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.