Swift Type Inference with Generic method

Question

I'm working on an SDK, and have developed a nice concise Combine pipeline method that accepts a generic parameter that's used to decode json with. Effectively, it's a re-usable combine pipeline for JSON -> Decodable . Works really well. Here's what that pipeline looks like:

func records<Record: Decodable>(forRequest request:RestRequest ) -> AnyPublisher<[Record], Never> {
return NetworkService.publisher(for: request)
  .tryMap({ (response) -> Data in
    response.asData()
  })
  .decode(type: Wrapper<Record>.self, decoder: JSONDecoder())
  .map({ (record) -> [Record] in
    record.records
  })
  .catch({ _ in
    Just([Record]())
  })
  .eraseToAnyPublisher()
}

Usage:

contactsCancellable = NetworkService.records(forRequest: request)
  .receive(on: RunLoop.main)
  .assign(to: \.contacts, on: self)

It's my understanding that Swift+Combine is inferring the generic parameter type from the assign(to:, on:) call.

But the powers that be want a non-Combine version, and I'm really struggling to figure out how to help Swift infer the type. I tried building a direct analog like this:

func fetchRecords<Record: Decodable>(forRequest request: RestRequest,
                   _ completionBlock: @escaping (Result<[Record], RestClientError>) -> Void) {

RestClient.shared.send(request: request) { result in
   switch result {
     case .success(let response):
       do {
          let decoder = JSONDecoder()
          let wrapper = try decoder.decode(Wrapper<Record>.self, from: response.asData())
          completionBlock(.success(wrapper.records))
       } catch {
          completionBlock(.success([Record]()))
       }
     case .failure(let err):
       completionBlock(.failure(err))
    }
  }
}

This compiles however, executing that method like this:

NetworkService.fetchRecords(forRequest: request) { records in
  print(records)
}

Results in a lovingly cryptic error Generic parameter 'Record' could not be inferred

How can I specify that generic Record 'type' - anything that conforms to Decodable, in this non-combine version?

Ps: Here's that Wrapper struct:

struct Wrapper<R: Decodable>: Decodable {
  var totalSize: Int
  var done: Bool
  var records: [R]
}

Answer 1

You could specify the generic type in the closure parameter list:

NetworkService.fetchRecords(forRequest: request) { (result: Result<[ConcreteRecordType], RestClientError>) { 
  switch result {
    case .success(let records):
       // "records" is of type [ConcreteRecordType]
       //...
    case .failure(let error):
       //...
  }
}

But that can be cumbersome having to provide the full Result type in the closure, so I recommend that you fill in the generic type information by accepting it as a parameter. (like the Decoder functions do.)

func fetchRecords<Record: Decodable>(ofType type: Record.Type, forRequest request: RestRequest, _ completionBlock: @escaping (Result<[Record], RestClientError>) -> Void) {
   //... same code...
}

Then, you'd call it like this:

NetworkService.fetchRecords(ofType: ConcreteRecordType.self, forRequest: request) { result in
  // No need to specify closure argument type :) 

  switch result {
    case .success(let records):
       // "records" is of type [ConcreteRecordType]
       //...
    case .failure(let error):
       //...
  }
}

Voila! The explicit type provided to fetchRecords cascades down to the closure argument type. No need to provide the type in the closure parameter list.