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function / @typed/fx

Fx.first

Returns the first value emitted by the Fx wrapped in an Option. If the Fx is empty, returns None.

Package version
2.0.0-beta.13
Category
Collecting values
Since
1.0.0
Member of
Fx

Follow one execution

Resolve with Some(firstValue) and stop the source. An empty source instead returns None.

Fx timelinefirst(source)first
Operatorfirst(source)
Read this diagram

Follow each lane from left to right. Events stacked vertically share a tick; the green cursor marks the current time across every lane.

  • A value
    The text inside the pill is the emitted value.
  • Work starts
    The raised chevron starts an inner run (^ in the source).
  • The run returns
    The vertical bar ends this lane’s run.
  • A cause is delivered
    The exclamation mark belongs to this lane.
  • Work is interrupted
    The cross marks cancellation of this run.
  • Current time
    The line and diamond move together across all lanes.
  • Happening now
    A highlighted event is at the current tick.
  • Still ahead
    Muted, dashed values have not happened yet.
  • Time continues
    The lane’s arrow is not a return marker. An empty stretch can be quiet work that is still running.

Illustrated ticks start at 0. At 1×, one illustrated tick takes one second; captions specify real durations when timing matters. A cause or interruption belongs to its lane, and other work may continue. Scroll horizontally to inspect the rest of a long timeline.

Running the Effect subscribes once and stops upstream after the first value. It returns None only when the source completes first; a source failure is still E. Interruption cleans up the active source.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

import { Fx } from "@typed/fx";

Access Fx.first through the imported Fx export. Its declaration below describes the member.

This public exposure is a re-export. Its import path is supported; declaration documentation is shared with the other public exposures below.

Signatures

export declare function first<A, E, R>(fx: Fx<A, E, R>): Effect<Option<A>, E, R>;

Why

first safely crosses from a possibly empty push source to one Effect value without inventing an error for the zero-emission case.

Ownership and lifetime

Running the Effect subscribes once and stops upstream after the first value. It returns None only when the source completes first; a source failure is still E. Interruption cleans up the active source.

Examples

import { Effect, Option } from "effect"
import { first, fromIterable } from "@typed/fx/Fx"

const program = first(fromIterable([10, 20])).pipe(
  Effect.map(Option.getOrElse(() => 0))
)

Other public imports

These import paths expose the same declaration. Each page retains its own public name and signature.

Source