Definitions

In CUE, schema are typically written as Definitions, using a :: instead of :. This tells CUE that they are to be used for validation and should not be output as data; it is okay for them to remain unspecified.

A definition also tells CUE the full set of allowed fields. In other words, definitions define “closed” structs. Including a ... in struct keeps it open.

Conn :: {
    address:  string
    port:     int
    protocol: string
    // ...    // uncomment this to allow any field
}

lossy: Conn & {
    address:  "1.2.3.4"
    port:     8888
    protocol: "udp"
    // foo: 2 // uncomment this to get an error
}

{
    "lossy": {
        "address": "1.2.3.4",
        "port": 8888,
        "protocol": "udp"
    }
}