[swift-evolution] Revisiting SE-0132 Rationalizing Sequence end-operation names

[Brent Royal-Gordon]

During the Swift 3 cycle, I proposed SE-0132, "Rationalizing Sequence end-operation names". It was rejected because it needed revision and there was no time to do so. Since then, part of the proposal—partial ranges and the `RangeExpression` slicing protocol—has been adopted in SE-0172, "One-sided Ranges". I''d like to reopen discussion of the rest of the proposal.

To refresh your memory, SE-0132 proposed systematically renaming a number of `Sequence` and `Collection` methods which operate on the beginning and end of a sequence. Many of these methods have names borrowed directly from functional programming; they use terminology in conflicting ways and don't follow our conventions for non-mutating method names. For example, consider the inconsistent and API Guideline-violating names of a few members which operate on the beginning of a sequence or collection:

	first			dropFirst()			removeFirst()
	prefix(_:)		dropFirst(_:)			removeFirst(_:)
	prefix(while:)	drop(while:)			—

These members could be renamed to form consistent "families" where a given term always meant the same thing:

	first			removingFirst()		removeFirst()
	prefix(_:)		removingPrefix(_:)		removePrefix(_:)
	prefix(while:)	removingPrefix(while:)	—

The main question in my mind about this plan is source stability. Back during Swift 3, we broke compatibility willy-nilly, but today we're being a little more circumspect. I believe these names meet the criteria of being actively harmful—they are difficult to discover, so developers don't use these members even when they should, and many of them sound like mutating methods or are unclear about their purpose—but that still doesn't tell us how we should treat the old names.

Basically, when should we introduce the new names?

	1. Swift 4.1 (or whatever pre-Swift 5 version the proposal ends up landing in)
	2. Swift 4.n (the version of Swift 5's compatibility mode for Swift 4)
	3. Swift 5

And when should we deprecate the old ones?

	1. Swift 4.1
	2. Swift 4.n
	3. Swift 5
	4. Swift 6
	5. Never

I'm also open to discussion about whether this should be done at all, whether any additional methods should be included (or included methods should be left alone), whether the now-obsolete `prefix(from:)` `prefix(upTo:)`, and `prefix(through:)` methods should be left alone, deprecated, or removed, and whether this should be done in this proposal or a different one.

The original proposal, which lists all affected methods and explains the logic behind them, is available at <https://github.com/apple/swift-evolution/blob/master/proposals/0132-sequence-end-ops.md>. Keep in mind that the parts about ranges have already been incorporated into Swift in a revised form, so you can ignore them.

I'll get cracking on an implementation once we figure out what I should implement.

Thanks!

-- 
Brent Royal-Gordon  
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