<A>; (start: number, deleteCount?: number): Array<A> };
with: (index: number, value: A) => Array<A>;
}
const emitLatch: Latch.Latchconst emitLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.openUnsafe(this: Latch): booleanOpens the latch synchronously, releasing all fibers waiting on it.
When to use
Use when synchronous code must open the latch immediately.
openUnsafe()
let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
arr: readonly [A, ...A[]](parameter) arr: {
0: A;
length: number;
toString: () => string;
toLocaleString: { (): string; (locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string };
concat: { (...items: Array<ConcatArray<A>>): Array<A>; (...items: Array<A | ConcatArray<A>>): Array<A> };
join: (separator?: string) => string;
slice: (start?: number, end?: number) => Array<A>;
indexOf: (searchElement: A, fromIndex?: number) => number;
lastIndexOf: (searchElement: A, fromIndex?: number) => number;
every: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): this is readonly S[]; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): boolean };
some: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => boolean;
forEach: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => void, thisArg?: any) => void;
map: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => U, thisArg?: any) => Array<U>;
filter: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): Array<S>; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): Array<A> };
reduce: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
reduceRight: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
find: { (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findIndex: (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
entries: () => ArrayIterator<[number, A]>;
keys: () => ArrayIterator<number>;
values: () => ArrayIterator<A>;
includes: (searchElement: A, fromIndex?: number) => boolean;
flatMap: (callback: (this: This, value: A, index: number, array: Array<A>) => U | ReadonlyArray<U>, thisArg?: This | undefined) => Array<U>;
flat: (this: A, depth?: D | undefined) => Array<FlatArray<A, D>>;
at: (index: number) => A | undefined;
findLast: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findLastIndex: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
toReversed: () => Array<A>;
toSorted: (compareFn?: ((a: A, b: A) => number) | undefined) => Array<A>;
toSpliced: { (start: number, deleteCount: number, ...items: Array<A>): Array<A>; (start: number, deleteCount?: number): Array<A> };
with: (index: number, value: A) => Array<A>;
}
let emitAtMs: numberemitAtMs = const clock: Clockconst clock: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
}
Clock.currentTimeMillisUnsafe(): numberReturns the current time in milliseconds unsafely.
When to use
Use to read millisecond time synchronously when you already have a Clock
service and can accept non-effectful access.
currentTimeMillisUnsafe() + const durationMs: numberdurationMs
return import EffectEffect.const void: Effect.Effect<void, never, never>(alias) const void: {
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;
}
Returns an effect that succeeds with void.
void
}),
import EffectEffect.const forever: <
Arg extends
| Effect<any, any, any>
| {
readonly disableYield?:
| boolean
| undefined
}
| undefined = {
readonly disableYield?: boolean | undefined
}
>(
effectOrOptions?: Arg,
options?:
| {
readonly disableYield?:
| boolean
| undefined
}
| undefined
) => [Arg] extends [
Effect<infer _A, infer _E, infer _R>
]
? Effect<never, _E, _R>
: <A, E, R>(
self: Effect<A, E, R>
) => Effect<never, E, R>
Repeats this effect forever (until the first error).
Example (Repeating forever)
import { Console, Effect, Fiber } from "effect"
const task = Effect.gen(function*() {
yield* Console.log("Task running...")
yield* Effect.sleep("1 second")
})
// This will run forever, printing every second
const program = task.pipe(Effect.forever)
// This will run forever, without yielding every iteration
const programNoYield = task.pipe(Effect.forever({ disableYield: true }))
// Run for 5 seconds then interrupt
const timedProgram = Effect.gen(function*() {
const fiber = yield* Effect.forkChild(program)
yield* Effect.sleep("5 seconds")
yield* Fiber.interrupt(fiber)
})
forever({ disableYield: truedisableYield: true }),
import EffectEffect.const onError: {
<E, X, R2>(
cleanup: (
cause: Cause.Cause<E>
) => Effect<X, never, R2>
): <A, R>(
self: Effect<A, E, R>
) => Effect<A, E, R2 | R>
<A, E, R, X, R2>(
self: Effect<A, E, R>,
cleanup: (
cause: Cause.Cause<E>
) => Effect<X, never, R2>
): Effect<A, E, R2 | R>
}
cause_: Cause.Cause<Cause.Done<void> | E>(parameter) cause_: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
let cause:
| Cause.Cause<Cause.Done | E>
| undefined
cause_: Cause.Cause<Cause.Done<void> | E>(parameter) cause_: {
reasons: ReadonlyArray<Reason<E>>;
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;
}
let emitAtMs: numberemitAtMs = const clock: Clockconst clock: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
}
Clock.currentTimeMillisUnsafe(): numberReturns the current time in milliseconds unsafely.
