Proposing new arrow characters with Bidi_Mirrored=Yes

[Nitai Sasson]

On Wednesday, 9 April 2025 at 14:45, Eli Zaretskii via Unicode unicode at corp.unicode.org wrote:

>> The problem is this replacement is done (as far as I know) outside of any rendering context, when the text is just a sequence of character codes. It's not reasonable to know which direction the text goes. Sometimes it's completely impossible, if the text direction depends on context that isn't available at the time of replacement.
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> The above is strictly speaking inaccurate. Any serious text rendering nowadays requires a shaping engine, such as HarfBuzz, and ligation of "->" into "→" would be done by such a shaper in cooperation with a font that supports ligatures. The shaping engine is aware of the bidi context and the script of the text it shapes, so it could in principle mirror the arrow.

I have seen this somewhere once, and it's pretty cool, but if you're a web developer and not a font designer, this isn't how you'd implement this. Instead you'd do this:https://github.com/discourse/discourse/blob/70998b73dbe29aa2ff7017d0f4c18a57d278e209/app/assets/javascripts/discourse-markdown-it/src/features/custom-typographer-replacements.js#L48-L51
// Custom arrows
.replace(/(^|\s)-{1,2}>(\s|$)/gm, "\u0020\u2192\u0020")
.replace(/(^|\s)<-{1,2}(\s|$)/gm, "\u0020\u2190\u0020") .replace(/(^|\s)<-{1,2}>(\s|$)/gm, "\u0020\u2194\u0020")
https://github.com/discourse/discourse/blob/70998b73dbe29aa2ff7017d0f4c18a57d278e209/app/assets/javascripts/discourse-markdown-it/src/features/custom-typographer-replacements.js#L48-L51

I think you'll agree that within this scope, it's difficult or impossible to determine the direction of the text. I do not think this approach for arrow replacement is inherently wrong or invalid.

> The question is whether the cases where mirroring is the correct rendering by far outweigh the non-mirroring cases. If yes, the shaper could mirror and expect text which wants to avoid that use directional override controls. If not, mirroring will not make sense.

I'm afraid I'm not following this - are you talking about in the shaper, with ligatures? There is no question about the desired correct output: the string "->" with ligature should appear as an arrow pointing in the same direction as the non-ligature "arrow" does. This means it should mirror if the greater-than sign is mirrored.

I have never looked into the fine details of how these things are implemented, and I can't find a font that explicitly supports both arrow ligatures and bidi text. But I did find Fira Code, an ASCII font that supports these ligatures and I was horrified by the results for "א -> ב":

https://fonts.google.com/specimen/Fira+Code?preview.text=A%20-%3E%20B,%20%D7%90%20-%3E%20%D7%91
This renders differently between Firefox and Chromium. Neither result is good. I've embedded screenshots here: [https://meta.discourse.org/t/wrong-arrow-direction-in-rtl-text-contexts/360760/11](https://meta.discourse.org/t/wrong-arrow-direction-in-rtl-text-contexts/360760/11?u=neatnit)

Ligatures are not a viable solution, and even if they were, this would not make an explicit forward-arrow character (or character combination) redundant.

This is getting off-topic, but what could be the source of this difference? I see these possibilities:

- Both implementations have (different) bugs and don't implement the Unicode spec correctly.
- Unicode specs are vague, and leave room for interpretation in this case
- Unicode has precise specs about this, one of the implementations is accurate, and the Unicode spec did not properly account for this case
- This just isn't supposed to work if the font doesn't explicitly support bidi (I hope this is not the case)

P.S. is it appropriate to send screenshots to this mailing list?

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