"Expression failed to parse, unknown error" only in Xcode 10.2.1 playgrounds

Using Swift
,
1

Hello!
I was experimenting with the VIPER architecture in a Xcode playground, and I encountered this error:
Expression failed to parse, unknown error

The strange thing is that the code builds successfully inside an Xcode project. Did I find a bug?
Here's the code (I tried on Xcode 10.2.1 only):


protocol VIPERVC: UIViewController {
    associatedtype Interactor: VIPERInteractor
        where Interactor.Presenter.ViewController == Self
    
    static var storyboardName: String { get }
    static var storyboardIdentifier: String { get }
    
    var interactor: Interactor! { get set }
}

extension VIPERVC {
    typealias Presenter = Interactor.Presenter
}

protocol VIPERInteractor {
    associatedtype Presenter: VIPERPresenter
        where Presenter.ViewController.Interactor == Self
    
    init(_ repo: VIPEREntityRepository)
    
    var presenter: Presenter! { get set }
}

extension VIPERInteractor {
    typealias ViewController = Presenter.ViewController
}

protocol VIPERPresenter {
    associatedtype ViewController: VIPERVC
        where ViewController.Interactor.Presenter == Self
    
    init(_ vc: ViewController)
    
    var vc: ViewController! { get set }
}

extension VIPERPresenter {
    typealias Interactor = ViewController.Interactor
}

class TestEntity {
    func provideString() -> String {
        return "TestString"
    }
}

struct VIPEREntityRepository {
    private let testEntity = TestEntity()
    
    init() { }
    
    func provideEntity<T>() -> T {
        let mirror = Mirror(reflecting: self)
        guard let entity = mirror.children.first(where: { $0.value is T })
            else {
                let message = "Dependency with type: \(T.self) not found.\nAvailable dependencies:\n"
                let dependencies = mirror.children.map { "\($0.label ?? "nil"): \(type(of: $0.value))" }
                fatalError(message + dependencies.joined(separator: "\n")) }
        return entity.value as! T
    }
}

class TestViewController: UIViewController, VIPERVC {
    class var storyboardName: String { return "TestStoryboard" }
    class var storyboardIdentifier: String { return "TestVC" }
    
    let mainLabel: UILabel = UILabel(frame: .init(origin: .zero, size: .init(width: 300, height: 60)))
    
    typealias Interactor = TestInteractor
    
    var interactor: TestInteractor! = nil
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.view.addSubview(mainLabel)
    }
    
    func pressButton() {
        interactor.retrieveString()
    }
}

class TestInteractor: VIPERInteractor {
    typealias Presenter = TestPresenter
    
    var presenter: TestPresenter! = nil
    
    private let repo: VIPEREntityRepository
    private lazy var entity: TestEntity = repo.provideEntity()
    
    required init(_ repo: VIPEREntityRepository) {
        self.repo = repo
    }
    
    func retrieveString() {
        let string = entity.provideString()
        presenter.updateLabel(withString: string)
    }
}

class TestPresenter: VIPERPresenter {
    typealias ViewController = TestViewController
    
    required init(_ vc: ViewController) {
        self.vc = vc
    }
    
    unowned var vc: TestViewController!
    
    func updateLabel(withString str: String) {
        vc.mainLabel.text = str
    }
}

struct ModuleBuilder<VC: VIPERVC> {
    
    func build(_ repo: VIPEREntityRepository) -> VC {
        //        let storyboard = UIStoryboard(name: VC.storyboardName, bundle: .main)
        //        let vc = storyboard.instantiateViewController(withIdentifier: VC.storyboardIdentifier) as! VC
        let vc = VC.init()
        
        var interactor = VC.Interactor(repo)
        var presenter = VC.Presenter(vc)
        
        vc.interactor = interactor
        interactor.presenter = presenter
        presenter.vc = vc
        
        return vc
    }
}
2

Are you using a Swift 5.1 language feature? Playgrounds supports up to 5.0 at this point.

3

I'm not using any Swift 5.1 feature, nor even Swift 5.0.
This code compiles successfully in Xcode 10.2.1, set to both Swift 5 or Swift 4.2, it doesn't work just in the playground.

4

Does Playgrounds highlight where it is failing to parse?

5

No, it doesn't. The error message I posted above appears in the "console" area in the bottom, and also the "Errors" panel on the left sidebar doesn't show anything.

