[SR-3444] map, filter on RangeReplaceableCollection should return Self

[airspeedswift]

Previous ID	SR-3444
Radar	None
Original Reporter	@airspeedswift
Type	Improvement

Additional Detail from JIRA

Votes	1
Component/s	Standard Library
Labels	Improvement, StarterProposal, swift-evolution-proposal-needed
Assignee	None
Priority	Medium

md5: 05669e9e4e1c642618be3a82e1ececb1

Issue Description:

Currently, map and filter on Sequence return an Array, even though the implementation use only capabilities of Array that are available on RangeReplaceableCollection* (an empty init, reserveCapacity, and append).

This means that RangeReplaceableCollection could implement a version of map that returned the same type. For example, a Deque could return another Deque. This is probably what most users would expect for these types.

This would only apply to RRCs that could hold the mapped-to element type. Also note that, in the case of map, this would only apply to transformations that return the same element type – to represent a transformation to another type would require higher-kinder types. And sequences and non-RRC collections, like Set would need to return arrays.

*(this isn't completely true: it creates a ContiguousArray and then turns it into an Array. But hopefully this optimization could be removed at some point)

[Dante-Broggi]

Is SE-0174 (the proposal that links to this report) going to be updated and implemented soon?

It is probably the simplest of the still-unimplemented accepted proposals.

[benrimmington]

In case anyone was thinking of implementing this proposal, its status may have changed, from Accepted to Withdrawn.

https://forums.swift.org/t/se-0232-remove-some-customization-points-from-the-standard-librarys-collection-hierarchy/17265/11

[airspeedswift]

To clarify: this Jira proposes amending RRC to return Self. That was implemented as part of SE-0174. The part that wasn't was adding an associated type to Sequence to represent the filtered type. That's what I'm now thinking should be withdrawn.

The map part still needs HKTs.