Using a custom error type with Effect.task

[abdulajet]

Hi folks,

I am trying to use async/await with the new Effect.task method to load some data. Looking at the source Effect.task either supports a Never or Error type for the second generic parameter on Effect.

Here is some code:

struct AsyncLoginClient {
    var user: (String) -> Effect<Follow, Error>
}

extension AsyncLoginClient {
    static let live = AsyncLoginClient(
        user: { username in
            Effect.task {
                let url = URL(string: Constants.baseURL + "/user?username=\(username)")!
                let (data, _) = try await URLSession.shared.data(for: URLRequest(url: url))
                return try JSONDecoder().decode(Follow.self, from: data)
            }
        })
}

I think this matches the example in the comments above the Effect.task source. But then because Error doesn't conform to Equatable my enum of actions can no longer conform to Equatable:

enum LoginAction {
    case loginResponse(Result<Follow, Error>)
}

I guess I can make the Follow on my effect optional and do error handling that way but that would be less than ideal.

I have got this working:

struct LoginClient {
    var user: (String) -> Effect<Follow, Failure>
    
    struct Failure: Error, Equatable {}
}

extension LoginClient {
    static let live = LoginClient(
        user: { username in
            return Effect.task {
                let url = URL(string: Constants.baseURL + "/user?username=\(username)")!
                let (data, _) = try await URLSession.shared.data(for: URLRequest(url: url))
                return try JSONDecoder().decode(Follow.self, from: data)
            }
            .mapError { error in
                return LoginClient.Failure()
            }
            .eraseToEffect()
        })
}

But the map swallows up my errors. I guess I can switch to an error enum rather than a struct ?

Is this the recommended?

[mbrandonw]

But the map swallows up my errors.

Can you explain a bit more what you mean by this? It seems to me that this code should do what you want. It turns any Swift.Error emitted by the task to a LoginClient.Failure, but the publisher still fails. It isn't ignored in anyway.

[abdulajet]

Ok so I am on the correct track with the mapError.

Can you explain a bit more what you mean by this?

Sorry, in its current state any throw/error will become a LoginClient.Failure So I will want to flesh out my struct and return useful info rather than the empty struct.

I just wanted to check that I was on the the right path, thank you!

[mbrandonw]

Ah yeah I see. You can definitely fill out the struct to hold onto some information from the error, such as the localizedDescription. You could also turn it into an enum to differentiate between the two types of errors that are possible your code, which is network failure or decoding failure:

enum Failure: Error {
  case decoding
  case network
}

That would only really be necessary if you really do need to tell the difference between those errors in your feature, which is often not the case.

[abdulajet]

Amazing thank you!