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580 lines
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580 lines
20 KiB
Plaintext
@c -*-texinfo-*-
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@c This is part of the GNU Emacs Lisp Reference Manual.
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@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
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@c See the file elisp.texi for copying conditions.
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@setfilename ../info/macros
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@node Macros, Loading, Functions, Top
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@chapter Macros
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@cindex macros
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@dfn{Macros} enable you to define new control constructs and other
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language features. A macro is defined much like a function, but instead
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of telling how to compute a value, it tells how to compute another Lisp
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expression which will in turn compute the value. We call this
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expression the @dfn{expansion} of the macro.
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Macros can do this because they operate on the unevaluated expressions
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for the arguments, not on the argument values as functions do. They can
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therefore construct an expansion containing these argument expressions
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or parts of them.
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If you are using a macro to do something an ordinary function could
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do, just for the sake of speed, consider using an inline function
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instead. @xref{Inline Functions}.
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@menu
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* Simple Macro:: A basic example.
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* Expansion:: How, when and why macros are expanded.
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* Compiling Macros:: How macros are expanded by the compiler.
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* Defining Macros:: How to write a macro definition.
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* Backquote:: Easier construction of list structure.
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* Problems with Macros:: Don't evaluate the macro arguments too many times.
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Don't hide the user's variables.
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@end menu
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@node Simple Macro
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@section A Simple Example of a Macro
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Suppose we would like to define a Lisp construct to increment a
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variable value, much like the @code{++} operator in C. We would like to
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write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
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Here's a macro definition that does the job:
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@findex inc
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@example
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@group
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(defmacro inc (var)
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(list 'setq var (list '1+ var)))
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@end group
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@end example
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When this is called with @code{(inc x)}, the argument @code{var} has
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the value @code{x}---@emph{not} the @emph{value} of @code{x}. The body
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of the macro uses this to construct the expansion, which is @code{(setq
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x (1+ x))}. Once the macro definition returns this expansion, Lisp
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proceeds to evaluate it, thus incrementing @code{x}.
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@node Expansion
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@section Expansion of a Macro Call
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@cindex expansion of macros
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@cindex macro call
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A macro call looks just like a function call in that it is a list which
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starts with the name of the macro. The rest of the elements of the list
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are the arguments of the macro.
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Evaluation of the macro call begins like evaluation of a function call
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except for one crucial difference: the macro arguments are the actual
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expressions appearing in the macro call. They are not evaluated before
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they are given to the macro definition. By contrast, the arguments of a
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function are results of evaluating the elements of the function call
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list.
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Having obtained the arguments, Lisp invokes the macro definition just
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as a function is invoked. The argument variables of the macro are bound
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to the argument values from the macro call, or to a list of them in the
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case of a @code{&rest} argument. And the macro body executes and
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returns its value just as a function body does.
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The second crucial difference between macros and functions is that the
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value returned by the macro body is not the value of the macro call.
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Instead, it is an alternate expression for computing that value, also
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known as the @dfn{expansion} of the macro. The Lisp interpreter
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proceeds to evaluate the expansion as soon as it comes back from the
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macro.
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Since the expansion is evaluated in the normal manner, it may contain
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calls to other macros. It may even be a call to the same macro, though
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this is unusual.
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You can see the expansion of a given macro call by calling
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@code{macroexpand}.
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@defun macroexpand form &optional environment
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@cindex macro expansion
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This function expands @var{form}, if it is a macro call. If the result
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is another macro call, it is expanded in turn, until something which is
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not a macro call results. That is the value returned by
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@code{macroexpand}. If @var{form} is not a macro call to begin with, it
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is returned as given.
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Note that @code{macroexpand} does not look at the subexpressions of
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@var{form} (although some macro definitions may do so). Even if they
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are macro calls themselves, @code{macroexpand} does not expand them.
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The function @code{macroexpand} does not expand calls to inline functions.
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Normally there is no need for that, since a call to an inline function is
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no harder to understand than a call to an ordinary function.
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If @var{environment} is provided, it specifies an alist of macro
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definitions that shadow the currently defined macros. Byte compilation
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uses this feature.
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@smallexample
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@group
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(defmacro inc (var)
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(list 'setq var (list '1+ var)))
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@result{} inc
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@end group
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@group
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(macroexpand '(inc r))
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@result{} (setq r (1+ r))
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@end group
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@group
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(defmacro inc2 (var1 var2)
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(list 'progn (list 'inc var1) (list 'inc var2)))
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@result{} inc2
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@end group
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@group
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(macroexpand '(inc2 r s))
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@result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.}
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@end group
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@end smallexample
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@end defun
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@node Compiling Macros
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@section Macros and Byte Compilation
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@cindex byte-compiling macros
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You might ask why we take the trouble to compute an expansion for a
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macro and then evaluate the expansion. Why not have the macro body
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produce the desired results directly? The reason has to do with
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compilation.
