令人困惑的 Swift 错误：表达式类型不明确，没有更多上下文

Question

I'm getting this confusing error: Type of expression is ambiguous without more context in my function below and I have no clue what is causing it. I think it has something to do with my three let constants, because when I made some changes there the error appeared.

private func FetchOriginCoordinates(completion: @escaping ([NominationStructure]) -> ()) {
        let locationUrl = "https://nominatim.openstreetmap.org/search?country=Sweden&city=Stockholm&street=\(self.origin.input)&format=json"
        
        let encodedLink = locationUrl.addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)
        let encodedURL = URL(string: encodedLink!)
        
        URLSession.shared.dataTask(with: locationUrl!) {data, response, error in
            if let data = data {
                do {
                    if let decodedJson = try? JSONDecoder().decode([NominationStructure].self, from: data) {
                        
                        DispatchQueue.main.async {
                            completion (decodedJson)
                        }
                    }
                } catch {
                    print(error)
                }
            }
        }.resume() // This is where the error appears 
    }

Answer 1

The error in Swift most of the times is not really helpful. Your issue is that you are trying to pass a string instead of an URL to your data task method. Besides that you should always handle the error instead of ignoring it. I would also change the method declaration to include the error in your completion handler:

Note: It is Swift naming convention to name your methods starting with a lowercase letter.

private func fetchOriginCoordinates(completion: @escaping ([Place]?, Error?) -> ()) {
    let url = URL(string:  "your APY string")!
    URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data else {
            completion(nil, error)
            return
        }
        do {
            let decodedJson = try JSONDecoder().decode([Place].self, from: data)
            DispatchQueue.main.async {
                completion(decodedJson, nil)
            }
        } catch {
            completion(nil, error)
        }
    }.resume()
}

---

edit/update:

To properly encode your URL you should use URLComponents and add the address components to its query items:

---

private func fetchOriginCoordinates(for street: String,
                                country: String = "Sweden",
                                city: String = "Stockholm",
                                format: String = "json",
                                completion: @escaping ([Place]?, Error?) -> ()) {
    var urlComp = URLComponents()
    urlComp.scheme = "https"
    urlComp.host = "nominatim.openstreetmap.org"
    urlComp.path = "/search"
    urlComp.queryItems = [
        .init(name: "country", value: country),
        .init(name: "city", value: city),
        .init(name: "street", value: street),
        .init(name: "format", value: format)
    ]
    guard let url = urlComp.url else {
        completion(nil, URL.Error.invalidCharacterFound(street))
        return
    }
    URLSession.shared.dataTask(with: url) { data, response, error in
        guard let data = data else {
            completion(nil, error)
            return
        }
        do {
            let decodedJson = try
                JSONDecoder().decode([Place].self, from: data)
            DispatchQueue.main.async {
                completion(decodedJson, nil)
            }
        } catch {
            completion(nil, error)
        }
    }.resume()
}

---

Usage:

struct Place: Codable {
    let placeID: Int
    let licence: String
    let osmType: OsmType
    let osmID: Int
    let boundingbox: [String]
    let lat, lon, displayName: String
    let rootClass: Class
    let type: TypeEnum
    let importance: Double

    enum CodingKeys: String, CodingKey {
        case placeID = "place_id", licence, osmType = "osm_type", osmID = "osm_id", boundingbox, lat, lon, displayName = "display_name", rootClass = "class", type, importance
    }
}

---

enum OsmType: String, Codable {
    case way
}

---

enum Class: String, Codable {
    case highway
}

---

enum TypeEnum: String, Codable {
    case residential, secondary, unclassified
}

---

extension URL {
    enum Error: Swift.Error {
        case invalidCharacterFound(String)
    }
}

---

---

fetchOriginCoordinates(for: "Valhallavägen") { results, error in
    guard let results = results else {
        print("error:", error ?? "nil")
        return
    }
    for result in results {
        print(result)
    }
}

---

This will print:

Place(placeID: 100830896, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 41510350, boundingbox: ["59.3431062", "59.3438379", "18.0792506", "18.0821732"], lat: "59.3434756", lon: "18.0807072", displayName: "Valhallavägen, Ladugårdsgärdet, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 114 36, Sweden", rootClass: Class.highway, type: TypeEnum.secondary, importance: 0.6138276880810758)
Place(placeID: 110921085, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 95305155, boundingbox: ["59.3473458", "59.3475855", "18.0653022", "18.0654864"], lat: "59.3475219", lon: "18.0653483", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 114 24, Sweden", rootClass: Class.highway, type: TypeEnum.secondary, importance: 0.6138276880810758)
Place(placeID: 98613676, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 33057170, boundingbox: ["59.3458032", "59.3459006", "18.0692504", "18.0695413"], lat: "59.3459006", lon: "18.0692504", displayName: "Valhallavägen, Lärkstaden, Östermalm, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 100 41, Sweden", rootClass: Class.highway, type: TypeEnum.secondary, importance: 0.6138276880810758)
Place(placeID: 97771827, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 32125128, boundingbox: ["59.3435989", "59.3440214", "18.0767179", "18.0788456"], lat: "59.3437642", lon: "18.0781052", displayName: "Valhallavägen, Villastaden, Östermalm, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 114 36, Sweden", rootClass: Class.highway, type: TypeEnum.secondary, importance: 0.6138276880810758) Place(placeID: 217216781, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 621715886, boundingbox: ["59.3400363", "59.3400593", "18.0943109", "18.0944135"], lat: "59.3400593", lon: "18.0943109", displayName: "Valhallavägen, Ladugårdsgärdet, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 115 24, Sweden", rootClass: Z9BD81329FEBF6EFE 22788E03DDEAF0AFZ.highway, type: TypeEnum.secondary, importance: 0.6138276880810758) Place(placeID: 91379768, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 16620413, boundingbox: ["59.3500575", "59.3502804", "18.0588563", "18.0593429"], lat: "59.3500575", lon: "18.0593429", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 113 54, Sweden", rootClass: Class.highway, type: TypeEnum.secondary, importance: 0.6138276880810758)
Place(placeID: 122081536, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 137883176, boundingbox: ["59.3467668", "59.3469813", "18.0671955", "18.0678302"], lat: "59.3468409", lon: "18.0676219", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 100 41, Sweden", rootClass: Class.highway, type: TypeEnum.residential, importance: 0.6138276880810758) Place(placeID: 210614450, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 571062827, boundingbox: ["59.3469813", "59.3471276", "18.067152", "18.0671955"], lat: "59.3469813", lon: "18.0671955", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 114 24, Sweden", rootClas s: Class.highway, type: TypeEnum.residential, importance: 0.6138276880810758)
Place(placeID: 100832986, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 40886504, boundingbox: ["59.3459069", "59.3467668", "18.0678302", "18.0702113"], lat: "59.3464817", lon: "18.0686787", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 100 41, Sweden", rootClass: Class.highway, type: TypeEnum.unclassified, importance: 0.6138276880810758)
Place(placeID: 172222242, licence: "Data © OpenStreetMap contributors, ODbL 1.0. https://osm.org/copyright" , osmType: OsmType.way, osmID: 345280785, boundingbox: ["59.3453872", "59.3458192", "18.0701008", "18.0715971"], lat: "59.3457283", lon: "18.0702244", displayName: "Valhallavägen, Ruddammen, Norra Djurgården, Östermalms stadsdelsområde, Stockholm, Stockholms kommun, Stockholm County, 114 26, Sweden", rootClass: Class.highway, type: TypeEnum.unclassified, importance: 0.6138276880810758)