ios - 为什么无法编译递归 PromiseKit 函数？

Question

我有下一个递归 Promise 函数调用:

 func getPlaces(location: Location) -> Promise<[Geosearch]> {
   return Promise().then {
     URLSession.shared.dataTask(.promise, with: url)
   }.compactMap { result in
     let json = try JSONDecoder().decode(JsonPlaces.self, from: result.data)
     let places = json.query.geosearch
     return places
   }
 }

 func getAtLeastOnePlace(location: Location) -> Promise<[Geosearch]> {
   return getPlaces(location).then{ places in      //error here
     if hasNonVisitedPlaces(places: places) {
        return Promise.value(places)
     } else {
        return getPlaces(location)
     }
   }
 }

 func hasNonVisitedPlaces(places: [Geosearch]) -> Bool {
     return places.count > 0
 }

但是编译器构建失败并出现错误“无法将‘Promise<_.T>’类型的返回表达式转换为‘Promise<[Geosearch]>’返回类型。错误在哪里？

---

这似乎是编译器错误。此函数已正确编译:

 func getAtLeastOnePlace(location: Location) -> Promise<[Geosearch]> {
   return getPlaces(location).then{ places in                            
      return Promise.value(places)
   }
 }

还有这个功能:

 func getAtLeastOnePlace(location: Location) -> Promise<[Geosearch]> {
   return getPlaces(location).then{ places in                            
      return getPlaces(location)
   }
 }

但是这个函数导致了一个错误:

 func getAtLeastOnePlace(location: Location) -> Promise<[Geosearch]> {
   return getPlaces(location).then{ places in      //error here
     if hasNonVisitedPlaces(places: places) {
        return Promise.value(places)
     } else {
        return getPlaces(location)
     }
   }
 }

Answer 1

有时我们需要显式提供返回类型，因此请尝试以下操作。

func getAtLeastOnePlace(location: Location) -> Promise<[Geosearch]> {
   return getPlaces(location).then{ places -> Promise<[Geosearch]> in
     if hasNonVisitedPlaces(places: places) {
        return Promise.value(places)
     } else {
        return getPlaces(location)
     }
   }
 }