[swift-evolution] [swift-evolution-announce] [Review] SE-0045: Add scan, prefix(while:), drop(while:), and iterate to the stdlib
jgroff at apple.com
Thu Apr 28 13:48:59 CDT 2016
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.
> On Apr 28, 2016, at 11:11 AM, Chris Lattner <clattner at apple.com> 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:
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