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
 }

但是编译器编译失败，并显示错误“无法将类型'Promise <_。T>'的返回表达式转换为类型'Promise <[Geosearch]>'。错误在哪里？

似乎是编译器错误。该函数已正确编译：

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

此功能也是如此：

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

但是此函数导致错误：

 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)
     }
   }
 }

为什么不能编译递归PromiseKit函数？

1 个答案: