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variable / @typed/fx/Fx

filterMapLoop

Loops over an Fx with an accumulator, producing an optional new value for each element. If the function returns None, the element is filtered out.

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

Follow one execution

Loops over an Fx with an accumulator, producing an optional new value for each element. If the function returns None, the element is filtered out.

Fx timelinefilterMapLoop can update state without emitting a valuefilterMapLoop
OperatorfilterMapLoop(0, everyOther)
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 seed is copied into each run and retained only until that run ends. The callback is pure, and source failure, services, completion, and interruption are preserved.

Source implementation · Learn the surrounding model

Compare all Fx timelines →

Import

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

Signatures

export declare const filterMapLoop: {
    <B, A, C>(seed: B, f: (acc: B, a: A) => readonly [
        Option.Option<C>,
        B
    ]): <E, R>(self: Fx<A, E, R>) => Fx<C, E, R>;
    <A, E, R, B, C>(self: Fx<A, E, R>, seed: B, f: (acc: B, a: A) => readonly [
        Option.Option<C>,
        B
    ]): Fx<C, E, R>;
};

Why

filterMapLoop combines per-run state, mapping, and filtering in one ordered step. Every input updates the accumulator and may emit one Some; None still commits the returned next state.

Ownership and lifetime

The seed is copied into each run and retained only until that run ends. The callback is pure, and source failure, services, completion, and interruption are preserved.

Examples

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

const everyOther = Fx.fromIterable([1, 2, 3]).pipe(
  Fx.filterMapLoop(0, (index, value) => [index % 2 === 0 ? Option.some(value) : Option.none(), index + 1]),
)

Other public imports

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

Source