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A program file, or Lisp file, contains a series of expressions that can be read and
evaluated by the Lisp interpreter. These expressions can include function or macro
definitions, variables and their values, properties of variables, and so on. How to save Interlisp-D expressions on these files is explained in Chapter 7. Loading a file is
explained in the Simple Commands for Manipulating Files section below.
Not all files, however, have Lisp expressions stored on them. For example, TEdit files store text; sketches are stored on files made with the package Sketch , or can be
incorporated into TEdit files. These files are not loaded directly into the environment, but are accessed with the package used to create them, such as TEdit or Sketch.
When you name a file, there are conventions that you should follow. These conventions allow you to tell the type of file by the extension to its name.