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When a macro call appears in a Lisp program being compiled, the Lisp
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compiler calls the macro definition just as the interpreter would, and
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receives an expansion. But instead of evaluating this expansion, it
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compiles the expansion as if it had appeared directly in the program.
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As a result, the compiled code produces the value and side effects
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intended for the macro, but executes at full compiled speed. This would
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not work if the macro body computed the value and side effects
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itself---they would be computed at compile time, which is not useful.
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In order for compilation of macro calls to work, the macros must be
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defined in Lisp when the calls to them are compiled. The compiler has a
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special feature to help you do this: if a file being compiled contains a
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@code{defmacro} form, the macro is defined temporarily for the rest of
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the compilation of that file. To use this feature, you must define the
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macro in the same file where it is used and before its first use.
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Byte-compiling a file executes any @code{require} calls at top-level
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in the file. This is in case the file needs the required packages for
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proper compilation. One way to ensure that necessary macro definitions
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are available during compilation is to require the files that define
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them (@pxref{Named Features}). To avoid loading the macro definition files
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when someone @emph{runs} the compiled program, write
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@code{eval-when-compile} around the @code{require} calls (@pxref{Eval
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During Compile}).
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@node Defining Macros
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@section Defining Macros
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A Lisp macro is a list whose @sc{car} is @code{macro}. Its @sc{cdr} should
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be a function; expansion of the macro works by applying the function
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(with @code{apply}) to the list of unevaluated argument-expressions
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from the macro call.
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It is possible to use an anonymous Lisp macro just like an anonymous
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function, but this is never done, because it does not make sense to pass
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an anonymous macro to functionals such as @code{mapcar}. In practice,
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all Lisp macros have names, and they are usually defined with the
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special form @code{defmacro}.
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@defspec defmacro name argument-list body-forms@dots{}
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@code{defmacro} defines the symbol @var{name} as a macro that looks
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like this:
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@example
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(macro lambda @var{argument-list} . @var{body-forms})
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@end example
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This macro object is stored in the function cell of @var{name}. The
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value returned by evaluating the @code{defmacro} form is @var{name}, but
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usually we ignore this value.
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The shape and meaning of @var{argument-list} is the same as in a
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function, and the keywords @code{&rest} and @code{&optional} may be used
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(@pxref{Argument List}). Macros may have a documentation string, but
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any @code{interactive} declaration is ignored since macros cannot be
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called interactively.
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@end defspec
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@node Backquote
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@section Backquote
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@cindex backquote (list substitution)
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@cindex ` (list substitution)
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@findex `
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Macros often need to construct large list structures from a mixture of
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constants and nonconstant parts. To make this easier, use the macro
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@samp{`} (often called @dfn{backquote}).
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Backquote allows you to quote a list, but selectively evaluate
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elements of that list. In the simplest case, it is identical to the
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special form @code{quote} (@pxref{Quoting}). For example, these
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two forms yield identical results:
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@example
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@group
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`(a list of (+ 2 3) elements)
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@result{} (a list of (+ 2 3) elements)
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@end group
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@group
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'(a list of (+ 2 3) elements)
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@result{} (a list of (+ 2 3) elements)
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@end group
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@end example
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@findex , @r{(with Backquote)}
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The special marker @samp{,} inside of the argument to backquote
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indicates a value that isn't constant. Backquote evaluates the
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argument of @samp{,} and puts the value in the list structure:
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@example
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@group
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(list 'a 'list 'of (+ 2 3) 'elements)
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@result{} (a list of 5 elements)
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@end group
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@group
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`(a list of ,(+ 2 3) elements)
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@result{} (a list of 5 elements)
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@end group
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@end example
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@findex ,@@ @r{(with Backquote)}
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@cindex splicing (with backquote)
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You can also @dfn{splice} an evaluated value into the resulting list,
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using the special marker @samp{,@@}. The elements of the spliced list
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become elements at the same level as the other elements of the resulting
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list. The equivalent code without using @samp{`} is often unreadable.
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Here are some examples:
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@example
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@group
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(setq some-list '(2 3))
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@result{} (2 3)
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@end group
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@group
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(cons 1 (append some-list '(4) some-list))
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@result{} (1 2 3 4 2 3)
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@end group
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@group
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`(1 ,@@some-list 4 ,@@some-list)
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@result{} (1 2 3 4 2 3)
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@end group
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@group
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(setq list '(hack foo bar))
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@result{} (hack foo bar)
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@end group
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@group
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(cons 'use
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(cons 'the
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(cons 'words (append (cdr list) '(as elements)))))
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@result{} (use the words foo bar as elements)
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@end group
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@group
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`(use the words ,@@(cdr list) as elements)
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@result{} (use the words foo bar as elements)
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@end group
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@end example
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@quotation
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Before Emacs version 19.29, @samp{`} used a different syntax which
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required an extra level of parentheses around the entire backquote
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construct. Likewise, each @samp{,} or @samp{,@@} substition required an
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extra level of parentheses surrounding both the @samp{,} or @samp{,@@}
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and the following expression. The old syntax required whitespace
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between the @samp{`}, @samp{,} or @samp{,@@} and the following
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expression.
