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

[David Hart]

> On 31 Jan 2017, at 07:23, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I guess I missed that discussion. This "feature" does more harm than good IMHO.

Indeed. I find this behavior very surprising and goes against Swift's safe-by-default and explicitness philosophy.

I'd argue removing it.

>> 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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