<A, E, R, X, AS extends A, ES, RS>(
effect: Effect.Effect<A, E, R>,
schedule: Schedule.Schedule<X, AS, ES, RS>
): Stream<A, E | ES, R | RS>Creates a stream from an effect producing a value of type A, which is
repeated using the specified schedule.
Example (Repeating an effect with a schedule)
import { Console, Effect, Schedule, Stream } from "effect"
const program = Effect.gen(function*() {
const stream = Stream.fromEffectSchedule(
Effect.succeed("ping"),
Schedule.recurs(2)
)
const result = yield* Stream.runCollect(stream)
yield* Console.log(result)
})
Effect.runPromise(program)
// Output: [ "ping", "ping", "ping" ]export const const fromEffectSchedule: <
A,
E,
R,
X,
AS extends A,
ES,
RS
>(
effect: Effect.Effect<A, E, R>,
schedule: Schedule.Schedule<X, AS, ES, RS>
) => Stream<A, E | ES, R | RS>
Creates a stream from an effect producing a value of type A, which is
repeated using the specified schedule.
Example (Repeating an effect with a schedule)
import { Console, Effect, Schedule, Stream } from "effect"
const program = Effect.gen(function*() {
const stream = Stream.fromEffectSchedule(
Effect.succeed("ping"),
Schedule.recurs(2)
)
const result = yield* Stream.runCollect(stream)
yield* Console.log(result)
})
Effect.runPromise(program)
// Output: [ "ping", "ping", "ping" ]
fromEffectSchedule = <function (type parameter) A in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>A, function (type parameter) E in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>E, function (type parameter) R in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>R, function (type parameter) X in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>X, function (type parameter) AS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>AS extends function (type parameter) A in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>A, function (type parameter) ES in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>ES, function (type parameter) RS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>RS>(
effect: Effect.Effect<A, E, 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) A in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>A, function (type parameter) E in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>E, function (type parameter) R in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>R>,
schedule: Schedule.Schedule<X, AS, ES, RS>(parameter) schedule: {
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 ScheduleSchedule.interface Schedule<out Output, in Input = unknown, out Error = never, out Env = never>A Schedule defines a strategy for repeating or retrying effects based on some policy.
Example (Defining retry and repeat schedules)
import { Console, Data, Effect, Schedule } from "effect"
class NetworkError extends Data.TaggedError("NetworkError")<{
readonly attempt: number
}> {}
// Basic retry schedule - retry up to 3 times with exponential backoff
const retrySchedule = Schedule.max([
Schedule.exponential("100 millis"),
Schedule.recurs(3)
])
// Basic repeat schedule - repeat every 30 seconds forever
const repeatSchedule: Schedule.Schedule<number, unknown, never> = Schedule
.spaced("30 seconds")
const program = Effect.gen(function*() {
let attempts = 0
const result1 = yield* Effect.retry(
Effect.gen(function*() {
attempts++
if (attempts < 3) {
return yield* Effect.fail(new NetworkError({ attempt: attempts }))
}
return "Success"
}),
retrySchedule
)
console.log(result1) // "Success"
yield* Console.log("heartbeat").pipe(
Effect.repeat(repeatSchedule.pipe(Schedule.upTo({ times: 5 })))
)
})
The Schedule namespace contains types and utilities for working with schedules.
Schedule<function (type parameter) X in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>X, function (type parameter) AS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>AS, function (type parameter) ES in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>ES, function (type parameter) RS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>RS>
): 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, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>A, function (type parameter) E in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>E | function (type parameter) ES in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>ES, function (type parameter) R in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>R | function (type parameter) RS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>RS> =>
const fromPull: <A, E, R, EX, RX>(
pull: Effect.Effect<
Pull.Pull<
Arr.NonEmptyReadonlyArray<A>,
E,
void,
R
>,
EX,
RX
>
) => Stream<A, Pull.ExcludeDone<E> | EX, R | RX>
Creates a stream from a pull effect, such as one produced by Stream.toPull.
Details
A pull effect yields chunks on demand and completes when the upstream stream ends.
See Stream.toPull for a matching producer.
