<OutElem, OutErr, OutDone, Env>(
self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>
): Effect.Effect<Option.Option<OutElem>, OutErr, Env>Runs a channel until the first output element is available, returning it in
an Option.
Details
Returns Option.some with the first output element, or Option.none if the
channel completes without emitting output.
export const const runHead: <
OutElem,
OutErr,
OutDone,
Env
>(
self: Channel<
OutElem,
OutErr,
OutDone,
unknown,
unknown,
unknown,
Env
>
) => Effect.Effect<
Option.Option<OutElem>,
OutErr,
Env
>
Runs a channel until the first output element is available, returning it in
an Option.
Details
Returns Option.some with the first output element, or Option.none if the
channel completes without emitting output.
runHead = <function (type parameter) OutElem in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutElem, function (type parameter) OutErr in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutErr, function (type parameter) OutDone in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutDone, function (type parameter) Env in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>Env>(
self: Channel<
OutElem,
OutErr,
OutDone,
unknown,
unknown,
unknown,
Env
>
(parameter) self: {
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 Channel<out OutElem, out OutErr = never, out OutDone = void, in InElem = unknown, in InErr = unknown, in InDone = unknown, out Env = never>A Channel is a nexus of I/O operations, which supports both reading and
writing. A channel may read values of type InElem and write values of type
OutElem. When the channel finishes, it yields a value of type OutDone. A
channel may fail with a value of type OutErr.
Details
Channels are the foundation of Streams: both streams and sinks are built on
channels. Most users shouldn't have to use channels directly, as streams and
sinks are much more convenient and cover all common use cases. However, when
adding new stream and sink operators, or doing something highly specialized,
it may be useful to use channels directly.
Channels compose in a variety of ways:
- Piping: One channel can be piped to another channel, assuming the
input type of the second is the same as the output type of the first.
- Sequencing: The terminal value of one channel can be used to create
another channel, and both the first channel and the function that makes
the second channel can be composed into a channel.
- Concatenating: The output of one channel can be used to create other
channels, which are all concatenated together. The first channel and the
function that makes the other channels can be composed into a channel.
Example (Typing channels)
import type { Channel } from "effect"
// A channel that outputs numbers and requires no environment
type NumberChannel = Channel.Channel<number>
// A channel that outputs strings, can fail with Error, completes with boolean
type StringChannel = Channel.Channel<string, Error, boolean>
// A channel with all type parameters specified
type FullChannel = Channel.Channel<
string, // OutElem - output elements
Error, // OutErr - output errors
number, // OutDone - completion value
number, // InElem - input elements
string, // InErr - input errors
boolean, // InDone - input completion
{ db: string } // Env - required environment
>
Channel<function (type parameter) OutElem in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutElem, function (type parameter) OutErr in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutErr, function (type parameter) OutDone in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutDone, unknown, unknown, unknown, function (type parameter) Env in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>Env>
): 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<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) OutElem in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutElem>, function (type parameter) OutErr in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutErr, function (type parameter) Env in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>Env> =>
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(() => {
let let head: Option.Option<OutElem>head = import OptionOption.const none: <A = never>() => Option<A>Creates an Option representing the absence of a value.
When to use
Use to represent a missing or uninitialized value, such as returning "no
result" from a function.
Details
- Returns
Option<never>, which is a subtype of Option<A> for any A
- Always returns the same singleton instance
Example (Creating an empty Option)
import { Option } from "effect"
// ┌─── Option<never>
// ▼
const noValue = Option.none()
console.log(noValue)
// Output: { _id: 'Option', _tag: 'None' }
none<function (type parameter) OutElem in <OutElem, OutErr, OutDone, Env>(self: Channel<OutElem, OutErr, OutDone, unknown, unknown, unknown, Env>): Effect.Effect<Option.Option<OutElem>, OutErr, Env>OutElem>()
return const runWith: <
OutElem,
OutErr,
OutDone,
Env,
EX,
RX,
AH = OutDone,
EH = never,
RH = never
>(
self: Channel<
OutElem,
OutErr,
OutDone,
unknown,
unknown,
unknown,
Env
>,
f: (
pull: Pull.Pull<OutElem, OutErr, OutDone>
) => Effect.Effect<void, EX, RX>,
onHalt?: (
leftover: OutDone
) => Effect.Effect<AH, EH, RH>
) => Effect.Effect<
AH,
Pull.ExcludeDone<EX> | EH,
Env | RX | RH
>
self: Channel<
OutElem,
OutErr,
OutDone,
unknown,
unknown,
unknown,
Env
>
(parameter) self: {
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; <…;
}
pull: Pull.Pull<
OutElem,
OutErr,
OutDone,
never
>
(parameter) 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;
}
pull: Pull.Pull<
OutElem,
OutErr,
OutDone,
never
>
(parameter) 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;
}
Pipeable.pipe<Pull.Pull<OutElem, OutErr, OutDone, never>, Effect.Effect<Option.Option<OutElem>, any, never>, Effect.Effect<void, unknown, unknown>>(this: Pull.Pull<OutElem, OutErr, OutDone, never>, ab: (_: Pull.Pull<OutElem, OutErr, OutDone, never>) => Effect.Effect<Option.Option<OutElem>, any, never>, bc: (_: Effect.Effect<Option.Option<OutElem>, any, never>) => Effect.Effect<void, unknown, unknown>): Effect.Effect<...> (+21 overloads)pipe(
import EffectEffect.const asSome: <A, E, R>(
self: Effect<A, E, R>
) => Effect<Option<A>, E, R>
Maps the success value of an Effect to Some, preserving failures.
Example (Wrapping success in Option.some)
import { Effect } from "effect"
const program = Effect.asSome(Effect.succeed(42))
Effect.runPromise(program).then(console.log)
// { _id: 'Option', _tag: 'Some', value: 42 }
asSome,
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>
}
head_: Option.Option<OutElem>head_) => {
let head: Option.Option<OutElem>head = head_: Option.Option<OutElem>head_
return import CauseCause.done()
})
), () => 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(let head: Option.Option<OutElem>head))
})