Is nested function efficient?

Question

In programming languages like Scala or Lua, we can define nested functions such as

function factorial(n)
  function _fac(n, acc)
    if n == 0 then
      return          


        

Answer 1

Yes (or it used to), as evidenced by Lua's effort to reuse function values when execution passes through a function definition multiple times.

Lua 5.2 Changes

Equality between function values has changed. Now, a function definition may not create a new value; it may reuse some previous value if there is no observable difference to the new function.

Since you have coded (assuming Lua) a function assigned to a global or local declared at a higher scope, you could code the short-circuit yourself (presuming no other code sets it to anything other than nil or false):

function factorial(n)
  _fac = _fac or function (n, acc)
  …
  end
  …
end