[swift-evolution] try? shouldn't work on non-method-call

[John McCall]

> On Aug 18, 2016, at 10:56 AM, Davor Jankolija <djankolija at gmail.com> wrote:
>> On 18 Aug 2016, at 19:46, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> 
>>> On Aug 18, 2016, at 10:15 AM, Davor Jankolija via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>>> This is a warning right now — do you think it should be an error?
>>>> 
>>>> Slavas-MacBook-Pro:~ slava$ cat ttt.swift 
>>>> func f() {}
>>>> 
>>>> func g() {
>>>> try f()
>>>> try? f()
>>>> }
>>>> 
>>>> Slavas-MacBook-Pro:~ slava$ swiftc ttt.swift 
>>>> ttt.swift:4:3: warning: no calls to throwing functions occur within 'try' expression
>>>> try f()
>>>> ^
>>>> ttt.swift:5:8: warning: no calls to throwing functions occur within 'try' expression
>>>> try? f()
>>>>     ^
>>> 
>>> IMHO at least, this should be an error. As a side note I do think that try should have greater precedence than as, even though this does potentially limit calls that would use only one try on a series of throwing statement that can be handled using parentheses. The benefit is that try is probably called in the vast majority of situations (that’s my experience at least) on a single throw statement and having it have higher precedance would avoid some surprises as most developers probably already assume that try wil be executed before as.
>> 
>> The design of ordinary 'try' and 'try!' are that they includes the entire expression to the right, spanning casts and other binary operators, and I think that's clearly the right rule for them.  It's not as clearly right for 'try?', but making 'try' and 'try?' be parsed differently would be horribly confusing.
>> 
>> John.
> 
> I see what you’re saying and I have to agree for the most part, it’s just that now you basically get a ‘try’ executed on the result of the ‘as?’ expression, which although is correct now that I look at your explanation is somewhat unintuitive, again this is just my opinion.
> 
> P.S.
> It would seem then, that the best way and probably simplest is to remember and place parentheses around the ‘try call()’ and then attempt a cast?

In your case, yes, I think the right solution is probably to parenthesize the try?.

John.
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