CLHS: Section 25.1.1

25.1.1 Top level loop

The top level loop is the Common Lisp mechanism by which the user normally interacts with the Common Lisp system. This loop is sometimes referred to as the Lisp read-eval-print loop because it typically consists of an endless loop that reads an expression, evaluates it and prints the results.

The top level loop is not completely specified; thus the user interface is implementation-defined. The top level loop prints all values resulting from the evaluation of a form. The next figure lists variables that are maintained by the Lisp read-eval-print loop.

*    +    /    -  
**   ++   //      
***  +++  ///

Figure 25-1. Variables maintained by the Read-Eval-Print Loop