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variable / @typed/fx/Fx

mergeLeft

Merges two Fx streams and emits only values from the left stream. Both streams run concurrently; completion when both complete.

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

Follow one execution

The server still runs and can fail, but only local values are delivered; wait for both completions.

Fx timelinemergeLeft(local, server)mergeLeft
OperatormergeLeft(local, server)
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.

Both sources are acquired together and owned by the consuming run. Interruption cancels both and normal completion waits for both. A Cause from either source is delivered to the Sink but does not intrinsically cancel its sibling; right successes remain suppressed by the filter.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

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

Why

mergeLeft keeps the right producer’s lifetime and failure signal while suppressing its values. Every left value is emitted once; right emissions only affect scheduling.

Ownership and lifetime

Both sources are acquired together and owned by the consuming run. Interruption cancels both and normal completion waits for both. A Cause from either source is delivered to the Sink but does not intrinsically cancel its sibling; right successes remain suppressed by the filter.

Examples

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

const program = Effect.gen(function* () {
  const failures = yield* Ref.make(0)
  yield* Fx.mergeLeft(Fx.succeed("visible"), Fx.fail("hidden side failed")).run(
    Sink.make(() => Ref.update(failures, (n) => n + 1), () => Effect.void)
  )
  return yield* Ref.get(failures) // 1; failure is still delivered
})

Other public imports

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

Source