Why Rethrowing functions in Swift? - Purpose & Use Cases

The Big Idea

What if you could pass errors up the chain without writing extra error handling code everywhere?

The Scenario

Imagine you have a function that calls another function which might throw an error. You want to pass the error up without handling it yourself. Doing this manually means writing extra code to catch and rethrow errors everywhere.

The Problem

This manual approach is slow and error-prone because you must write repetitive try-catch blocks just to pass errors along. It clutters your code and makes it harder to read and maintain.

The Solution

Rethrowing functions let you declare that your function can throw errors only if the function it calls throws. This way, you don't write extra error handling code, and errors flow naturally up the call chain.

Before vs After

✗ Before

func perform(action: () throws -> Void) throws {
    do {
        try action()
    } catch {
        throw error
    }
}

✓ After

func perform(action: () throws -> Void) rethrows {
    try action()
}

What It Enables

It enables clean, concise error propagation without unnecessary boilerplate, making your code easier to write and understand.

Real Life Example

When writing a sorting function that takes a comparison closure which might throw errors, rethrowing lets your sorting function pass those errors up without extra catch blocks.

Key Takeaways

Manual error passing requires repetitive try-catch blocks.

Rethrowing functions automatically propagate errors from called functions.

This leads to cleaner and more maintainable code.