[swift-evolution] Warn about unused Optional.some(())

[Daniel Duan]

I guess I missed that discussion. This "feature" does more harm than good IMHO.

> On Jan 30, 2017, at 10:16 PM, Charlie Monroe via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Jan 31, 2017, at 1:03 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> 
>> Sent from my iPad
>> 
>>> On Jan 30, 2017, at 5:25 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>>> On Jan 30, 2017, at 2:58 PM, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> Right now, expressions that evaluates to Optional<()>, Optional<Optional<()>>… gets special treatment when it’s unused. For example:
>>>> 
>>>> func f(s: String) {}
>>>> let s: String = “”
>>>> s.map(f) // no warning here, even tho the resulting type is `Optional<()>` and unused.
>>>> 
>>>> func g() throws {}
>>>> try? g() // no warnings here neither.
>>>> 
>>>> This is convenient, but encourages composing map/filter/reduce, etc with side-effect-ful functions, which we have found a few cases of in our production code recently. Granted, these cases could’ve been caught with more careful code reviews. But we wouldn’t have missed them if this “feature” didn’t exist.
>>>> 
>>>> I think we should remove the special treatment so that code in the example above would generate a warning about `()?` being unused. Users can silence it manually by assigning the result to `_`. 
>>>> 
>>>> OTOH, this would undermine the convenience of `try?` when the throwing function don’t return anything.
>>> 
>>> IMHO, using ‘try?’ to ignore an error result, instead of just turning it into an optional, is an anti-pattern, and forcing users to write ‘_ = try? foo()’ might not be so bad…
>> 
>> +1
> 
> Isn't this how it was in Swift 2.x and the first versions of 3.0? I believe this was changed only recently - which I personally found as good news. In some cases you simply do not care about the error result since it has no impact if the call fails and typing "_ =" seemed like boilerplate...
> 
> If I recall correctly, this was discussed here on the list and changed to the current behavior.
> 
> 
>> 
>>> 
>>>> 
>>>> What do y’all think?
>>>> 
>>>> Daniel Duan
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