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This syntax is still accepted, but no longer recommended except for
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compatibility with old Emacs versions.
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@end quotation
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@node Problems with Macros
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@section Common Problems Using Macros
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The basic facts of macro expansion have counterintuitive consequences.
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This section describes some important consequences that can lead to
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trouble, and rules to follow to avoid trouble.
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@menu
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* Argument Evaluation:: The expansion should evaluate each macro arg once.
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* Surprising Local Vars:: Local variable bindings in the expansion
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require special care.
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* Eval During Expansion:: Don't evaluate them; put them in the expansion.
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* Repeated Expansion:: Avoid depending on how many times expansion is done.
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@end menu
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@node Argument Evaluation
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@subsection Evaluating Macro Arguments Repeatedly
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When defining a macro you must pay attention to the number of times
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the arguments will be evaluated when the expansion is executed. The
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following macro (used to facilitate iteration) illustrates the problem.
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This macro allows us to write a simple ``for'' loop such as one might
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find in Pascal.
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@findex for
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@smallexample
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@group
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(defmacro for (var from init to final do &rest body)
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"Execute a simple \"for\" loop.
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For example, (for i from 1 to 10 do (print i))."
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(list 'let (list (list var init))
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(cons 'while (cons (list '<= var final)
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(append body (list (list 'inc var)))))))
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@end group
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@result{} for
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@group
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(for i from 1 to 3 do
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(setq square (* i i))
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(princ (format "\n%d %d" i square)))
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@expansion{}
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@end group
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@group
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(let ((i 1))
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(while (<= i 3)
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(setq square (* i i))
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(princ (format "%d %d" i square))
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(inc i)))
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@end group
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@group
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@print{}1 1
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@print{}2 4
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@print{}3 9
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@result{} nil
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@end group
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@end smallexample
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@noindent
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(The arguments @code{from}, @code{to}, and @code{do} in this macro are
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``syntactic sugar''; they are entirely ignored. The idea is that you
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will write noise words (such as @code{from}, @code{to}, and @code{do})
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in those positions in the macro call.)
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Here's an equivalent definition simplified through use of backquote:
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@smallexample
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@group
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(defmacro for (var from init to final do &rest body)
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"Execute a simple \"for\" loop.
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For example, (for i from 1 to 10 do (print i))."
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`(let ((,var ,init))
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(while (<= ,var ,final)
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,@@body
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(inc ,var))))
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@end group
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@end smallexample
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Both forms of this definition (with backquote and without) suffer from
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the defect that @var{final} is evaluated on every iteration. If
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@var{final} is a constant, this is not a problem. If it is a more
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complex form, say @code{(long-complex-calculation x)}, this can slow
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down the execution significantly. If @var{final} has side effects,
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executing it more than once is probably incorrect.
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@cindex macro argument evaluation
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A well-designed macro definition takes steps to avoid this problem by
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producing an expansion that evaluates the argument expressions exactly
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once unless repeated evaluation is part of the intended purpose of the
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macro. Here is a correct expansion for the @code{for} macro:
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@smallexample
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@group
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(let ((i 1)
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(max 3))
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(while (<= i max)
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(setq square (* i i))
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(princ (format "%d %d" i square))
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(inc i)))
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@end group
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@end smallexample
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Here is a macro definition that creates this expansion:
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@smallexample
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@group
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(defmacro for (var from init to final do &rest body)
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"Execute a simple for loop: (for i from 1 to 10 do (print i))."
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`(let ((,var ,init)
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(max ,final))
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(while (<= ,var max)
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,@@body
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(inc ,var))))
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@end group
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@end smallexample
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Unfortunately, this introduces another problem.
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@ifinfo
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Proceed to the following node.
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@end ifinfo
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@node Surprising Local Vars
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@subsection Local Variables in Macro Expansions
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@ifinfo
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In the previous section, the definition of @code{for} was fixed as
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follows to make the expansion evaluate the macro arguments the proper
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number of times:
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@smallexample
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@group
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(defmacro for (var from init to final do &rest body)
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"Execute a simple for loop: (for i from 1 to 10 do (print i))."