6

I created a demo project on Github to help find out what's wrong:

This project includes a working app that uses the code that triggers the playground error, and a playground file that misbehaves on the same code, producing the unknown error.

7

I pasted your code into a new Playground in the current shipping version of Playgrounds and (after adding import UIKit) it compiled without error. Are you using the beta?

1 Like
8

I'm not using anything related to the latest Beta, I've installed them in the past weeks but had to remove everything from my current machine. I just double checked everything:
Xcode 10.2.1 (10E1001)
both Swift 4.2 and Swift 5

I looked into the DiagnosticReports folder and I found the following log with the correct timing of my last try, I'm not able to read it though.

Process:               lldb-rpc-server [7305]
Path:                  /Applications/Xcode.app/Contents/SharedFrameworks/LLDBRPC.framework/Versions/A/Resources/lldb-rpc-server
Identifier:            lldb-rpc-server
Version:               2
Code Type:             X86-64 (Native)
Parent Process:        Xcode [7279]
Responsible:           lldb-rpc-server [7305]
User ID:               502

Date/Time:             2019-07-01 14:04:47.674 +0200
OS Version:            Mac OS X 10.14.5 (18F132)
Report Version:        12
Bridge OS Version:     3.5 (16P5125)
Anonymous UUID:        EE5EBBB8-BD54-DF23-19B2-269F1C9CFF77


Time Awake Since Boot: 20000 seconds

System Integrity Protection: disabled

Crashed Thread:        7  <lldb.process.internal-state(pid=7421)>

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Termination Signal:    Segmentation fault: 11
Termination Reason:    Namespace SIGNAL, Code 0xb
Terminating Process:   exc handler [7305]

VM Regions Near 0:
--> 
    __TEXT                 000000010499d000-0000000104a1f000 [  520K] r-x/rwx SM=COW  /Applications/Xcode.app/Contents/SharedFrameworks/LLDBRPC.framework/Versions/A/Resources/lldb-rpc-server

Thread 0:: Connection::RunLoop() packet reading thread  Dispatch queue: com.apple.main-thread
0   libsystem_kernel.dylib        	0x00007fff71c2cef2 read + 10
1   lldb-rpc-server               	0x00000001049a74b1 rpc_common::RPCConnectionSocket::Read(std::__1::basic_string<unsigned char, std::__1::char_traits<unsigned char>, std::__1::allocator<unsigned char> >&, bool) + 173
2   lldb-rpc-server               	0x00000001049adfba rpc_common::Connection::ReceivePacket(rpc_common::RPCPacket&) + 148
3   lldb-rpc-server               	0x00000001049adae7 rpc_common::Connection::RunLoop() + 203
4   lldb-rpc-server               	0x00000001049ea3f7 main + 471
5   libdyld.dylib                 	0x00007fff71af63d5 start + 1

Thread 1:
0   libsystem_pthread.dylib       	0x00007fff71ce93f0 start_wqthread + 0

