[swift-evolution] Generic Subscripts

[Slava Pestov]

> On Jan 11, 2017, at 10:53 AM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Jan 11, 2017, at 12:32 PM, Erica Sadun via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Jan 11, 2017, at 12:26 AM, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> 
>>> On Jan 10, 2017, at 11:40 AM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> On Jan 10, 2017, at 10:34 AM, Michael Ilseman via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>> [Forgot to CC swift-evolution the first time]
>>>>> 
>>>>> When this came up last, it was seen as more so a bug in the current implementation, rather than an explicit choice. There's no need for a proposal, just a JIRA: https://bugs.swift.org/browse/SR-115?jql=text%20~%20%22Generic%20subscript%22 <https://bugs.swift.org/browse/SR-115?jql=text%20~%20%22Generic%20subscript%22> 
>>>> 
>>>> It’s a nontrivial new user-facing feature with new syntax in the language, so it’ll need a proposal. ‘twould be good for the proposal to link to the JIRA ticket.
>>>> 
>>>> I’ve only heard positive reactions toward this feature, and it’s something that the standard library could make good use of.
>>> 
>>> +1, this would be clearly great to happen.
>>> 
>>> -Chris
>> 
>> 
>> I apologize for adding to this topic rather than starting a new one, but I figure people interested in subscripts would be more likely to see my question:
>> 
>> Is there a good reason subscripts cannot throw? Right now you can create a [safe: index] subscript to return an optional but you can't create one that returns an unwrapped value or throws.
> 
> These topics do tend to come up together they are the two unnecessary limitations subscripts have right now.  I hope both make it into Swift 4.  I’m sure we’ll see a proposal after phase 2 opens up.

Another limitation is that subscripts cannot have default arguments. I don’t believe this even requires an evolution proposal, it’s a quirk of the implementation and can be fixed by cleaning up some duplicated code paths in SILGen. If anyone is interested in looking at this I can point you to the code in question.

Slava

> 
>> 
>> Thanks, -- E
>> 
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