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14. Evaluation

The evaluation of expressions in XEmacs Lisp is performed by the Lisp interpreter---a program that receives a Lisp object as input and computes its value as an expression. How it does this depends on the data type of the object, according to 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.

14.1 Introduction to Evaluation  Evaluation in the scheme of things.
14.2 Eval  How to invoke the Lisp interpreter explicitly.
14.3 Kinds of Forms  How various sorts of objects are evaluated.
14.4 Quoting  Avoiding evaluation (to put constants in the program).
14.5 Multiple values  Functions may return more than one result.


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14.1 Introduction to Evaluation

The Lisp interpreter, or evaluator, is the program that computes the value of an expression that 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 that 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 section 23.3 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 section 25. Command Loop.

Evaluation is a recursive process. That is, evaluation of a form may call eval 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.

Evaluation of a function call ultimately calls the function specified in it. See section 17. Functions and Commands. The execution of the function may itself work by evaluating the function definition; or the function may be a Lisp primitive implemented in C, or it may be a byte-compiled function (see section 21. Byte Compilation).

The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables.(2) 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 section 16. Variables.

Evaluation of a form may create new environments for recursive evaluation by binding variables (see section 16.3 Local Variables). These environments are temporary and vanish 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).

The details of what evaluation means for each kind of form are described below (see section 14.3 Kinds of Forms).


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14.2 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.

Please note: it is generally cleaner and more flexible to call functions that are stored in data structures, rather than to evaluate expressions stored in data structures. Using functions provides the ability to pass information to them as arguments.

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 section 20. 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 section 14.3 Kinds of 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
(eval 'foo)
     => bar

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

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 during the evaluation.

You can use the variable load-read-function to specify a function for eval-region to use instead of read for reading expressions. See section 20.1 How Programs Do Loading.

eval-region always returns nil.

Command: eval-buffer buffer &optional stream
This is like eval-region except that it operates on the whole contents of buffer.

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"). This counts internal uses of those functions, such as for calling the functions mentioned in Lisp expressions, and recursive evaluation of function call arguments and function body forms.

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 1000. 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 section 16.3 Local Variables.

Variable: values
The value of this variable is a list of the values returned by all the expressions that were read from buffers (including the minibuffer), evaluated, and printed. The elements are ordered 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-most-recent
;;   before this example.
(nth 3 values)
     => 1


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14.3 Kinds of Forms

A Lisp object that is intended to be evaluated is called a form. How XEmacs evaluates a form depends on its data type. XEmacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". This section describes all three kinds, starting with "all other types" which are self-evaluating forms.

14.3.1 Self-Evaluating Forms  Forms that evaluate to themselves.
14.3.2 Symbol Forms  Symbols evaluate as variables.
14.3.3 Classification of List Forms  How to distinguish various sorts of list forms.
14.3.4 Symbol Function Indirection  When a symbol appears as the car of a list, we find the real function via the symbol.
14.3.5 Evaluation of Function Forms  Forms that call functions.
14.3.6 Lisp Macro Evaluation  Forms that call macros.
14.3.7 Special Operators  "Special operators" are idiosyncratic primitives, most of them extremely important. Also known as special forms.
14.3.8 Autoloading  Functions set up to load files containing their real definitions.


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14.3.1 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 there's no way to write them textually. It is possible to construct Lisp expressions containing these types by means of a Lisp program. Here is an example:

 
;; Build an expression containing a buffer object.
(setq buffer (list 'print (current-buffer)))
     => (print #<buffer eval.texi>)
;; Evaluate it.
(eval buffer)
     -| #<buffer eval.texi>
     => #<buffer eval.texi>


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14.3.2 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 16. Variables.

In the following example, we set the value of a symbol with setq. Then we evaluate the symbol, and get back the value that setq stored.

 
(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; you cannot set or bind them to any other values. Thus, these two symbols act like self-evaluating forms, even though eval treats them like any other symbol.


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14.3.3 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 remaining list elements constitute the arguments for the function, macro, or special operator.

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 such as Scheme.


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14.3.4 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 it obtains a non-symbol. See section 17.3 Naming a Function, 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, in which case the subroutine symbol-function signals 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 operator, 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 section 17.8 Accessing Function Cell Contents). 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

Executing the function itself evaluates its body; 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 object
This function returns the meaning of object as a function. If object is a symbol, then it finds object's function definition and starts over with that value. If object is not a symbol, then it returns object itself.

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

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


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14.3.5 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)

The first step in evaluating a function call is to evaluate the remaining elements of the list from left to right. The results are the actual argument values, one value for each list element. The next step is to call the function with this list of arguments, effectively using the function apply (see section 17.5 Calling Functions). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (see section 17.2 Lambda Expressions); then the forms in the function body are evaluated in order, and the value of the last body form becomes the value of the function call.


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14.3.6 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, to be 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.

Ordinary evaluation of a macro call finishes by evaluating the expansion. However, the macro expansion is not necessarily evaluated right away, or at all, because other programs also expand macro calls, and they may or may not evaluate the expansions.

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 computed when the expansion is computed.

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 section 18. Macros, for a complete description of XEmacs Lisp macros.