Thread 2:
0   libsystem_kernel.dylib        	0x00007fff71c2b22a mach_msg_trap + 10
1   libsystem_kernel.dylib        	0x00007fff71c2b76c mach_msg + 60
2   com.apple.CoreFoundation      	0x00007fff45bc2bee __CFRunLoopServiceMachPort + 328
3   com.apple.CoreFoundation      	0x00007fff45bc215c __CFRunLoopRun + 1612
4   com.apple.CoreFoundation      	0x00007fff45bc18be CFRunLoopRunSpecific + 455
5   com.apple.CoreFoundation      	0x00007fff45bc16d2 CFRunLoopRun + 40
6   com.apple.DebugSymbols        	0x00007fff5a83bd2b SpotlightQueryThread(void*) + 475
7   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
8   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
9   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 3:: RPC packet thread for client tid 000512c4 (332484)
0   libsystem_kernel.dylib        	0x00007fff71c2e86a __psynch_cvwait + 10
1   libsystem_pthread.dylib       	0x00007fff71ced56e _pthread_cond_wait + 722
2   libc++.1.dylib                	0x00007fff6ed28a0a std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 18
3   lldb-rpc-server               	0x00000001049b1102 Packets::ReadThread() + 222
4   lldb-rpc-server               	0x00000001049b101f Packets::RunReadThread(void*) + 9
5   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
6   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
7   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 4:: RPC packet thread for client tid 00051049 (331849)
0   libsystem_kernel.dylib        	0x00007fff71c2e86a __psynch_cvwait + 10
1   libsystem_pthread.dylib       	0x00007fff71ced56e _pthread_cond_wait + 722
2   libc++.1.dylib                	0x00007fff6ed28a0a std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 18
3   lldb-rpc-server               	0x00000001049b1102 Packets::ReadThread() + 222
4   lldb-rpc-server               	0x00000001049b101f Packets::RunReadThread(void*) + 9
5   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
6   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
7   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 5:: RPC packet thread for client tid 000512c3 (332483)
0   libsystem_kernel.dylib        	0x00007fff71c2e86a __psynch_cvwait + 10
1   libsystem_pthread.dylib       	0x00007fff71ced56e _pthread_cond_wait + 722
2   libc++.1.dylib                	0x00007fff6ed28a0a std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 18
3   lldb-rpc-server               	0x00000001049b1102 Packets::ReadThread() + 222
4   lldb-rpc-server               	0x00000001049b101f Packets::RunReadThread(void*) + 9
5   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
6   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
7   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 6:: <lldb.process.gdb-remote.async>
0   libsystem_kernel.dylib        	0x00007fff71c2e86a __psynch_cvwait + 10
1   libsystem_pthread.dylib       	0x00007fff71ced56e _pthread_cond_wait + 722
2   libc++.1.dylib                	0x00007fff6ed28a0a std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 18
3   com.apple.LLDB.framework      	0x000000010c0401ab lldb_private::Listener::GetEventInternal(lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&, lldb_private::Broadcaster*, lldb_private::ConstString const*, unsigned int, unsigned int, std::__1::shared_ptr<lldb_private::Event>&) + 235
4   com.apple.LLDB.framework      	0x000000010c0404c3 lldb_private::Listener::GetEvent(std::__1::shared_ptr<lldb_private::Event>&, lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&) + 33
5   com.apple.LLDB.framework      	0x000000010c1bcd46 lldb_private::process_gdb_remote::ProcessGDBRemote::AsyncThread(void*) + 262
6   com.apple.LLDB.framework      	0x000000010c26f93b lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) + 105
7   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
8   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
9   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 7 Crashed:: <lldb.process.internal-state(pid=7421)>
0   com.apple.LLDB.framework      	0x000000010c2e5e13 lldb_private::ThreadList::WillResume() + 177
1   com.apple.LLDB.framework      	0x000000010c2963be lldb_private::Process::PrivateResume() + 252
2   com.apple.LLDB.framework      	0x000000010c29c91f lldb_private::Process::ShouldBroadcastEvent(lldb_private::Event*) + 849
