[Emacs-diffs] Changes to macros.texi

[Glenn Morris]

CVSROOT:        /sources/emacs
Module name:    emacs
Changes by:     Glenn Morris <gm>       07/09/06 04:21:36

Index: macros.texi
===================================================================
RCS file: macros.texi
diff -N macros.texi
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ macros.texi 6 Sep 2007 04:21:36 -0000       1.1
@@ -0,0 +1,752 @@
address@hidden -*-texinfo-*-
address@hidden This is part of the GNU Emacs Lisp Reference Manual.
address@hidden Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 2001, 
2002,
address@hidden   2003, 2004, 2005, 2006, 2007  Free Software Foundation, Inc.
address@hidden See the file elisp.texi for copying conditions.
address@hidden ../info/macros
address@hidden Macros, Customization, Functions, Top
address@hidden Macros
address@hidden macros
+
+  @dfn{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 @dfn{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.  @xref{Inline Functions}.
+
address@hidden
+* Simple Macro::            A basic example.
+* Expansion::               How, when and why macros are expanded.
+* Compiling Macros::        How macros are expanded by the compiler.
+* Defining Macros::         How to write a macro definition.
+* Backquote::               Easier construction of list structure.
+* Problems with Macros::    Don't evaluate the macro arguments too many times.
+                              Don't hide the user's variables.
+* Indenting Macros::        Specifying how to indent macro calls.
address@hidden menu
+
address@hidden Simple Macro
address@hidden A Simple Example of a Macro
+
+  Suppose we would like to define a Lisp construct to increment a
+variable value, much like the @code{++} operator in C.  We would like to
+write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
+Here's a macro definition that does the job:
+
address@hidden inc
address@hidden
address@hidden
+(defmacro inc (var)
+   (list 'setq var (list '1+ var)))
address@hidden group
address@hidden example
+
+  When this is called with @code{(inc x)}, the argument @var{var} is the
+symbol @address@hidden the @emph{value} of @code{x}, as it would
+be in a function.  The body of the macro uses this to construct the
+expansion, which is @code{(setq x (1+ x))}.  Once the macro definition
+returns this expansion, Lisp proceeds to evaluate it, thus incrementing
address@hidden
+
address@hidden Expansion
address@hidden Expansion of a Macro Call
address@hidden expansion of macros
address@hidden macro call
+
+  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 @code{&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 @dfn{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
address@hidden
+
address@hidden macroexpand form &optional environment
address@hidden macro expansion
+This function expands @var{form}, if it is a macro call.  If the result
+is another macro call, it is expanded in turn, until something which is
+not a macro call results.  That is the value returned by
address@hidden  If @var{form} is not a macro call to begin with, it
+is returned as given.
+
+Note that @code{macroexpand} does not look at the subexpressions of
address@hidden (although some macro definitions may do so).  Even if they
+are macro calls themselves, @code{macroexpand} does not expand them.
+
+The function @code{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 @var{environment} is provided, it specifies an alist of macro
+definitions that shadow the currently defined macros.  Byte compilation
+uses this feature.
+
address@hidden
address@hidden
+(defmacro inc (var)
+    (list 'setq var (list '1+ var)))
+     @result{} inc
address@hidden group
+
address@hidden
+(macroexpand '(inc r))
+     @result{} (setq r (1+ r))
address@hidden group
+
address@hidden
+(defmacro inc2 (var1 var2)
+    (list 'progn (list 'inc var1) (list 'inc var2)))
+     @result{} inc2
address@hidden group
+
address@hidden
+(macroexpand '(inc2 r s))
+     @result{} (progn (inc r) (inc s))  ; @address@hidden not expanded here.}
address@hidden group
address@hidden smallexample
address@hidden defun
+
+
address@hidden macroexpand-all form &optional environment
address@hidden expands macros like @code{macroexpand}, but
+will look for and expand all macros in @var{form}, not just at the
+top-level.  If no macros are expanded, the return value is @code{eq}
+to @var{form}.
+
+Repeating the example used for @code{macroexpand} above with
address@hidden, we see that @code{macroexpand-all} @emph{does}
+expand the embedded calls to @code{inc}:
+
address@hidden
+(macroexpand-all '(inc2 r s))
+     @result{} (progn (setq r (1+ r)) (setq s (1+ s)))
address@hidden smallexample
+
address@hidden defun
+
address@hidden Compiling Macros
address@hidden Macros and Byte Compilation
address@hidden byte-compiling macros
+
+  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 @code{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 @code{defmacro} in the same file where it
+is used, and before its first use.
