[swift-evolution] [Review] SE-0065 A New Model for Collections and Indices

Tony Parker anthony.parker at apple.com
Wed Apr 13 17:19:59 CDT 2016

> On Apr 13, 2016, at 12:57 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> on Wed Apr 13 2016, Dave Abrahams <swift-evolution at swift.org> wrote:
>>> Reverse is the best opposite we have of advance, so it makes sense to
>>> me. 
>> Oh, I get it.
>>> Or we could use retreat. =) There are other pairs of words that work
>>> as well, like “increment/decrement”. 
>> Yeah, unfortunately those carry an incorrect implication when the
>> indices are numbers, because, e.g. the collection might be offsetting
>> the number by 2 for each position.  One could of course argue that using
>> numbers that way as indices was a bad design choice.
>> I'll have to think about that idea again.  We considered and rejected it
>> for a reason, but it might not be a really strong one.  Thanks for
>> bringing it up.
> ...and having talked it over at lunch, now I remember why we rejected
> it: there's no good way to make a nonmutating version.
>  let x = c.incremented(i)   // reads like an assertion about the past
>  let y = c.incrementing(i)  // reads like it has side-effects and returns c, or
>                             // a new version of c

In fact, it does return a new version* of c; just like this:

let s2 = myString.appending(“foo”)

*new version: the result is related to the argument

This works out great:

let next = c.incrementing(first)

- Tony

> APIs where the receiver returns a modified version of an argument don't
> lend themselves to verb forms.
> -- 
> Dave
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