Re: adventures with iterators
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- Subject: Re: adventures with iterators
- From: steve donovan <steve.j.donovan@...>
- Date: Sun, 2 Dec 2012 20:09:17 +0200
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.