doc/tutorial/basics/0_intro/43_schema.txt

cue export schema.cue
cmp stdout expect-stdout-cue

-- frontmatter.toml --
title = "Definitions"
description = ""

-- text.md --
In CUE, schemas are typically written as Definitions.
A definition is a field which identifier starts with
a `#` or `_#`.
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.

-- schema.cue --
#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
}

-- expect-stdout-cue --
{
    "lossy": {
        "address": "1.2.3.4",
        "port": 8888,
        "protocol": "udp"
    }
}