# filter

Filters elements of an Fx using a predicate function.

## Signatures

```ts
export declare const filter: {
    <A>(f: (a: A) => boolean): <E, R>(self: Fx<A, E, R>) => Fx<A, E, R>;
    <A, E, R>(self: Fx<A, E, R>, f: (a: A) => boolean): Fx<A, E, R>;
};
```

## Why

`filter` keeps the source's push timing and order while allowing zero or one output for each
input. The predicate is synchronous, so it adds neither failures nor service requirements.

## Ownership and lifetime

No resource or buffer is acquired. The predicate runs while the source invokes the downstream
sink; source failure, completion, and interruption are forwarded unchanged.

## Examples

```ts
import { Fx } from "@typed/fx"

const even = Fx.fromIterable([1, 2, 3, 4]).pipe(Fx.filter((n) => n % 2 === 0))
```