Some conservative Lua metamethod extension proposals

[Sergey Zakharchenko <doublef.mobile@...>]

Hello list,

I have some half-baked ideas on conservative but potentially useful
adjustments to metamethod protocols. No Lua implementation code
changes required (just yet). It's all about convention. "We cannot
wait for Lua creators' favour, our aim is to help ourselves" (slightly
evil grin).

* Keys-only __pairs semantics.

If __pairs is invoked wth a second argument equal to the special value
false, the caller is not interested in values (only in keys), so the
returned iterator may returned any value, including nil, or no value
at all, for them.

* Key-adjusting __newindex semantics.

If __newindex is invoked with a special fourth argument (what kind of?
equal to false?), then the metamethod is expected (but not required)
to perform insertion (a la table.insert) if the vale is non-nil, or
removal (a la table.remove) if the value is nil. This may or may not
include:
- adjusting indices of other values;
- raising an error if adjustment is not possible (e.g. the key is a
string and already used) or there is no such element to remove;
- supporting nil as key;
- returning the (actual) key used.

Tell me if you find these interesting and worth developing further.

Best regards,

-- 
DoubleF