In CUE, schemas are typically written as definitions. A definition is a field whose identifier starts with # or _#.

Because CUE knows that definitions are used for validation, they aren’t output as data.
It’s normal for definitions to specify fields that don’t have concrete values, such as types.

A definition also tells CUE the complete set of allowed fields in a struct.
Referring to a definition closes any structs and lists recursively. Structs and lists can be left open using ....

message: "Hello, \(#Name)!"
#Name:   "world"

#A: {
	n: int
	// Uncomment this to allow any field.
	// ...
}

valid: #A & {n: 3}
invalid: #A & {N: 3}

$ cue eval -ic file.cue
message: "Hello, world!"
valid: {
    n: 3
}
invalid: {
    N: _|_ // invalid.N: field not allowed
    n: int
}