Swift Generic func calling a Generic func - swift

Question

I have an interesting compile error when using one generic to call another. This post is a bit long but hopefully, it's not longer than needed to describe the problem.
I have defined a generic func that works great. I use it a lot and the pattern of use is often the same. I was trying to implement a new generic func that nests the existing generic func, but I'm getting a compile-time error.
For some context, here is how my API works with the generic now. My REST API makes a call the films() func, which hits the StarWarsAPI (swapi.co) and returns a list of all the Star Wars films they have in their database, as follows:
StarWarsAPI.shared.films(){ (films, error) in
for film in films {
print(film.title)
}
}
The films() function calls a generic (restCall()) which works great. Here is the definition of films():
public func films(completion: #escaping (_ films:[Film]?, _ error:StarWarsError?) -> Void) {
guard StarWarsAPI.isOperational else {
return completion(nil,StarWarsError.starWarsAPINotOperational)
}
restCall(fetchUrl: filmsUrl!, modelType: FilmResult()) { (filmResults, error ) in
completion(filmResults?.results, error)
}
}
Where the restCall (the generic) is defined as follows: (Note I'm using the Swift 4 Codable API)
public func restCall<T: Codable>(fetchUrl: URL, modelType: T, completion: #escaping (_ modelObject: T?, _ error:StarWarsError?) -> Void){
var fetchRequest = URLRequest(url: fetchUrl, cachePolicy: .useProtocolCachePolicy, timeoutInterval: 10.0)
fetchRequest.httpMethod = "GET"
fetchRequest.allHTTPHeaderFields = [
"content-type": "application/json",
"cache-control": "no-cache",
]
let session = URLSession.shared
let fetchDataTask = session.dataTask(with: fetchRequest) { (data, response, error) in
DispatchQueue.main.async { // return to main thread
var modelObject:T?
do {
let jsonDecoder = JSONDecoder()
modelObject = try jsonDecoder.decode(T.self, from: data)// FIXME: Something about decoding the People object is going wrong.
return completion(modelObject, nil)
}catch let error as NSError {
completion(nil, StarWarsError.decodingModelError(error: error))
}
}
}
fetchDataTask.resume()
}
So the above works great. I use it for rest functions films(), people(), ships(), etc. I use the same pattern for each rest call. I want to create a generic that I can use instead of explicit films(), people(), etc.
I've been trying to get the following to work with little success:
public func fetchAll<T: Result>(result:T, completionAll: #escaping (_ result:T?, _ error:StarWarsError?) -> Void) {
restCall(fetchUrl: result.urlPath!, modelType: T) { (finalResults, error ) in
completionAll(finalResults!, error)
}
}
Where Result type is the base type and is defined as follows:
public class Result {
var urlPath:URL?
}
public class FilmResult: Result, Codable {
var count:Int?
var next:String?
var previous:String?
var results:[Film]?
}
The error I'm getting is shown in the screenshot below - hopefully, it's clear.
Any help you can provide would be much appreciated!

Answer 1

Your call has to be
this was T previously ───┐
restCall(fetchUrl: result.urlPath!, modelType: result) { (finalResults, error ) in
Note the result instead of T.
Relatively minimal code to reproduce:
public func restCall<T>(fetchUrl: URL, modelType: T, completion: #escaping (_ modelObject: T?, _ error:String?) -> Void) { }
public func fetchAll<T>(result:T, completionAll: #escaping (_ result:T?, _ error:String?) -> Void) {
┌── should be result
restCall(fetchUrl: URL(string: "asdasd")!, modelType: T) { (finalResults, error ) in
completionAll(finalResults, error)
}
}