Example (Creating a stream from a pull effect)
import { Console, Effect, Stream } from "effect"
const program = Effect.scoped(
Effect.gen(function*() {
const source = Stream.make(1, 2, 3)
const pull = yield* Stream.toPull(source)
const stream = Stream.fromPull(Effect.succeed(pull))
const values = yield* Stream.runCollect(stream)
yield* Console.log(values)
})
)
Effect.runPromise(program)
// Output: [1, 2, 3]
fromPull(import EffectEffect.const gen: {
<Eff extends Effect<any, any, any>, AEff>(
f: () => Generator<Eff, AEff, never>
): Effect<
AEff,
[Eff] extends [never]
? never
: [Eff] extends [
Effect<infer _A, infer E, infer _R>
]
? E
: never,
[Eff] extends [never]
? never
: [Eff] extends [
Effect<infer _A, infer _E, infer R>
]
? R
: never
>
<Self, Eff extends Effect<any, any, any>, AEff>(
options: { readonly self: Self },
f: (this: Self) => Generator<Eff, AEff, never>
): Effect<
AEff,
[Eff] extends [never]
? never
: [Eff] extends [
Effect<infer _A, infer E, infer _R>
]
? E
: never,
[Eff] extends [never]
? never
: [Eff] extends [
Effect<infer _A, infer _E, infer R>
]
? R
: never
>
}
const step: (
input: AS
) => Pull.Pull<
Schedule.Metadata<X, AS>,
ES,
X,
RS
>
import ScheduleSchedule.const toStepWithMetadata: <
Output,
Input,
Error,
Env
>(
schedule: Schedule<Output, Input, Error, Env>
) => Effect<
(
input: Input
) => Pull.Pull<
Metadata<Output, Input>,
Error,
Output,
Env
>,
never,
Env
>
Extracts a step function from a Schedule that sleeps for each computed
delay and returns metadata for the completed step.
When to use
Use to drive a schedule manually while preserving the computed output,
delay, input, attempt, and elapsed timing metadata for each step.
Details
The returned step reads the current time from Clock when invoked, calls the
schedule step with that timestamp and input, sleeps for the returned
duration, and then yields Metadata.
toStepWithMetadata(schedule: Schedule.Schedule<X, AS, ES, RS>(parameter) schedule: {
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; <…;
}
let s: As = yield* import EffectEffect.const provideService: {
<I, S>(service: Context.Key<I, S>): {
(implementation: S): <A, E, R>(
self: Effect<A, E, R>
) => Effect<A, E, Exclude<R, I>>
<A, E, R>(
self: Effect<A, E, R>,
implementation: S
): Effect<A, E, Exclude<R, I>>
}
<I, S>(
service: Context.Key<I, S>,
implementation: S
): <A, E, R>(
self: Effect<A, E, R>
) => Effect<A, E, Exclude<R, I>>
<A, E, R, I, S>(
self: Effect<A, E, R>,
service: Context.Key<I, S>,
implementation: S
): Effect<A, E, Exclude<R, I>>
}
effect: Effect.Effect<A, E, 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 ScheduleSchedule.const CurrentMetadata: Context.Reference<
Schedule.Metadata<unknown, unknown>
>
const CurrentMetadata: {
key: string;
Service: {
output: Output;
duration: Duration.Duration;
input: Input;
attempt: number;
start: number;
now: number;
elapsed: number;
elapsedSincePrevious: number;
};
defaultValue: () => Shape;
of: (this: void, self: Schedule.Metadata<unknown, unknown>) => Schedule.Metadata<unknown, unknown>;
context: (self: Schedule.Metadata<unknown, unknown>) => Context.Context<never>;
use: (f: (service: Schedule.Metadata<unknown, unknown>) => Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
useSync: (f: (service: Schedule.Metadata<unknown, unknown>) => 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;
}
Context reference containing metadata for the currently running schedule step.
Details
Repeat, retry, stream, and channel scheduling operations provide this service
to effects run between schedule steps. The default value contains undefined
input and output values, zero duration, and zeroed timing fields before any
schedule step has produced metadata.
CurrentMetadata, import ScheduleSchedule.const CurrentMetadata: Context.Reference<
Schedule.Metadata<unknown, unknown>
>
const CurrentMetadata: {
key: string;
Service: {
output: Output;
duration: Duration.Duration;
input: Input;
attempt: number;
start: number;
now: number;
elapsed: number;
elapsedSincePrevious: number;
};
defaultValue: () => Shape;
of: (this: void, self: Schedule.Metadata<unknown, unknown>) => Schedule.Metadata<unknown, unknown>;
context: (self: Schedule.Metadata<unknown, unknown>) => Context.Context<never>;
use: (f: (service: Schedule.Metadata<unknown, unknown>) => Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
useSync: (f: (service: Schedule.Metadata<unknown, unknown>) => 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;
}
Context reference containing metadata for the currently running schedule step.