+
+  Byte-compiling a file executes any @code{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 (@pxref{Named Features}).  To avoid loading the macro definition files
+when someone @emph{runs} the compiled program, write
address@hidden around the @code{require} calls (@pxref{Eval
+During Compile}).
+
address@hidden Defining Macros
address@hidden Defining Macros
+
+  A Lisp macro is a list whose @sc{car} is @code{macro}.  Its @sc{cdr} should
+be a function; expansion of the macro works by applying the function
+(with @code{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 @code{mapcar}.  In practice,
+all Lisp macros have names, and they are usually defined with the
+special form @code{defmacro}.
+
address@hidden defmacro name argument-list address@hidden
address@hidden defines the symbol @var{name} as a macro that looks
+like this:
+
address@hidden
+(macro lambda @var{argument-list} . @var{body-forms})
address@hidden example
+
+(Note that the @sc{cdr} of this list is a function---a lambda expression.)
+This macro object is stored in the function cell of @var{name}.  The
+value returned by evaluating the @code{defmacro} form is @var{name}, but
+usually we ignore this value.
+
+The shape and meaning of @var{argument-list} is the same as in a
+function, and the keywords @code{&rest} and @code{&optional} may be used
+(@pxref{Argument List}).  Macros may have a documentation string, but
+any @code{interactive} declaration is ignored since macros cannot be
+called interactively.
address@hidden defspec
+
+  The body of the macro definition can include a @code{declare} form,
+which can specify how @key{TAB} should indent macro calls, and how to
+step through them for Edebug.
+
address@hidden declare @address@hidden
address@hidden of declare}
+A @code{declare} form is used in a macro definition to specify various
+additional information about it.  Two kinds of specification are
+currently supported:
+
address@hidden @code
address@hidden (debug @var{edebug-form-spec})
+Specify how to step through macro calls for Edebug.
address@hidden Macro Calls}.
+
address@hidden (indent @var{indent-spec})
+Specify how to indent calls to this macro.  @xref{Indenting Macros},
+for more details.
address@hidden table
+
+A @code{declare} form only has its special effect in the body of a
address@hidden form if it immediately follows the documentation
+string, if present, or the argument list otherwise.  (Strictly
+speaking, @emph{several} @code{declare} forms can follow the
+documentation string or argument list, but since a @code{declare} form
+can have several @var{specs}, they can always be combined into a
+single form.)  When used at other places in a @code{defmacro} form, or
+outside a @code{defmacro} form, @code{declare} just returns @code{nil}
+without evaluating any @var{specs}.
address@hidden defmac
+
+  No macro absolutely needs a @code{declare} form, because that form
+has no effect on how the macro expands, on what the macro means in the
+program.  It only affects secondary features: indentation and Edebug.
+
address@hidden Backquote
address@hidden Backquote
address@hidden backquote (list substitution)
address@hidden ` (list substitution)
address@hidden `
+
+  Macros often need to construct large list structures from a mixture of
+constants and nonconstant parts.  To make this easier, use the @samp{`}
+syntax (usually called @dfn{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 @code{quote} (@pxref{Quoting}).  For example, these
+two forms yield identical results:
+
address@hidden
address@hidden
+`(a list of (+ 2 3) elements)
+     @result{} (a list of (+ 2 3) elements)
address@hidden group
address@hidden
+'(a list of (+ 2 3) elements)
+     @result{} (a list of (+ 2 3) elements)
address@hidden group
address@hidden example
+
address@hidden , @r{(with backquote)}
+The special marker @samp{,} inside of the argument to backquote
+indicates a value that isn't constant.  Backquote evaluates the
+argument of @samp{,} and puts the value in the list structure:
+
address@hidden
address@hidden
+(list 'a 'list 'of (+ 2 3) 'elements)
+     @result{} (a list of 5 elements)
address@hidden group
address@hidden
+`(a list of ,(+ 2 3) elements)
+     @result{} (a list of 5 elements)
address@hidden group
address@hidden example
+
+  Substitution with @samp{,} is allowed at deeper levels of the list
+structure also.  For example:
+
address@hidden
address@hidden
+(defmacro t-becomes-nil (variable)
+  `(if (eq ,variable t)
+       (setq ,variable nil)))
address@hidden group
+
address@hidden
+(t-becomes-nil foo)
+     @equiv{} (if (eq foo t) (setq foo nil))
address@hidden group
address@hidden example
+
address@hidden ,@@ @r{(with backquote)}
address@hidden splicing (with backquote)
+  You can also @dfn{splice} an evaluated value into the resulting list,
+using the special marker @samp{,@@}.  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 @samp{`} is often unreadable.
