<A>(self: Subscription<A>): Effect.Effect<A>Takes a single message from the subscription. If no messages are available, this will suspend until a message becomes available.
Example (Taking a message)
import { Effect, Fiber, PubSub } from "effect"
const program = Effect.gen(function*() {
const pubsub = yield* PubSub.bounded<string>(10)
yield* Effect.scoped(Effect.gen(function*() {
const subscription = yield* PubSub.subscribe(pubsub)
// Start a fiber to take a message (will suspend)
const takeFiber = yield* Effect.forkChild(
PubSub.take(subscription)
)
// Publish a message
yield* PubSub.publish(pubsub, "Hello")
// The take will now complete
const message = yield* Fiber.join(takeFiber)
console.log("Received:", message) // "Hello"
}))
})subscriptions
Source effect/PubSub.ts:116019 lines
export const const take: <A>(
self: Subscription<A>
) => Effect.Effect<A>
Takes a single message from the subscription. If no messages are available,
this will suspend until a message becomes available.
Example (Taking a message)
import { Effect, Fiber, PubSub } from "effect"
const program = Effect.gen(function*() {
const pubsub = yield* PubSub.bounded<string>(10)
yield* Effect.scoped(Effect.gen(function*() {
const subscription = yield* PubSub.subscribe(pubsub)
// Start a fiber to take a message (will suspend)
const takeFiber = yield* Effect.forkChild(
PubSub.take(subscription)
)
// Publish a message
yield* PubSub.publish(pubsub, "Hello")
// The take will now complete
const message = yield* Fiber.join(takeFiber)
console.log("Received:", message) // "Hello"
}))
})
take = <function (type parameter) A in <A>(self: Subscription<A>): Effect.Effect<A>A>(self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
interface Subscription<out A>A subscription represents a consumer's connection to a PubSub, allowing them to take messages.
Example (Taking messages from a subscription)
import { Effect, PubSub } from "effect"
const program = Effect.gen(function*() {
const pubsub = yield* PubSub.bounded<string>(10)
// Subscribe within a scope for automatic cleanup
yield* Effect.scoped(Effect.gen(function*() {
const subscription: PubSub.Subscription<string> = yield* PubSub.subscribe(
pubsub
)
yield* PubSub.publishAll(pubsub, ["msg1", "msg2", "msg3"])
// Take individual messages
const message = yield* PubSub.take(subscription)
console.log(message) // "msg1"
// Take multiple messages
const messages = yield* PubSub.takeUpTo(subscription, 1)
console.log(messages) // ["msg2"]
const allMessages = yield* PubSub.takeAll(subscription)
console.log(allMessages) // ["msg3"]
}))
})
Subscription<function (type parameter) A in <A>(self: Subscription<A>): Effect.Effect<A>A>): 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>(self: Subscription<A>): Effect.Effect<A>A> =>
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 (self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<out A>.shutdownFlag: MutableRef.MutableRef<boolean>(property) Subscription<out A>.shutdownFlag: {
current: T;
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;
}
MutableRef<boolean>.current: booleancurrent) {
return import EffectEffect.const interrupt: Effect<never>const interrupt: {
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 is immediately interrupted.
Example (Creating an interrupted effect)
import { Effect } from "effect"
const program = Effect.gen(function*() {
return yield* Effect.interrupt
yield* Effect.succeed("This won't execute and is unreachable")
})
Effect.runPromise(program).catch(console.error)
// Throws: InterruptedException
interrupt
}
if (self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<A>.replayWindow: PubSub.ReplayWindow<A>(property) Subscription<A>.replayWindow: {
take: () => A | undefined;
takeN: (n: number) => Array<A>;
takeAll: () => Array<A>;
remaining: number;
}
PubSub<in out A>.ReplayWindow<A>.remaining: numberremaining > 0) {
const const message: NonNullable<A>message = self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<A>.replayWindow: PubSub.ReplayWindow<A>(property) Subscription<A>.replayWindow: {
take: () => A | undefined;
takeN: (n: number) => Array<A>;
takeAll: () => Array<A>;
remaining: number;
}
PubSub<in out A>.ReplayWindow<A>.take(): A | undefinedtake()!
