For example, constant time subscripting, or improved length calculations, are made possible by encodings other than utf-8.
But when performance isn't critical, utf-8 should be the default. I don't see a reason for any other encoding.
If you're hoping that a fixed offset gives you a user-percieved character boundary, then you're not handling composed characters or zero-width-joiners or any number of other things that may cause a grapheme cluster to be composed of multiple UTF code points.
The "fixed" size of code points in encodings like UTF-32 are just that: code points. Whether a code point corresponds with anything useful, like the boundary of a visible character, will always require linear-time indexing of the string, in any encoding.
(*) Approximately nothing. If you're in a position where you've somehow already vetted that the text is of a subset of human languages where you're guaranteed to never have grapheme clusters that occupy more than a single code point, then you maybe have a use case for this, but I'd argue you really just have a bunch of bugs waiting to happen.
What about UTF-256? Maybe not today, maybe not tomorrow, but someday...
E.g., Ruby runtimes scan the bytes in a string and then cache data about them in value called a code range. Knowing the code range, you can optimize many operations to not require additional linear scans of the string. Knowing a UTF-8 string consists only of ASCII characters can allow operations to be just as fast as if the string truly were ASCII-only (Ruby supports 100+ string encodings). And that fact is used throughout the core library to provide fast implementations of many operations (upcase, downcase, capitalize, gsub, substring, and so on). Moreover, a JIT can generate extremely tight code in those situations. Having to take a linear pass through the string to discover codepoint boundaries incurs a huge performance cost. While all strings could be treated uniformly and use Unicode tables for case mapping and such, the extra overhead is brutal. It has a measurable impact on string-heavy applications, such as template rendering and text processing.
In the most general case, yes, you know nothing about the string and can't make any assumptions. You can't even be sure the byte sequence is valid UTF-8. But, very often you do know properties of those strings. And you can manage boundaries where strings with known properties are joined with strings with unknown properties (e.g., variable interpolation in a template file).
With the exception of conversion from numbers (which has its own optimizations that are likely equally applicable in UTF8 since Arabic numbers are just ASCII anyway), I’d say all of your examples sound like bugs waiting to happen.
Why shouldn’t string literals be allowed to contain complex emoji? Why should identifiers disallow them? Why should there be a company policy around putting complex emoji places?
Just saying “let’s just declare things such that strings aren’t allowed to have multi-code point grapheme clusters” sounds great until you accidentally let that assumption leak into a place where a user wants to use an emoji and can’t make it match their skin tone.
I’d also say that such restrictions are putting the cart before the horse; the typical reasons for restricting the allowed character set, are precisely because you want to make lazy assumptions about things like string offsets. Saying that such assumptions are a good thing because you have these restrictions in place, seems like circular logic to me.
I didn't say they shouldn't, just that many do not and you know that at parse time.
> Why should identifiers disallow them?
I don't write the language specs. Many languages don't allow classes, methods, variables, etc. to have complex grapheme clusters in them.
> Why should there be a company policy around putting complex emoji places?
Performance. Code sanity. Indexing. Ease of typing. Again, I'm not the one writing the policies. But, they exist.
> Just saying “let’s just declare things such that strings aren’t allowed to have multi-code point grapheme clusters” sounds great until you accidentally let that assumption leak into a place where a user wants to use an emoji and can’t make it match their skin tone.
I'm making a clear distinction between situations where you have user-supplied data and data under control of the language runtime, developer-created files, or those just adhering well-defined file formats. These are all strings and commonly consist of simple codepoints; indeed, many times they're just ASCII characters.
I addressed user-supplied values when I wrote "And you can manage boundaries where strings with known properties are joined with strings with unknown properties (e.g., variable interpolation in a template file)." TruffleRuby, for example, uses ropes as its underlying structure, so if you have a template written using all ASCII characters (rather common) and interpolate a user-supplied value, you can put the user string in one rope, the template in others and link them all together into a tree with ConcatRopes. The template ropes still know they only have simple codepoints and operations on those parts can be fast. The user variable only knows it's a generic UTF-8 string and operations on that string, if any, can go down the slower path. Oftentimes, there are no operations to perform on that user string other than to display it. Its mere presence doesn't need to adversely affect the rest of the template.
> I’d also say that such restrictions are putting the cart before the horse; the typical reasons for restricting the allowed character set, are precisely because you want to make lazy assumptions about things like string offsets. Saying that such assumptions are a good thing because you have these restrictions in place, seems like circular logic to me.
I'm not making any assumptions. I've spent an awful lot of time optimizing string performance in the context of a Ruby runtime and your initial claim of constant-time subscripting being a myth doesn't match my experience. Ruby allows as complex of a string as you want, but the reality is there are many situations where strings, by either by restrictions or de facto, will not have multi-codepoint grapheme clusters. In many situations you'll have strings with all the codepoints in the ASCII range. If the only information the runtime records when parsing a string is that "this is a UTF-8" string and then operates on all UTF-8 strings uniformly, you leave a lot of performance on the table. The best performing situation is when you don't have to deal with variable-width codepoints in a UTF-8 string. UTF-16 and UTF-32 aren't terribly common in Ruby, but they exist as valid encodings (well, UTF-16BE/UTF-16LE and UTF-32BE/UTF-32LE) and have simpler execution paths than UTF-8 for many use cases.
Source code manipulation is frequently Unicode aware but doesn't care about combinations or things outside of a strict subset of Unicode to modify lexing control flow.
Being able to store (and later refer to) character offsets in the source code is a plus because they'll only ever occur in places where the strict subset is enforced.
This is especially true of languages with line-only comments, etc, where different writing systems being used won't affect the error message information.
Like I said, there are a few useful cases where having a fixed width encoding is beneficial. It's less helpful to the discussion to assert you know better for every case, ever.
It doesn't add anything over storing byte offsets into an UTF-8 string.
Assuming you mean different encoding forms of Unicode (rather than entirely different and far less comprehensive character sets, such as ASCII or Latin-1), there are very few use cases where "subscripting" or "length calculations" would benefit significantly from using a different encoding form, because it is rare that individual Unicode code points are the most appropriate units to work with.
(If you're happy to sacrifice support for most of the world's writing systems in favour of raw performance for a limited subset of scripts and text operations, that's different.)