Swift flatMap和泛型

Question

I'm trying to use flatMap to build a Resource<T> in Swift, but keep getting a strange error, and only works when I force the cast.

Resource<T> :

public struct Resource<T> {
    let record: CKRecord
    let parser: [String: AnyObject] -> T?
}

Working code:

public func buildResource<T>(resource: Resource<T>) -> T? {
    var dataJson: [String: AnyObject] = [:]
    dataJson["recordID"] = resource.record.recordID
    for name in resource.record.attributeKeys {
        dataJson[name] = resource.record[name]
    }
    return (dataJson as? [String: AnyObject]).flatMap(resource.parser)
}

The code above gives a warning that the casts always succeeds, which is true. But when I try to remove the cast like so:

public func buildResource<T>(resource: Resource<T>) -> T? {
    var dataJson: [String: AnyObject] = [:]
    dataJson["recordID"] = resource.record.recordID
    for name in resource.record.attributeKeys {
        dataJson[name] = resource.record[name]
    }
    return dataJson.flatMap(resource.parser)
}

It gives the following error: 'flatMap' produces '[S.Generator.Element]', not the expected contextual result type 'T'? .

The parser is a struct init like so:

struct Example {

    let name: String
    let id: Int
}

extension Example {

    init?(dataJson: [String: AnyObject]) {
        guard let name = dataJson["name"] as? String else {
            return nil
        }
        guard let id = dataJson["id"] as? Int else {
            return nil
        }
        self.name = name
        self.id = id
        return
    }

}

Any ideas how to fix this or a different approach? The idea here is to transform any CKRecord into a struct easily without needing to write a lot of boilerplate code.

Answer 1

Daniel Hall's answer is right but, by doing this, you'll be forced to change your parser init signature to receive (String, AnyObject) .

The best option would be to create another init with this signature and parsing it to your json signature's init , still being able to create this struct from a raw json .

extension Example {

    init?(json: [String: AnyObject]) {
        guard let name = json["name"] as? String else {
            return nil
        }
        guard let id = json["id"] as? Int else {
            return nil
        }
        self.name = name
        self.id = id
      return
    }

    init (tuple : (String, AnyObject)) {
        var json : [String : AnyObject] = [:]
        json["name"] = tuple.0
        json["id"] = tuple.1
        self.init(json: json)!
    }
}

EDIT

As you're creating your dataJson as a [String : AnyObject] , you don't need to do a flatMap on it, you can just return resource.parser(json: dataJson)

Answer 2

Looks like you have the wrong signature for your parser function. The entire json dictionary is of type [String : AnyObject] , but the individual elements in that dictionary when enumerated with flatMap() are of type (String, AnyObject) , not [String : AnyObject] .

Try changing this:

public struct Resource<T> {
    let record: CKRecord
    let parser: [String: AnyObject] -> T?
}

to this:

public struct Resource<T> {
    let record: CKRecord
    let parser: (String, AnyObject) -> T?
}