Revisit whether a missing file should throw an error or default to empty in file providers

[czechboy0]

When setting up a config provider hierarchy, you might treat config files as optional - read them if they're there, but ignore if they're missing. This allows a nice linear setup like this:

let config = ConfigReader(providers: [
    EnvironmentVariablesProvider(),
    JSONProvider(filePath: "config.json")
])

The above will throw an error if config.json is missing, so today you need to handle it manually with:

var providers: [any ConfigProvider] = [
    EnvironmentVariablesProvider()
]
if FileManager.default.fileExists(atPath: "config.json") {
    providers.append(JSONProvider(filePath: "config.json"))
}
let config = ConfigReader(providers: providers)

While not the end of the world, the verbosity and the repeating of the path makes this slightly painful.

Do we want to change the file providers to instead handle missing files gracefully, for example using an optional flag in the initializer that specifies the behavior?

let config = ConfigReader(providers: [
    EnvironmentVariablesProvider(),
    JSONProvider(filePath: "config.json", isRequired: true), // will throw if file is missing or malformed
    JSONProvider(filePath: "optional-config.json", isRequired: false), // will become an empty provider if file is missing or malformed
])

We'd still default isRequired: Bool = true, but folks can make it more graceful by setting it to false.

Opinions?

[czechboy0]

cc @josephnoir

[czechboy0]

Current thinking:

• make it configurable using a boolean parameter, default to required
• however, even in optional mode, still throw if the file is malformed (but present)