Why would a lua module set its table's __index as itself?

Question

I noticed a lua module setting the returned table's __index as itself

local M = {
  _VERSION = "1.0.0"
}
M.__index = M

function M.do()
end

return M

What does setting a table's __index as itself accomplish?

Later, you would use the module

local m = require("m")
m.do()

In the code you've provided, it does nothing. `M` is never used as a metatable, so the `__index` field isn't particularly relevant. It's just another key in the table. Can you provide a more complete example? — Nicol Bolas, Sep 06 '18 at 15:07
It sounds likely that 'it does nothing' is the correct answer. — joeforker, Sep 06 '18 at 15:21
It's more likely that you're incorrectly copying something you saw in a module. That's why I want to see a complete example. — Nicol Bolas, Sep 06 '18 at 15:22
It is https://github.com/zmartzone/lua-resty-openidc/blob/master/lib/resty/openidc.lua#L76 — joeforker, Sep 06 '18 at 17:47

score 3 · Answer 1 · answered Sep 06 '18 at 15:23

3

It is usually done to avoid creating a separate metatable to be used in objects created by the library:

function M.new()
    return setmetatable({},M)
end

I do this all the time in my libraries. It is somewhat lazy.

answered Sep 06 '18 at 15:23

lhf

1 Answers1