+Here are some examples:
+
address@hidden
address@hidden
+(setq some-list '(2 3))
+     @result{} (2 3)
address@hidden group
address@hidden
+(cons 1 (append some-list '(4) some-list))
+     @result{} (1 2 3 4 2 3)
address@hidden group
address@hidden
+`(1 ,@@some-list 4 ,@@some-list)
+     @result{} (1 2 3 4 2 3)
address@hidden group
+
address@hidden
+(setq list '(hack foo bar))
+     @result{} (hack foo bar)
address@hidden group
address@hidden
+(cons 'use
+  (cons 'the
+    (cons 'words (append (cdr list) '(as elements)))))
+     @result{} (use the words foo bar as elements)
address@hidden group
address@hidden
+`(use the words ,@@(cdr list) as elements)
+     @result{} (use the words foo bar as elements)
address@hidden group
address@hidden example
+
+In old Emacs versions, before version 19.29, @samp{`} used a different
+syntax which required an extra level of parentheses around the entire
+backquote construct.  Likewise, each @samp{,} or @samp{,@@} substitution
+required an extra level of parentheses surrounding both the @samp{,} or
address@hidden,@@} and the following expression.  The old syntax required
+whitespace between the @samp{`}, @samp{,} or @samp{,@@} and the
+following expression.
+
+This syntax is still accepted, for compatibility with old Emacs
+versions, but support for it will soon disappear.
+
address@hidden Problems with Macros
address@hidden Common Problems Using Macros
+
+  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.
+
address@hidden
+* Wrong Time::             Do the work in the expansion, not in the macro.
+* Argument Evaluation::    The expansion should evaluate each macro arg once.
+* Surprising Local Vars::  Local variable bindings in the expansion
+                              require special care.
+* Eval During Expansion::  Don't evaluate them; put them in the expansion.
+* Repeated Expansion::     Avoid depending on how many times expansion is done.
address@hidden menu
+
address@hidden Wrong Time
address@hidden Wrong Time
+
+  The most common problem in writing macros is doing some of the
+real work prematurely---while expanding the macro, rather than in the
+expansion itself.  For instance, one real package had this macro
+definition:
+
address@hidden
+(defmacro my-set-buffer-multibyte (arg)
+  (if (fboundp 'set-buffer-multibyte)
+      (set-buffer-multibyte arg)))
address@hidden example
+
+With this erroneous macro definition, the program worked fine when
+interpreted but failed when compiled.  This macro definition called
address@hidden during compilation, which was wrong, and
+then did nothing when the compiled package was run.  The definition
+that the programmer really wanted was this:
+
address@hidden
+(defmacro my-set-buffer-multibyte (arg)
+  (if (fboundp 'set-buffer-multibyte)
+      `(set-buffer-multibyte ,arg)))
address@hidden example
+
address@hidden
+This macro expands, if appropriate, into a call to
address@hidden that will be executed when the compiled
+program is actually run.
+
address@hidden Argument Evaluation
address@hidden Evaluating Macro Arguments Repeatedly
+
+  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.
+
address@hidden for
address@hidden
address@hidden
+(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)))))))
address@hidden group
address@hidden for
+
address@hidden
+(for i from 1 to 3 do
+   (setq square (* i i))
+   (princ (format "\n%d %d" i square)))
address@hidden
address@hidden group
address@hidden
+(let ((i 1))
+  (while (<= i 3)
+    (setq square (* i i))
+    (princ (format "\n%d %d" i square))
+    (inc i)))
address@hidden group
address@hidden
+
+     @print{}1       1
+     @print{}2       4
+     @print{}3       9
address@hidden nil
address@hidden group
address@hidden smallexample
+
address@hidden
+The arguments @code{from}, @code{to}, and @code{do} in this macro are
+``syntactic sugar''; they are entirely ignored.  The idea is that you
+will write noise words (such as @code{from}, @code{to}, and @code{do})
+in those positions in the macro call.
