Functions, locals, and return values

The idea

A function is a value that can be stored in a variable, passed to another function, or returned as a result. Define a local function when its name belongs to one block or module. Parameters are local variables initialized from the call’s arguments. Reassigning a numeric parameter does not change the caller’s variable. A return statement ends the current call and sends results back to its caller.

Lua functions can return several values, and an assignment can capture them in separate variables. Missing results become nil. This is useful for returning a useful value together with an explanation of failure. Keep each result’s meaning stable across successful and unsuccessful paths. Names inside a local block can shadow outer names, so inspect the nearest visible declaration when tracing a program.

local function divide(a, b)
  if b == 0 then return nil, "zero divisor" end
  return a / b, nil
end
local value, problem = divide(8, 0)
print(value == nil)
print(problem)

true
zero divisor

The early return produces two results. The caller receives absence in value and a diagnostic string in problem, without attempting division.