Closed structs

Struct is the most important composite type in CUE.

A struct may be open or closed. A closed struct may only be merged with structs that have fields that it defines to be allowed. In other words, closing a struct is equivalent to requiring that all other values be undefined.

A closed struct can be created using the close builtin, but are more commonly defined using a definition, defined next.

a: close({
    field: int
})

b: a & {
    feild: 3
}

a: {
    field: int
}
b: {
    field: int
    feild: _|_ // b.feild: field not allowed
}