Joe Groff on Twitter: "@rob_rix @cocoaphony @an0 @raganwald @AirspeedSwift I don't think '+' is implemented to check for unique referencing at all."

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Airspeed Velocity ‏@AirspeedSwift 1 Aug 2015

Your reminder that building arrays with reduce, while fun, is accidentally quadratic.pic.twitter.com/5hmrpQkKBS
111 retweets 124 likes
Reginald deBRAIDEWAD ‏@raganwald 1 Aug 2015

@AirspeedSwift @rob_rix Building arrays with reduce and an On append operation is quadratic. #NotAllBuildngArraysWIthReduce
1 retweet 9 likes
an0 ‏@an0 1 Aug 2015

@raganwald @AirspeedSwift @rob_rix odd. Unless it's list, concat should be O(n) with n=postfix length=1, and no copy should happen. So why?
Rob Napier ‏@cocoaphony 1 Aug 2015

@an0 Each step of the reduce generates a new Array in Swift, requiring an O(n) copy to be made. @raganwald @AirspeedSwift @rob_rix
Rob Napier ‏@cocoaphony 1 Aug 2015

@an0 The + operator on Array has to make a copy because Arrays can't do partial sharing like Lists. @raganwald @AirspeedSwift @rob_rix
2 likes
an0 ‏@an0 1 Aug 2015

@cocoaphony @raganwald @AirspeedSwift @rob_rix A + B can not be implemented by mutating A when A is unshared?
Rob Napier ‏@cocoaphony 1 Aug 2015

@an0 The "A" in this case was passed to the closure. Hard to discover it's unshared. @raganwald @AirspeedSwift @rob_rix
Rob Napier ‏@cocoaphony 1 Aug 2015

@an0 Not saying it's totally impossible for compiler to detect, but Swift doesn't encourage the pattern. @raganwald @AirspeedSwift @rob_rix
Rob Rix‎ ‏@rob_rix 1 Aug 2015

@cocoaphony @an0 @raganwald @AirspeedSwift I think it’s a missed ARC optimization. isUniquelyReferenced() should be able to deal.
2 likes

Joe Groff

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