23AF HORIZONTAL LINE EXTENSION: glyph or variation selector?

[Whistler, Ken]

Ilya,

U+23AF is *definitely* not a variation selector at all.

It is part of a set of bracket pieces (and other graphic pieces)
in the range U+239B..U+23B1.

See discussion of the topic at:

http://www.unicode.org/forum/viewtopic.php?f=35&t=206

See also Section 2.13 of UTR #25:

http://www.unicode.org/reports/tr25/

which discusses the use of these symbol pieces. It does not
specifically talk about the arrow extender pieces, focusing
instead on the bracket pieces, but the principles are the same.

These glyphic pieces of symbols are only relevant and useful
in the context of mathematical typesetting programs like TeX.

The set of box drawing characters in the U+2500 block were
encoded for compatibility with old character sets that did
character cell graphics.

So the two are different, but neither set is of much current
relevance for general text currently using arrows.

--Ken

> Subject: 23AF HORIZONTAL LINE EXTENSION: glyph or variation selector?
> 
> Current (and 7.0.0-tobe) versions do not say much:
> 
>   23AF	HORIZONTAL LINE EXTENSION
> 	* used for extension of arrows
> 	x (vertical line extension - 23D0)
> 
> If it is intended to be a variation selector (possibly prepended
> instead of appended!), then using it with ⇒ should give longer double
> arrow, and using it with ↦ should give a longer variant of ↦.
> 
> If it is a glyph, then what is the difference with U+2500 ─ ?  It
> looks like then any vertical-positioning distinction is trivially
> understood from context…
> 
> Any thoughts?  Thanks,
> Ilya