situations that need manual metamethod retry (Re: [ANN] Lua 5.4.0 (beta-rc2) now available)

[nobody <nobody+lua-list@...>]

On 22/10/2019 15.25, Roberto Ierusalimschy wrote:
> > > The string library uses metamethods that try to coerce strings to numbers
> > > in all arithmetic operations. If the conversion fails, the library calls
> > > the metamethod of the other operand (if present) or it raises an error.
> > Actually it only tries the metamethod of the right side, to prevent a
> > "ping-pong" loop with some other metamethod that applies the same logic.
> In general, other metamethods do not need (and do not have) this
> behavior. (At least mine don't.)
OK, agree.

Just for completeness:  It *is* necessary to manually retry the right
side's metamethod when working with multiple levels of partially
compatible types a < b < c, e.g. the string situation of number < string
< (whatever a user may define), or a different (maybe simpler?) example:
numbers < complex numbers < vectors. If vectors treat non-vectors as
constants, number-anything works as usual or Lua uses the right side's
metamethod, vector-complex / vector-number work as expected, but
complex-vector needs to manually fall back to the right side (just like
Lua does for strings).

-- nobody