CUE’s most important composite type is the struct, also known as a map. Its members are called fields.

A regular field (field: value) must be made concrete if it is exported to a data format such as JSON or YAML.

An optional field constraint (field?: value) only restricts the field’s value if it is specified elsewhere as a regular field. An optional field with a value of bottom (_|_) means the field cannot be specified.

A required field constraint (field!: value) requires that the field be specified as a regular field if it’s exported.

A: {
	foo!:  int    // required
	bar?:  string // optional
	baz:   float  // regular
	quux?: _|_    // cannot be specified
}

A: {
	bar: "42"
	baz: 42.0
}

$ cue export file.cue
A.foo: field is required but not present:
    ./file.cue:2:2