3   com.apple.LLDB.framework      	0x000000010c2992d0 lldb_private::Process::HandlePrivateEvent(std::__1::shared_ptr<lldb_private::Event>&) + 210
4   com.apple.LLDB.framework      	0x000000010c29d209 lldb_private::Process::RunPrivateStateThread(bool) + 865
5   com.apple.LLDB.framework      	0x000000010c29c9b2 lldb_private::Process::PrivateStateThread(void*) + 22
6   com.apple.LLDB.framework      	0x000000010c26f93b lldb_private::HostNativeThreadBase::ThreadCreateTrampoline(void*) + 105
7   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
8   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
9   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 8:: RPC packet thread for client tid 00051b0e (334606)
0   libsystem_kernel.dylib        	0x00007fff71c3261a __select + 10
1   com.apple.LLDB.framework      	0x000000010c47a47b SelectHelper::Select() + 895
2   com.apple.LLDB.framework      	0x000000010bfa8685 lldb_private::ConnectionFileDescriptor::BytesAvailable(lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&, lldb_private::Status*) + 253
3   com.apple.LLDB.framework      	0x000000010bfa8284 lldb_private::ConnectionFileDescriptor::Read(void*, unsigned long, lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&, lldb::ConnectionStatus&, lldb_private::Status*) + 112
4   com.apple.LLDB.framework      	0x000000010c02972b lldb_private::Communication::ReadFromConnection(void*, unsigned long, lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&, lldb::ConnectionStatus&, lldb_private::Status*) + 45
5   com.apple.LLDB.framework      	0x000000010c0292d8 lldb_private::Communication::Read(void*, unsigned long, lldb_private::Timeout<std::__1::ratio<1l, 1000000l> > const&, lldb::ConnectionStatus&, lldb_private::Status*) + 174
6   com.apple.LLDB.framework      	0x000000010c1a748f lldb_private::process_gdb_remote::GDBRemoteCommunication::WaitForPacketNoLock(StringExtractorGDBRemote&, lldb_private::Timeout<std::__1::ratio<1l, 1000000l> >, bool) + 273
7   com.apple.LLDB.framework      	0x000000010c37308c lldb_private::process_gdb_remote::GDBRemoteClientBase::SendPacketAndWaitForResponseNoLock(llvm::StringRef, StringExtractorGDBRemote&) + 106
8   com.apple.LLDB.framework      	0x000000010c372f9c lldb_private::process_gdb_remote::GDBRemoteClientBase::SendPacketAndWaitForResponse(llvm::StringRef, StringExtractorGDBRemote&, bool) + 76
9   com.apple.LLDB.framework      	0x000000010c1b9b12 lldb_private::process_gdb_remote::ProcessGDBRemote::DoDestroy() + 598
10  com.apple.LLDB.framework      	0x000000010c292eba lldb_private::Process::Destroy(bool) + 238
11  com.apple.LLDB.framework      	0x0000000108e9a3c9 lldb::SBProcess::Kill() + 101
12  lldb-rpc-server               	0x00000001049a0e45 rpc_server::_ZN4lldb9SBProcess4KillEv::HandleRPCCall(rpc_common::Connection&, rpc_common::RPCStream&, rpc_common::RPCStream&) + 47
13  lldb-rpc-server               	0x00000001049accd3 rpc_common::Connection::PrivateHandleRPCPacket(rpc_common::RPCPacket&, rpc_common::RPCPacket&, bool&) + 469
14  lldb-rpc-server               	0x00000001049ad9d4 rpc_common::Connection::HandleRPCPacket(rpc_common::RPCPacket&) + 62
15  lldb-rpc-server               	0x00000001049b12ba Packets::ProcessPackets() + 254
16  lldb-rpc-server               	0x00000001049b10e2 Packets::ReadThread() + 190
17  lldb-rpc-server               	0x00000001049b101f Packets::RunReadThread(void*) + 9
18  libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
19  libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
20  libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 9:: RPC packet thread for client tid 00051ae5 (334565)
0   libsystem_kernel.dylib        	0x00007fff71c2e86a __psynch_cvwait + 10
1   libsystem_pthread.dylib       	0x00007fff71ced56e _pthread_cond_wait + 722
2   libc++.1.dylib                	0x00007fff6ed28a0a std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) + 18
3   lldb-rpc-server               	0x00000001049b1102 Packets::ReadThread() + 222
4   lldb-rpc-server               	0x00000001049b101f Packets::RunReadThread(void*) + 9
5   libsystem_pthread.dylib       	0x00007fff71cea2eb _pthread_body + 126
6   libsystem_pthread.dylib       	0x00007fff71ced249 _pthread_start + 66
7   libsystem_pthread.dylib       	0x00007fff71ce940d thread_start + 13

