Completion for the Lua eval prompt

This isn't very useful, maybe, but it's kinda cool that it is even possible!

Use this function as the completion_callback argument to awful.prompt.run.

function lua_completion (line, cur_pos, ncomp)
   -- Only complete at the end of the line, for now
   if cur_pos ~= #line + 1 then
      return line, cur_pos
   end

   -- We're really interested in the part following the last (, [, comma or space
   local lastsep = #line - (line:reverse():find('[[(, ]') or #line)
   local lastidentifier
   if lastsep ~= 0 then
      lastidentifier = line:sub(lastsep + 2)
   else
      lastidentifier = line
   end

   local environment = _G

   -- String up to last dot is our current environment
   local lastdot = #lastidentifier - (lastidentifier:reverse():find('.', 1, true) or #lastidentifier)
   if lastdot ~= 0 then
      -- We have an environment; for each component in it, descend into it
      for env in lastidentifier:sub(1, lastdot):gmatch('([^.]+)') do
         if not environment[env] then
            -- Oops, no such subenvironment, bail out
            return line, cur_pos
         end
         environment = environment[env]
      end
   end

   local tocomplete = lastidentifier:sub(lastdot + 1)
   if tocomplete:sub(1, 1) == '.' then
      tocomplete = tocomplete:sub(2)
   end

   local completions = {}
   for k, v in pairs(environment) do
      if type(k) == "string" and k:sub(1, #tocomplete) == tocomplete then
         table.insert(completions, k)
      end
   end

   if #completions == 0 then
      return line, cur_pos
   end
   
   while ncomp > #completions do
      ncomp = ncomp - #completions
   end

   local str = ""
   if lastdot + lastsep ~= 0 then
      str = line:sub(1, lastsep + lastdot + 1)
   end
   str = str .. completions[ncomp]
   cur_pos = #str + 1
   return str, cur_pos
end