next: Disjunctions

## Structs

Struct is the most important composite type in CUE. It’s 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.

structs.cue

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

\$ cue eval -c structs.cue

b: {
foo: 3
}