''' Purpose: introduce modules. the module defines two functions add() and
             negate().  they are available to other python files that
             import harb.py
'''
# new keyword def starts in column 1
# we define functions with parentheses - the name of this function is add(), and it has 2 parameters, a and b
# if we get any number of input arguments that is not 2, the function will not run

def add( a, b ) :              # function named add(), parameters named a and b ( names don't super matter! )
   ''' Returns value of a + b
   '''
   # the parameters are initialized before the code starts running to the values that we pass in during invokation
   # this is called pass by value, when we pass copies of the inputs in. the function cannot change the original

   result = a + b              # compute value of interest

   return result               # function hands back its result
   # once we hit the return statement, we exit the function and go back to the program that this was invoked in,
   # likely inger.py
   # any code after a return will not be run

# can we change the meaning of these functions in inger.py? no. they are defined here, when we call them inger.py is
# paused and we come to harb.py where the code of the function is. even if we defined a different add() function in
# inger.py , since we specifically call harb.add() in inger.py, we will still run this function

# you can run code in modules! you can define functions and then use them in the same file

def negate( x ) :              # function named negate(), parameter named x
   ''' Returns inverse of x
   '''

   result = -x                 # compute value of interest

   return result               # function hands back the inverse