Function

Default Return

Lua function returns nil as default.

print((function() end)() == nil) -- true

Variadic Parameter

Variadic parameter in lua does not have name and default type, it's a special identifier ... that has to be expanded to a table using { ... }

Such identifier can be used in:

• { ... }: direct expansion to a table
• { a, ..., b }: append extra items around inside a table
• select('#', ...) and select(idx, ...) to get length or pick one item from args without intermediate expansion

local function foo(...)
  local args = { ... }
  print(table.concat(args, ', '))
end

foo('hello', 'world') -- hello, world

Iterate over variadic without expansion
local function foo(...)
  for idx = 1, select('#', ...) do
    _ = select(idx, ...)
  end
end

Multi-Returns

• unpack(list: table, start?: integer, end?: integer): splatter items in successive indices from a table
• select(start: integer, ...any): ...unknown: slice a splat from start into another splat
  • select(symbol: '#', ...any): ...unknown: retrieve a length of the splat
• { <expr> }: collect splats from <expr>, a function call for example.

  local function foo(...) return unpack { ... } end
  _ = { foo(1, 2, 3) } -- [1, 2, 3]

Iterator Function

Iterator in lua can be implemented by closure, which scopes local fields in its containing factory function, and operates iteration within the created iterator function.

• store state in outer scope
• return nil to cease the iteration
• consume by for .. in statement(the first return being nil would stop the loop)

--- my ipairs impl at home
---@generic T
---@param array T[]
---@return fun(): integer, T
local function iter_array(array)
  -- store state in outer scope
  local idx = 0 -- [!code highlight]
  local current = nil -- [!code highlight]

  return function()
    idx = idx + 1
    if idx <= #array then
      current = rawget(array, idx)
      return idx, current
    end

    return nil, nil -- return nil to cease the iteration
  end
end

for idx, item in iter_array { 1, 2, 3 } do
  print(idx, item)
end