<B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (
self: SubscriptionRef<A>
) => Effect.Effect<B, E, R>
<A, B, E, R>(
self: SubscriptionRef<A>,
modify: (a: A) => Effect.Effect<readonly [B, A], E, R>
): Effect.Effect<B, E, R>Modifies the SubscriptionRef atomically with an effectful function that
computes a return value and a new value, notifying subscribers of the
change.
Example (Modifying with an effect)
import { Effect, SubscriptionRef } from "effect"
const program = Effect.gen(function*() {
const ref = yield* SubscriptionRef.make(10)
const result = yield* SubscriptionRef.modifyEffect(
ref,
(n) => Effect.succeed([`Doubled from ${n}`, n * 2] as const)
)
console.log(result)
const newValue = yield* SubscriptionRef.get(ref)
console.log("New value:", newValue)
})export const const modifyEffect: {
<B, A, E, R>(
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
): (
self: SubscriptionRef<A>
) => Effect.Effect<B, E, R>
<A, B, E, R>(
self: SubscriptionRef<A>,
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
): Effect.Effect<B, E, R>
}
Modifies the SubscriptionRef atomically with an effectful function that
computes a return value and a new value, notifying subscribers of the
change.
Example (Modifying with an effect)
import { Effect, SubscriptionRef } from "effect"
const program = Effect.gen(function*() {
const ref = yield* SubscriptionRef.make(10)
const result = yield* SubscriptionRef.modifyEffect(
ref,
(n) => Effect.succeed([`Doubled from ${n}`, n * 2] as const)
)
console.log(result)
const newValue = yield* SubscriptionRef.get(ref)
console.log("New value:", newValue)
})
modifyEffect: {
<function (type parameter) B in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>B, function (type parameter) A in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>A, function (type parameter) E in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>E, function (type parameter) R in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>R>(
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
a: Aa: function (type parameter) A in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>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<readonly [function (type parameter) B in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>B, function (type parameter) A in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>A], function (type parameter) E in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>E, function (type parameter) R in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>R>
): (self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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 SubscriptionRef<in out A>A mutable reference whose updates are serialized and published to
subscribers.
When to use
Use to observe the current value and subsequent updates as a
stream.
The SubscriptionRef namespace containing type definitions associated with
subscription references.
SubscriptionRef<function (type parameter) A in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>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) B in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>B, function (type parameter) E in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>E, function (type parameter) R in <B, A, E, R>(modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): (self: SubscriptionRef<A>) => Effect.Effect<B, E, R>R>
<function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A, function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R>(
self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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 SubscriptionRef<in out A>A mutable reference whose updates are serialized and published to
subscribers.
When to use
Use to observe the current value and subsequent updates as a
stream.
The SubscriptionRef namespace containing type definitions associated with
subscription references.
SubscriptionRef<function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A>,
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
a: Aa: function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>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<readonly [function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A], function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R>
): import EffectEffect.interface Effect<out A, out E = never, out R = never>The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
Effect<function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R>
} = dual<(...args: Array<any>) => any, <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>) => Effect.Effect<B, E, R>>(arity: 2, body: <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>) => Effect.Effect<B, E, R>): ((...args: Array<any>) => any) & (<A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>) => Effect.Effect<B, 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, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A, function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R>(
self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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 SubscriptionRef<in out A>A mutable reference whose updates are serialized and published to
subscribers.
When to use
Use to observe the current value and subsequent updates as a
stream.
The SubscriptionRef namespace containing type definitions associated with
subscription references.
SubscriptionRef<function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A>,
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
a: Aa: function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>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<readonly [function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) A in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>A], function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R>
): import EffectEffect.interface Effect<out A, out E = never, out R = never>The Effect interface defines a value that lazily describes a workflow or
job. The workflow requires some context R, and may fail with an error of
type E, or succeed with a value of type A.
When to use
Use when you need to represent a lazy, composable workflow that can require
services, fail with a typed error, or succeed with a typed value.
Details
Effect values model resourceful interaction with the outside world,
including synchronous, asynchronous, concurrent, and parallel interaction.
They use a fiber-based concurrency model, with built-in support for
scheduling, fine-grained interruption, structured concurrency, and high
scalability.
To run an Effect value, you need a Runtime, which is a type that is
capable of executing Effect values.
Effect<function (type parameter) B in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>B, function (type parameter) E in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>E, function (type parameter) R in <A, B, E, R>(self: SubscriptionRef<A>, modify: (a: A) => Effect.Effect<readonly [B, A], E, R>): Effect.Effect<B, E, R>R> =>
self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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; <…;
}
SubscriptionRef<in out A>.semaphore: Semaphore.Semaphore(property) SubscriptionRef<in out A>.semaphore: {
resize: (this: Semaphore, permits: number) => Effect.Effect<void>;
withPermits: (this: Semaphore, permits: number) => <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
withPermit: <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<A, E, R>;
withPermitsIfAvailable: (this: Semaphore, permits: number) => <A, E, R>(self: Effect.Effect<A, E, R>) => Effect.Effect<Option.Option<A>, E, R>;
take: (this: Semaphore, permits: number) => Effect.Effect<number>;
release: (this: Semaphore, permits: number) => Effect.Effect<number>;
releaseAll: Effect.Effect<number>;
}
Semaphore.withPermit<A, E, R>(self: Effect.Effect<A, E, R>): Effect.Effect<A, E, R>Runs an effect with the given number of permits and releases the permits
when the effect completes.
When to use
Use to run an effect while holding exactly one semaphore permit.
Details
This function acquires the specified number of permits before executing
the provided effect. Once the effect finishes, the permits are released.
If insufficient permits are available, the function will wait until they
are released by other tasks.
withPermit(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(() =>
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>
}
modify: (
a: A
) => Effect.Effect<readonly [B, A], E, R>
self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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; <…;
}
SubscriptionRef<A>.value: Avalue), ([b: Bb, newValue: AnewValue]) => {
const setUnsafe: <A>(
self: SubscriptionRef<A>,
value: A
) => void
self: SubscriptionRef<A>(parameter) self: {
value: A;
semaphore: Semaphore.Semaphore;
pubsub: PubSub.PubSub<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; <…;
}
newValue: AnewValue)
return b: Bb
})
)))