Correctly Iterate Variadic Parameters in Lua

Our logger functionality exposes a simple interface, which doesn’t limit the number of items you can send it.

function M.info(...)
function M.error(...)
-- etc.

To send it to the syslog service, we need to convert all the items into a single string. Normally, you would convert the variadic parameter (...) to an array and use ipairs to iterate over it:

local texts = {}
for index, item in ipairs({...}) do
  texts[#texts + 1] = tostring(item)
end
local result = table.concat(texts, " ")

However, ipairs has a quirk : a nil value will cause it to stop the iteration over the array.

logger.info("foo", nil, "bar")
-- logs "foo"

How to fix this? Using select!

• select("#", ...) retrieves the actual number of items hidden in the variadic parameter.
• select(i, ...) will retrieve the item in index i (remember they start at 1 in Lua) from the variadic parameter.

For example -

local reduce_to_string = function(...)
  local texts = {}
  for index = 1, select('#', ...) do
    texts[#texts + 1] = tostring(select(index, ...))
  end
  return table.concat(texts, " ")
end