When to use
Use to read millisecond time synchronously when you already have a Clock
service and can accept non-effectful access.
currentTimeMillisUnsafe()
const emitLatch: Latch.Latchconst emitLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.openUnsafe(this: Latch): booleanOpens the latch synchronously, releasing all fibers waiting on it.
When to use
Use when synchronous code must open the latch immediately.
openUnsafe()
const endLatch: Latch.Latchconst endLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.openUnsafe(this: Latch): booleanOpens the latch synchronously, releasing all fibers waiting on it.
When to use
Use when synchronous code must open the latch immediately.
openUnsafe()
return import EffectEffect.const void: Effect.Effect<void, never, never>(alias) const void: {
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;
}
Returns an effect that succeeds with void.
void
}),
import EffectEffect.const forkIn: {
(
scope: Scope,
options?: {
readonly startImmediately?:
| boolean
| undefined
readonly uninterruptible?:
| boolean
| "inherit"
| undefined
}
): <A, E, R>(
self: Effect<A, E, R>
) => Effect<Fiber<A, E>, never, R>
<A, E, R>(
self: Effect<A, E, R>,
scope: Scope,
options?: {
readonly startImmediately?:
| boolean
| undefined
readonly uninterruptible?:
| boolean
| "inherit"
| undefined
}
): Effect<Fiber<A, E>, never, R>
}
scope: Scope.Scope(parameter) scope: {
strategy: "sequential" | "parallel";
state: State.Open | State.Closed | State.Empty;
}
const sleepLoop: Effect.Effect<
readonly [A, ...A[]],
Cause.Done<void> | E,
R
>
const sleepLoop: {
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.const suspend: <A, E, R>(
effect: LazyArg<Effect<A, E, R>>
) => Effect<A, E, R>
Creates an Effect lazily, delaying construction until it is needed.
When to use
Use when you need to defer the evaluation of an effect until it is required.
Details
suspend takes a thunk that represents an effect and delays creating it
until the suspended effect is evaluated. This is useful for optimizing
expensive computations, managing circular dependencies such as recursive
functions, and helping TypeScript unify return types when branches construct
different effects. Any side effects or scoped captures inside the thunk are
re-executed on each invocation.
Example (Lazily evaluating side effects)
import { Effect } from "effect"
let i = 0
const bad = Effect.succeed(i++)
const good = Effect.suspend(() => Effect.succeed(i++))
console.log(Effect.runSync(bad)) // Output: 0
console.log(Effect.runSync(bad)) // Output: 0
console.log(Effect.runSync(good)) // Output: 1
console.log(Effect.runSync(good)) // Output: 2
Example (Suspending recursive Fibonacci evaluation)
import { Effect } from "effect"
const blowsUp = (n: number): Effect.Effect<number> =>
n < 2
? Effect.succeed(1)
: Effect.zipWith(blowsUp(n - 1), blowsUp(n - 2), (a, b) => a + b)
// console.log(Effect.runSync(blowsUp(32)))
// crash: JavaScript heap out of memory
const allGood = (n: number): Effect.Effect<number> =>
n < 2
? Effect.succeed(1)
: Effect.zipWith(
Effect.suspend(() => allGood(n - 1)),
Effect.suspend(() => allGood(n - 2)),
(a, b) => a + b
)
console.log(Effect.runSync(allGood(32)))
// Output: 3524578
Example (Helping TypeScript infer recursive effect types)
import { Effect } from "effect"
// Without suspend, TypeScript may struggle with type inference.
// Inferred type:
// (a: number, b: number) =>
// Effect<never, Error, never> | Effect<number, never, never>
const withoutSuspend = (a: number, b: number) =>
b === 0
? Effect.fail(new Error("Cannot divide by zero"))
: Effect.succeed(a / b)
// Using suspend to unify return types.
