CUE merges the concepts of values and types. In CUE, types are values.

A field can be specified with:

  • a concrete value such as "foo", 42, or true - something that could be represented in JSON,
  • a type such as int or string,
  • or something in between the two such as >=500, or !="foo" - not concrete, but more specific than a basic type.

The following examples show a CUE schema; a typical CUE constraint that refines the schema; and some concrete values that satisfy both the constraint and, therefore, the schema.

CUE schema
municipality: {
	name:    string
	pop:     int
	capital: bool
CUE constraint
largeCapital: {
	name:    string
	pop:     >5M
	capital: true
Concrete values
kinshasa: {
	name:    "Kinshasa"
	pop:     16.32M
	capital: true

With CUE, we generally start with a broad definition of a schema describing all possible instances and then progressively narrow down these definitions for a particular use case until a concrete data instance remains.