[swift-evolution] It's the little things..

[Jordan Rose]

> On Dec 13, 2016, at 20:43, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Dec 13, 2016, at 9:51 AM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com>> wrote:
>> 
>> On Dec 12, 2016, at 6:58 PM, David Sweeris via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> On Dec 12, 2016, at 16:15, John Holdsworth via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>> I’d like to raise again the idea of optionality when referencing a key or
>>>> calling a function could be possible using a ? i.e instead of
>>>> 
>>>>  let a = key != nil ? dict[key] : nil
>>>> 
>>>> you could just write:
>>>> 
>>>>  let a = dict[key?]
>>>> 
>>>> or even 
>>>> 
>>>>  let a = func( arg: argumentThatMayBeNull? ) // not called if argument is nil
>>> 
>>> The first part is pretty easy to add in an extension:
>>> 
>>> extension Dictionary {
>>>   subscript(_ key:Key?) -> Value? {
>>>       return key != nil ? self[key!] : nil
>>>   }
>>> }
>>> 
>>> At least I think that works... I'm on my phone so I can't test it.
>> 
>> You can do something like this, but I’d recommend labeling the subscript.  The problem comes up when you have a dictionary that has an optional key:   When you use “myDict[nil]”, you may get one or the other, but you probably mean one specifically.  
> I don’t think that’s an issue in the stdlib, because `Optional` doesn’t conform to `Hashable` and, AFAIK, no other stdlib types conform to `ExpressibleByNilLiteral`. Custom types could conform to both, though, and according to a playground, that does indeed lead to some confusing code:
> struct Foo : ExpressibleByNilLiteral, Hashable {...}
> extension Dictionary { subscript(_ key:Key?) -> Value? { return key != nil ? self[key!] : nil } }
> var bar = [Foo:Int]()
> bar[nil] //calls `Foo.init(nilLiteral:())`, and tries to look up the new `Foo` in `bar` using the stdlib's subscript
> bar[nil as Foo?] //passes `Optional<Foo>.none, which uses the extension's subscript

I wouldn't be surprised if there's a proposal to make Optional conditionally conform to Hashable if/when we get conditional conformances.

Jordan

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