// Inferred type:
// (a: number, b: number) => Effect<number, Error, never>
const withSuspend = (a: number, b: number) =>
Effect.suspend(() =>
b === 0
? Effect.fail(new Error("Cannot divide by zero"))
: Effect.succeed(a / b)
)
suspend(function function (local function) loop(): Pull.Pull<Arr.NonEmptyReadonlyArray<A>, E, void, R>loop(): import PullPull.interface Pull<out A, out E = never, out Done = void, out R = never>An effectful pull step that either produces a value, fails with E, or
signals completion with Cause.Done<Done>.
When to use
Use to model one low-level pull step when a consumer repeatedly evaluates an
effect that may emit a value, fail normally, or signal normal completion
through Cause.Done.
Details
Pull represents completion in the error channel so low-level stream
consumers can distinguish ordinary failures from end-of-input and carry a
leftover value when needed.
Pull<import ArrArr.type NonEmptyReadonlyArray<A> = readonly [
A,
...A[]
]
A readonly array guaranteed to have at least one element.
When to use
Use when non-emptiness must be tracked at the type level while preventing mutation.
Many Array module functions accept or return this type.
Example (Typing a non-empty array)
import type { Array } from "effect"
const nonEmpty: Array.NonEmptyReadonlyArray<number> = [1, 2, 3]
const head: number = nonEmpty[0] // guaranteed to exist
NonEmptyReadonlyArray<function (type parameter) A in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>A>, function (type parameter) E in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>E, void, function (type parameter) R in <A, E, R>(self: Stream<A, E, R>, duration: Duration.Input): Stream<A, E, R>R> {
const const now: numbernow = const clock: Clockconst clock: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
}
Clock.currentTimeMillisUnsafe(): numberReturns the current time in milliseconds unsafely.
When to use
Use to read millisecond time synchronously when you already have a Clock
service and can accept non-effectful access.
currentTimeMillisUnsafe()
const const timeMs: numbertimeMs = let emitAtMs: numberemitAtMs < const now: numbernow ? const durationMs: numberdurationMs : var Math: MathAn intrinsic object that provides basic mathematics functionality and constants.
Math.Math.min(...values: number[]): numberReturns the smaller of a set of supplied numeric expressions.
min(const durationMs: numberdurationMs, let emitAtMs: numberemitAtMs - const now: numbernow)
return 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>
}
import EffectEffect.const raceFirst: {
<A2, E2, R2>(
that: Effect<A2, E2, R2>,
options?: {
readonly onWinner?: (options: {
readonly fiber: Fiber<any, any>
readonly index: number
readonly parentFiber: Fiber<any, any>
}) => void
}
): <A, E, R>(
self: Effect<A, E, R>
) => Effect<A | A2, E | E2, R | R2>
<A, E, R, A2, E2, R2>(
self: Effect<A, E, R>,
that: Effect<A2, E2, R2>,
options?: {
readonly onWinner?: (options: {
readonly fiber: Fiber<any, any>
readonly index: number
readonly parentFiber: Fiber<any, any>
}) => void
}
): Effect<A | A2, E | E2, R | R2>
}
import EffectEffect.const sleep: (
duration: Duration.Input
) => Effect<void>
Returns an effect that suspends the current fiber for the specified duration
without blocking a JavaScript thread.
Example (Pausing without blocking)
import { Console, Effect } from "effect"
const program = Effect.gen(function*() {
yield* Console.log("Start")
yield* Effect.sleep("2 seconds")
yield* Console.log("End")
})
Effect.runFork(program)
// Output: "Start" (immediately)
// Output: "End" (after 2 seconds)
sleep(const timeMs: numbertimeMs), const endLatch: Latch.Latchconst endLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.await: Effect.Effect<void>(property) Latch.await: {
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;
}
Waits for the latch to be opened or released.
When to use
Use to suspend until the latch allows the current fiber to continue.
await), () => {
const const now: numbernow = const clock: Clockconst clock: {
currentTimeMillisUnsafe: () => number;
currentTimeMillis: Effect<number>;
currentTimeNanosUnsafe: () => bigint;
currentTimeNanos: Effect<bigint>;
sleep: (duration: Duration.Duration) => Effect<void>;
}
Clock.currentTimeMillisUnsafe(): numberReturns the current time in milliseconds unsafely.
