Programming to interfaces is at the core of flexible structure.

The more important point comes from a program design perspective. Here, "programming to an interface" means focusing your design on what the code is doing, not how it does it. This is a vital distinction that pushes your design towards correctness and flexibility.

This assumes that the module has been given a well-defined, stable description (the interface in the sense of information hiding).

Source Code is a UX to interface with the raw machine.

Rule of Silence: When a program has nothing surprising to say, it should say nothing.

Write programs to handle text streams, because that is a universal interface.

The gem provides a command line utility for checking passwords.