Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See section Inline Functions.
Suppose we would like to define a Lisp construct to increment a
variable value, much like the ++ operator in C. We
would like to write (inc x) and have the effect of
(setq x (1+ x)). Here's a macro definition that does
the job:
(defmacro inc (var) (list 'setq var (list '1+ var)))
When this is called with (inc x), the argument
var is the symbol x---not the
value of x, as it would be in a function. The
body of the macro uses this to construct the expansion, which is
(setq x (1+ x)). Once the macro definition returns
this expansion, Lisp proceeds to evaluate it, thus incrementing
x.
A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.
Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.
Having obtained the arguments, Lisp invokes the macro definition
just as a function is invoked. The argument variables of the macro
are bound to the argument values from the macro call, or to a list
of them in the case of a &rest argument. And the
macro body executes and returns its value just as a function body
does.
The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.
Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.
You can see the expansion of a given macro call by calling
macroexpand.
macroexpand. If form
is not a macro call to begin with, it is returned as given. Note that macroexpand does not look at the
subexpressions of form (although some macro definitions
may do so). Even if they are macro calls themselves,
macroexpand does not expand them.
The function macroexpand does not expand calls to
inline functions. Normally there is no need for that, since a call
to an inline function is no harder to understand than a call to an
ordinary function.
If environment is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature.
(defmacro inc (var)
(list 'setq var (list '1+ var)))
=> inc
(macroexpand '(inc r))
=> (setq r (1+ r))
(defmacro inc2 (var1 var2)
(list 'progn (list 'inc var1) (list 'inc var2)))
=> inc2
(macroexpand '(inc2 r s))
=> (progn (inc r) (inc s)) ; inc not expanded here.
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must
already be defined in Lisp when the calls to them are compiled. The
compiler has a special feature to help you do this: if a file being
compiled contains a defmacro form, the macro is
defined temporarily for the rest of the compilation of that file.
To make this feature work, you must put the defmacro
in the same file where it is used, and before its first use.
Byte-compiling a file executes any require calls at
top-level in the file. This is in case the file needs the required
packages for proper compilation. One way to ensure that necessary
macro definitions are available during compilation is to require
the files that define them (see section Features). To avoid loading the macro
definition files when someone runs the compiled program,
write eval-when-compile around the
require calls (see section Evaluation During Compilation).
A Lisp macro is a list whose CAR is macro. Its CDR
should be a function; expansion of the macro works by applying the
function (with apply) to the list of unevaluated
argument-expressions from the macro call.
It is possible to use an anonymous Lisp macro just like an
anonymous function, but this is never done, because it does not
make sense to pass an anonymous macro to functionals such as
mapcar. In practice, all Lisp macros have names, and
they are usually defined with the special form
defmacro.
defmacro defines
the symbol name as a macro that looks like this: (macro lambda argument-list . body-forms)
(Note that the CDR of this list is a function--a lambda
expression.) This macro object is stored in the function cell of
name. The value returned by evaluating the
defmacro form is name, but usually we
ignore this value.
The shape and meaning of argument-list is the same as
in a function, and the keywords &rest and
&optional may be used (see section Other Features of Argument Lists).
Macros may have a documentation string, but any
interactive declaration is ignored since macros cannot
be called interactively.
Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the ``' syntax (usually called backquote).
Backquote allows you to quote a list, but selectively evaluate
elements of that list. In the simplest case, it is identical to the
special form quote (see section Quoting). For example, these two forms
yield identical results:
`(a list of (+ 2 3) elements)
=> (a list of (+ 2 3) elements)
'(a list of (+ 2 3) elements)
=> (a list of (+ 2 3) elements)
The special marker `,' inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of `,' and puts the value in the list structure:
(list 'a 'list 'of (+ 2 3) 'elements)
=> (a list of 5 elements)
`(a list of ,(+ 2 3) elements)
=> (a list of 5 elements)
Substitution with `,' is allowed at deeper levels of the list structure also. For example:
(defmacro t-becomes-nil (variable)
`(if (eq ,variable t)
(setq ,variable nil)))
(t-becomes-nil foo)
== (if (eq foo t) (setq foo nil))
You can also splice an evaluated value into the resulting list, using the special marker `,@'. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ``' is often unreadable. Here are some examples:
(setq some-list '(2 3))
=> (2 3)
(cons 1 (append some-list '(4) some-list))
=> (1 2 3 4 2 3)
`(1 ,@some-list 4 ,@some-list)
=> (1 2 3 4 2 3)
(setq list '(hack foo bar))
=> (hack foo bar)
(cons 'use
(cons 'the
(cons 'words (append (cdr list) '(as elements)))))
=> (use the words foo bar as elements)
`(use the words ,@(cdr list) as elements)
=> (use the words foo bar as elements)
In old Emacs versions, before version 19.29, ``' used a different syntax which required an extra level of parentheses around the entire backquote construct. Likewise, each `,' or `,@' substitution required an extra level of parentheses surrounding both the `,' or `,@' and the following expression. The old syntax required whitespace between the ``', `,' or `,@' and the following expression.
This syntax is still accepted, for compatibility with old Emacs versions, but we recommend not using it in new programs.
The basic facts of macro expansion have counterintuitive consequences. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.
When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a simple "for" loop such as one might find in Pascal.
