List processing

The two elements of a computer program are the computations (the actions we want done) and the data (the things we want the actions done upon). The computations are defined using expressions in a computer language, combined to form procedures, which are in turn combined to form compound procedures and eventually programs. The ability to combine simple expressions into procedures is the key to using computer programs to model processes in the real world. Data is defined in a similar way: compound data objects are built from simple parts, like numbers, and combined to represent real-world objects that have complex properties. Compound procedures and compound data are used for the same purposes: to improve the modularity of the program and to raise the conceptual level of its design. One of the simplest and most widespread form of compound data is the list.