Structs

Struct is the most important composite type in CUE. Its members are called fields.

A struct field may be optional. One can use an optional field to indicate what the type should be if it were specified. A regular (or required) field, on the other hand, must be made concrete for a configuration to be converted to, say, JSON.

It is okay for an optional field to be bottom (_|_). This just means that field may not be specified.

#a: {
    foo?: int
    bar?: string
    baz?: string
}
b: #a & {
    foo:  3
    baz?: 2  // baz?: _|_
}

b: {
    foo: 3
}