Using async let with synchronous code

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I’m sure that you’ve already used async let.

It’s a very powerful feature that lets you run several asynchronous functions concurrently, with very little additional syntax required:

But did you know that it has a hidden feature?

Take a look at this synchronous Swift function, that performs a computation that will take a few seconds to complete:

To run this function asynchronously, we could make the function async or wrap its call site into a Task.

However, there’s actually an even simpler solution.

And that’s because using async let is not limited to asynchronous functions!

You can totally use this syntax to call a regular synchronous function:

And when you do, the execution of this function will be dispatched to a background thread!

The best part is that we didn’t had to make any change to the code of the function or to use error-prone APIs like a Task.

That’s all for this article, as always I hope that you’ve enjoyed discovering this new trick!

Here’s the code if you want to experiment with it:

import Foundation

func performTimeConsumingTask() -> Int {
    var result = 0
    for i in 1...100_000_000 {
        result += i
    }
    return result
}

async let result = performTimeConsumingTask()

print("result: \(await result)")