Troubleshooting.md

Troubleshooting

Compile Errors

99% of questions about compile issues with PromiseKit can be solved by either:

0. Check Your Handler

return firstly {
      URLSession.shared.dataTask(.promise, with: url)
}.compactMap {
    JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
}.then { dict in
    User(dict: dict)
}

Swift (unhelpfully) says:

Cannot convert value of type '([String : Any]) -> User' to expected argument type '([String : Any]) -> _'

What’s the real problem? then must return a Promise, you wanted map:

return firstly {
      URLSession.shared.dataTask(.promise, with: url)
}.compactMap {
    JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
}.map { dict in
    User(dict: dict)
}

1. Specifying Closure Parameters and Return Types

Please try it.

We made great effort to reduce the need for this with PromiseKit 6, but like normal functions in Swift (eg. Array.map) that return a generic type, if the closure body is longer than one line you may need to tell Swift what returns.

Tip: Sometimes you can force a one liner with semi-colons.

For example:

func _() -> Promise<Void> {
    return firstly {
        proc.launch(.promise)      // proc: Foundation.Process
    }.then {
        when(fulfilled: p1, p2)    // both p1 & p2 are `Promise<Void>`
    }
}

Fails to compile with:

Cannot invoke 'then' with an argument list of type '(() -> _)

Highlighting the then. The fix is to specify the closure parameter of the then:

func _() -> Promise<Void> {
    return firstly {
        proc.launch(.promise)
    }.then { _ in
        when(fulfilled: p1, p2)
    }
}

Why is this the fix? Well Process.launch(.promise) returns Promise<(String, String)> and we were ignoring this value in our then, if we’d used $0 or named the parameter we’d be fine.

Swift’s diagnostics were unhelpful at every stage trying to troubleshoot this example. You’re on your own :(

2. Move Code To A Temporary Inline Function

Take the code out of the closure and put it in a standalone function, now Swift will tell you the real error message. For example:

func doStuff() {
    firstly {
        foo()
    }.then {
        let bar = bar()
        let baz = baz()
        when(fulfilled: bar, baz)
    }
}

Becomes:

func doStuff() {
    func fluff() -> Promise<…> {
        let bar = bar()
        let baz = baz()
        when(fulfilled: bar, baz)  
    }

    firstly {
        foo()
    }.then {
        fluff()
    }
}

So an inline function is all you need. Now Swift will tell you the real error message. Here that you forgot a return.

You Copied Code Off The Internet That Doesn’t Work

Swift changed a lot over the years and thus PromiseKit has had to change to keep up. Your code is probably for an older PromiseKit. Read the definitions of the functions. Doing this with Xcode is easy (option click or command click). All PromiseKit functions are documented and provide examples.

Context type for closure argument expects 1 argument, which cannot be implicitly ignored

You have a then you want a done.

Other Issues

Pending Promise Deallocated!

If you see this warning you have a path in your Promise initializer where the promise is not sealed:

Promise<String> { seal in
    task { value, error in
        if let value = value as? String {
            seal.fulfill(value)
        } else if let error = error {
            seal.reject(error)
        }
    }
}

There are two missing paths here and if either occur the promise will soon be deallocated without resolving. This will show itself as a bug in your app, probably the awful: infinite spinner.

So let’s be thorough:

Promise<String> { seal in
    task { value, error in
        if let value = value as? String {
            fulfill(value)
        } else if let error = error {
            reject(error)
        } else if value != nil {
            reject(MyError.valueNotString)
        } else {
            // should never happen, but we have an `PMKError` for task being called with `nil`, `nil`
            reject(PMKError.invalidCallingConvention)
        }
    }
}

If this seems tedious it shouldn’t. You would have to be this thorough withoutpromises too, the difference is without promises you wouldn’t get a warning in the console letting you know your mistake!

Slow Compilation / Compiler Cannot Solve in Reasonable Time

Add return types to your closures.

My Promise Never Resolves

Check your asynchronous task even starts. You’d be surprised how often this is the cause. For example if you are using URLSession (without our extension, but don’t do that, use our extension, it’s thorough) did you forget to call resume on the task? If so it never starts, so of course, it never finishes either.

Result of call to 'foo(on:_:)' is unused

Promise deliberately avoids the @discardableResult annotation because the unused result warning is your hint that you have not handled the error in your chain. So do one of these:

1. Add a catch
2. return the promise (thus punting the error handling to the caller)
3. Use cauterize() to silence the warning.

Obviously do 1. or 2. in preference to 3.