[swift-users] Subclass instance passed to closure expecting parent class “cannot convert value of type to expected argument type”

[Mark Lacey]

> On Sep 28, 2016, at 5:43 PM, Eric Conner via swift-users <swift-users at swift.org> wrote:
> 
> Hello all!
> 
> I hope it's not frowned upon to repost a question here from Stack Overflow, but I have not found help there.  In any case, the email gets some nice formatting out of it :-D.
> 
> I am trying to create a subclass with a generic type that can be passed into a completion block, but am getting the following error:
> 
> Cannot convert value of type '(ModelListResponse) -> ()' to expected argument type '(APIResponse) -> ()'
> Parent class:
> 
> import ObjectMapper
> 
> class APIResponse: Mappable {
>     var status: String!
>     var message: String?
> 
>     required init?(map: Map) {}
> 
>     func mapping(map: Map) {
>         status <- map["status"]
>         message <- map["message"]
>     }
> }
> Subclass:
> 
> import ObjectMapper
> 
> class ModelListResponse<T: Mappable>: APIResponse {
>     var data: ModelList<T>?
> 
>     required init?(map: Map) {
>         super.init(map: map)
>     }
> 
>     override func mapping(map: Map) {
>         super.mapping(map: map)
>         data <- map["data"]
>     }
> }
> Then I have a function definition:
> 
>  func _GET(path: String, parameters: Parameters, completion:@escaping (APIResponse) -> ())
> And I'm trying to call it like so:
> 
> // error is on this line
> self._GET(path: "/rest/posts", parameters: parameters) { (response: ModelListResponse<Post>) in
>     // ...
> }
> But here I get the error about how ModelListResponse cannot be converted. Isn't a ModelListResponse an APIResponse?  I don't understand why this does not work.
> 
> 

Below is a simplified version of what you’re attempting to do, which will hopefully shed some light. 

In your example, you’ve got a function, _GET() which expects to be passed a function that can take an APIReponse (which it could then invoke methods on). You’re instead passing in a function that can take a ModelListResponse<Post>, and that function would expect to be able to invoke methods on a ModelListResponse<Post>, but the caller (_GET()), might pass in an APIRespons that is not a ModelListResponse<Post>.

Mark


class A {}
class B : A {
  func gotcha() {}
}

func apply(_ f: (A) -> (), _ a: A) { f(a) }

func processB(_ b: B) -> () { b.gotcha() }

apply(processB, A()) // cannot convert value of type '(B) -> ()' to expected argument type '(A) -> ()'





> I managed to get around the issue by using a generic. Though I'm not sure why this works and the above code does not. Here is my working declaration:
> 
> func _GET<T: Mappable>(path: String, parameters: Parameters, completion:@escaping (T) -> ()) { }
> Thanks!
> 
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> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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