Question about chaining with `all()`

[Johnny Sung]

Hi all,
I have using kovenant framework with some questions
I would like to fetch a table of contents and fetch the content of the table
simultaneously, then output a mixed list with table content title and content itself.

But I faced the problems for coding with kovenant.

For example, I have MySubject and MySubjectDetails class.

class MySubject {
    var id: String = ""
    var name: String = ""
}

class MySubjectDetails {
    var id: String = ""
    var name: String = ""
    var data: List<String> = ArrayList()
}

and APICalls class (part of code is pseudo code),

object APICalls {
    fun getSubjects(): Promise<List<MySubject>, Exception> {
        val deferred = deferred<List<MySubject>, Exception>()
        // ... API calls ...
        return deferred.promise
    }

    fun getSubjectById(id: String): Promise<MySubjectDetail, Exception> {
        val deferred = deferred<MySubjectDetail, Exception>()
        // ... API calls ...
        return deferred.promise
    }
}

I may fetch the data like is:

APICalls.getSubjects().then { subjects ->
    val promises = subjects.map {
        APICalls.getSubjectById(it.id)
    }
    all(promises)
}.then { promise: Promise<List<MySubjectDetail>, Exception> ->
    //       ^~~~~~ What's this?
}

I will get a result with Promise<List<MySubjectDetail>, Exception> type then I can't continue the code.
I expected it will be a combined list (List<MySubjectDetail>) but it comes up a Promise<List<MySubjectDetail>, Exception> object thats confused me.

How can I do or fix the code? Thanks.