[swift-evolution] [Review] SE-0047 Defaulting non-Void functions so they warn on unused results

Andrey Tarantsov andrey at tarantsov.com
Wed Mar 23 15:04:46 CDT 2016

> What is your evaluation of the proposal?

-0.5 if the annotation is verbose (much longer than @discardable).
+0.5 if the annotation is pleasant and concise, like @discardable

> Is the problem being addressed significant enough to warrant a change to Swift?

The warn-by-default behavior is mostly useless. The only reason to make the change is because “if we were designing Swift from scratch, this would probably be a slightly better default”.

Most non-void functions are called for their result, and nobody ever forgets to use that result; if they do, it's like if they forgot to call the function altogether — trivial to find, not useful as a compiler diagnostic at all.

The new default is better for:

- (A) classes that provide both mutating and non-mutating methods;
- (B) methods where forgetting to use the result produces a bug (a download task that needs to be resumed, an alert that needs to be displayed, a listener that needs to be stored somewhere, etc).

The old default is better for:

- (C) fluid APIs and other similar DSL scenarios;
- (D) methods that are mainly invoked for their side effect and return a value “just in case”, like removing/adding elements, scheduling tasks, ...

I've just scanned the entire Swift codebase I wrote since that fateful WWDC'14 (~20 kLOC excluding comments and blanks). I only have a handful of classes/methods in each category above, and annotating them one way or another is a trivial matter. Some of them *are not* currently annotated with @warn_unused_result, which is a point in favor of this proposal.

> Does this proposal fit well with the feel and direction of Swift?

I think the safe-by-default, explicit-opt-out behavior is a better default for Swift, although, like I've said, in practice it doesn't really matter much.

Perhaps it's most useful for newcomers; you can easily skip over @warn_unused_result when learning Swift, but you won't be able ignore @discardable.

> If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?

Haven't ever seen this in a language.

Golang had a (not very informative) discussion on this topic at https://groups.google.com/forum/#!topic/golang-nuts/ksfgSWxJcCo

> How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

Closer to an in-depth study; looked at my Swift codebase, and read through all the discussions.

On a side note, let me once again point to a dangerous trend in this mailing list: not looking at (or reporting on) how potential changes affect actual, specific, production code bases. We need a lot more of that in our reviews.



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