# filterMap

Filter and map entries into a read-only derived trie.

## Signatures

```ts
export declare const filterMap: {
    <V>(f: (value: V, key: string) => Option.Option<V>): <E, R>(ref: RefTrie<V, E, R>) => RefSubject.Computed<Trie.Trie<V>, E, R>;
    <V, E, R>(ref: RefTrie<V, E, R>, f: (value: V, key: string) => Option.Option<V>): RefSubject.Computed<Trie.Trie<V>, E, R>;
};
```

## Why

Creates a read-only Computed trie that can transform or omit each entry; it leaves the writable
source trie unchanged.

## Ownership and lifetime

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