> Off the top of my head, I don't know of a terminal that actually implements the entire (very complex) set of Unicode text rendering behaviors

There are at least two problems with this:

First, nobody actually seems to know how bidirectional text should interact with terminal control sequences, or indeed how it should be typeset on a terminal in the first place (where are the paragraph boundaries?). There is the pre-Unicode bi-directional support mode (BDSM, I kid you not) in ECMA-48[1] and TR/53[2], which AFAIK nobody implements nor cares about, and which doesn’t seem to actually; there are terminal emulators made by bidi-language users[3], which AFAIK nobody has written down the behaviour of; there is the Freedesktop bidi terminal spec[4], which is a draft and AFAIK nobody implements yet either but at least some people care about; finally, there are bidi-language users who say that spec is a mistake[5].

Second, aside from bidi and a smattering of other things such as emoji, there is no detailed “Unicode rendering behaviour”, there are only standards specific to font formats, the most recent being OpenType, which is dubiously compatible across implementations, decently documented only through painstaking reverse engineering (sometimes in words[6], sometimes only in Freetype library code), and generally full of snakes[7]. And it has no notion of monospace font—only of a (proportional) font where all Lat/Cyr/Grk characters just happen to have the same advance.

AFAICT that is not an oversight or negligence, but rather a concession to the fact that there are scripts which don’t really have a notion of monospace in the typographic tradition and in fact are written such that it’s extremely unclear what monospace would even mean—certainly not one or two cells per codepoint (e.g. Burmese or Tibetan; apparently there are Arabic monospace fonts[8] but I’ve no idea how the hell they work). Not coincidentally, those are the scripts where you really, really need that shaper, otherwise nothing looks anywhere close to correct.

[1] https://www.ecma-international.org/publications-and-standard...

[2] https://www.ecma-international.org/publications-and-standard...

[3] https://news.ycombinator.com/item?id=8086417

[4] https://terminal-wg.pages.freedesktop.org/bidi/

[5] http://litcave.rudi.ir/

[6] https://github.com/n8willis/opentype-shaping-documents

[7] https://litherum.blogspot.com/2019/03/addition-font.html

[8] https://news.ycombinator.com/item?id=10395464

Even the Han version, though, does not seem to me to be the sort of “what does it even mean?” problem like those I listed above; more like what you want the input to be. You can make your terminal keep language state, e.g. using the deprecated language tags. Pro: some form of this likely already needs to happen for bidi support; similar to what HTML does. Con: no text file or program ever did this; your nice UTF-8-only terminal is now stateful and goes mad after `head /dev/urandom`. Alternatively, you can require the driving program to emit variation selectors for each Han character. Pro: the state and the ensuing madness is now limited; you can still pretend you’re looking at a stream of characters. Con: no text file or program ever did this; neither does HTML although it theorerically could.

[1] https://commons.wikimedia.org/wiki/File:Cyrillic_alternates....

[2] https://www.twardoch.com/download/polishhowto/kreska.html

Does any terminal provide a way to display (say) two consecutive lines in different fonts?

Because according to Unicode (and for once I don't particularly disagree), Chinese versus Japanese ideographs are the same characters, drawn in two different fonts.

Many years ago I thought the highlighted text was a light hearted joke. Now I see what it is actually quite serious. (And now I understand why, among all people, a Japanese philosopher brought up the issue)

I personally think Unicode didn't go essentialist enough though. There's lots of obscure CJK characters in unicode that, when you look up a dictionary, just says it's another form of a common character. What I heard from people in the know was that the CJK committee basically made an exception for the Japanese because they were threatening to withdraw from the Unicode effort unless their demands were met, and for the most part they got what they wanted.

The "Chinese versus Japanese ideographs are the same characters, drawn in two different fonts" is actually a minority of those cases, which I personally interpret as clerical errors instead of a philosophical disagreement.

For example if Unicode were actually thorough with the essentialist philosophy, 關 and 関 would have the same code point.

Unification was done originally because early versions of Unicode were bit-constrained and there simply was not enough space to have separate codepoints for each language. This would need to be "undone" in some future version of Unicode and text encoders would need to make the backward incompatible change (in terms of rendering in unknown downstream software) of using new codepoints if this were to be done. Then the "proper" font would be deterministic.

This goes beyond just bidirectional text. The traditional behavior of text in a terminal is based around two key assumptions, both of which break down catastrophically when dealing with non-ASCII text:

1) The state of a terminal can be represented as a set of cells, each of which has exactly one glyph in it and can be drawn independently from any other cell.

2) Printing a character will write a glyph to the cell the cursor is in and move the cursor to the right by one cell (or down to the next line).