When to use
Use to read millisecond time synchronously when you already have a Clock
service and can accept non-effectful access.
currentTimeMillisUnsafe()
if (const now: numbernow < let emitAtMs: numberemitAtMs) {
return function (local function) loop(): Pull.Pull<Arr.NonEmptyReadonlyArray<A>, E, void, R>loop()
} else if (let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
const emitLatch: Latch.Latchconst emitLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.closeUnsafe(this: Latch): booleanCloses the latch synchronously so future waiters suspend again.
When to use
Use when synchronous code must close the latch immediately.
closeUnsafe()
const pullLatch: Latch.Latchconst pullLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.closeUnsafe(this: Latch): booleanCloses the latch synchronously so future waiters suspend again.
When to use
Use when synchronous code must close the latch immediately.
closeUnsafe()
const const eff: Effect.Effect<
[A, ...A[]],
never,
never
>
const eff: {
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.const succeed: <A>(value: A) => Effect<A>Creates an Effect that always succeeds with a given value.
When to use
Use when an effect should complete successfully with a specific value without any errors
or external dependencies.
Example (Creating a successful effect)
import { Effect } from "effect"
// Creating an effect that represents a successful scenario
//
// ┌─── Effect<number, never, never>
// ▼
const success = Effect.succeed(42)
succeed(import ArrArr.const of: <A>(a: A) => NonEmptyArray<A>Wraps a single value in a NonEmptyArray.
Example (Creating a single-element array)
import { Array } from "effect"
console.log(Array.of(1)) // [1]
of(import ArrArr.const lastNonEmpty: <A>(
self: NonEmptyReadonlyArray<A>
) => A
Returns the last element of a NonEmptyReadonlyArray directly (no Option
wrapper).
When to use
Use to get the last element without Option wrapping when the array is known
to be non-empty.
Example (Getting the last of a non-empty array)
import { Array } from "effect"
console.log(Array.lastNonEmpty([1, 2, 3, 4])) // 4
lastNonEmpty(let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
let lastArr: {
0: A;
length: number;
toString: () => string;
toLocaleString: { (): string; (locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string };
concat: { (...items: Array<ConcatArray<A>>): Array<A>; (...items: Array<A | ConcatArray<A>>): Array<A> };
join: (separator?: string) => string;
slice: (start?: number, end?: number) => Array<A>;
indexOf: (searchElement: A, fromIndex?: number) => number;
lastIndexOf: (searchElement: A, fromIndex?: number) => number;
every: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): this is readonly S[]; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): boolean };
some: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => boolean;
forEach: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => void, thisArg?: any) => void;
map: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => U, thisArg?: any) => Array<U>;
filter: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): Array<S>; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): Array<A> };
reduce: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
reduceRight: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
find: { (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findIndex: (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
entries: () => ArrayIterator<[number, A]>;
keys: () => ArrayIterator<number>;
values: () => ArrayIterator<A>;
includes: (searchElement: A, fromIndex?: number) => boolean;
flatMap: (callback: (this: This, value: A, index: number, array: Array<A>) => U | ReadonlyArray<U>, thisArg?: This | undefined) => Array<U>;
flat: (this: A, depth?: D | undefined) => Array<FlatArray<A, D>>;
at: (index: number) => A | undefined;
findLast: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findLastIndex: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
toReversed: () => Array<A>;
toSorted: (compareFn?: ((a: A, b: A) => number) | undefined) => Array<A>;
toSpliced: { (start: number, deleteCount: number, ...items: Array<A>): Array<A>; (start: number, deleteCount?: number): Array<A> };
with: (index: number, value: A) => Array<A>;
}
let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
var undefinedundefined
return const eff: Effect.Effect<
[A, ...A[]],
never,
never
>
const eff: {
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;
}
let cause:
| Cause.Cause<Cause.Done | E>
| undefined
import EffectEffect.const failCause: <E>(
cause: Cause.Cause<E>
) => Effect<never, E>
Creates an Effect that represents a failure with a specific Cause.
When to use
Use when you already have a full Cause and need to preserve defects,
interruptions, annotations, or combined failures in the effect's failure
channel.
Details
This function allows you to create effects that fail with complex error
structures, including multiple errors, defects, interruptions, and more.
