Fix Publisher.decode for JSONDecoder and TopLevelDecoder 🛠

Publisher+CustomDecoder.swift

@available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
extension Publisher where Output == URLSession.DataTaskPublisher.Output {
    public func decode<Item, Coder>(type: Item.Type, decoder: Coder) -> Publishers.TryMap<Self, Item> where Item: Decodable, Coder: TopLevelDecoder, Coder.Input == Data {
        return tryMap { try decoder.decode(type, from: $0.data) }
    }
}

If you look at the original code, you will see the problem: Self.Output == Coder.Input

@available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
extension Publisher {

    /// Decodes the output from upstream using a specified `TopLevelDecoder`.
    /// For example, use `JSONDecoder`.
    public func decode<Item, Coder>(type: Item.Type, decoder: Coder) -> Publishers.Decode<Self, Item, Coder> where Item : Decodable, Coder : TopLevelDecoder, Self.Output == Coder.Input
}

JSONDecoder is extended by the TopLevelDecoder protocol.

@available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
public protocol TopLevelDecoder {

    associatedtype Input

    func decode<T>(_ type: T.Type, from: Self.Input) throws -> T where T : Decodable
}

@available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
extension JSONDecoder : TopLevelDecoder {
}

As a result, TopLevelDecoder.Input == Data. And then there is an error (in Publisher.decode):

error: instance method 'decode(type:decoder:)' requires the types 'URLSession.DataTaskPublisher.Output' (aka '(data: Data, response: URLResponse)') and 'Data' be equivalent
    .decode(type: Object.self, decoder: JSONDecoder())

Example code with this error:

struct Repo: Codable, CustomStringConvertible {
    let full_name: String
    
    var description: String { return full_name }
}

let url = URL(string: "https://api.github.com/repositories")!
var request = URLRequest(url: url)
request.httpMethod = "GET"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")

URLSession.shared.dataTaskPublisher(for: request)
    .decode(type: [Repo].self, decoder: JSONDecoder()) // error: Instance method 'decode(type:decoder:)' requires the types 'URLSession.DataTaskPublisher.Output' (aka '(data: Data, response: URLResponse)') and 'Data' be equivalent
    .sink { (repos) in
        print("\(repos)")
    }

Someone solves the problem like this:

URLSession.shared.dataTaskPublisher(for: request)
    .tryMap({ return $0.data })
    .decode(type: [Repo].self, decoder: JSONDecoder())
    .sink { (repos) in
        print("\(repos)")
    }

Or like this:

class MyDecoder: TopLevelDecoder {
    private let jsonDecoder = JSONDecoder()

    typealias Input = URLSession.DataTaskPublisher.Output

    func decode<T: Decodable>(_ type: T.Type, from data: Input) throws -> T {
        return try self.jsonDecoder.decode(type, from: data.data)
    }
}

URLSession.shared.dataTaskPublisher(for: request)
    .decode(type: [Repo].self, decoder: MyDecoder())
    .sink { (repos) in
        print("\(repos)")
    }

But these are not fun solutions to the problem. That's why this gist appeared. Good luck! 😉😜

Discussion of the problem 🤠