[swift-evolution] Feature proposal: Range operator with step

[Erica Sadun]

> On Apr 6, 2016, at 2:17 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>> Guidance:
>> 
>> When using odd integer literals to produce an even number sequence,
>> prefer the `...` operator to the `..<` operator and change your ending
>> literal to an even number.
> 
> I don't think you can fix counterintuitive behavior with guidance.  
> 
> (1..<199).striding(by: -2) is the first way I'd reach for to express
> 197, 195, ..., 3, 1

Yes, but you can with warnings and fixits. 

* The compiler should issue a warning for any use of 

(n..<m).striding(by: v | v < 0) | n, m, v : Integer

with a fixit of "replace  (n..<m) with (n...m - 1)" regardless of
whether n or m is known at compile time 

* If v cannot be known at compile time, I think the compiler should
always prefer ... to ..<.

* The compiler should not allow

(n..<m).striding(by: v | v < 0) | n, m, v: floating point types

where v is known at compile time to be a negative constant. There should 
also be a runtime precondition that raises a fatal error should a negative 
v be used with a half-open interval.

-- E


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