Learning iOS Development

Understanding Discardable Results in Swift

A Valuable Tool in Swift For Code Readability and Managing Functions that Return Values

Photo by Alexander Schimmeck on Unsplash

Swift is known for its strong emphasis on type safety and code readability. One feature that aligns with these principles is the @discardableResult attribute. This attribute allows you to mark a function's return value as "discardable," meaning that you can choose to ignore the return value without generating a compiler warning. In this blog, we'll explore what a discardable result is, why it's useful, and how to use it effectively in Swift.

What is a Discardable Result?

In Swift, when you call a function that returns a value, the compiler expects you to do something with that value. However, there are scenarios where you may not need the return value, and discarding it doesn’t represent a problem. Examples include functions that perform actions without needing their result immediately, such as logging or notification.

The @discardableResult attribute is used to indicate to the Swift compiler that the return value of a function can be safely ignored, eliminating the warning that would typically occur when you don't use a value returned by a function.

Here’s the basic syntax for using @discardableResult:

@discardableResult
func someFunction() -> Int {
    // Function implementation
    return 42
}

In this example, we’ve marked someFunction() as @discardableResult, which tells Swift that it's okay to call this function and not use its return value.

When to Use @discardableResult

The @discardableResult attribute is particularly useful in the following scenarios:

1. Logging and Debugging

Imagine you have a logging function that records events in your application. You might not always need to do anything with the result of the logging function, as the primary goal is to log information for debugging or monitoring purposes.

@discardableResult
func logEvent(message: String) -> Bool {
    // Log the event
    print(message)
    
    // Return true if the event was logged successfully
    return true
}

In this case, marking the return value as discardable allows you to log events without cluttering your code with unnecessary variables.

2. Fluent Interfaces

When building fluent interfaces, you might have methods that chain together but don’t require capturing intermediate results. The @discardableResult attribute makes it clear that these methods can be used without assigning their results to variables.

@discardableResult
func addToCart(item: String) -> ShoppingCart {
    // Add the item to the shopping cart
    self.items.append(item)
    
    // Return the updated shopping cart
    return self
}

With @discardableResult, you can chain method calls like this:

let cart = ShoppingCart()
    .addToCart(item: "Product 1")
    .addToCart(item: "Product 2")

3. Notifications and Callbacks

Functions that trigger notifications, callbacks, or delegate methods often return values indicating success or failure. However, in some cases, you may not need to check the return value immediately. Marking these return values as discardable can improve code clarity.

@discardableResult
func sendNotification(message: String) -> Bool {
    // Send a notification
    NotificationCenter.default.post(name: .notificationName, object: nil, userInfo: ["message": message])
    
    // Return true if the notification was sent successfully
    return true
}

Using @discardableResult, you can send notifications without having to explicitly capture the return value.

Conclusion

The @discardableResult attribute in Swift is a valuable tool for improving code readability and managing functions that return values you might not always need to use. It allows you to eliminate compiler warnings when you choose to ignore a return value intentionally, making your code cleaner and more expressive. By using @discardableResult judiciously in your codebase, you can strike a balance between type safety and practicality, ensuring that your code remains clear and efficient.