Two Roles: Writer and Consumer of Functions

The remainder of this section covers finer points about functions that you might skip on a first reading.

We are only doing tiny examples so far to get the basic idea of functions. In much larger programs, functions are useful to manage complexity, splitting things up into logically related, modest sized pieces. Programmers are both writers of functions and consumers of the other functions called inside their functions. It is useful to keep those two roles separate:

The user of an already written function needs to know:

1. the name of the function

2. the order and meaning of parameters

3. what is returned or produced by the function

How this is accomplished is not relevant at this point. For instance, you use the work of the C# development team, calling functions that are built into the language. You need know the three facts about the functions you call. You do not need to know exactly how the function accomplishes its purpose.

On the other hand when you write a function you need to figure out exactly how to accomplish your goal, name relevant variables, and write your code, which brings us to the next section.

The jargon for these parts are the interface (for the consumer) and the implementation (for the programmer, who must be sure to satisfy the public interface).