[swift-evolution] Generic Subscripts

[Karl Wagner]

> On 12 Jan 2017, at 22:37, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> 
>> On Jan 12, 2017, at 9:53 AM, Chris Eidhof via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> Ok, I've got a draft up as a gist: https://gist.github.com/chriseidhof/6c681677d44903045587bf75fb17eb25 <https://gist.github.com/chriseidhof/6c681677d44903045587bf75fb17eb25>
>> 
>> Before I submit it, could someone let me know if adding generics to subscripts would influence the ABI? ( still feel pretty clueless in that area).
> 
> It won’t change the ABI of existing subscript calls, but if the standard library introduces new generic subscripts that replace older non-generic subscripts, it will impact ABI.
> 
> Slava

Why are subscripts so different, anyway? One would think they are basically sugared functions, but they don’t support so many things that regular functions support. Not just syntax stuff, either - they also don’t support @inline(__always) for some reason…

Where generic subscripts are concerned, there are a couple of different things to express:
- Generic parameter  (I can understand various co-ordinates for the data)
- Generic return type (I can construct your preferred representation of the data)
- Generic setter type (I can set the data using various compatible types):

protocol MeaningfulToFoo {}
protocol ConstructableFromFoo {}

struct Foo {
    subscript<Index>(index: Index) where Index: SignedInteger {
        get<T> where T: ConstructableFromFoo { return T(self) }
        set<U> where T: MeaningfulToFoo      { self.someProperty = newValue.someData }
    }
}

The syntax looks a bit awkward, though, IMO. I’m wondering if it might be better to have some kind of combined subscript + property behaviours (remember those?) and allow those to be generic instead. Subscripts and properties are very similar anyway - they are both bundles of functions to represent getting and setting data (not just regular-old “get” and “set”, either, but also magic stuff like “mutableAddressWithPinnedNativeOwner”). The only difference is that property getters can’t have parameters — which is something I would also like to see lifted one day (I believe I’ve even seen people asking for “named subscripts” due to the lack of this =P)

- Karl

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