PromiseKit `Result.. is unused warning` in a chained sequence with a for loop

Question

I have this annoying warning and I'm not able to get rid of it.
Let's say that I have a promise that sends a command to a bluetooth le device, to do a procedure I need to send these commands synchronously.
So what you can do in PromiseKit 6 is create a for loop and append each command using a then to the previous promise. The issue with that approach is that the compiler trigger a warning about an Result of call to 'then(on:flags:_:)' is unused, is like is missing the catch. I'm aware that I can use a cauterize() , but in this case I guess I will lost errors thrown while executing each promise. A when is not viable because it will execute concurrently while I need synchronously.
Here a small sample used with strings, in my real application they are NOT string.

 static func promise_startProcedure(with commands: [String]) -> Promise<Void> {
        let promise = Promise()
        for command in commands {
            promise.then { //<-- WARNING SHOW HERE
                Promise.value(command)
            }
        }
        return promise
    }

Is there a way to get rid of this warning?
[UPDATE]
I did a mistake, the answer was not correct bu pointed me to the right direction.

static func promise_startProcedure(with commands: [String]) -> Promise<Void> {
            var promise = Promise()
            for command in commands {
                promise = promise.then { 
                    Promise.value(command)
                }
            }
            return promise
        }

@Kamran of course, as I said this is just a little example, this procedure is chained with other promises and at the end there is a `catch` — Andrea, Apr 29 '19 at 11:41

score 3 · Accepted Answer · edited Dec 26 '21 at 05:51

3

Try this. I just added Underscore to suppress the warning and did not get that warning.

static func promise_startProcedure(with commands: [String]) -> Promise<Void> {
    let promise = Promise()
    for command in commands {
        _ = promise.then {
            Promise.value(command)
        }
    }
    return promise
}

edited Dec 26 '21 at 05:51

Phil Dukhov

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answered Apr 29 '19 at 12:55

Anand

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score 1 · Answer 2 · answered Apr 29 '19 at 10:39

1

You can add @discardableResult to the declaration of the method. From the docs:

For example, the discardableResult attribute on a function declaration indicates that, although the function returns a value, the compiler shouldn’t generate a warning if the return value is unused.

answered Apr 29 '19 at 10:39

J. Doe

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It won't work, the warning is shown here `promise.then {` but thanks for pointing it, I will edit the question – Andrea Apr 29 '19 at 11:43

PromiseKit `Result.. is unused warning` in a chained sequence with a for loop

2 Answers2