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Matias Guijarro wrote:
I would like to Python docstrings...
> I think a possible solution would be to use the debug library
> (debug.getinfo) to find the source file and to extract a kind of
> "docstring" out of the function code.

This can be done using weak tables in a way that avoids source processing and debug:

-- docstring.lua
local M = {}
local docstrings = setmetatable({}, {__mode = 'kv'})

function M.set(obj, str)
  docstrings[obj] = str
  return obj
function M.get(obj) return docstrings[obj] end
function print(M.get(obj)) end

-- optional syntax simplification
local mt = {__concat = function(self, obj)
                          return M.set(obj, self[1]) end}
setmetatable(M, {__call = function(_, str)
  return setmetatable({str}, mt)

return M

-- test.lua (example)
local docstring = require 'docstring'

local square =
  docstring[[Square a number.]] ..
    return x * x

assert(square(5) == 25)
assert(docstring.get(square) == 'Square a number.') -- prints "Square a number."
docstring.set(square, 'calcular el cuadrado de un número')
assert(docstring.get(square) == 'calcular el cuadrado de un número')
assert(docstring.get(print) == nil)

One reason '..' is chosen is because it is right associative, which allows additional decorators (besides docstrings) to be applied to an object as well.