Swift - cannot specialize in nonequivalent definitions

Question

I have a function that is responsible for executing HTTP requests in an application. Basically, it sets all the necessary headers, timeouts, etc. Etc .... Then, when the request is completed, I run 2 functions (provided by the developer): whenSuccess / whenError (depending on the result of the call) and whenComplete (regardless of the result of the call). I want the latter to be able to get the result of the whenSuccess function.

I have a doRequest function declared as

 private func doRequest<S>(whenSuccess: (_ json: JSON?)->(S?), whenError: (_ description: String)->(), whenComplete: Optional<(S?)->()>) { // yada yada yada } 

So, the developer provides a function that receives JSON and returns a generic one. If whenError is whenError , whenComplete is called with the nil parameter.

I call it with

 doRequest<Int>(whenSuccess: { (json) -> (Int?) in //Cannot specialize a non-generic definition return 5 }, whenError: { (error) in }) { (success) in } 

I get an error message:

It is impossible to specialize not a general definition

Any idea if this can be done and how?

Answer 1

In Swift, you should not explicitly specialize common functions. Instead, general functions automatically become specialized by type inference from their arguments.

The call you are trying to make should look like this:

 doRequest(whenSuccess: { json -> Int? in //... }, whenError: { error in //... }, whenComplete: { success in //... })