# RefIterable.countBy

Count elements satisfying a predicate.

## Signatures

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

## Why

Makes count by a live projection of the iterable; consumers can sample it now or observe it
without copying the source state.

## Ownership and lifetime

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