# union

Merge another HashMap into this one.

## Signatures

```ts
export declare const union: {
    <K, V>(that: HashMap.HashMap<K, V>): <E, R>(ref: RefHashMap<K, V, E, R>) => Effect.Effect<HashMap.HashMap<K, V>, E, R>;
    <K, V, E, R>(ref: RefHashMap<K, V, E, R>, that: HashMap.HashMap<K, V>): Effect.Effect<HashMap.HashMap<K, V>, E, R>;
};
```

## Why

Combines bulk hash map changes in one committed value, giving subscribers one coherent update
rather than a partially applied sequence.

## Ownership and lifetime

Running union performs one serialized hash map transition and resolves with its committed value.
It acquires no resource; failures and services remain those of the source ref.