[swift-evolution] Reduce with inout

[Matthew Johnson]

> On Jan 24, 2017, at 8:12 AM, Chris Eidhof <chris at eidhof.nl> wrote:
> 
> But if we want to add "copyOf" we should do that to every method that takes a struct? Also, what if you pass in an object?
> 
> I see the concern, but I don't think adding `copyOf` will increase clarity. That said, I'm open to suggestions.

I’m not really trying to advocate for “copyOf”.  I think we should really have something more concise in a case like this.  My main point is that just using `mutating` is inaccurate and therefore probably not the right solution.

Your question about passing in an object (I assume you mean a reference type here) is a good one.  In an ideal world that would probably not be allowed as it doesn’t really make sense to replace the reference during reduction.  Unfortunately I don’t think we have a way to prevent that at the moment.

> 
> On Tue, Jan 24, 2017 at 2:43 PM, Xiaodi Wu <xiaodi.wu at gmail.com <mailto:xiaodi.wu at gmail.com>> wrote:
> It's only verbose if the words aren't needed! The shortest way to describe something with sufficient accuracy can never be verbose, let alone undesirable, and I highly agree with this concern. We already have names of this form, such as `FloatingPoint.init(signOf:magnitudeOf:)`.

I mostly agree with this Xiaodi, but I also think that commonly known and frequently used methods like `reduce` deserve to be as concise as possible.  That is less a concern with less common and less frequently used signatures.

> 
> On Tue, Jan 24, 2017 at 07:33 Matthew Johnson via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
> 
> Sent from my iPad
> 
> On Jan 24, 2017, at 1:54 AM, Chris Eidhof via swift-evolution <swift-evolution at swift.org <mailto: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 <https://github.com/apple/swift-evolution/pull/587>
>> 
>> On Mon, Jan 23, 2017 at 8:27 AM, Russ Bishop <xenadu at gmail.com <mailto:xenadu at gmail.com>> wrote:
>> 
>>> On Jan 22, 2017, at 10:56 PM, Chris Eidhof <chris at eidhof.nl <mailto: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 <mailto: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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> 
> 
> -- 
> Chris Eidhof

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