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(const message: NonNullable<A>message)
}
const const message:
| typeof MutableList.Empty
| A
self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<out A>.pollers: MutableList.MutableList<Deferred.Deferred<any>>(property) Subscription<out A>.pollers: {
head: MutableList.Bucket<A> | undefined;
tail: MutableList.Bucket<A> | undefined;
length: number;
}
MutableList<Deferred<any, never>>.length: numberlength === 0
? self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<A>.subscription: PubSub.BackingSubscription<A>(property) Subscription<A>.subscription: {
isEmpty: () => boolean;
size: () => number;
poll: () => typeof MutableList.Empty | A;
pollUpTo: (n: number) => Array<A>;
unsubscribe: () => void;
}
PubSub<in out A>.BackingSubscription<A>.poll(): typeof MutableList.Empty | Apoll()
: import MutableListMutableList.const Empty: typeof MutableList.EmptyDefines the unique symbol used to represent an empty result when taking elements from a MutableList.
This symbol is returned by take when the list is empty, allowing for safe type checking.
When to use
Use to detect that take returned no element before handling the result as a
list item.
Example (Checking for empty results)
import { MutableList } from "effect"
const list = MutableList.make<string>()
// Take from empty list returns Empty symbol
const result = MutableList.take(list)
console.log(result === MutableList.Empty) // true
// Safe pattern for checking emptiness
const processNext = (queue: MutableList.MutableList<string>) => {
const item = MutableList.take(queue)
if (item === MutableList.Empty) {
console.log("Queue is empty")
return null
}
return item.toUpperCase()
}
// Compare with other empty results
MutableList.append(list, "hello")
const next = MutableList.take(list)
console.log(next !== MutableList.Empty) // true, got "hello"
const empty = MutableList.take(list)
console.log(empty === MutableList.Empty) // true, list is empty
The type of the Empty symbol, used for type checking when taking elements from a MutableList.
This provides compile-time safety when checking for empty results.
Example (Handling empty results type-safely)
import { MutableList } from "effect"
const list = MutableList.make<number>()
// Type-safe handling of empty results
const takeAndDouble = (
queue: MutableList.MutableList<number>
): number | null => {
const item: number | MutableList.Empty = MutableList.take(queue)
if (item === MutableList.Empty) {
return null
}
// TypeScript knows item is number here
return item * 2
}
console.log(takeAndDouble(list)) // null (empty list)
MutableList.append(list, 5)
console.log(takeAndDouble(list)) // 10
// Type guard function
const isEmpty = (
result: number | MutableList.Empty
): result is MutableList.Empty => {
return result === MutableList.Empty
}
const value = MutableList.take(list)
if (isEmpty(value)) {
console.log("List is empty")
} else {
console.log("Got value:", value)
}
Empty
if (const message:
| typeof MutableList.Empty
| A
import MutableListMutableList.const Empty: typeof MutableList.EmptyDefines the unique symbol used to represent an empty result when taking elements from a MutableList.
This symbol is returned by take when the list is empty, allowing for safe type checking.
When to use
Use to detect that take returned no element before handling the result as a
list item.
Example (Checking for empty results)
import { MutableList } from "effect"
const list = MutableList.make<string>()
// Take from empty list returns Empty symbol
const result = MutableList.take(list)
console.log(result === MutableList.Empty) // true
// Safe pattern for checking emptiness
const processNext = (queue: MutableList.MutableList<string>) => {
const item = MutableList.take(queue)
if (item === MutableList.Empty) {
console.log("Queue is empty")
return null
}
return item.toUpperCase()
}
// Compare with other empty results
MutableList.append(list, "hello")
const next = MutableList.take(list)
console.log(next !== MutableList.Empty) // true, got "hello"
const empty = MutableList.take(list)
console.log(empty === MutableList.Empty) // true, list is empty
The type of the Empty symbol, used for type checking when taking elements from a MutableList.
This provides compile-time safety when checking for empty results.