Details
Repeat, retry, stream, and channel scheduling operations provide this service
to effects run between schedule steps. The default value contains undefined
input and output values, zero duration, and zeroed timing fields before any
schedule step has produced metadata.
CurrentMetadata.Reference<Metadata<unknown, unknown>>.defaultValue: () => Schedule.Metadata<unknown, unknown>defaultValue())
let let initial: booleaninitial = true
const const pull: Pull.Pull<
readonly [A, ...A[]],
E | ES,
void,
R | RS
>
const 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;
}
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(() => const step: (
input: AS
) => Pull.Pull<
Schedule.Metadata<X, AS>,
ES,
X,
RS
>
let s: As as function (type parameter) AS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>AS)).Pipeable.pipe<Effect.Effect<Schedule.Metadata<X, AS>, ES | Cause.Done<X>, RS>, Effect.Effect<A, E | ES | Cause.Done<X>, RS | Exclude<R, never>>, Effect.Effect<[A, ...A[]], E | ES | Cause.Done<X>, RS | Exclude<R, never>>>(this: Effect.Effect<...>, ab: (_: Effect.Effect<Schedule.Metadata<X, AS>, ES | Cause.Done<X>, RS>) => Effect.Effect<A, E | ES | Cause.Done<X>, RS | Exclude<...>>, bc: (_: Effect.Effect<...>) => Effect.Effect<...>): Effect.Effect<...> (+21 overloads)pipe(
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>
}
meta: Schedule.Metadata<X, AS>(parameter) meta: {
output: Output;
duration: Duration.Duration;
input: Input;
attempt: number;
start: number;
now: number;
elapsed: number;
elapsedSincePrevious: number;
}
import EffectEffect.const provideService: {
<I, S>(service: Context.Key<I, S>): {
(implementation: S): <A, E, R>(
self: Effect<A, E, R>
) => Effect<A, E, Exclude<R, I>>
<A, E, R>(
self: Effect<A, E, R>,
implementation: S
): Effect<A, E, Exclude<R, I>>
}
<I, S>(
service: Context.Key<I, S>,
implementation: S
): <A, E, R>(
self: Effect<A, E, R>
) => Effect<A, E, Exclude<R, I>>
<A, E, R, I, S>(
self: Effect<A, E, R>,
service: Context.Key<I, S>,
implementation: S
): Effect<A, E, Exclude<R, I>>
}
effect: Effect.Effect<A, E, 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 ScheduleSchedule.const CurrentMetadata: Context.Reference<
Schedule.Metadata<unknown, unknown>
>
const CurrentMetadata: {
key: string;
Service: {
output: Output;
duration: Duration.Duration;
input: Input;
attempt: number;
start: number;
now: number;
elapsed: number;
elapsedSincePrevious: number;
};
defaultValue: () => Shape;
of: (this: void, self: Schedule.Metadata<unknown, unknown>) => Schedule.Metadata<unknown, unknown>;
context: (self: Schedule.Metadata<unknown, unknown>) => Context.Context<never>;
use: (f: (service: Schedule.Metadata<unknown, unknown>) => Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
useSync: (f: (service: Schedule.Metadata<unknown, unknown>) => 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;
}
Context reference containing metadata for the currently running schedule step.
Details
Repeat, retry, stream, and channel scheduling operations provide this service
to effects run between schedule steps. The default value contains undefined
input and output values, zero duration, and zeroed timing fields before any
schedule step has produced metadata.
CurrentMetadata, meta: Schedule.Metadata<X, AS>(parameter) meta: {
output: Output;
duration: Duration.Duration;
input: Input;
attempt: number;
start: number;
now: number;
elapsed: number;
elapsedSincePrevious: number;
}
import EffectEffect.const map: {
<A, B>(f: (a: A) => B): <E, R>(
self: Effect<A, E, R>
) => Effect<B, E, R>
<A, E, R, B>(
self: Effect<A, E, R>,
f: (a: A) => B
): Effect<B, E, R>
}
next: Anext) => {
let s: As = next: Anext
return 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(next: Anext)
})
) as 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, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>A>, function (type parameter) E in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>E | function (type parameter) ES in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>ES, void, function (type parameter) R in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>R | function (type parameter) RS in <A, E, R, X, AS extends A, ES, RS>(effect: Effect.Effect<A, E, R>, schedule: Schedule.Schedule<X, AS, ES, RS>): Stream<A, E | ES, R | RS>RS>
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 initial: booleaninitial) {
let initial: booleaninitial = false
return 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(let s: As))
}
return const pull: Pull.Pull<
readonly [A, ...A[]],
E | ES,
void,
R | RS
>
const 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;
}