# Sink.filterMapLoop

Threads pure state through successes and optionally forwards a derived value.

## Signatures

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

## Why

It combines consumer-side state, filtering, and mapping in one ordered transition.

## Ownership and lifetime

Each adapted sink owns its seed. Every input updates state synchronously; `Some` emits once and
`None` emits nothing. Failures pass through. Reusing the sink continues from its retained seed;
create a new sink to reset. The adapter acquires no resources.

## Examples

```ts
import { Effect, Option } from "effect"
import * as Sink from "@typed/fx/Sink"
const target = Sink.make(Effect.logError, Effect.log)
const everyOther = Sink.filterMapLoop(target, false, (skip, n: number) => [skip ? Option.none() : Option.some(n), !skip])
```