How to write an extension for Result Enum - swift

Question

I have represented the results of my API requests using Result Enum with generic type as follow Result<T: Resource, Error>
In some cases I do not want to discuss whether the request has succeed or failed but I still need the returned resource as nullable value.
I tried to write the extension as the code below:
extension Result<T, Error> where T: Resource {
var value: Any? {
switch self {
case .success(let resource):
return resource.value
case .failure:
return null
}
}
}
However I got these compiler errors:
Constrained extension must be declared on the unspecialized generic type 'Result' with constraints specified by a 'where' clause
Use of undeclared type 'T'

Answer 1

You can simply create extension as below,
extension Result {
var value: Any? {
switch self {
case .success(let resource):
return resource
case .failure:
return nil
}
}
}
As in the enum definition, T is always Resource so you don't need to specify in the extension.
But if Resource is some protocol and you want to put a constraint on value variable in declaration then you can specify that in the extension using where clause as below,
extension Result where T == someSubTypeOfResource {
var value: Any? {
switch self {
case .success(let resource):
return resource
case .failure:
return nil
}
}
}

Answer 2

Write the type constraint as follows:
enum Result<T, Error> {
case success(T)
case failure(Error)
}
struct Resource {
let value: String
}
extension Result where T == Resource {
var value: Any? {
switch self {
case .success(let resource):
return resource.value
case .failure:
return nil
}
}
}

Answer 3

You can simply do it like,
enum Result<T> where T: Resource {
case success(T)
case failure(Error?)
}
Create the enum Result with case success accepting a generic associated value of type Resource and another case failure with Error? as its associated value.
Since you're using the generic param of type Resource, I assume Resource is a protocol having value as one of its properties.
protocol Resource {
var value: String {get set}
}
Now, the extension Result goes like,
extension Result {
var value: Any? {
switch self {
case let .success(resource):
return resource.value
case let .failure(error):
return error
}
}
}
Now return error in case failure and resource.value for case success.
There is no need to specify the generic parameter T with where clause in the extension again since you did that already while defining the enum Result.
Usage:
struct R1: Resource {
var value: String
}
let r1 = R1(value: "R1 is of type Resource")
let result = Result<R1>.success(r1)
print(result.value)