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variable / @typed/fx/Fx

prepend

Prepends a value to the beginning of an Fx.

Package version
2.0.0-beta.7
Category
Combining sources
Since
1.0.0

Follow one execution

Deliver the start value before subscribing to the source.

Fx timelineprepend(start)prepend
Operatorprepend(start)
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.

The value is emitted exactly once before the source run starts, then every source success or failure is delivered. Interruption or a defect in the Sink’s prepended-value handler prevents the source from starting. The operation acquires no resource and retains source failure, services, and lifetime.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

export declare const prepend: {
    <B>(value: B): <A, E, R>(fx: Fx<A, E, R>) => Fx<B | A, E, R>;
    <A, E, R, B>(fx: Fx<A, E, R>, value: B): Fx<B | A, E, R>;
};

Why

prepend emits an initial marker through the same Sink before the source is observed, without allocating a separate collection or state holder.

Ownership and lifetime

The value is emitted exactly once before the source run starts, then every source success or failure is delivered. Interruption or a defect in the Sink’s prepended-value handler prevents the source from starting. The operation acquires no resource and retains source failure, services, and lifetime.

Examples

import { Fx } from "@typed/fx"
import { Effect } from "effect"

const framed = Fx.prepend(Fx.fromIterable([1, 2]), "start")
Effect.runPromise(Fx.collectAll(framed)).then(console.log)
// ["start", 1, 2]

Other public imports

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

Source