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14.3.7 Special Operators

A special operator (historically, and less logically, a special form) is a primitive function specially marked so that its arguments are not all evaluated. Most special operators define control structures or perform variable bindings--things which functions cannot do.

Each special operator 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 operators in XEmacs Lisp with a reference to where each is described.

and
see section 15.3 Constructs for Combining Conditions

catch
see section 15.5.1 Explicit Nonlocal Exits: catch and throw

cond
see section 15.2 Conditionals

condition-case
see section 15.5.3.3 Writing Code to Handle Errors

defconst
see section 16.5 Defining Global Variables

defmacro
see section 18.4 Defining Macros

defun
see section 17.4 Defining Functions

defvar
see section 16.5 Defining Global Variables

function
see section 17.7 Anonymous Functions

if
see section 15.2 Conditionals

interactive
see section 25.3 Interactive Call

let
let*
see section 16.3 Local Variables

or
see section 15.3 Constructs for Combining Conditions

prog1
prog2
progn
see section 15.1 Sequencing

quote
see section 14.4 Quoting

save-current-buffer
see section 41.3 Excursions

save-excursion
see section 41.3 Excursions

save-restriction
see section 41.4 Narrowing

save-selected-window
see section 41.3 Excursions

save-window-excursion
see section 38.16 Window Configurations

setq
see section 16.7 How to Alter a Variable Value

setq-default
see section 16.9.2 Creating and Deleting Buffer-Local Bindings

unwind-protect
see section 15.5 Nonlocal Exits

while
see section 15.4 Iteration

with-output-to-temp-buffer
see section 52.8 Temporary Displays

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


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14.3.8 Autoloading

The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into XEmacs. It specifies which file contains the definition. When an autoload object appears as a symbol's function definition, calling that symbol as a function automatically loads the specified file; then it calls the real definition loaded from that file. See section 20.2 Autoload.


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14.4 Quoting

The special operator quote returns its single argument, as written, without evaluating it. This provides a way to include constant symbols and lists, which are not self-evaluating objects, in a program. (It is not necessary to quote self-evaluating objects such as numbers, strings, and vectors.)

Special Operator: quote object
This special operator returns object, without evaluating it.

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 section 17.7 Anonymous Functions), which causes an anonymous lambda expression written in Lisp to be compiled, and ``' (see section 18.5 Backquote), which is used to quote only part of a list, while computing and substituting other parts.


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14.5 Multiple values

Under XEmacs, expressions can return zero or more results, using the values and values-list functions. Results other than the first are typically discarded, but special operators are provided to access them.

Function: values arguments...
This function returns arguments as multiple values. Callers will always receive the first element of arguments, but must use various special operators, described below, to access other elements of arguments.

The idiom (values (function-call argument)), with one argument, is the normal mechanism to avoid passing multiple values to the calling form where that is not desired.

XEmacs implements the Common Lisp specification when it comes to the exact details of when to discard and when to preserve multiple values; see Common Lisp the Language or the Common Lisp hyperspec for more details. The most important thing to keep in mind is when multiple values are passed as an argument to a function, all but the first are discarded.

Function: values-list argument
This function returns the elements of the lst argument as multiple values.

Macro: multiple-value-bind (var...) values-form forms...
This macro evaluates values-form, which may return multiple values. It then binds the vars to these respective values, as if by let, and then executes the body forms. If there are more vars than values, the extra vars are bound to nil. If there are fewer vars than values, the excess values are ignored.

Macro: multiple-value-setq (var...) form
This macro evaluates form, which may return multiple values. It then sets the vars to these respective values, as if by setq. Extra vars or values are treated the same as in multiple-value-bind.

Special Operator: multiple-value-call function forms...
This special operator evaluates function, discarding any multiple values. It then evaluates forms, preserving any multiple values, and calls function as a function with the results. Conceptually, this function is a version of apply'that by-passes the multiple values infrastructure, treating multiple values as intercalated lists.

Macro: multiple-value-list form
This macro evaluates form and returns a list of the multiple values given by it.

Special Operator: multiple-value-prog1 first body...
This special operator evaluates the form first, then the forms body. It returns the value given by first, preserving any multiple values. This is identical to prog1, except that prog1 always discards multiple values.

Macro: nth-value n form
This macro evaluates form and returns the nth value it gave. n must be an integer of value zero or more. If form gave insufficient multiple values, nth-value returns nil.

Variable: multiple-values-limit
This constant describes the exclusive upper bound on the number of multiple values that values accepts and that multiple-value-bind, etc. will consume.

To take full advantage of multiple values, Emacs Lisp code must have been compiled by XEmacs 21.5 or later, which is not yet true of the XEmacs packages. Matched values and multiple-value-bind calls will work in code included in the XEmacs packages when run on 21.5, though the following incantation may be necessary at the start of your file, until appropriate code is included in XEmacs 21.4:

 
(eval-when-compile (when (eq 'list (symbol-function 'values))
                     (define-compiler-macro values (&rest args)
                       (cons 'list args))
                     (define-compiler-macro values-list (list) list)))

Such code cannot, unfortunately, rely on XEmacs to discard multiple values where that is appropriate.


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