Proper way to err out of PromiseKit

Question

What is the proper way to throw an error from a function like this:

    func fetch(by id: String, page: Int = 1) -> Promise<ProductReviewBase> {
    // call api
    guard let url = URL(string: "") else {
        return Promise { _ in return IntegrationError.invalidURL }
    }

    return query(with: url)

}

I'm confused whether to make this a function that throws an error, or return a promise that returns an error. Thanks

Answer 1

I really hate interfaces that mix metaphors. If you are going to return a promise, then use the promise's error system. If you want more justification than my hatred, then visualize what it would look like at the call site:

do {
    (try fetch(by: id))
        .then {
            // do something
        }
        .catch { error in 
            // handle error
        }
}
catch {
    // handle error
}

vs

fetch(by: id)
    .then {
        // do something
    }
    .catch { error in 
        // handle error
    }

The latter looks a whole lot cleaner.

Here's the best way (IMO) to write your example function:

func fetch(by id: String, page: Int = 1) -> Promise<ProductReviewBase> {
    guard let url = URL(string: "") else { return Promise(error: IntegrationError.invalidURL) }
    return query(with: url)
}