Generic Encodable could not be inferred - ios

Question

I have an API class with the following function:
static func JSONPostRequest<EncodableType:Codable, DecodableType:Codable>(endpoint: String, jsonData: EncodableType, callback: #escaping (DecodableType) -> Void, clientErrorCallback: #escaping (Error) -> Void, responseErrorCallback: #escaping (URLResponse) -> Void, requestHeaders: [String: String]?) {
// Encoding the data into JSON
var jsonData: Data = Data();
do {
jsonData = try JSONEncoder().encode(jsonData)
}
catch {
print("JSON Encode")
return ;
}
// Setup the request
var request: URLRequest = URLRequest(url: URL(string: self.endpoint + self.apiVersion + "/" + endpoint)!)
request.httpMethod = "POST"
request.httpBody = jsonData
// Set the custom headers
if(requestHeaders != nil) {
for (headerKey, headerValue) in requestHeaders! {
request.setValue(headerValue, forHTTPHeaderField: headerKey)
}
}
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
// Was there an error in request?
if error != nil {
clientErrorCallback(error!)
return
}
// Response code is 2XX?
guard let httpResponse = response as? HTTPURLResponse, (200...299).contains(httpResponse.statusCode) else {
responseErrorCallback(response!)
return
}
// Has mime type fine?
guard let mime = response!.mimeType, mime == "application/json" else {
print("Wrong MIME type!")
return
}
let decoder = JSONDecoder()
let loginResponse = try! decoder.decode(DecodableType.self, from: data!)
callback(loginResponse)
}
task.resume()
}
But when I would like to call it from my login view ... :
API.JSONPostRequest(
endpoint: "login",
jsonData: LoginRequest(username: self.username, password: self.password),
callback: { loginResponse in
// success callback
if loginResponse.success && loginResponse.token != "" {
callback(loginResponse)
}
else {
// backend logic error
}
}, clientErrorCallback: { error in
// client error
}, responseErrorCallback: { urlResponse in
// response error
}, requestHeaders: headers
)
... I get the following error message:
Generic parameter 'EncodableType' could not be inferred
Here's my LoginRequest implementation:
struct LoginRequest: Codable {
var username: String;
var password: String;
}
The error message shows near the first line of the function call, near "API.JSONPostRequest(".
What could be the problem here?

Answer 1

You didn't provide a type for loginResponse, and there's no way from this code to guess what it is. Looking at the code you've posted, I can't figure out what the response is supposed to be (nothing you've posted here has a .success or .token value).
I assume you meant to add the following code:
struct LoginResponse: Codable {
var success: Bool
var token: String
}
And I assume somewhere there is a variable callback of type (LoginResponse) -> Void.
With that type, you'll still need to let the compiler know it's what you expect. It has no way to guess of the infinite number of types this could return, that you want LoginResponse:
...
callback: { (loginResponse: LoginResponse) in
...
Alternately, you can adjust your type signature to pass the expected type, for example:
static func JSONPostRequest<Request, Response>(endpoint: String,
jsonData: Request,
returning: Response.Type = Response.self,
callback: #escaping (Response) -> Void,
clientErrorCallback: #escaping (Error) -> Void,
responseErrorCallback: #escaping (URLResponse) -> Void,
requestHeaders: [String: String]?)
where Request: Encodable, Response: Decodable {
This would allow you to pass returning: LoginResponse.self, which can be nicer than embedding it in the closure:
...
returning: LoginResponse.self,
callback: { loginResponse in
...
(That said, what I'm discussing here fixes the fact that DecodableType is actually ambiguous. The fact that you're getting an error on EncodableType suggests that you have other type-mismatches. I recommend simplifying this to a minimal example that actually demonstrates the problem in a playground. As you've written it, I had to guess at a bunch of extra code.)