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

[Daniel Duan]

> On Feb 6, 2017, at 10:58 AM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> I think I see Alex's point here. Optional chaining is still intended to be a substitute for Objective-C's nil-swallowing, and therefore foo?.bar() should not warn if 'bar' has a discardable result, even though there is semantic information about whether the method was actually called. I think that of the three things under consideration here:
> 
> 1. foo?.bar() should not warn
> 2. foo.map(baz) should warn
> 3. Ternaries should be consistent with non-ternaries
> 

I 100% agree with this analysis.

> #1 is the most important, at least to me. The Swift 3 change was to sacrifice #2 in favor of #3, which I'm not sure I would have done, but I wouldn't want to sacrifice #1 in favor of #2.
> 
> I wouldn't mind the model of the type being '@discardableResult Optional<Void>' or whatever, but I think that's probably more work than anyone wants to sign up for.

I’ll give this a go and report back. *crosses fingers*

> 
> Jordan
> 
> 
>> On Jan 31, 2017, at 08:16, Daniel Duan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Good to know the history. If I were to fix the inconsistency, I'd add the warning to optional chaining instead. 
>> 
>> Deliberately make the compiler give us *less* information for esthetic reasons feels wrong to me. As I mentioned in the original email, this has cost us a few unnoticed bad patterns slipping into our production. That's the opposite of what this type of warning is supposed to achieve.
>> 
>>> On Jan 31, 2017, at 1:07 AM, Alex Hoppen <alex at ateamer.de <mailto:alex at ateamer.de>> wrote:
>>> 
>>> This was a deliberate change between Swift 3 beta 1 and beta 2 after a friend of mine pointed the following inconsistency out to me:
>>> 
>>> struct Foo {
>>> func bar() {}
>>> }
>>> let foo: Foo? = Foo()
>>> foo?.bar() // Does not create a warning
>>> true ? foo?.bar() : foo?.bar()  // expression of type '()?' is unused
>>> 
>>> After some offline discussion at WWDC with the Swift team we decided to move to a consistent model where ()?, ()??, … is always discardable since we didn't want to take the convenience of foo?.bar() away (something that regularly occurs with weak variables, e.g. captures in closures).
>>> 
>>> So much for the history of this feature.
>>> 
>>> – Alex
>>> 
>>> 
>>>> On 30 Jan 2017, at 22:58, Daniel Duan via swift-evolution <swift-evolution at swift.org <mailto: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.
>>>> 
>>>> What do y’all think?
>>>> 
>>>> Daniel Duan
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>>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>> 
>> 
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