[swift-evolution] [Review] SE-0105: Removing Where Clauses from For-In Loops

[Kevin Ballard]

On Fri, Jun 24, 2016, at 11:03 AM, Erica Sadun via swift-evolution wrote:
> 
> > On Jun 24, 2016, at 11:26 AM, Karl Wagner via swift-evolution <swift-evolution at swift.org> wrote:
> > 
> > -1
> > 
> > I've followed this discussion since the beginning, and I feel the usage is clear given that for...in is a *data-driven* loop
> > 
> > I wouldn't mind renaming to "if" if there is some confusion, but I think we use "where" consistently in the language (and Dave Abrahams proposal for closure argument names takes this further), so could be argued that it's already clear.
> > 
> > Perhaps we should use "where" consistently for data-filtering operations, and possibly rename the generic constraint specifier. If we wanted to be really rigorously consistent.
> > 
> > Karl
> 
> I'd much rather keep `where` for protocol constraints and rename data-filtering in for-in if kept. My suggestions in the proposal are "if", "unless", "until", and "while". 

None of these keywords make sense as a replacement.

if - This only makes sense if it's a condition that's checked before the loop starts and causes the entire loop to be skipped.
unless - This has the same problem as if, though it inverts the boolean condition.
until - This keyword implies that iteration stops when the condition fails, which is a change in behavior from the where clause (and IMO less useful than the current where behavior).
while - Same as until, though it inverts the boolean condition.

I really think we should just keep the where here. I don't think there's any problem using it both here and in protocol constraints. It means basically the same thing in both cases, that the thing being modified applies to all cases (e.g. all types, or all elements of the sequence) where the condition holds true.

-Kevin