Separating user interface code

The first program I wrote on a salary was scientific calculation software in Fortran. As I was writing, I noticed that the code running the primitive menu system differed in style from the code carrying out the calculations. So I separated the routines for these tasks, which paid off when I was asked to create higher-level tasks that did several of the individual menu steps. I could just write a routine that called the calculation routines directly without involving the menus. Thus, I learned for myself a design principle that's served me well in software development: Keep your user interface code separate from everything else. It's a simple rule, embodied into more than one application framework, but it's often not followed, which causes quite a bit of trouble.