Re: comparative functions and "same metamethod" gotcha

[Javier Guerra Giraldez <javier@...>]

On Wed, Jun 4, 2014 at 11:45 PM, Andrew Starks <andrew.starks@trms.com> wrote:
> return setmetatable({}, {
> --for brevity, I'm only including the function in question.
> __eq = function(a, b) return a.url == b.url end ,
>
> })


this is a common idiom that i don't understand why is so common.  to
create an object, you create an empty table and set a metatable.
that's ok.  in this case, the methods don't have upvalues, so they
should be the same closures, or at least, they could be.   but why do
you create a new metatable for each object instance?

wouldn't it be much better to do something like this?

local _mt = {
  __eq = function(a,b) return a.url == b.url end,
}

function constructor(url)
    .......
    return setmetatable({}, _mt)
end

here the metatable is defined once and used for every object.  you
could also use it as a 'class test', by comparing it:
getmetatable(obj)==knownmetatable

at least, i guess it could eliminate Andrew's problem

-- 
Javier