A program should be light and agile, its subroutines connected like a strings of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little nor too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity.
A program should follow the ‘Law of Least Astonishment’. What is this law? It is simply that the program should always respond to the user in the way that astonishes him least.
A program, no matter how complex, should act as a single unit. The program should be directed by the logic within rather than by outward appearances.
If the program fails in these requirements, it will be in a state of disorder and confusion. The only way to correct this is to rewrite the program.
– Geoffrey James, “The Tao of Programming”