Thread 7 crashed with X86 Thread State (64-bit):
  rax: 0x0000000000000000  rbx: 0x000000010c7e240a  rcx: 0x00007fff71c2ef4a  rdx: 0x0000000000000000
  rdi: 0x00007f7f2f0f4180  rsi: 0x000000010c7b13b9  rbp: 0x0000700005b65ca0  rsp: 0x0000700005b65b90
   r8: 0x0000700005b65708   r9: 0x0000000000000000  r10: 0x00007fff71c2ef4a  r11: 0x0000000000000296
  r12: 0x00007f7f2f7274c0  r13: 0x000000010c7e2582  r14: 0x000000010c7e253c  r15: 0x00007f7f2f727460
  rip: 0x000000010c2e5e13  rfl: 0x0000000000010246  cr2: 0x0000000108cb9000
  
Logical CPU:     0
Error Code:      0x0200005d
Trap Number:     133

...
9

Might be due to the associatedtype in your protocols. Does it work if you remove it?

1 Like
10

I’ve just realized you meant a Playground in XCode and not Swift Playgrounds in iOS (which is where I tried it successfully). Why did Apple name two different things the same name? :man_shrugging:

11

I expected your suggestion to result in a working playground... but it didn't! :exploding_head:
I commented out the lines as following, but I still get the same error.
Here's the commented code:

import UIKit

protocol VIPERVC: UIViewController {
//    associatedtype Interactor: VIPERInteractor
//        where Interactor.Presenter.ViewController == Self
    
    static var storyboardName: String { get }
    static var storyboardIdentifier: String { get }
    
//    var interactor: Interactor! { get set }
}

extension VIPERVC {
//    typealias Presenter = Interactor.Presenter
}

protocol VIPERInteractor {
//    associatedtype Presenter: VIPERPresenter
//        where Presenter.ViewController.Interactor == Self
    
    init(_ repo: VIPEREntityRepository)
    
//    var presenter: Presenter! { get set }
}

extension VIPERInteractor {
//    typealias ViewController = Presenter.ViewController
}

protocol VIPERPresenter {
//    associatedtype ViewController: VIPERVC
//        where ViewController.Interactor.Presenter == Self
    
//    init(_ vc: ViewController)
//
//    var vc: ViewController! { get set }
}

extension VIPERPresenter {
//    typealias Interactor = ViewController.Interactor
}

class TestEntity {
    func provideString() -> String {
        return "TestString"
    }
}

struct VIPEREntityRepository {
    private let testEntity = TestEntity()
    
    init() { }
    
    func provideEntity<T>() -> T {
        let mirror = Mirror(reflecting: self)
        guard let entity = mirror.children.first(where: { $0.value is T })
            else {
                let message = "Dependency with type: \(T.self) not found.\nAvailable dependencies:\n"
                let dependencies = mirror.children.map { "\($0.label ?? "nil"): \(type(of: $0.value))" }
                fatalError(message + dependencies.joined(separator: "\n")) }
        return entity.value as! T
    }
}

class TestViewController: UIViewController, VIPERVC {
    class var storyboardName: String { return "Main" }
    class var storyboardIdentifier: String { return "TestVC" }
    
    @IBOutlet var mainLabel: UILabel!
    
    typealias Interactor = TestInteractor
    
    var interactor: TestInteractor! = nil
    
    @IBAction func pressButton() {
        interactor.retrieveString()
    }
}

class TestInteractor: VIPERInteractor {
    typealias Presenter = TestPresenter
    
    var presenter: TestPresenter! = nil
    
    private let repo: VIPEREntityRepository
    private lazy var entity: TestEntity = repo.provideEntity()
    
    required init(_ repo: VIPEREntityRepository) {
        self.repo = repo
    }
    
    func retrieveString() {
        let string = entity.provideString()
        presenter.updateLabel(withString: string)
    }
}

class TestPresenter: VIPERPresenter {
    typealias ViewController = TestViewController
    
    required init(_ vc: ViewController) {
        self.vc = vc
    }
    
    unowned var vc: TestViewController!
    
    func updateLabel(withString str: String) {
        vc.mainLabel.text = str
    }
}

struct ModuleBuilder<VC: VIPERVC> {
    
//    func build(_ repo: VIPEREntityRepository) -> VC {
//        let storyboard = UIStoryboard(name: VC.storyboardName, bundle: .main)
//        let vc = storyboard.instantiateViewController(withIdentifier: VC.storyboardIdentifier) as! VC
//
//        var interactor = VC.Interactor(repo)
//        var presenter = VC.Presenter(vc)
//
//        vc.interactor = interactor
//        interactor.presenter = presenter
//        presenter.vc = vc
//
//        return vc
//    }
}

@toph42 In defense of Apple's naming: "iPlaygrounds" sounds horrible :sweat_smile:

12

I've found the offending line:

func updateLabel(withString str: String) {
        vc.mainLabel.text = str // This triggers the parsing error
}

If I change it adding a question mark everything goes fine (also works with all the associatedtypes!):

func updateLabel(withString str: String) {
        vc.mainLabel?.text = str // This works just fine!
}

This totally looks like a Playground bug to me. Who can we @ at about playgrounds?

13

You can file a bug report on bugs.swift.org

14

I further simplified the error sample code and I submitted the bug: [SR-11054] “Expression failed to parse, unknown error” only in Xcode 10.2.1 playgrounds · Issue #53446 · apple/swift · GitHub

Now the offending code is reduced to this:

class TestViewController: UIViewController {
    var mainLabel: UILabel!
}

class TestPresenter {
    init(_ vc: TestViewController) {
        self.vc = vc
    }
    
    var vc: TestViewController!
    
    func updateLabel(withString str: String) {
        vc.mainLabel.text = str
    }
}
2 Likes