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variable / @typed/fx/Fx

concat

Concatenates two Fx streams: runs the first to completion, then runs the second. Emits all values from the first stream in order, then all values from the second stream in order.

Completion: The concatenated stream completes when the second stream completes (the first must complete before the second starts).

Failures: A source Cause is delivered to the Sink. Because Fx.run is infallible, delivery alone does not suppress the continuation: the right source is run after the left run returns.

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

Follow one execution

Subscribe to live only after the cached run returns.

Fx timelineconcat(cached, live)concat
Operatorconcat(cached, live)
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.

Only one source is active at a time. Interruption stops the active source and prevents later acquisition. Failure is Sink delivery rather than a failing Fx.run Effect, so it does not by itself prevent the right source from being acquired.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

export declare const concat: {
    <A2, E2, R2>(that: Fx<A2, E2, R2>): <A, E, R>(self: Fx<A, E, R>) => Fx<A | A2, E | E2, R | R2>;
    <A, E, R, A2, E2, R2>(self: Fx<A, E, R>, that: Fx<A2, E2, R2>): Fx<A | A2, E | E2, R | R2>;
};

Why

concat makes subscription order explicit: the right source is not acquired until the left run returns. It preserves every value and each source’s internal order. A failure callback can be followed by continuation values when the Sink handles the Cause without interrupting the run.

Ownership and lifetime

Only one source is active at a time. Interruption stops the active source and prevents later acquisition. Failure is Sink delivery rather than a failing Fx.run Effect, so it does not by itself prevent the right source from being acquired.

Examples

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

const program = Effect.gen(function* () {
  const deliveries = yield* Ref.make<Array<string>>([])
  const sink = Sink.make(
    () => Ref.update(deliveries, (xs) => [...xs, "failure"]),
    (value: string) => Ref.update(deliveries, (xs) => [...xs, value])
  )
  yield* Fx.concat(Fx.fail("left failed"), Fx.succeed("right still runs")).run(sink)
  return yield* Ref.get(deliveries) // ["failure", "right still runs"]
})

Other public imports

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

Source