Example (Handling empty results type-safely)
import { MutableList } from "effect"
const list = MutableList.make<number>()
// Type-safe handling of empty results
const takeAndDouble = (
queue: MutableList.MutableList<number>
): number | null => {
const item: number | MutableList.Empty = MutableList.take(queue)
if (item === MutableList.Empty) {
return null
}
// TypeScript knows item is number here
return item * 2
}
console.log(takeAndDouble(list)) // null (empty list)
MutableList.append(list, 5)
console.log(takeAndDouble(list)) // 10
// Type guard function
const isEmpty = (
result: number | MutableList.Empty
): result is MutableList.Empty => {
return result === MutableList.Empty
}
const value = MutableList.take(list)
if (isEmpty(value)) {
console.log("List is empty")
} else {
console.log("Got value:", value)
}
Empty) {
return const pollForItem: <A>(
self: Subscription<A>
) => Effect.Effect<A, never, never>
self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<out A>.strategy: PubSub.Strategy<any>(property) Subscription<out A>.strategy: {
shutdown: Effect.Effect<void>;
handleSurplus: (pubsub: PubSub.Atomic<any>, subscribers: PubSub.Subscribers<any>, elements: Iterable<any>, isShutdown: MutableRef.MutableRef<boolean>) => Effect.Effect<boolean>;
onPubSubEmptySpaceUnsafe: (pubsub: PubSub.Atomic<any>, subscribers: PubSub.Subscribers<any>) => void;
completePollersUnsafe: (pubsub: PubSub.Atomic<any>, subscribers: PubSub.Subscribers<any>, subscription: PubSub.BackingSubscription<any>, pollers: MutableList.MutableList<Deferred.Deferred<any, never>>) => void;
completeSubscribersUnsafe: (pubsub: PubSub.Atomic<any>, subscribers: PubSub.Subscribers<any>) => void;
}
PubSub<in out A>.Strategy<any>.onPubSubEmptySpaceUnsafe(pubsub: PubSub<in out A>.Atomic<any>, subscribers: PubSub.Subscribers<any>): voidDescribes how subscribers should signal to publishers waiting for space
to become available in the PubSub that space may be available.
onPubSubEmptySpaceUnsafe(self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<out A>.pubsub: PubSub.Atomic<any>(property) Subscription<out A>.pubsub: {
capacity: number;
isEmpty: () => boolean;
isFull: () => boolean;
size: () => number;
publish: (value: any) => boolean;
publishAll: (elements: Iterable<any>) => Array<any>;
slide: () => void;
subscribe: () => PubSub.BackingSubscription<any>;
replayWindow: () => PubSub.ReplayWindow<any>;
}
self: Subscription<A>(parameter) self: {
pubsub: PubSub.Atomic<any>;
subscribers: PubSub.Subscribers<any>;
subscription: PubSub.BackingSubscription<A>;
pollers: MutableList.MutableList<Deferred.Deferred<any>>;
shutdownHook: Latch.Latch;
shutdownFlag: MutableRef.MutableRef<boolean>;
strategy: PubSub.Strategy<any>;
replayWindow: PubSub.ReplayWindow<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; <…;
}
Subscription<out A>.subscribers: PubSub.Subscribers<any>(property) Subscription<out A>.subscribers: {
clear: () => void;
delete: (key: PubSub.BackingSubscription<any>) => boolean;
forEach: (callbackfn: (value: Set<MutableList.MutableList<Deferred.Deferred<any, never>>>, key: PubSub.BackingSubscription<any>, map: Map<PubSub.BackingSubscription<any>, Set<MutableList.MutableList<Deferred.Deferred<any, never>>>>) => void, thisAr…;
get: (key: PubSub.BackingSubscription<any>) => Set<MutableList.MutableList<Deferred.Deferred<any, never>>> | undefined;
has: (key: PubSub.BackingSubscription<any>) => boolean;
set: (key: PubSub.BackingSubscription<any>, value: Set<MutableList.MutableList<Deferred.Deferred<any, never>>>) => PubSub.Subscribers<any>;
size: number;
entries: () => MapIterator<[PubSub.BackingSubscription<any>, Set<MutableList.MutableList<Deferred.Deferred<any, never>>>]>;
keys: () => MapIterator<PubSub.BackingSubscription<any>>;
values: () => MapIterator<Set<MutableList.MutableList<Deferred.Deferred<any, never>>>>;
}
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(const message: Amessage)
}
})Referenced by 3 symbols