[swift-evolution] Proposal: Contiguous Variables (A.K.A. Fixed Sized Array Type)

[Joe Groff]

> On Jan 28, 2016, at 4:04 PM, Haravikk <e-mail at haravikk.me> wrote:
> 
> 
>> On 28 Jan 2016, at 22:37, Joe Groff via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> 
>>> On Jan 28, 2016, at 2:36 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>>> 
>>> I like this idea, but the syntax seems dangerously close to a call site for  "func *(lhs: Int, rhs: Any.Type)"  (which is obviously ill-advised, but it is allowed).
>>> 
>>> Maybe we could take advantage of something which would be very invalid under the current grammar, namely (n T) rather than (n * T):
>>> 
>>>    let values: (4 Int) = (1, 2, 3, 4)
>> 
>> Sure, or we could lift (4 x Int) from LLVM IR's syntax.
> 
> How about:
> 
> 	let values:Int[4] = (1,2,3,4)
> 
> While it looks a bit like a subscript, it doesn’t make sense in a type declaration at present, so could be a good way to define restrictions of this type (we could even extend it to collections later). If the similarity is too close then:
> 
> 	let values:(Int[4]) = (1,2,3,4)
> 
> Could work too? Just some alternatives anyway, as I like the idea.

This kind of syntax doesn't compose well with other type productions. If you parse Int[N][M] naively as (Int[N])[M], then you end up with an array of M (array of N (Int)), which ends up subscripting in the opposite order, array[0..<M][0..<N]. C works around this by flipping the order of multiple array indices in a type declaration, so int [n][m] is really (int [m]) [n], but this doesn't work well for Swift, which has other postfix type productions—how would Int[N]?[M] parse? Choosing a prefix notation for fixed-sized array bounds is better IMO to avoid these pitfalls.

-Joe


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