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
(list 'let (list (list var init))
(cons 'while (cons (list '<= var final)
(append body (list (list 'inc var)))))))
=> for
(for i from 1 to 3 do
(setq square (* i i))
(princ (format "\n%d %d" i square)))
==>
(let ((i 1))
(while (<= i 3)
(setq square (* i i))
(princ (format "%d %d" i square))
(inc i)))
-|1 1
-|2 4
-|3 9
=> nil
The arguments from, to, and
do in this macro are "syntactic sugar"; they are
entirely ignored. The idea is that you will write noise words (such
as from, to, and do) in
those positions in the macro call.
Here's an equivalent definition simplified through use of backquote:
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
For example, (for i from 1 to 10 do (print i))."
`(let ((,var ,init))
(while (<= ,var ,final)
,@body
(inc ,var))))
Both forms of this definition (with backquote and without)
suffer from the defect that final is evaluated on every
iteration. If final is a constant, this is not a
problem. If it is a more complex form, say
(long-complex-calculation x), this can slow down the
execution significantly. If final has side effects,
executing it more than once is probably incorrect.
A well-designed macro
definition takes steps to avoid this problem by producing an
expansion that evaluates the argument expressions exactly once
unless repeated evaluation is part of the intended purpose of the
macro. Here is a correct expansion for the for
macro:
(let ((i 1)
(max 3))
(while (<= i max)
(setq square (* i i))
(princ (format "%d %d" i square))
(inc i)))
Here is a macro definition that creates this expansion:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
`(let ((,var ,init)
(max ,final))
(while (<= ,var max)
,@body
(inc ,var))))
Unfortunately, this fix introduces another problem, described in the following section.
The new definition of for has a new problem: it
introduces a local variable named max which the user
does not expect. This causes trouble in examples such as the
following:
(let ((max 0))
(for x from 0 to 10 do
(let ((this (frob x)))
(if (< max this)
(setq max this)))))
The references to max inside the body of the
for, which are supposed to refer to the user's binding
of max, really access the binding made by
for.
The way to correct this is to use an uninterned symbol instead
of max (see section Creating and Interning Symbols). The
uninterned symbol can be bound and referred to just like any other
symbol, but since it is created by for, we know that
it cannot already appear in the user's program. Since it is not
interned, there is no way the user can put it into the program
later. It will never appear anywhere except where put by
for. Here is a definition of for that
works this way:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(let ((tempvar (make-symbol "max")))
`(let ((,var ,init)
(,tempvar ,final))
(while (<= ,var ,tempvar)
,@body
(inc ,var)))))
This creates an uninterned symbol named max and
puts it in the expansion instead of the usual interned symbol
max that appears in expressions ordinarily.
Another problem can happen if the macro definition itself
evaluates any of the macro argument expressions, such as by calling
eval (see section Eval). If the argument is supposed to
refer to the user's variables, you may have trouble if the user
happens to use a variable with the same name as one of the macro
arguments. Inside the macro body, the macro argument binding is the
most local binding of this variable, so any references inside the
form being evaluated do refer to it. Here is an example:
(defmacro foo (a)
(list 'setq (eval a) t))
=> foo
(setq x 'b)
(foo x) ==> (setq b t)
=> t ; and b has been set.
;; but
(setq a 'c)
(foo a) ==> (setq a t)
=> t ; but this set a, not c.
It makes a difference whether the user's variable is named
a or x, because a conflicts
with the macro argument variable a.
Another problem with calling eval in a macro
definition is that it probably won't do what you intend in a
compiled program. The byte-compiler runs macro definitions while
compiling the program, when the program's own computations (which
you might have wished to access with eval) don't occur
and its local variable bindings don't exist.
To avoid these problems, don't evaluate an argument expression while computing the macro expansion. Instead, substitute the expression into the macro expansion, so that its value will be computed as part of executing the expansion. This is how the other examples in this chapter work.
Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.
Therefore, you should avoid side effects in computation of the macro expansion, unless you really know what you are doing.
One special kind of side effect can't be avoided: constructing Lisp objects. Almost all macro expansions include constructed lists; that is the whole point of most macros. This is usually safe; there is just one case where you must be careful: when the object you construct is part of a quoted constant in the macro expansion.
If the macro is expanded just once, in compilation, then the object is constructed just once, during compilation. But in interpreted execution, the macro is expanded each time the macro call runs, and this means a new object is constructed each time.
In most clean Lisp code, this difference won't matter. It can matter only if you perform side-effects on the objects constructed by the macro definition. Thus, to avoid trouble, avoid side effects on objects constructed by macro definitions. Here is an example of how such side effects can get you into trouble:
(defmacro empty-object ()
(list 'quote (cons nil nil)))
(defun initialize (condition)
(let ((object (empty-object)))
(if condition
(setcar object condition))
object))
If initialize is interpreted, a new list
(nil) is constructed each time initialize
is called. Thus, no side effect survives between calls. If
initialize is compiled, then the macro
empty-object is expanded during compilation, producing
a single "constant" (nil) that is reused and altered
each time initialize is called.
One way to avoid pathological cases like this is to think of
empty-object as a funny kind of constant, not as a
memory allocation construct. You wouldn't use setcar
on a constant such as '(nil), so naturally you won't
use it on (empty-object) either.