<S, A, E = never, R = never>(
s: S,
f: (
s: S
) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>
): Stream<A, E, R>Creates a stream by repeatedly evaluating an effectful page function.
When to use
Use to consume paginated APIs where each step returns a batch of values together with an optional next state.
Details
This is similar to unfold, but each step can emit zero or more values and independently decide whether another state should be requested.
Example (Paginating stream state)
import { Console, Effect, Option, Stream } from "effect"
const stream = Stream.paginate(0, (n: number) =>
Effect.succeed(
[
[n],
n < 3 ? Option.some(n + 1) : Option.none<number>()
] as const
))
Effect.runPromise(Stream.runCollect(stream)).then(console.log)
// Output: [ 0, 1, 2, 3 ]export const const paginate: <
S,
A,
E = never,
R = never
>(
s: S,
f: (
s: S
) => Effect.Effect<
readonly [ReadonlyArray<A>, Option.Option<S>],
E,
R
>
) => Stream<A, E, R>
Creates a stream by repeatedly evaluating an effectful page function.
When to use
Use to consume paginated APIs where each step returns a batch of values
together with an optional next state.
Details
This is similar to
unfold
, but each step can emit zero or more values
and independently decide whether another state should be requested.
Example (Paginating stream state)
import { Console, Effect, Option, Stream } from "effect"
const stream = Stream.paginate(0, (n: number) =>
Effect.succeed(
[
[n],
n < 3 ? Option.some(n + 1) : Option.none<number>()
] as const
))
Effect.runPromise(Stream.runCollect(stream)).then(console.log)
// Output: [ 0, 1, 2, 3 ]
paginate = <function (type parameter) S in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>S, function (type parameter) A in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>A, function (type parameter) E in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>E = never, function (type parameter) R in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>R = never>(
s: Ss: function (type parameter) S in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>S,
f: (
s: S
) => Effect.Effect<
readonly [ReadonlyArray<A>, Option.Option<S>],
E,
R
>
s: Ss: function (type parameter) S in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>S
) => 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<readonly [interface ReadonlyArray<T>ReadonlyArray<function (type parameter) A in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>A>, import OptionOption.type Option<A> = Option.None<A> | Option.Some<A>The Option data type represents optional values. An Option<A> is either
Some<A>, containing a value of type A, or None, representing absence.
When to use
Use to represent initial values that may not yet exist
- Returning from partial functions (not defined for all inputs)
- Managing optional fields in data structures
Namespace containing utility types for Option.
When to use
Use to access type-level helpers associated with Option.
Option<function (type parameter) S in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>S>], function (type parameter) E in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>E, function (type parameter) R in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], 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 <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>A, function (type parameter) E in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>E, function (type parameter) R in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>R> =>
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 sync: <A>(
thunk: LazyArg<A>
) => Effect<A>
Creates an Effect that represents a synchronous side-effectful computation.
When to use
Use when you need to wrap a synchronous side-effectful operation that is not
expected to throw.
Details
The provided function is evaluated lazily when the effect runs.
Gotchas
The function must not throw. If it throws, the thrown value is treated as a
defect, not as a typed failure. Use try when throwing is expected.
Example (Capturing synchronous logging in an Effect)
import { Effect } from "effect"
const log = (message: string) =>
Effect.sync(() => {
console.log(message) // side effect
})
// ┌─── Effect<void, never, never>
// ▼
const program = log("Hello, World!")
sync(() => {
let let state: Sstate = s: Ss
let let done: booleandone = false
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(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 <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>A>, function (type parameter) E in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>E, void, function (type parameter) R in <S, A, E = never, R = never>(s: S, f: (s: S) => Effect.Effect<readonly [ReadonlyArray<A>, Option.Option<S>], E, R>): Stream<A, E, R>R> {
if (let done: booleandone) return import CauseCause.const done: <A = void>(
value?: A
) => Effect.Effect<never, Done<A>>
Creates an Effect that fails with a Done error. Shorthand for
Effect.fail(Cause.Done(value)).
When to use
Use when you model stream or queue completion through the error channel.
Example (Failing with Done)
import { Cause, Effect } from "effect"
const program = Cause.done("finished")
Effect.runPromiseExit(program).then((exit) => {
console.log(exit._tag) // "Failure"
})
done()
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>
}
f: (
s: S
) => Effect.Effect<
readonly [ReadonlyArray<A>, Option.Option<S>],
E,
R
>
let state: Sstate), ([a: readonly A[]a, s: Option.Option<S>s]) => {
if (import OptionOption.const isNone: <A>(
self: Option<A>
) => self is None<A>
Checks whether an Option is None (absent).
When to use
Use when you need to branch on an absent Option before accessing .value.
Details
- Acts as a type guard, narrowing to
None<A>
Example (Checking for None)
import { Option } from "effect"
console.log(Option.isNone(Option.some(1)))
// Output: false
console.log(Option.isNone(Option.none()))
// Output: true
isNone(s: Option.Option<S>s)) {
let done: booleandone = true
} else {
let state: Sstate = s: Option.Some<S>(parameter) s: {
_tag: "Some";
_op: "Some";
value: A;
valueOrUndefined: A;
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;
}
Some<S>.value: Svalue
}
if (!import ArrArr.const isReadonlyArrayNonEmpty: <A>(
self: ReadonlyArray<A>
) => self is NonEmptyReadonlyArray<A>
Checks whether a ReadonlyArray is non-empty, narrowing the type to
NonEmptyReadonlyArray.
When to use
Use when you need to prove a readonly array has at least one element without
requiring mutable array methods afterward.
Example (Checking for a non-empty readonly array)
import { Array } from "effect"
console.log(Array.isReadonlyArrayNonEmpty([])) // false
console.log(Array.isReadonlyArrayNonEmpty([1, 2, 3])) // true
isReadonlyArrayNonEmpty(a: readonly A[]a)) return function (local function) loop(): Pull.Pull<Arr.NonEmptyReadonlyArray<A>, E, void, R>loop()
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(a: readonly [A, ...A[]](parameter) a: {
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>;
}