One thing I've been thinking about with regards to the existing `reduce`
operation is whether it would be better expressed in Swift as taking its
closure as (inout State, Element) -> Void rather than (State, Element) ->
State. Doing so would avoid many of the accidentally-quadratic issues with the
current formulation of reduce:
arrayOfArrays.reduce([], combine: +) // quadratic temporary arrays
arrayOfArrays.inPlaceReduce([], combine: +=) // can be linear by
appending arrays in-place
Thanks to the scoped semantics of `inout`, there's no hazard of the mutable
state reference being escaped, so the inout form is isomorphic to the
traditional pure form of reduce.
Now `scan`-ing to generate an array of intermediate arrays is inherently
quadratic, since each intermediate array shows up as a distinct copy in the
resulting collection. However, if someone used `scan` to produce a sequence of
tree data structures instead of flat arrays, it could still be interesting to
share structure among the intermediate states collected by `scan` by performing
an in-place operation to generate new values instead of an out-of-place
operation. It might be interesting to consider a similar signature change to
`scan` for these same reasons.
-Joe
> On Apr 28, 2016, at 11:11 AM, Chris Lattner <[email protected]> wrote:
>
> Hello Swift community,
>
> The review of "SE-0045: Add scan, prefix(while:), drop(while:), and iterate
> to the stdlib" begins now and runs through May 3. The proposal is available
> here:
>
>
> https://github.com/apple/swift-evolution/blob/master/proposals/0045-scan-takewhile-dropwhile.md
>
> Reviews are an important part of the Swift evolution process. All reviews
> should be sent to the swift-evolution mailing list at
>
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
> or, if you would like to keep your feedback private, directly to the review
> manager.
>
> What goes into a review?
>
> The goal of the review process is to improve the proposal under review
> through constructive criticism and, eventually, determine the direction of
> Swift. When writing your review, here are some questions you might want to
> answer in your review:
>
> * What is your evaluation of the proposal?
> * Is the problem being addressed significant enough to warrant a change
> to Swift?
> * Does this proposal fit well with the feel and direction of Swift?
> * If you have you used other languages or libraries with a similar
> feature, how do you feel that this proposal compares to those?
> * How much effort did you put into your review? A glance, a quick
> reading, or an in-depth study?
>
> More information about the Swift evolution process is available at
>
> https://github.com/apple/swift-evolution/blob/master/process.md
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