In a previous post, I discussed how Swift’s failable initializers could be problematic. Specifically, I argued that their ease of use could persuade or encourage us to revert to old (bad) Objective-C habits of returning
init. Initialization is usually not the right place to fail. We should aim to avoid optionals as much as possible to reduce having to handle this absence of values. Recently, @danielgomezrico asked a great question about a possible use case for a failable initializer — parsing JSON. Given this problem’s popularity in the Swift community, I thought sharing my response here would be helpful.
Suppose we have a JSON object that contains the data for a model object. Should we write a failable initializer for this model that receives the JSON, and fails if there is problem with parsing or validation? This scenario would look similar to the following:
This rather straightforward, but is using
init? the best solution? There are some issues here that we need to address. First, the model knows everything. It knows that JSON is a thing, that JSON exists. It shouldn’t. A model should be a dumb container (preferably immutable) that holds data. Even worse, the model knows how to parse the JSON. This means the model knows how JSON is generally structured and how it works, but more specifically it knows how itself is represented as JSON.
What if the structure or keys in the JSON change? Then we would have to update our model. What if we are using Core Data, and our model is an
NSManagedObject subclass? Then we would have to stand up an entire Core Data stack just to unit test the JSON parsing. What if the service from which we receive the JSON changes and instead we receive XML? Then we would need a new initializer,
init?(xml: XML), and the model would know all about XML.
This design has put our model in a fragile position.
The issues above can be addressed by removing the model’s dependency on JSON (or XML) and creating single-purpose objects for each step of the process: (1) validating the JSON, (2) parsing the JSON, and (3) constructing the model.
The first step is creating a generic validator object. We’ll use a phantom type to ensure that a validator can only validate the JSON for a specific type of model. We initialize the validator with a closure that receives JSON and returns a
Bool indicating whether or not it is valid. This closure is saved as a property on the validator.
The combination of a phantom type and a closure property enable us to construct many unique validators, while maintaining a single generic interface through which validation occurs. In other words, we do not have to create many different concrete validators (or validator subclasses) for many different models. Additionally, in this example you can see how this brings type-safety and readability to the validator. We know that this validator is for
Next, we’ll define a JSON parser protocol, and implement a concrete parser. The protocol will allow us to reference parsers throughout our code in a generic way, while enabling each concrete parser to know about parsing a specific type of model. The parser will receive JSON, parse it, and return a model. We’ll also add a new (more proper) designated initializer to
MyModel that uses dependency injection and remove the old one,
init?(json: JSON). This parser assumes that the JSON has already been validated.
Now we can put everything together. Once we receive JSON, we can validate it. If validation fails, then we are finished and there is no need to continue. With this solution, no failable initializers are required.
This is much better. We have divided the problem into smaller subproblems and addressed each one individually. Even better, we can now unit test each component in isolation. However, because we are using Swift we can make this better. We can combine all the steps above into a top-level generic function. This function receives each of the components above — JSON, a validator, and a parser — and returns a model.
Suffering from angle-brack-T blindness? Me too. Let’s break this down. The type parameter
T specifies the model type. The validator must be a validator for
T, and the function returns an optional
T?. The type parameter
P specifies the parser type, and the
where clause enforces that the parser
P parses models of type
T. If validation fails, then the function returns
nil, otherwise it will parse the JSON and return a model.
Note: Depending on your architecture, it may not be possible nor worthwhile to have separate validation and parsing steps for JSON. If so, these two steps could be combined into the parser object and everything will still work wonderfully.
When to fail
As you can see, we have a clear separation of concerns, easily testable code, and no failable initializers. Of course there will be situations where a solution like this is not possible, namely, resource loading. For example, a
NSBundle represents an actual resource on disk. If a resource does not exist, then the class that represents it cannot be instantiated and using a failable initializer is perfect. In this situation,
init? has exactly the semantics we want. But for models and similar instances, using
init? is probably not a good idea.
Next time you find yourself wanting to write
init? instead of
init, there is likely a way to avoid it with a more thoughtful design that will push optionals and failure states further toward the edges of your object graph. Remember, where we choose to fail does matter.