On Wed, Dec 17, 2008 at 4:33 PM, Mark Hamburg wrote:

So, now we can write:
label _angle, _radius
function Point( x, y )
return {
_angle = math.atan2( y, x ),
_radius = math.sqrt( y * y + x * x ),
coordinates = function( self )

` return self._radius *
``math.cos( self._angle
`), self._radius * math.sin( self._angle )
end
}
end

You can achieve a similar effect with "inside-out objects"[1-2] via
Lua's support for weak keys. Example:
local radius = setmetatable({}, {__mode = 'k'})
local angle = setmetatable({}, {__mode = 'k'})
local function Point(x,y)
local self = {}
angle [self] = math.atan2(y,x)
radius[self] = math.sqrt(y*y + x*x)
function self:coordinates()
return radius[self] * math.cos( angle[self] ),
radius[self] * math.sin( angle[self] )
end
return self
end
This is at a cost of one table per attribute per class rather than one
table per object.
Also, since radius and angle are lexical variables, means of
compile-time detection of undefined variables[3] can detect
mispellings[2]. For example, "radus[self]" shows up as an access to
the global "radus" in a "luac -p -l example.lua | grep ETGLOBAL".