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Pre-RFC: Rename SVf_UTF8 et al.

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Felipe Gasper
August 18, 2021 17:17
Pre-RFC: Rename SVf_UTF8 et al.
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Per recent IRC discussion …

PROBLEM: The naming of Perl’s “UTF-8 flag” is a continual source of confusion regarding the flag’s significance. Some think it indicates whether a given PV stores text versus binary. Some think it means that the PV is valid UTF-8. Still others likely hold other inaccurate views.

The problem here is the naming. For example, consider `perl -e'my $foo = "é"'`. In this code $foo is a “UTF-8 string” by virtue of the fact that its code points (assuming use of a UTF-8 terminal) correspond to the bytes that encode “é” in UTF-8. The “UTF-8 flag”, however, is likely *not* set on this string. By contrast, consider `perl -Mutf8 -e'my $foo = "é"'`. Here $foo has the “UTF-8 flag” set, but $foo is NOT a “UTF-8 string” because its code points (in this case, only 1) aren’t valid UTF-8.

The fact that quite often a “UTF-8 string” lacks the “UTF-8 flag”, and a “UTF-8-flagged” string is (usually) *not* a “UTF-8 string”, makes little sense except to the “highly initiated”.

Another problem is “UTF-8” doesn’t really describe the “upgraded” format. This format is what Perl historically called “lax UTF-8” and is now widely called “generalized UTF-8”, which includes unpaired surrogates and code points above Unicode’s maximum.

PROPOSAL: Rename the following identifiers in code and documentation, leaving macros for the old ones as aliases:
- SVf_UTF8        -> SVf_PVUPGRADED
- SvUTF8          -> Sv_PVUPGRADED
- SvUTF8_on       -> Sv_PVUPGRADED_on
- SvUTF8_off      -> Sv_PVUPGRADED_off
- SvPOK_only_UTF8 -> SvPOK_only_UPGRADED

Note that flags like REFCOUNTED_HE_KEY_UTF8 do not need a rename because these indicate an actual (if incomplete/invalidated) UTF-8 decoding step.

BENEFITS: Over time, this rename will minimize the confusion between Perl’s upgraded-PV storage format versus UTF-8. The rename may also compel current users of the language who hold mistaken mental models of the flag’s purpose to reexamine their understanding, hopefully for the better.

POTENTIAL COMPLICATIONS: The mismatch between amended documentation and existing documentation may cause confusion; it should, though, be an auspicious confusion that eventually clarifies rather than misleads.
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