PromiseKit firstly around code, not function call

Question

I don't want to write a separate function to return a Promise in my firstly call. I just want to write this:

firstly
{
    return Promise<Bool>
    { inSeal in
        var isOrderHistory = false
        let importTester = CSVImporter<String>(url: url)
        importTester?.startImportingRecords(structure:
            { (inFieldNames) in
                if inFieldNames[2] == "Payment Instrument Type"
                {
                    isOrderHistory = true
                }
            }, recordMapper: { (inRecords) -> String in
                return ""   //  Don't care
            }).onFinish
            { (inItems) in
                inSeal.resolve(isOrderHistory)
            }
    }
}
.then
{ inIsOrderHistory in
    if inIsOrderHistory -> Void
    {
    }
    else
    {
...

But I'm getting something wrong. ImportMainWindowController.swift:51:5: Ambiguous reference to member 'firstly(execute:)'

None of the example code or docs seems to cover this (what I thought was a) basic use case. In the code above, the CSVImporter operates on a background queue and calls the methods asynchronously (although in order).

I can't figure out what the full type specification should be for Promise or firstly, or what.

Deepika · Accepted Answer · 2018-09-20T23:13:52.243

According to my understanding, since you are using then in the promise chain, it is also meant to return a promise and hence you are getting this error. If you intend not to return promise from your next step, you can directly use done after firstly.

Use below chain if you want to return Promise from then

firstly {
     Promise<Bool> { seal in
        print("hello")
        seal.fulfill(true)
     }
}.then { (response) in
    Promise<Bool> { seal in
            print(response)
            seal.fulfill(true)
    }
}.done { _ in
    print("done")
}.catch { (error) in
        print(error)
}

If you do not want to return Promise from then, you can use chain like below.

firstly {
            Promise<Bool> { seal in
                print("hello")
                seal.fulfill(true)
            }
        }.done { _ in
                print("done")
        }.catch { (error) in
                print(error)
        }

I hope it helped.

Updated:

In case you do not want to return anything and then mandates to return a Promise, you can return Promise<Void> like below.

firstly {
    Promise<Bool> { seal in
        print("hello")
        seal.fulfill(true)
    }
}.then { (response) -> Promise<Void> in
            print(response)
            return Promise()
}.done { _ in
        print("done")
}.catch { (error) in
        print(error)
}

Yes, that seems to be the issue. But now I'm finding I really do want a `.then` that returns nothing, because I want the semantic clarity of doing something else in the `.done` section. The `.then` sets a property on the object. Obviously it can return that result instead, but it seems like I should be able to have `.then` that don't return anything. Maybe I'm just being picky? — Rick, Sep 20 '18 at 22:51
You can use a Promise. I have updated the answer. I hope this helps — Deepika, Sep 20 '18 at 23:14

PromiseKit firstly around code, not function call

1 Answers1