Re: COW/lazy copying of lua tables

[Axel Kittenberger <axkibe@...>]

This needs Lua 5.2, with 5.1 or LuaJIT you're mostly out of luck (as
far I know). Also note this will make a copy of original at the moment
of write, not of the moment made the COW, so if you change the
original before, it changes in background. But I suppose you consider
that one immutable.

function makeCOW(o)
    return setmetatable({}, {
        __index= o,
        __pairs = function() return pairs(o) end,
        __ipairs = function() return ipairs(o) end,
        __newindex =
            function(t, k, v)
                setmetatable(t, nil)
                for ck, cv in pairs(o) do t[ck] = cv end
                t[k] = v
            end,
    })
end

local org = {1,2,3,4,5,6,7,8,9}
local cpy = makeCOW(org);

for k, v in ipairs(cpy) do print(k, v) end
print('----')
cpy[2] = 5
for k, v in ipairs(cpy) do print(k, v) end