Thanks much, ikegami and Ken, for the additional explanations and ideas. Super helpful!

Expanding on ikegami's explanation of why a "compatibility composition" might be ambiguous: I also see that, for instance, U+2168 ROMAN NUMERAL NINE has a compatibility decomposition into the capital letters "I" and "X," but even if no other Unicode character has that particular decomposition, that certainly doesn't mean that any "I" followed by an "X" represents the roman numeral and should thus be "compatibility composed" into it.

So yes, I see now why the concept is fraught with peril—in general. But a string like "3\N{FRACTION SLASH}8" seems to have an unambiguous meaning that is always equivalent to U+215C VULGAR FRACTION THREE EIGHTHS. So it seems a compatibility_compose_where_it_makes_sense() function could be written. But it would require judgment calls for every possible "compatibility composition," potentially not all of which would be clear-cut, so I can see why no one's rushing to implement it.

In reply to Re^2: Unicode vulgar fraction composition by raygun
in thread Unicode vulgar fraction composition by raygun

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