[swift-evolution] [Pre-proposal] Forward/Reverse Only Indexing Methods

[Haravikk]

So for Swift 3 we’re going to have the great new indexing model that performs index manipulation through the collection to which an index belongs.

However, it retains one of the things I didn’t like about the old model, which is that the distinction between forward/backward only types is a bit fuzzy, since the single advancedBy() method, now the index(:offsetBy:) method, was used for both forward and backward movement, which seems contradictory compared to the forward/backward only single-step methods.

Anyway, I’m wondering what people’s thoughts would be on tweaking the formula slightly such that there are methods that only work in a particular direction, i.e- we’d have three main variations of the methods like so:

    public func index(_ index:Index, advancedBy:Index.Distance) -> Index { … } // Available on forward and bidirectional collections
    public func index(_ index:Index, reversedBy:Index.Distance) -> Index { … } // Available on reverse and bidirectional collections
    public func index(_ index:Index, offsetBy:Index.Distance) -> Index { … } // Available only on bidirectional collections

(note, the naming isn’t definite, as reversed may not be clear enough, it’s just an example for now)

There are three reasons I’d prefer this:

The first is that I can pass the same distance into either of the first two methods, and any negation etc. is handled internally. In essence I shouldn’t have to handle negative distances at all when working with the first two methods. So if I’m working with a step size of 5, I can just pass that into the appropriate method, I never have to do anything with it the value itself.

The second benefit is that there should be no uncertainty about the capabilities of the type you’re using; if it doesn’t have the index(:reversedBy:) method then you can’t go backwards, same as index(before:) and index(after:).

The third and main benefit is that the methods are just more explicit about what they do, and what direction you can go in; passing negatives into either of the first two would produce errors outright, allowing you to pick on mistakes in these cases.

The other main thing is that offsetBy doesn’t indicate whether a type supports forward-only offsets, you have to read the documentation to determine this either in the method itself or the type, whereas the presence or absence of the first two variants are pretty clear.

Currently the offsetBy, and the previous advancedBy(), methods require forward-only types to produce fatal errors if handed a negative distance, and vice versa for backward-only types, which can only produce errors at runtime, whereas the presence or absence of the first two methods can be handled during development. You could still pass a negative value and end up with a runtime error instead of course, but for the types of common uses they’re intended for you should be unlikely to produce one.

The offsetBy form would still exist for bidirectional collections, but would only really be used when you need to do more complex index/distance manipulation outside of the type where a calculation might produce either positive or negative values (e.g- if you're calculating the distance and don’t know where two indices are in relation to each other), the rest of the time you should try to use the more specific, single-direction forms as they clarify your intent and can help to catch mistakes if you’ve incorrectly generated a distance for example.


Just curious what other people’s thoughts are about this?

I intended to mention this a lot sooner (to change advancedBy), but then I find out about the new indexing model so thought I’d wait until afterwards, then completely forgot =)
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