Main Actor Isolation Error in Xcode Playground for Simple Swift Code

Question

I’m new to Swift and running into an issue with actor isolation in Xcode Playgrounds. When I try to run the following simple code, I get the error message about actor isolation, and I’m not sure why.

error: Testone.xcplaygroundpage:7:35: main actor-isolated var 'connectionSuccess' can not be referenced from a nonisolated context
    if let newConnectionSuccess = connectionSuccess {
                                  ^

Testone.xcplaygroundpage:4:5: note: var declared here
var connectionSuccess: Bool? = nil
    ^

Testone.xcplaygroundpage:6:6: note: add '@MainActor' to make global function 'checkConnection()' part of global actor 'MainActor'
func checkConnection() -> Bool {
     ^
@MainActor

Code Swift v6.0 and Xcode v16

import Foundation

var connectionSuccess: Bool? = nil

func checkConnection() -> Bool {
    if let newConnectionSuccess = connectionSuccess {
        return newConnectionSuccess
    }
    
    return false
}

checkConnection()

I'm not working with threads or doing anything complicated, just trying to check the value of an optional Bool. The error suggests adding @MainActor, but I’ve seen similar code in a tutorial without this requirement. Is this something new in Swift? Appreciate your help with this.

Thanks @Sweeper I want to understand why this is happening in my environment but not for others. While adding MainActor would fix it, I’m curious why the actor isolation is being applied here. I'm not working with threads or UI-related code, so I’m curious why connectionSuccess is being isolated to the MainActor. Is there something about Xcode Playgrounds or my setup that could triggers this? — Archibald
– Archibald, Commented Oct 9, 2024 at 14:47

Sweeper · Accepted Answer · 2024-10-09 14:58:26Z

Is this something new in Swift?

Kind of? Complete concurrency checking is now mandatory in Swift 6, whereas previously you need to opt-in with the -strict-concurrency=complete option.

The problem here is that connectionSuccess is implicitly isolated to the main actor because it is variable declared at the top level that allows statements (like in a playground/Swift script/main.swift etc).

checkConnection is not isolated to the main actor (it's not isolated to any actor), so it cannot synchronously access connectionSuccess.

So you should just add @MainActor to checkConnection as the error suggests.

You can lower the Swift version of a playground by going into the .playground folder, and find the contents.xcplayground file. It looks like this

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<playground version='7.0' target-platform='macos' swift-version='6' buildActiveScheme='true' importAppTypes='true'/>

Just change the swift-version='6' part to your desired Swift version.

Thanks @Sweeper for the explanation. Downgrading the Swift version seems to have worked. Found this github.com/swiftlang/swift-evolution/blob/main/proposals/… and sharing as a helpful reference. I’m also wondering now if this stricter concurrency model will make migrating older code a bit difficult.

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