How to parse a deeply nested objects with dynamic fallbacks?

[MrJohz]

I have a configuration object that I've loaded from JSON somewhere, and I want to validate/parse it into the correct format. However, in some situations, the configuration object may be missing certain paths, and I want to provide a fallback to load those paths from environment variables or similar. For example, I might have a schema looking roughly like this:

{
  system: {
    database: { connectionString: "string" },
    authApi: { hostName: "string" },
  },
}

And I want the following cases to parse correctly:

// all attributes present on configuration object
{
  system: {
    database: { connectionString: "abcdef" },
    authApi: { hostName: "ghijkl" },
  },
}

// missing specific key, but a default value is available via an environment variable
env SYSTEM_DATABASE_CONNECTIONSTRING="abcdef"
{
  system: {
    database: {}, // missing specific key
    authApi: { hostName: "ghijkl" },
  },
}

// missing a whole namespace, but a default value is available via an environment variable
env SYSTEM_AUTHAPI_HOSTNAME="ghijkl"
{
  system: {
    database: { connectionString: "abcdef" },
    // missing whole `authApi" namespace
  },
}

// missing entire configuration, but default values are available
env SYSTEM_DATABASE_CONNECTIONSTRING="abcdef"
env SYSTEM_AUTHAPI_HOSTNAME="ghijkl"
{}

I could handle this by having an ArkType parser with everything optional, and then constructing a new object based on the parsed value and any additional sources of information. But with ArkType, I like that I declare my types once and I don't need to duplicate the work. Is there a way to declare an ArkType parser that can fill in the blanks dynamically like this?

[ssalbdivad]

It seems like what you're looking for is defaultable properties:

https://arktype.io/docs/objects#defaultable