Make it possible to catch specific error types and chain error handling

[gwynne]

Example:

func getUser(req: Request) -> Future<ResponseRepresentable> {
    return User.find(req.parameters.next()).map(to: PublicUser.self) { user in
        return try user.makePublic()
    }.catchMap(UserError.self, to: Response.self) { error in
        guard case .makePublicFailed = error else { throw error } 
        
        return req.makeResponse().withEncodedContent(MyErrorResponse())
    }
}

Of particular interest is how to handle catchMap() blocks returning a different type than the last map() block.

[calebkleveter]

Why call it catchMap? If I am correct, we want to mirror do/catch syntax so it feels more natural. I am more in favor of changing (or overloading) the current catch method to handle throwing and returning.

[jdmcd]

I actually prefer catchMap. It still returns a value (in @gwynne's example, return req.makeResponse().withEncodedContent(MyErrorResponse())) which I think is reason enough to let it follow the format of the current vapor map and flatMap methods. Especially if it's going to have similar overloads like to: .

Regular catch might get confusing with Vapor's catch, as the Swift catch doesn't explicitly return a value.

[calebkleveter]

Correct, it doesn't return a value, but I can return a value from a method from inside the catch block (or throw, for that matter).

[jdmcd]

Of course, but the method proposed here requires some kind of return, since it's chained on to the map or flatMap above it. Personally I think it's clearer when it follows the same pattern

[kdawgwilk]

I would prefer to keep things as similar to RxSwift as we can so the transition is easy for iOS devs using RxSwift to come over to Vapor 3: catchError && catchErrorJustReturn