[swift-evolution] Reduce with inout

[Matthew Johnson]

Sent from my iPad

> On Jan 24, 2017, at 1:54 AM, Chris Eidhof via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I've thought about it for a few days, and really like `reduce(mutating:_)`.

I'm not a fan of this.  It reads in a way that makes it seem like the parameter should be inout, but it isn't.  A variation of reduce where the initial value parameter *is* inout is perfectly sensible (whether or not we want it in the standard library).  With that in mind, I don't think we should use this name.  

Unfortunately I don't have a better suggestion.  I think it was Brent who suggested "mutatingCopyOf" which is more accurate, but also undesirably verbose.

> I've updated the PR, and am now happy for this to go into review.
> 
> https://github.com/apple/swift-evolution/pull/587
> 
>> On Mon, Jan 23, 2017 at 8:27 AM, Russ Bishop <xenadu at gmail.com> wrote:
>> 
>>> On Jan 22, 2017, at 10:56 PM, Chris Eidhof <chris at eidhof.nl> wrote:
>>> 
>>> Not as a direct reply to Russ, but just to reiterate: to me, there are two clear benefits of using the `inout` version of reduce:
>>> 
>>> 1. The performance (currently discussed at length)
>>> 2. Readability (because we can use mutating methods on `inout` arguments).
>>> 
>>> Even if the compiler were to optimize the unnecessary copy of `return arr + [el]` away, there are still a lot of other mutable methods that you might want to use within the reduce closure. So I think the proposal is still very valid even if the compiler optimizations would magically appear tomorrow.
>>> 
>>> To push this proposal forward a little bit, I'd like to come up with a good name. It seems like we shouldn't overload `reduce`, but choose a different name, so that we don't stress the typechecker. Any other suggestions?
>>> 
>>> On Mon, Jan 23, 2017 at 7:11 AM, Russ Bishop <xenadu at gmail.com> wrote:
>>> -- 
>>> Chris Eidhof
>> 
>> 
>> Sorry for the derail!
>> 
>> reduce(mutating:_:) { } is still my favorite; You can take mutating to mean we will copy the value now but mutate it later.
>> 
>> 
>> Some alternatives:
>> 
>> reduce(forMutating:_:) { }
>> 
>> reduce(forInout:_:) { }
>> 
>> reduce(initial:_:) { }
>> 
>> reduce(copying:mutate:) { }
>> 
>> // just kidding...
>> reduce(copyForLaterMutating:_:) { }
>> 
>> 
>> 
>> It should definitely be some form of reduce. 
>> 
>> Russ
> 
> 
> 
> -- 
> Chris Eidhof
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