Rethrowing functions in Swift - Mini Project: Build & Apply

Rethrowing Functions in Swift

📖 Scenario: Imagine you are building a small app that processes tasks. Some tasks might fail and throw errors. You want to write a function that accepts another function as input and calls it. If the input function throws an error, your function should pass that error up to the caller.

🎯 Goal: You will create a rethrowing function in Swift that calls a throwing function passed as a parameter. This will help you understand how to handle errors in higher-order functions.

📋 What You'll Learn

Create a throwing function called taskThatMightFail that throws an error of type TaskError.

Create a rethrowing function called performTask that accepts a throwing function parameter and rethrows errors.

Call performTask with taskThatMightFail inside a do-catch block to handle errors.

Print "Task succeeded" if no error occurs, or print the error message if it fails.

💡 Why This Matters

🌍 Real World

Rethrowing functions are useful when you write utility functions that call other functions which might throw errors. This helps keep error handling clean and flexible.

💼 Career

Understanding rethrowing functions is important for Swift developers working on apps that handle errors gracefully, such as network requests, file operations, or user input validation.

1

Define the error type and a throwing function

Create an enum called TaskError that conforms to Error with a case failed. Then create a function called taskThatMightFail that throws TaskError.failed.

Swift

// Your code here

Need a hint?

Define an enum for errors and a function that always throws that error.

2

Create a rethrowing function

Create a function called performTask that accepts a parameter task of type () throws -> Void. Mark performTask with rethrows and call task() inside it.

Swift

enum TaskError: Error {
    case failed
}

func taskThatMightFail() throws {
    throw TaskError.failed
}

// Your code here

Need a hint?

Use rethrows to let performTask throw only if task throws.

3

Call the rethrowing function inside a do-catch block

Write a do-catch block where you call performTask with taskThatMightFail as the argument. In the catch block, print "Caught error: \(error)".

Swift

enum TaskError: Error {
    case failed
}

func taskThatMightFail() throws {
    throw TaskError.failed
}

func performTask(task: () throws -> Void) rethrows {
    try task()
}

// Your code here

Need a hint?

Use do and catch to handle errors from performTask.

4

Modify the throwing function to sometimes succeed and print success message

Change taskThatMightFail to accept a Boolean parameter shouldFail. If shouldFail is true, throw TaskError.failed, otherwise do nothing. Then call performTask with a closure that calls taskThatMightFail(shouldFail: false). In the do block, print "Task succeeded".

Swift

enum TaskError: Error {
    case failed
}

// Your code here

Need a hint?

Use a closure to call taskThatMightFail with shouldFail: false and print success if no error.