Evaluation
**********

The evaluation of expressions in Emacs Lisp is performed by the Lisp interpreter---a program that receives a Lisp object as input and computes its value as an expression.  The value is computed in a fashion that depends on the data type of the object, following rules described in this chapter.  The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function eval.

Introduction to Evaluation
==========================

The Lisp interpreter, or evaluator, is the program which computes the value of an expression which is given to it.  When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body.  Thus, running any Lisp program really means running the Lisp interpreter.

How the evaluator handles an object depends primarily on the data type of the object.

A Lisp object which is intended for evaluation is called an expression or a form.  The fact that expressions are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages.  Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.

It is very common to read a Lisp expression and then evaluate the expression, but reading and evaluation are separate activities, and either can be performed alone.  Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself.  It is up to the caller of read whether this object is a form to be evaluated, or serves some entirely different purpose.  See Input Functions.

Do not confuse evaluation with command key interpretation.  The editor command loop translates keyboard input into a command (an interactively callable function) using the active keymaps, and then uses call-interactively to invoke the command.  The execution of the command itself involves evaluation if the command is written in Lisp, but that is not a part of command key interpretation itself. See Command Loop.

Evaluation is a recursive process.  That is, evaluation of a form may cause eval to be called again in order to evaluate parts of the form.  For example, evaluation of a function call first evaluates each argument of the function call, and then evaluates each form in the function body.  Consider evaluation of the form (car x): the subform x must first be evaluated recursively, so that its value can be passed as an argument to the function car.

The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables. (This definition of ``environment'' is specifically not intended to include all the data which can affect the result of a program.)  Whenever the form refers to a variable without creating a new binding for it, the value of the binding in the current environment is used.  See Variables.

Evaluation of a form may create new environments for recursive evaluation by binding variables (see Local Variables).  These environments are temporary and will be gone by the time evaluation of the form is complete.  The form may also make changes that persist; these changes are called side effects.  An example of a form that produces side effects is (setq foo 1).

Finally, evaluation of one particular function call, byte-code, invokes the byte-code interpreter on its arguments.  Although the byte-code interpreter is not the same as the Lisp interpreter, it uses the same environment as the Lisp interpreter, and may on occasion invoke the Lisp interpreter.  (see Byte Compilation.)

The details of what evaluation means for each kind of form are described below (see Forms).

Eval
====

Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run.  On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as after reading a form from text being edited or getting one from a property list.  On these occasions, use the eval function.

The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values.  Loading a file also does evaluation (see Loading).

 -- Function: eval form
     This is the basic function for performing evaluation.  It evaluates FORM in the current environment and returns the result.  How the evaluation proceeds depends on the type of the object (see Forms).

     Since eval is a function, the argument expression that appears in a call to eval is evaluated twice: once as preparation before eval is called, and again by the eval function itself. Here is an example:

          (setq foo 'bar)
               => bar

          (setq bar 'baz)
               => baz
          ;; eval receives argument bar, which is the value of foo
          (eval foo)
               => baz

     The number of currently active calls to eval is limited to max-lisp-eval-depth (see below).

 -- Command: eval-current-buffer &optional stream
     This function evaluates the forms in the current buffer.  It reads forms from the buffer and calls eval on them until the end of the buffer is reached, or until an error is signaled and not handled.

     If STREAM is supplied, the variable standard-output is bound to STREAM during the evaluation (see Output Functions).

     eval-current-buffer always returns nil.

 -- Command: eval-region start end &optional stream
     This function evaluates the forms in the current buffer in the region defined by the positions START and END.  It reads forms from the region and calls eval on them until the end of the region is reached, or until an error is signaled and not handled.

     If STREAM is supplied, standard-output is bound to it for the duration of the command.

     eval-region always returns nil.

 -- Variable: max-lisp-eval-depth
     This variable defines the maximum depth allowed in calls to eval, apply, and funcall before an error is signaled (with error message "Lisp nesting exceeds max-lisp-eval-depth").  eval is called recursively to evaluate the arguments of Lisp function calls and to evaluate bodies of functions.

     This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.

     The default value of this variable is 200.  If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached.

     max-specpdl-size provides another limit on nesting. See Local Variables.

 -- Variable: values
     The value of this variable is a list of values returned by all expressions which were read from buffers (including the minibuffer), evaluated, and printed.  The elements are in order, most recent first.

          (setq x 1)
               => 1

          (list 'A (1+ 2) auto-save-default)
               => (A 3 t)

          values
               => ((A 3 t) 1 ...)

