Re: adventures with iterators

[steve donovan <steve.j.donovan@...>]

On Sat, Dec 1, 2012 at 8:32 PM, Sven Olsen <sven2718@gmail.com> wrote:
> Sadly, in the cases where I actually need an insertion-safe iteration, my
> tables have nontrivial __newindex metamethods.

Here's a fairly conventional solution, but packaged conveniently:

local null = {}

local modproxy_mt = {
    __newindex = function (self,k,v)
        if v == nil then v = null end
        self.entries[k] = v
    end;

    commit = function(self)
        local t = self.T
        for k,v in pairs(self.entries) do
            if v == null then v = nil end
            t[k] = v
        end
    end
}
modproxy_mt.__index = modproxy_mt

function modproxy (T)
    return setmetatable({ T = T, entries = {} }, modproxy_mt)
end

Ok, the essential trick is to use a unique key 'null' as a placeholder
for nil. With this in place, modproxy can be used like this:

local T = {one=1,two=2,three=4}

local pt = modproxy(T)

for k,v in pairs(T) do
    if v == 2 then
        pt[k] = nil
    end
    if v == 1 then
        pt[k..'-x'] = 1
    end
end

pt:commit()

for k,v in pairs(T) do print(k,v) end

BTW, I don't care for 'modproxy', but naming things is hard ;)

steve d.