<K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>Returns an iterable of all keys currently stored in the RcMap.
When to use
Use to inspect which keys currently have stored resources in an RcMap.
Details
If the RcMap has been closed, the effect is interrupted.
Example (Listing keys)
import { Effect, RcMap } from "effect"
Effect.gen(function*() {
const map = yield* RcMap.make({
lookup: (key: string) => Effect.succeed(`value-${key}`)
})
// Add some resources to the map
yield* RcMap.get(map, "foo")
yield* RcMap.get(map, "bar")
yield* RcMap.get(map, "baz")
// Get all keys currently in the map
const allKeys = yield* RcMap.keys(map)
console.log(allKeys) // ["foo", "bar", "baz"]
}).pipe(Effect.scoped)export const const keys: <K, A, E>(
self: RcMap<K, A, E>
) => Effect.Effect<Iterable<K>>
Returns an iterable of all keys currently stored in the RcMap.
When to use
Use to inspect which keys currently have stored resources in an RcMap.
Details
If the RcMap has been closed, the effect is interrupted.
Example (Listing keys)
import { Effect, RcMap } from "effect"
Effect.gen(function*() {
const map = yield* RcMap.make({
lookup: (key: string) => Effect.succeed(`value-${key}`)
})
// Add some resources to the map
yield* RcMap.get(map, "foo")
yield* RcMap.get(map, "bar")
yield* RcMap.get(map, "baz")
// Get all keys currently in the map
const allKeys = yield* RcMap.keys(map)
console.log(allKeys) // ["foo", "bar", "baz"]
}).pipe(Effect.scoped)
keys = <function (type parameter) K in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>K, function (type parameter) A in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>A, function (type parameter) E in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>E>(self: RcMap<K, A, E>(parameter) self: {
lookup: (key: K) => Effect.Effect<A, E, Scope.Scope>;
context: Context.Context<never>;
scope: Scope.Scope;
idleTimeToLive: (key: K) => Duration.Duration;
capacity: number;
state: State<K, A, 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; <…;
}
interface RcMap<in out K, in out A, in out E = never>An RcMap is a reference-counted map data structure that manages the lifecycle
of resources indexed by keys. Resources are lazily acquired and automatically
released when no longer in use.
When to use
Use to share scoped resources by key while automatically releasing them after
their last active reference is gone.
Example (Inspecting a reference-counted map)
import { Effect, RcMap } from "effect"
Effect.gen(function*() {
// Create an RcMap that manages database connections
const dbConnectionMap = yield* RcMap.make({
lookup: (dbName: string) =>
Effect.acquireRelease(
Effect.succeed(`Connection to ${dbName}`),
(conn) => Effect.log(`Closing ${conn}`)
),
capacity: 10,
idleTimeToLive: "5 minutes"
})
// The RcMap interface provides access to:
// - lookup: Function to acquire resources
// - capacity: Maximum number of resources
// - idleTimeToLive: Time before idle resources are released
// - state: Current state of the map
console.log(`Capacity: ${dbConnectionMap.capacity}`)
}).pipe(Effect.scoped)
RcMap<function (type parameter) K in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>K, function (type parameter) A in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>A, function (type parameter) E in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>E>): 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<interface Iterable<T, TReturn = any, TNext = any>Iterable<function (type parameter) K in <K, A, E>(self: RcMap<K, A, E>): Effect.Effect<Iterable<K>>K>> => {
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(() =>
self: RcMap<K, A, E>(parameter) self: {
lookup: (key: K) => Effect.Effect<A, E, Scope.Scope>;
context: Context.Context<never>;
scope: Scope.Scope;
idleTimeToLive: (key: K) => Duration.Duration;
capacity: number;
state: State<K, A, 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; <…;
}
RcMap<K, A, E>.state: State<K, A, E>state._tag: "Open" | "Closed"_tag === "Closed" ? 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 : 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 MutableHashMapMutableHashMap.const keys: <K, V>(
self: MutableHashMap<K, V>
) => Iterable<K>
Returns an iterable over the keys in the MutableHashMap.
When to use
Use to iterate over the keys currently stored in a mutable hash map.
Example (Reading keys)
import { MutableHashMap } from "effect"
const map = MutableHashMap.make(
["apple", 1],
["banana", 2],
["cherry", 3]
)
const allKeys = Array.from(MutableHashMap.keys(map))
console.log(allKeys) // ["apple", "banana", "cherry"]
// Useful for iteration or validation
const hasRequiredKeys = allKeys.includes("apple") && allKeys.includes("banana")
keys(self: RcMap<K, A, E>(parameter) self: {
lookup: (key: K) => Effect.Effect<A, E, Scope.Scope>;
context: Context.Context<never>;
scope: Scope.Scope;
idleTimeToLive: (key: K) => Duration.Duration;
capacity: number;
state: State<K, A, 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; <…;
}
RcMap<K, A, E>.state: State<K, A, E>(property) RcMap<K, A, E>.state: {
_tag: "Open";
map: MutableHashMap.MutableHashMap<K, Entry<A, E>>;
}
State<K, A, E>.Open<K, A, E>.map: MutableHashMap.MutableHashMap<K, Entry<A, E>>(property) State<K, A, E>.Open<K, A, E>.map: {
backing: Map<K, V>;
buckets: Map<number, NonEmptyArray<K>>;
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;
}