Kotlin initialization: how to fail

Question

I need to parse Firebase DataSnapshot (a JSON object) into a data class, whose properties include enum and list. So I prefer parsing it manually by passing the DataSnapshot into a secondary constructor. However, I want the construction to fail if some fields are missing in the DataSnapshot. In Swift, it has failable initializer that returns null so I know it has failed. How to do that in Kotlin?

The following is my attempt, but I assume you can't just return from a constructor, right?

data class Unit(val name: String, val type: UnitType, val components: List<Component>) {
    constructor(snapshot: DataSnapshot) {
        name = snapshot.child("name").value as? String ?: return
        val typeString = snapshot.child("type").value as? String ?: return
        type = UnitType.values().firstOrNull { it.abbrv == typeString } ?: return
        ...
    }

}

Answer 1

There is the require function which was designed for this purpose:

// ...
init {
    require(foo == "bar") {
        "$foo should be equal to 'bar'"
    }
}

There is also requireNotNull, check, error and some others. These are called Preconditions in Kotlin. There is no magic here, they are just some helper functions. You can check the actual source here, it is very useful. There is a related blog post about the topic here.

Answer 2

No, you can't just return from a constructor - it doesn't make much sense conceptually - but since I'm fairly naive on Kotlin syntax, I'll answer to a more broad paradigm here.

Your data class is not responsible for error checking or error handling. It is only responsible for storing data. Therefore, you should rely on a factory pattern to parse the result of a DataSnapshot and conditionally instantiate a new instance of Unit if and only if no fields are missing.