Example (Failing with a full Cause)
import { Cause, Effect } from "effect"
const program = Effect.failCause(
Cause.fail("Network error")
)
Effect.runPromiseExit(program).then(console.log)
// Output: { _id: 'Exit', _tag: 'Failure', cause: ... }
failCause(let cause:
| Cause.Cause<Cause.Done | E>
| undefined
let cause: {
reasons: ReadonlyArray<Reason<E>>;
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 (local function) loop(): Pull.Pull<Arr.NonEmptyReadonlyArray<A>, E, void, R>loop()
})
})
return import EffectEffect.const suspend: <A, E, R>(
effect: LazyArg<Effect<A, E, R>>
) => Effect<A, E, R>
Creates an Effect lazily, delaying construction until it is needed.
When to use
Use when you need to defer the evaluation of an effect until it is required.
Details
suspend takes a thunk that represents an effect and delays creating it
until the suspended effect is evaluated. This is useful for optimizing
expensive computations, managing circular dependencies such as recursive
functions, and helping TypeScript unify return types when branches construct
different effects. Any side effects or scoped captures inside the thunk are
re-executed on each invocation.
Example (Lazily evaluating side effects)
import { Effect } from "effect"
let i = 0
const bad = Effect.succeed(i++)
const good = Effect.suspend(() => Effect.succeed(i++))
console.log(Effect.runSync(bad)) // Output: 0
console.log(Effect.runSync(bad)) // Output: 0
console.log(Effect.runSync(good)) // Output: 1
console.log(Effect.runSync(good)) // Output: 2
Example (Suspending recursive Fibonacci evaluation)
import { Effect } from "effect"
const blowsUp = (n: number): Effect.Effect<number> =>
n < 2
? Effect.succeed(1)
: Effect.zipWith(blowsUp(n - 1), blowsUp(n - 2), (a, b) => a + b)
// console.log(Effect.runSync(blowsUp(32)))
// crash: JavaScript heap out of memory
const allGood = (n: number): Effect.Effect<number> =>
n < 2
? Effect.succeed(1)
: Effect.zipWith(
Effect.suspend(() => allGood(n - 1)),
Effect.suspend(() => allGood(n - 2)),
(a, b) => a + b
)
console.log(Effect.runSync(allGood(32)))
// Output: 3524578
Example (Helping TypeScript infer recursive effect types)
import { Effect } from "effect"
// Without suspend, TypeScript may struggle with type inference.
// Inferred type:
// (a: number, b: number) =>
// Effect<never, Error, never> | Effect<number, never, never>
const withoutSuspend = (a: number, b: number) =>
b === 0
? Effect.fail(new Error("Cannot divide by zero"))
: Effect.succeed(a / b)
// Using suspend to unify return types.
// Inferred type:
// (a: number, b: number) => Effect<number, Error, never>
const withSuspend = (a: number, b: number) =>
Effect.suspend(() =>
b === 0
? Effect.fail(new Error("Cannot divide by zero"))
: Effect.succeed(a / b)
)
suspend(() => {
if (let cause:
| Cause.Cause<Cause.Done | E>
| undefined
let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
const eff: Effect.Effect<
[A, ...A[]],
never,
never
>
const eff: {
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.const succeed: <A>(value: A) => Effect<A>Creates an Effect that always succeeds with a given value.
When to use
Use when an effect should complete successfully with a specific value without any errors
or external dependencies.
Example (Creating a successful effect)
import { Effect } from "effect"
// Creating an effect that represents a successful scenario
//
// ┌─── Effect<number, never, never>
// ▼
const success = Effect.succeed(42)
succeed(import ArrArr.const of: <A>(a: A) => NonEmptyArray<A>Wraps a single value in a NonEmptyArray.
Example (Creating a single-element array)
import { Array } from "effect"
console.log(Array.of(1)) // [1]
of(import ArrArr.const lastNonEmpty: <A>(
self: NonEmptyReadonlyArray<A>
) => A
Returns the last element of a NonEmptyReadonlyArray directly (no Option
wrapper).
When to use
Use to get the last element without Option wrapping when the array is known
to be non-empty.