     This variable is useful for referring back to values of forms recently evaluated.  It is generally a bad idea to print the value of values itself, since this may be very long.  Instead, examine particular elements, like this:

          ;; Refer to the most recent evaluation result.
          (nth 0 values)
               => (A 3 t)

          ;; That put a new element on,
          ;;   so all elements move back one.
          (nth 1 values)
               => (A 3 t)

          ;; This gets the element that was next-to-last
          ;;   before this example.
          (nth 3 values)
               => 1

Kinds of Forms
==============

A Lisp object that is intended to be evaluated is called a form. How Emacs evaluates a form depends on its data type.  Emacs has three different kinds of form that are evaluated differently: symbols, lists, and ``all other types''.  All three kinds are described in this section, starting with ``all other types'' which are self-evaluating forms.

Self-Evaluating Forms
---------------------

A self-evaluating form is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated.  Thus, the number 25 evaluates to 25, and the string "foo" evaluates to the string "foo". Likewise, evaluation of a vector does not cause evaluation of the elements of the vector---it returns the same vector with its contents unchanged.

     '123               ; An object, shown without evaluation.
          => 123

     123                ; Evaluated as usual---result is the same.
          => 123

     (eval '123)        ; Evaluated ``by hand''---result is the same.
          => 123

     (eval (eval '123)) ; Evaluating twice changes nothing.
          => 123

It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because it is inconvenient and not very useful; however, it is possible to put them inside Lisp programs when they are constructed from subexpressions rather than read.  Here is an example:

     ;; Build such an expression.
     (setq buffer (list 'print (current-buffer)))
          => (print #<buffer eval.texi>)

     ;; Evaluate it.
     (eval buffer)
          -| #<buffer eval.texi>
          => #<buffer eval.texi>

Symbol Forms
------------

When a symbol is evaluated, it is treated as a variable.  The result is the variable's value, if it has one.  If it has none (if its value cell is void), an error is signaled.  For more information on the use of variables, see Variables.

In the following example, we set the value of a symbol with setq.  When the symbol is later evaluated, that value is returned.

     (setq a 123)
          => 123

     (eval 'a)
          => 123

     a
          => 123

The symbols nil and t are treated specially, so that the value of nil is always nil, and the value of t is always t.  Thus, these two symbols act like self-evaluating forms, even though eval treats them like any other symbol.

Classification of List Forms
----------------------------

A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element.  These three kinds of forms are evaluated in different ways, described below.  The rest of the list consists of arguments for the function, macro or special form.

The first step in evaluating a nonempty list is to examine its first element.  This element alone determines what kind of form the list is and how the rest of the list is to be processed.  The first element is not evaluated, as it would be in some Lisp dialects including Scheme.

Symbol Function Indirection
---------------------------

If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol.  If the contents are another symbol, this process, called symbol function indirection, is repeated until a non-symbol is obtained.  See Function Names, for more information about using a symbol as a name for a function stored in the function cell of the symbol.

One possible consequence of this process is an infinite loop, in the event that a symbol's function cell refers to the same symbol.  Or a symbol may have a void function cell, causing a void-function error.  But if neither of these things happens, we eventually obtain a non-symbol, which ought to be a function or other suitable object.

More precisely, we should now have a Lisp function (a lambda expression), a byte-code function, a primitive function, a Lisp macro, a special form, or an autoload object.  Each of these types is a case described in one of the following sections.  If the object is not one of these types, the error invalid-function is signaled.

The following example illustrates the symbol indirection process.  We use fset to set the function cell of a symbol and symbol-function to get the function cell contents (see Function Cells).  Specifically, we store the symbol car into the function cell of first, and the symbol first into the function cell of erste.

     ;; Build this function cell linkage:
     ;;   -------------       -----        -------        -------
     ;;  | #<subr car> | <-- | car |  <-- | first |  <-- | erste |
     ;;   -------------       -----        -------        -------

     (symbol-function 'car)
          => #<subr car>

     (fset 'first 'car)
          => car

     (fset 'erste 'first)
          => first

     (erste '(1 2 3))   ; Call the function referenced by erste.
          => 1

By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.

     ((lambda (arg) (erste arg))
      '(1 2 3))
          => 1

After that function is called, its body is evaluated; this does involve symbol function indirection when calling erste.

The built-in function indirect-function provides an easy way to perform symbol function indirection explicitly.

