GNU Info

(librep.info)Evaluation

---

Next: Variables Prev: Symbols Up: The language

---

Enter node , (file) or (file)node

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

   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).

Symbol Forms

How variables are accessed

List Forms

Subroutine calls

Self-Evaluating Forms

Forms which don't get evaluated

Quoting

How to prevent evaluation of forms