Multiple Returns and Default Arguments

Returning several values

local function minMax(list)
  local lo, hi = list[1], list[1]
  for _, v in ipairs(list) do
    if v < lo then lo = v end
    if v > hi then hi = v end
  end
  return lo, hi
end

local low, high = minMax({4, 9, 1, 7})
print(low, high)
print(minMax({3, 8}))
print((minMax({3, 8})))
print(minMax({3, 8}), "end")

1	9
3	8
3
3	end

• return a, b returns two values; assign them to two variables.
• Extra parentheses keep only the first value.
• A call keeps all its values only when it's last in a list — minMax(...), "end" passes just the first result.

Returning nil, "error message" on failure is a common Lua convention (io.open does it).

Default arguments

Lua has no default-argument syntax. Use or:

local function connect(host, port, secure)
  port = port or 443
  if secure == nil then secure = true end
  local scheme = secure and "https" or "http"
  return scheme .. "://" .. host .. ":" .. port
end

print(connect("example.com"))
print(connect("localhost", 8080, false))

https://example.com:443
http://localhost:8080

port = port or 443 works for numbers and strings, but for booleans x or true would turn an explicit false into true — compare with nil instead.

Named arguments with a table

For many optional settings, pass one table:

local function createWindow(opts)
  local title = opts.title or "Untitled"
  local width = opts.width or 800
  local height = opts.height or 600
  return string.format("%s (%dx%d)", title, width, height)
end

print(createWindow{title = "Editor", width = 1024})
print(createWindow{})

Editor (1024x600)
Untitled (800x600)

When the only argument is a table or string literal, the parentheses can be dropped: createWindow{...}, require "json".