[swift-evolution] InternalString class for easy String manipulation

William Sumner prestonsumner at me.com
Wed Aug 17 15:57:09 CDT 2016

> On Aug 17, 2016, at 2:34 PM, Kenny Leung via swift-evolution <swift-evolution at swift.org> wrote:
>> William Sumner says:
>> Can you be more specific about the improvements you’d like to see? Based on an earlier message, you want to be able to use subscripting on strings to retrieve visual glyphs, but you can do this now via the .characters property, which presents a view of the string’s contents as a collection of extended grapheme clusters.
> I did not know about .characters. I would say this addresses the glyph portion of my issues.
> I still have a problem with not being able to index using simple integers to create subscripts or ranges. I wrote my own “split" function, and found it extremely frustrating to have to work with .Index types instead of being able to use integers for subscripts and ranges. Compared to other languages, this almost obviates the usefulness of subscripts altogether. I understand that there are performance implications with translating integer subscripts into actual indexes in the string, but I guess this is a case where even generating another view on the string doesn’t do enough (internally) to make it worthwhile. Perhaps if it did… Again, this is very beginner unfriendly. I guess I will amend my definition of beginner to not only include people new to programming, but people already experienced in languages besides Swift. Now that I think about it, NSString is as powerful as Swift.String when ti comes to Unicode, and it still allows integer based indexing.
> Another issue I have is that a String itself is not subscriptable, but that you have to get a view of it. I think String should have some default subscriptability that “does the right thing”, whatever that is decided to be.
> <heart-to-heart on>
> Now that we’re getting to the heart of the problem (thanks for the prompting me to think more deeply about it), Swift may be more frustrating to learn for experienced programmers coming from C, Objective-C, Java, Ruby, etc. You try to do the simplest think like index into a string, and then find out you can’t - you think to yourself, “I’ve been programming in Objective-C for 20 years. Why can’t I do this? Am I stupid? Is the Swift team purposely trying to make this hard for me?”
> I’ve been reading swift-evolution for a long time now, and a reason often given for design decisions is “term of art”. I believe that integer-based subscriptablilty is a term of art that should be supported.
> <heart-to-heart off>

Note that working with individual characters of a NSString can be unsafe because a visual glyph may be represented by multiple characters. NSString provides methods like rangeOfComposedCharacterSequencesForRange: to enable you to align your character retrievals along grapheme cluster boundaries.

You may be interested in this article by Mike Ash, which gives a rationale for the String API, including why indexes aren't simple integers: https://www.mikeash.com/pyblog/friday-qa-2015-11-06-why-is-swifts-string-api-so-hard.html

In short, these are not simple accesses but potentially expensive operations, and integer subscripting could give users the assumption that they’re accessing arrays with no performance overhead.


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