# filter

Filter the value inside the Option of a RefOption.

## Signatures

```ts
export declare const filter: {
    <A>(predicate: (a: A) => boolean): <E, R>(ref: RefOption<A, E, R>) => RefSubject.Computed<Option.Option<A>, E, R>;
    <A, E, R>(ref: RefOption<A, E, R>, predicate: (a: A) => boolean): RefSubject.Computed<Option.Option<A>, E, R>;
};
```

## Why

Projects option state with filter for both current reads and future pushes, avoiding a second
mutable cache of the derived value.

## Ownership and lifetime

The filter view retains no independent state. An Effect read samples the source once; Fx
observation follows later pushes and its observing Scope owns subscription cleanup.