# Composition Expression

A composition expression combines multiple values, constraints, or structures.

## `&`

`&` performs constraint-preserving composition.

```dcdl
Port = Int & >= 1 & <= 65535;
Config = MyConfig & { port = 8000; };
```

Because object fields present on only one side are retained, `&` has no direction.
Use [`as`](./ascription.md) to verify that the left side is narrower than the right and treat the right side as the field domain.

## `//`

`//` performs a right-biased structural patch.

```dcdl
Patched = Base // {
    feature_hoge.enable = false;
};
```

See [Operators](../operators.md) for details.