This Commented CUE demonstrates how to disallow specific data fields inside an otherwise open schema.

package example

// This schema is open because we do not know
// which additional fields might be present.
name!:    string
address!: string

// We must not allow our data model to contain
// people's ages, so we disallow the age field.
age?: error("field name forbidden")

name: Charlie Cartwright
address: Ripon, North Yorkshire
species: cat
age: 15.5

$ cue vet -c . data.yaml
age: field name forbidden:
    ./file.cue:10:7

Another common way to prevent the existence of unwanted data fields is to rely on the closedness property that comes from the use of definitions or the close built-in function. However, in situations such as the example above, closedness cannot be used to disallow specific fields because the schema author cannot specify each and every acceptable field - and therefore the schema must be left open.

Related content

• The CUE Tour: Definitions
• Glossary: the close built-in function
• Reference: The CUE Language Specification: the built-in function error