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variable / @typed/fx/Fx

scan

Scans the stream with a pure function, emitting the accumulated state after each element. Emits the initial value first, then for each input a emits f(state, a) and updates state.

Semantics align with Effect Stream’s scan: output is initial, f(initial, a1), f(..., a2), …

Package version
2.0.0-beta.7
Category
Stateful transforms
Since
1.0.0

Follow one execution

Scans the stream with a pure function, emitting the accumulated state after each element. Emits the initial value first, then for each input a emits f(state, a) and updates state. Semantics align with Effect Stream’s scan: output is initial, f(initial, a1), f(..., a2), …

Fx timelinescan emits its seed and every accumulated valuescan
Operatorscan(100, add)
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.

Each run owns an independent accumulator and releases it when the run ends. The pure reducer acquires no resources and preserves source failures, services, and interruption.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

export declare const scan: {
    <S, A>(initial: S, f: (s: S, a: A) => S): <E, R>(fx: Fx<A, E, R>) => Fx<S, E, R>;
    <A, E, R, S>(fx: Fx<A, E, R>, initial: S, f: (s: S, a: A) => S): Fx<S, E, R>;
};

Why

scan exposes every accumulated state, including initial before the source is run. For n inputs it emits n + 1 states in order, making incremental reduction observable.

Ownership and lifetime

Each run owns an independent accumulator and releases it when the run ends. The pure reducer acquires no resources and preserves source failures, services, and interruption.

Examples

import { Fx } from "@typed/fx"

const totals = Fx.fromIterable([1, 2, 3]).pipe(Fx.scan(0, (sum, n) => sum + n))

Other public imports

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

Source