Pseudo-code to show closures

gistfile1.txt

// The following function, "Calculate_average", will be the only
// public function in your library API 
FUNCTION Calculate_average ( N1, N2, ..., NN )
  LOCAL VARIABLE sum = 0
  LOCAL VARIABLE count = 0

  // Define a function named "sum_and_count" that isn't public API of your library,
  // this function is visible and usable only inside "Calculate_average"
  LOCAL FUNCTION sum_and_count(x)
    sum = sum + x        // The key of closures: here we can modify variables that are not in the scope of sum_and_count()
    count = count + 1
  END LOCAL FUNCTION

  // For each parameter of Calculate_average, we call "sum_and_count" with each element as parameter
  FOR_EACH N = N1, N2, ..., NN
      CALL_THE_FOLLOWING_FUNCTION sum_and_count ( N )

  // Wait a second, here we return "sum / count", but we didn't modify those
  // variables... oh yeah, sum_and_count() has modified them (a closure: a
  // subfunction that modified variables from other scope).
  RETURN num / count
END FUNCTION