[swift-evolution] [Review] SE-0180: String Index Overhaul

Xiaodi Wu xiaodi.wu at gmail.com
Wed Jun 14 09:55:58 CDT 2017

On Wed, Jun 14, 2017 at 09:26 Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> If we leave aside for a moment the nomenclature issue where everything in
> Foundation referring to a character is really referring to a Unicode
> scalar, Kevin’s example illustrates the whole problem in a nutshell,
> doesn’t it? In that example, we have a straightforward attempt to slice
> with a misaligned index. The totality of options here are:
> * return nil, an option the rejection of which is the premise of your
> proposal
> * return a partial character (i.e., \u{301}), an option which we haven’t
> yet talked about in this thread–seems like this could have simpler
> semantics, potentially yields garbage if the index is garbage but in the
> case of Kevin’s example actually behaves as the user might expect
> * return a whole character after “rounding down”–difficult semantics to
> define and explain, always results in a whole character but in the case of
> Kevin’s example gives an unexpected answer
> * returns a whole character after “rounding up”–difficult semantics to
> define and explain, always results in a whole character but when the index
> is misaligned would result in a character or range of characters in which
> the index is not found
> * trap–simple semantics, never returns garbage, obvious disadvantage that
> execution will not proceed
> No clearly perfect answer here. However, _if_ we hew strictly to the
> stated premise of your proposal that failable APIs are awkward enough to
> justify a change, and moreover that the awkwardness is truly “needless”
> because of the rarity of misaligned index usage, then at face value
> trapping should be a perfectly acceptable solution.
> That Kevin’s example raises the specter of trapping being a realistic
> occurrence in currently working code actually suggests a challenge to your
> stated premise. If we accept that this challenge is a substantial one, then
> it’s not clear to me that abandoning failable APIs should be ruled out from
> the outset.
> However, if this desire to remove failable APIs remains strong then I
> wonder if the undiscussed second option above is worth at least some
> consideration.

Having digested your revised proposed behavior a little better I see you’re
kind of getting at this exact issue, but I’m uncomfortable with how it’s so
tied to the underlying encoding, which is not guaranteed to be UTF-16 but
is assumed to be for the purposes of slicing. I’d like to propose an
alternative that attempts to deliver on what I’ve called the second option
above–somewhat similar:

A string index will notionally or actually keep track of the view in which
it was originally aligned, be it utf8, utf16, unicodeScalars, or
characters. A slicing operation str.xxx[idx] will behave as expected if idx
is not misaligned with respect to str.xxx. If it is misaligned, the
operation would instead be notionally String(str.yyy[idx...]).xxx.first!,
where yyy is the original view in which idx was known aligned–if idx is not
also misaligned with respect to str.yyy (as might be the case if idx was
returned from an operation on a different string). If it is still
misaligned, trap.

On Wed, Jun 14, 2017 at 08:49 Dave Abrahams <dabrahams at apple.com> wrote:
>> > On Jun 13, 2017, at 6:16 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>> >
>> > I’m coming to this conversation rather late, so forgive the naive
>> question:
>> >
>> > Your proposal claims that current code with failable APIs is needlessly
>> awkward and that most code only interchanges indices that are known to
>> succeed. So, why is it not simply a precondition of string slicing that the
>> index be correctly aligned? It seems like this would simplify the behavior
>> greatly.
>> Well, consider the case raised by Kevin Ballard if nothing else: that
>> code would start trapping.
>> -Dave
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