Unicode guide

This is a WIP page, take nothing here as final.

Background

Many modern programming languages have added or are in the processing of adding support for Unicode. Generally they do this by making their textual strings use Unicode values of some sort and ensure string operations work with these values.

This swaps one problem for another: Instead of writing code that only works with one type of string, programmers will now write code that only works with another type of string.

Having poor language abstractions is a problem, but it's not the problem. The main problem is that programmers just don't have a useful mental model for Unicode.

For example, if you search online for a Unicode tutorial you'll likely get the following information:

• Before Unicode there was ASCII
• ASCII stores a Latin character without complex encoding
• Unicode has characters from almost every written script
• It needs to be encoded and decoded using UTF-8, UTF-16 or UTF-32
• UTF-8 is used commonly for serving web pages

This isn't very useful knowledge in the grand scheme of what people do with strings in applications.

Likewise, sitting down and reading the standard will teach you everything you need to know about Unicode, but not how to really think about it when programming.

After thinking about it for a while I've built up what I thought is a useful model for Unicode strings: The layered model.

Unicode overview

Before I get in to the main article I'd like to provide a quick overview of what Unicode is.

Unicode defines the following:

• A large multilingual set of abstract characters (known just as 'characters')
• Properties for each character
• How to encode characters for storage
• How to normalize characters in to a canonical format
• How to segment text in to words, sentences, lines, and paragraphs
• How to map text between different cases
• How to order text for sorting
• How to match text for searching
• How to incorporate Unicode in to regular expressions

Many of these can be further tailored by locale-dependent rules and custom algorithms. The Unicode Common Locale Data Repository provides locale-specific information that aids in this tailoring.

I highly recommend reading the following resources

1. The Unicode Standard, Version 15.0 chapters 1, 2, 3, 4 and 23
2. Unicode Glossary
3. Unicode Technical Reports
4. Unicode Frequently Asked Questions

You might also find the following tools helpful:

• Unicode Utilities
• Unicode Code Charts
• Unicode Character Database

The layered model

- levels of abstraction

- indexing

- sort

- match

- search

- normalize

- serialize

- case map

- properties

- breaking/segmentation

- reversing

Level 1: Bytes

level 1: bytes. you can compare, search, splitting, sorting. your basic unit is the byte

filesystem/unix/C

Level 2: Code units

level 2: code units. your basic unit is the smallest unit of your unicode encoding: a byte for utf-8, a 16-bit int for UTF-16, a 32-bit int for UTF-32. you can compare, search, splitting, sort. to get to this point you have to handle endianness

windows

Level 3: Unicode scalars

level 3: unicode scalars. your basic unit is a number between 0x0 and 0x1fffff inclusive, with some ranges for surrogates not allowed. to get tho this point you have to decode utf-8, utf-16 or utf-32. you can compare, search, split, etc but it's important to note that these are just numbers. there's no meaning attached to them

python

Level 4: Unicode characters

level 4: unicode characters: your basic unit is a code point that your runtime recognizes and is willing to interpret using a copy of the unicode database. results vary according to the supported unicode version. you can normalize, compare, match, search, and splitting, case map strings. locale specific operations may be provided. to get these the runtime needs to check if the characters are supported.

???

Level 5: Segmented text

level 5: unicode texts: your basic unit is a string of unicode characters of some amount, such as a word, paragraph, grapheme cluster. to get these you need to convert from a string of unicode characters with breaking/segmentation rules

swift/raku

TODO:

languages/locales

Non-Unicode compatibility

- preserving data

- While writing this page I researched and documented Unicode support in various programming languages. You can see my notes here: Unicode guide/Implementations.