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

[Dave Abrahams]

on Wed Apr 06 2016, Erica Sadun <swift-evolution at swift.org> wrote:

>     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.

You can lessen the impact of any design choice by adding warnings and
fixits, but I'm pretty sure it's better to make a choice that doesn't
require them.

-- 
Dave