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@end group
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@group
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`(let ((,var ,init)
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(max ,final))
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(while (<= ,var max)
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,@@body
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(inc ,var))))
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@end group
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@end smallexample
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@end ifinfo
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The new definition of @code{for} has a new problem: it introduces a
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local variable named @code{max} which the user does not expect. This
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causes trouble in examples such as the following:
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@smallexample
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@group
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(let ((max 0))
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(for x from 0 to 10 do
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(let ((this (frob x)))
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(if (< max this)
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(setq max this)))))
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@end group
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@end smallexample
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@noindent
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The references to @code{max} inside the body of the @code{for}, which
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are supposed to refer to the user's binding of @code{max}, really access
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the binding made by @code{for}.
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The way to correct this is to use an uninterned symbol instead of
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@code{max} (@pxref{Creating Symbols}). The uninterned symbol can be
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bound and referred to just like any other symbol, but since it is
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created by @code{for}, we know that it cannot already appear in the
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user's program. Since it is not interned, there is no way the user can
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put it into the program later. It will never appear anywhere except
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where put by @code{for}. Here is a definition of @code{for} that works
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this way:
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@smallexample
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@group
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(defmacro for (var from init to final do &rest body)
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"Execute a simple for loop: (for i from 1 to 10 do (print i))."
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(let ((tempvar (make-symbol "max")))
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`(let ((,var ,init)
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(,tempvar ,final))
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(while (<= ,var ,tempvar)
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,@@body
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(inc ,var)))))
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@end group
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@end smallexample
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@noindent
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This creates an uninterned symbol named @code{max} and puts it in the
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expansion instead of the usual interned symbol @code{max} that appears
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in expressions ordinarily.
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@node Eval During Expansion
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@subsection Evaluating Macro Arguments in Expansion
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Another problem can happen if you evaluate any of the macro argument
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expressions during the computation of the expansion, such as by calling
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@code{eval} (@pxref{Eval}). If the argument is supposed to refer to the
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user's variables, you may have trouble if the user happens to use a
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variable with the same name as one of the macro arguments. Inside the
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macro body, the macro argument binding is the most local binding of this
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variable, so any references inside the form being evaluated do refer
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to it. Here is an example:
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@example
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@group
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(defmacro foo (a)
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(list 'setq (eval a) t))
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@result{} foo
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@end group
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@group
|
|
(setq x 'b)
|
|
(foo x) @expansion{} (setq b t)
|
|
@result{} t ; @r{and @code{b} has been set.}
|
|
;; @r{but}
|
|
(setq a 'c)
|
|
(foo a) @expansion{} (setq a t)
|
|
@result{} t ; @r{but this set @code{a}, not @code{c}.}
|
|
|
|
@end group
|
|
@end example
|
|
|
|
It makes a difference whether the user's variable is named @code{a} or
|
|
@code{x}, because @code{a} conflicts with the macro argument variable
|
|
@code{a}.
|
|
|
|
Another reason not to call @code{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 @code{eval}) don't occur and its local variable bindings don't
|
|
exist.
|
|
|
|
The safe way to work with the run-time value of an expression is to
|
|
put the expression into the macro expansion, so that its value is
|
|
computed as part of executing the expansion.
|
|
|
|
@node Repeated Expansion
|
|
@subsection How Many Times is the Macro Expanded?
|
|
|
|
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.
|
|
|
|
In particular, constructing objects is a kind of side effect. If the
|
|
macro is called once, then the objects are constructed only once. In
|
|
other words, the same structure of objects is used each time the macro
|
|
call is executed. In interpreted operation, the macro is reexpanded
|
|
each time, producing a fresh collection of objects each time. Usually
|
|
this does not matter---the objects have the same contents whether they
|
|
are shared or not. But if the surrounding program does side effects
|
|
on the objects, it makes a difference whether they are shared. Here is
|
|
an example:
|
|
|
|
@lisp
|
|
@group
|
|
(defmacro empty-object ()
|
|
(list 'quote (cons nil nil)))
|
|
@end group
|
|
|
|
@group
|
|
(defun initialize (condition)
|
|
(let ((object (empty-object)))
|
|
(if condition
|
|
(setcar object condition))
|
|
object))
|
|
@end group
|
|
@end lisp
|
|
|
|
@noindent
|
|
If @code{initialize} is interpreted, a new list @code{(nil)} is
|
|
constructed each time @code{initialize} is called. Thus, no side effect
|
|
survives between calls. If @code{initialize} is compiled, then the
|
|
macro @code{empty-object} is expanded during compilation, producing a
|
|
single ``constant'' @code{(nil)} that is reused and altered each time
|
|
@code{initialize} is called.
|
|
|
|
One way to avoid pathological cases like this is to think of
|
|
@code{empty-object} as a funny kind of constant, not as a memory
|
|
allocation construct. You wouldn't use @code{setcar} on a constant such
|
|
as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)}
|
|
either.
|