next: References and Scopes

## Templates

One of CUE’s most powerful features is the ability to specify optional fields in bulk. This allows one to specify constraints, or templates, to be unified with each field of a struct.

An optional field set is an expression in square brackets to specify to which fields to apply a constraint (currently only string, or all fields is supported). Using an alias in the square brackets binds the label of the matched field to the given identifier, which can then be used within the template.

templates.cue

// The following struct is unified with all elements in job.
// The name of each element is bound to Name and visible in the struct.
job: [Name=_]: {
name:     Name
replicas: uint | *1
command:  string
}

job: list: command: "ls"

job: nginx: {
command:  "nginx"
replicas: 2
}

\$ cue eval templates.cue

job: {
list: {
name:     "list"
replicas: 1
command:  "ls"
}
nginx: {
name:     "nginx"
replicas: 2
command:  "nginx"
}
}