The first assumption breaks down when dealing with full-width characters and ligatures/complex scripts, but can at least be papered over to handle full-width. The second assumption breaks down when exposed to virtually any interesting typographical feature (RTL, combining characters and ZWJ, shaped characters, etc). And I'm not sure it's possible to fix without some pretty substantial changes to how terminals operate -- standard terminal control sequences, and the code that uses them, are all built around these assumptions; introducing new behaviors like "the cursor doesn't always move from left to right" or "erasing the middle of a string might change how the rest of it displays" will break existing applications.

The ECMA standards are of absolutely no help in the matter. They were written in the early 1990s, before Unicode came onto the scene. Their idea of "international language support" was supporting both French and German.

A terminal can't defer rendering that "A" until another character shows up -- that'd break common use cases like echoing user input -- and it can't refuse to apply the combining mark just because it arrived later, because the input to a terminal is a stream of bytes, not characters, and that stream can be delayed or interrupted in any number of ways.

And that's without getting into other interactions with terminal features, like: what happens if you move the cursor, or change its attributes, between the base character and its combining mark? How do you apply a combining mark to the last character on a line, since printing that character moves the cursor to the next line? How do combining marks interact with backspace? And so on.

If you send a console command to move the cursor or do anything else to it, the simple answer is to make that a barrier between glyphs. If you output a combining character after doing that, the combining character goes into the next cell all by itself.

Backspace sent to a terminal should move the cursor left by one cell as usual. A backspace sent by a user would probably delete one glyph if no IMEs are involved. If an IME is involved that's a different problem than plain old combining characters.

Being the last character on a line isn't really any different from being in the middle of a line. The cursor auto-moves, but if the next bytes the terminal gets are combining characters then you can pretty easily shove them into the last-printed glyph.

Maybe. I'd be concerned about what effects that might have on applications which need to be able to display multiple streams of output at once, like tmux.

> Being the last character on a line isn't really any different from being in the middle of a line

You might be surprised. After printing the last character on a line, the cursor goes into a weird "wrapnext" state, where the cursor stays at the end of the line, but will wrap to the next line (potentially triggering scrolling) when the next character is output. This state is highly ephemeral; it can't be entered through control sequences, even by DECSC/DECRC.

[1] https://r12a.github.io/scripts/gujarati/gu.html

[2] https://ltr.wtf/explained/bidiintro.html#selections

[3] https://en.wikipedia.org/wiki/File:IME_demonstratie_-_Matsuo...

It's the whole concept that is problematic.

Unicode mixes way to many concepts in an unholy way. The result is insanity.

The original sin is of course that content and appearance / layout aren't strictly separated.

The whole idea that a text format should be "typewriter compatible" is also a huge part of the problem.

Human script is way too complex that one could pretend that a typewriter abstraction would suffice. But Unicode tries hard to keep this leaky abstraction (because "backwards compatibility"; even it's actually not compatible in all kinds of "corner cases", BIDI being only one). The result is the dumpster fire we've got (and that we likely never ever will get rid of, as it's "good enough" for the majority; I mean as long as you're using European scripts, and no scientific notation, math, and such…).

ASCII was sane (give or take minor annoyances like hexadecimal digits or DEL); the problem was it was too narrowly scoped for general-purpose use. In the context it was actually developed for (exclusively English text on machines that struggled (or didn't bother) to even render 256 different characters, nevermind thousands), ASCII was a broadly-reasonable character encoding that even had half the encoding space left over for something like UTF-8 to build a sensible, more general-purpose encoding in.

Unfortunately, we got specifically UTF-8, with the dumpster fire that is Unicode tied around its neck like a string of albatrosses.

I'd argue this is not in fact a modern approach but rather a pretty dated one.

Modern language string API design instead leans into 'views' of the underlying depending on what you are trying to do with it.

- Transmission and storage: UTF-8 bytes.

- Parsing: code points.

- Editing: grapheme clusters.

- Rendering: pixels.

It doesn't make sense to consider a random assortment of bytes to be a 'string', that's just an array of random bytes - and the language almost certainly already has a better type for that like a vector of 8-bit unsigned values or a slice type. Strings should be opaque - but always valid - encoded data to be sensible and differentiated from a slice type.

Zig seems to be even worse than Go, requiring null-termination instead of storing them as Pascal strings. This means that a valid substring requires a copy so that the substring can itself be null-terminated to be valid. Inner pointers are not valid and therefore substrings are very expensive relatively speaking.

No, Zig uses slices normally (ie ptr + len). Terminators are part of the type system for the (relatively rare, mostly related to C interop) cases where you really want a sentinel value at the end of your array.