Constraints specify what values are allowed. Constraints are values because all types are values.

Constraints can reduce boilerplate and simplify the specification of data. If a constraint specifies a field then the field and its value are present everywhere the constraint is unified, and don’t need to be repeated.

In this example, the cue export output includes the data that Viola is a human. This is because the viola field is specified as the unification of a struct which declares Viola’s name and age, and the person constraint which additionally declares the human field. This unification uses the explicit unification operator &.

person: {
	name:  string
	age:   int & >=0
	human: true // People are always humans

viola: person & {
	name: "Viola"
	age:  38
$ cue export file.cue -e viola
    "name": "Viola",
    "age": 38,
    "human": true

Unification succeeds because person’s name field constraint of string doesn’t conflict with the concrete value "Viola", and age’s unified constraint of int & >=0 doesn’t conflict with the value 38.