PromiseKit with Swift: terminate chain of promises

Question

I'm trying to use PromiseKit with Swift. I am not really familiar with it, and there doesn't seem to be much information on its usage with Swift.

I can't seem to figure out how to terminate a chain of promises. As long as the last (terminal) then block contains a single statement, everything is fine:

firstly {
    // ...
}.then { obj in
    self.handleResult(obj)
}.catch { error in
    self.handleError(error)
}

However, if I try to add another statement, compiler complains:

firstly {
    // ...
}.then { obj in
    self.handleResult(obj)
    self.doSomethingDifferent(obj)
}.catch { error in // compiler error: Missing return in a closure expected to return 'AnyPromise'
    self.handleError(error)
}

Obviously, the solution is to return another promise, but it doesn't make sense in the terminal block. Is there anything else I can do?

Answer 1

According to http://promisekit.org/PromiseKit-2.0-Released/, under Swift Compiler Issues section:

The Swift compiler will often error with then. To figure out the issue, first try specifying the full signature for your closures:

foo.then { x in
    doh()
    return bar()
}

// will need to be written as:

foo.then { obj -> Promise<Type> in
    doh()
    return bar()
}

// Because the Swift compiler cannot infer closure types very
// well yet. We hope this will be fixed.

// Watch out for  one-line closures though! Swift will
// automatically infer the types, which may confuse you:

foo.then {
    return bar()
}

So you'll have to change your code to:

firstly {
    // ...
}.then { obj -> Promise<WhateverTypeDoSomethingDifferentPromises> in
    self.handleResult(obj)
    self.doSomethingDifferent(obj)
}.catch { error in
    self.handleError(error)
}

Or you can use obj -> Void to stop the chain

Answer 2

Change it to:

firstly {
    // ...
}.then { obj -> Void in
    self.handleResult(obj)
    self.doSomethingDifferent(obj)
}.catch { error in
    self.handleError(error)
}