Default values and optional fields

When writing a Cue config, you will sometimes want to set default values in your package.

The most common way you'll encounter in our codebase is: key: type | *value:

defaultValue: string | *"foo"
defaultValue: bool | *false

You'll also encounter the *null default value, which is self explanatory:

// here, defaultValue either accepts a #PersonalDefinition, or stays null by default
defaultValue: #PersonalDefinition | *null

To test the type of defaultValue, you can directly do such assertion:

if defaultValue != "foo" | if defaultValue != false | if defaultValue != null {
    ...
}
 
if defaultValue == "foo" | if defaultValue == false | if defaultValue == null {
    ...
}

However, don't get confused with the optional fields. Optional fields check whether a key is concrete at the given scope in the DAG. You declare them with ? at the end of their name: foo?.

foo?: string // 1. declare foo. It remains undefined for now

foo: "bar" // 2. Now, `foo` gets concrete. The field isn't undefined anymore

To check on a field's concreteness, use the bottom value _|_:

if foo != _|_ { // if foo is not `undefined`
    ...
}
 
if foo == _|_ { // if foo is `undefined`
    ...
}