Example (Getting the last of a non-empty array)
import { Array } from "effect"
console.log(Array.lastNonEmpty([1, 2, 3, 4])) // 4
lastNonEmpty(let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
let lastArr: {
0: A;
length: number;
toString: () => string;
toLocaleString: { (): string; (locales: string | string[], options?: Intl.NumberFormatOptions & Intl.DateTimeFormatOptions): string };
concat: { (...items: Array<ConcatArray<A>>): Array<A>; (...items: Array<A | ConcatArray<A>>): Array<A> };
join: (separator?: string) => string;
slice: (start?: number, end?: number) => Array<A>;
indexOf: (searchElement: A, fromIndex?: number) => number;
lastIndexOf: (searchElement: A, fromIndex?: number) => number;
every: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): this is readonly S[]; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): boolean };
some: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => boolean;
forEach: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => void, thisArg?: any) => void;
map: (callbackfn: (value: A, index: number, array: ReadonlyArray<A>) => U, thisArg?: any) => Array<U>;
filter: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): Array<S>; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): Array<A> };
reduce: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
reduceRight: { (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A): A; (callbackfn: (previousValue: A, currentValue: A, currentIndex: number, array: ReadonlyArray<A>) => A, initialValue: A): A; (callbac…;
find: { (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findIndex: (predicate: (value: A, index: number, obj: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
entries: () => ArrayIterator<[number, A]>;
keys: () => ArrayIterator<number>;
values: () => ArrayIterator<A>;
includes: (searchElement: A, fromIndex?: number) => boolean;
flatMap: (callback: (this: This, value: A, index: number, array: Array<A>) => U | ReadonlyArray<U>, thisArg?: This | undefined) => Array<U>;
flat: (this: A, depth?: D | undefined) => Array<FlatArray<A, D>>;
at: (index: number) => A | undefined;
findLast: { (predicate: (value: A, index: number, array: ReadonlyArray<A>) => value is S, thisArg?: any): S | undefined; (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any): A | undefined };
findLastIndex: (predicate: (value: A, index: number, array: ReadonlyArray<A>) => unknown, thisArg?: any) => number;
toReversed: () => Array<A>;
toSorted: (compareFn?: ((a: A, b: A) => number) | undefined) => Array<A>;
toSpliced: { (start: number, deleteCount: number, ...items: Array<A>): Array<A>; (start: number, deleteCount?: number): Array<A> };
with: (index: number, value: A) => Array<A>;
}
let lastArr:
| Arr.NonEmptyReadonlyArray<A>
| undefined
var undefinedundefined
return const eff: Effect.Effect<
[A, ...A[]],
never,
never
>
const eff: {
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.const failCause: <E>(
cause: Cause.Cause<E>
) => Effect<never, E>
Creates an Effect that represents a failure with a specific Cause.
When to use
Use when you already have a full Cause and need to preserve defects,
interruptions, annotations, or combined failures in the effect's failure
channel.
Details
This function allows you to create effects that fail with complex error
structures, including multiple errors, defects, interruptions, and more.
Example (Failing with a full Cause)
import { Cause, Effect } from "effect"
const program = Effect.failCause(
Cause.fail("Network error")
)
Effect.runPromiseExit(program).then(console.log)
// Output: { _id: 'Exit', _tag: 'Failure', cause: ... }
failCause(let cause:
| Cause.Cause<Cause.Done | E>
| undefined
let cause: {
reasons: ReadonlyArray<Reason<E>>;
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 pullLatch: Latch.Latchconst pullLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.openUnsafe(this: Latch): booleanOpens the latch synchronously, releasing all fibers waiting on it.
When to use
Use when synchronous code must open the latch immediately.
openUnsafe()
return const emitLatch: Latch.Latchconst emitLatch: {
open: Effect.Effect<boolean>;
openUnsafe: (this: Latch) => boolean;
release: Effect.Effect<boolean>;
await: Effect.Effect<void>;
close: Effect.Effect<boolean>;
closeUnsafe: (this: Latch) => boolean;
whenOpen: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
isOpen: (this: Latch) => boolean;
}
Latch.whenOpen<A, E, R>(self: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>Runs the given effect only after the latch allows waiting fibers to
continue.
When to use
Use to gate an effect behind the latch signal.
whenOpen(const sleepLoop: Effect.Effect<
readonly [A, ...A[]],
Cause.Done<void> | E,
R
>
const sleepLoop: {
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;
}