 -- Function: indirect-function function
     This function returns the meaning of FUNCTION as a function.  If FUNCTION is a symbol, then it finds FUNCTION's function definition and starts over with that value.  If FUNCTION is not a symbol, then it returns FUNCTION itself.

     Here is how you could define indirect-function in Lisp:

          (defun indirect-function (function)
            (if (symbolp function)
                (indirect-function (symbol-function function))
              function))

Evaluation of Function Forms
----------------------------

If the first element of a list being evaluated is a Lisp function object, byte-code object or primitive function object, then that list is a function call.  For example, here is a call to the function +:

     (+ 1 x)

When a function call is evaluated, the first step is to evaluate the remaining elements of the list in the order they appear.  The results are the actual argument values, one argument from each element.  Then the function is called with this list of arguments, effectively using the function apply (see Calling Functions).  If the function is written in Lisp, the arguments are used to bind the argument variables of the function (see Lambda Expressions); then the forms in the function body are evaluated in order, and the result of the last one is used as the value of the function call.

Lisp Macro Evaluation
---------------------

If the first element of a list being evaluated is a macro object, then the list is a macro call.  When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro.  The macro definition computes a replacement form, called the expansion of the macro, which is evaluated in place of the original form.  The expansion may be any sort of form: a self-evaluating constant, a symbol or a list.  If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.

Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are evaluated when the expansion is evaluated.

For example, given a macro defined as follows:

     (defmacro cadr (x)
       (list 'car (list 'cdr x)))

an expression such as (cadr (assq 'handler list)) is a macro call, and its expansion is:

     (car (cdr (assq 'handler list)))

Note that the argument (assq 'handler list) appears in the expansion.

See Macros, for a complete description of Emacs Lisp macros.

Special Forms
-------------

A special form is a primitive function specially marked so that its arguments are not all evaluated.  Special forms define control structures or perform variable bindings---things which functions cannot do.

Each special form has its own rules for which arguments are evaluated and which are used without evaluation.  Whether a particular argument is evaluated may depend on the results of evaluating other arguments.

Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.

and
     see Combining Conditions

catch
     see Catch and Throw

cond
     see Conditionals

condition-case
     see Handling Errors

defconst
     see Defining Variables

defmacro
     see Defining Macros

defun
     see Defining Functions

defvar
     see Defining Variables

function
     see Anonymous Functions

if
     see Conditionals

interactive
     see Interactive Call

let
let*
     see Local Variables

or
     see Combining Conditions

prog1
prog2
progn
     see Sequencing

quote
     see Quoting

save-excursion
     see Excursions

save-restriction
     see Narrowing

save-window-excursion
     see Window Configurations

setq
     see Setting Variables

setq-default
     see Creating Buffer-Local

track-mouse
     see Mouse Tracking

unwind-protect
     see Nonlocal Exits

while
     see Iteration

with-output-to-temp-buffer
     see Temporary Displays

     Common Lisp note: here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.  setq, if, and catch are special forms in both Emacs Lisp and Common Lisp. defun is a special form in Emacs Lisp, but a macro in Common Lisp.  save-excursion is a special form in Emacs Lisp, but doesn't exist in Common Lisp.  throw is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).

Autoloading
-----------

The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs.  When an autoload object appears as a symbol's function definition and that symbol is used as a function, Emacs will automatically install the real definition (plus other associated code) and then call that definition. (see Autoload.)

Quoting
=======

The special form quote returns its single argument ``unchanged''.

 -- Special Form: quote object
     This special form returns OBJECT, without evaluating it.  This allows symbols and lists, which would normally be evaluated, to be included literally in a program.  (It is not necessary to quote numbers, strings, and vectors since they are self-evaluating.)

     Because quote is used so often in programs, Lisp provides a convenient read syntax for it.  An apostrophe character (') followed by a Lisp object (in read syntax) expands to a list whose first element is quote, and whose second element is the object.  Thus, the read syntax 'x is an abbreviation for (quote x).

     Here are some examples of expressions that use quote:

          (quote (+ 1 2))
               => (+ 1 2)

          (quote foo)
               => foo

          'foo
               => foo

          ''foo
               => (quote foo)

          '(quote foo)
               => (quote foo)

          ['foo]
               => [(quote foo)]

Other quoting constructs include function (see Anonymous Functions), which causes an anonymous lambda expression written in Lisp to be compiled, and ` (see Backquote), which is used to quote only part of a list, while computing and substituting other parts.