Should be map for the second one, and no need to specify return types for one-liners:

firstly {
    session.dataTask(.promise, with: url)
}.map(on: queue) { data, response in
    try JSONSerialization.jsonObject(with: data, options: []) as? [String : Any]
}.compactMap(on: queue) { imageDict in
    self.factory.getImage(fromDict: imageDict)
}

You're right about the return types, but I wanted to point out in the examples that the data is being pushed down the chain.

I suppose since we're using the 'try' it will either throw or create the dict, so it can be a .map. I just followed the original example set by: https://github.com/mxcl/PromiseKit/blob/master/Documentation/CommonPatterns.md

Oh PMK docs are wrong too then, whoops, will fix.

Showing return types is fine if that is your goal and your justification is good.

Oh my bad compactMap is correct here, didn’t see the as?

Cool, appreciate the input :) If it wasn't cast into an optional then .map would do just fine :) It would actually be clearer in the intent.