5.7 Evaluation

So far only the primitive data types have been discussed, and how the Lisp reader converts textual descriptions of these types into Lisp objects. Obviously there has to be a way of actually computing something—it would be difficult to write a useful program otherwise.

What sets Lisp apart from other languages is that in Lisp there is no difference between programs and data: a Lisp program is just a sequence of Lisp objects which will be evaluated as a program when required.

The subsystem which does this evaluation is called the Lisp evaluator and each expression to be evaluated is called a form. The evaluator (the function eval) examines the structure of the form that is applied to it and computes the value of that form within the current Lisp environment.

A form can be any type of data object; the only types which the evaluator treats specially are symbols (which describe variables) and lists (subroutine applications), anything else is returned as-is (and is called a self-evaluating form).

Function: eval form #!optional structure

This function computes and returns the value of form within the current module and dynamic environment, and a null lexical environment.

If the optional argument structure is non-nil, then form is evaluated in that structure.

However, eval is rarely explicitly invoked, except in the read-eval-print loop. Lisp provides many other methods of evaluation that are usually much more suitable within a program.

Variable: max-lisp-depth

This variable limits the number of nested calls to eval. If more than this many nested calls to eval exist, an error is signalled. The intention is to detect infinite recursion before hitting the stack size limit (causing a segmentation fault).