+
+Here's an equivalent definition simplified through use of backquote:
+
address@hidden
address@hidden
+(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))))
address@hidden group
address@hidden smallexample
+
+Both forms of this definition (with backquote and without) suffer from
+the defect that @var{final} is evaluated on every iteration.  If
address@hidden is a constant, this is not a problem.  If it is a more
+complex form, say @code{(long-complex-calculation x)}, this can slow
+down the execution significantly.  If @var{final} has side effects,
+executing it more than once is probably incorrect.
+
address@hidden macro argument evaluation
+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 @code{for} macro:
+
address@hidden
address@hidden
+(let ((i 1)
+      (max 3))
+  (while (<= i max)
+    (setq square (* i i))
+    (princ (format "%d      %d" i square))
+    (inc i)))
address@hidden group
address@hidden smallexample
+
+Here is a macro definition that creates this expansion:
+
address@hidden
address@hidden
+(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))))
address@hidden group
address@hidden smallexample
+
+  Unfortunately, this fix introduces another problem,
+described in the following section.
+
address@hidden Surprising Local Vars
address@hidden Local Variables in Macro Expansions
+
address@hidden
+  In the previous section, the definition of @code{for} was fixed as
+follows to make the expansion evaluate the macro arguments the proper
+number of times:
+
address@hidden
address@hidden
+(defmacro for (var from init to final do &rest body)
+  "Execute a simple for loop: (for i from 1 to 10 do (print i))."
address@hidden group
address@hidden
+  `(let ((,var ,init)
+         (max ,final))
+     (while (<= ,var max)
+       ,@@body
+       (inc ,var))))
address@hidden group
address@hidden smallexample
address@hidden ifnottex
+
+  The new definition of @code{for} has a new problem: it introduces a
+local variable named @code{max} which the user does not expect.  This
+causes trouble in examples such as the following:
+
address@hidden
address@hidden
+(let ((max 0))
+  (for x from 0 to 10 do
+    (let ((this (frob x)))
+      (if (< max this)
+          (setq max this)))))
address@hidden group
address@hidden smallexample
+
address@hidden
+The references to @code{max} inside the body of the @code{for}, which
+are supposed to refer to the user's binding of @code{max}, really access
+the binding made by @code{for}.
+
+The way to correct this is to use an uninterned symbol instead of
address@hidden (@pxref{Creating Symbols}).  The uninterned symbol can be
+bound and referred to just like any other symbol, but since it is
+created by @code{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 @code{for}.  Here is a definition of @code{for} that works
+this way:
+
address@hidden
address@hidden
+(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)))))
address@hidden group
address@hidden smallexample
+
address@hidden
+This creates an uninterned symbol named @code{max} and puts it in the
+expansion instead of the usual interned symbol @code{max} that appears
+in expressions ordinarily.
+
address@hidden Eval During Expansion
address@hidden Evaluating Macro Arguments in Expansion
+
+  Another problem can happen if the macro definition itself
+evaluates any of the macro argument expressions, such as by calling
address@hidden (@pxref{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:
+
address@hidden
address@hidden
+(defmacro foo (a)
+  (list 'setq (eval a) t))
+     @result{} foo
address@hidden group
address@hidden
+(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}.}
+
address@hidden group
address@hidden example
+
+  It makes a difference whether the user's variable is named @code{a} or
address@hidden, because @code{a} conflicts with the macro argument variable
address@hidden
+
+  Another problem with calling @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.
+
+  To avoid these problems, @strong{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.
+
address@hidden Repeated Expansion
address@hidden 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.
+
+  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, @strong{avoid side effects on
+objects constructed by macro definitions}.  Here is an example of how
+such side effects can get you into trouble:
+
address@hidden
address@hidden
+(defmacro empty-object ()
+  (list 'quote (cons nil nil)))
address@hidden group
+
address@hidden
+(defun initialize (condition)
+  (let ((object (empty-object)))
+    (if condition
+        (setcar object condition))
+    object))
address@hidden group
address@hidden lisp
+
address@hidden
+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
address@hidden is called.
+
+One way to avoid pathological cases like this is to think of
address@hidden 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.
+
address@hidden Indenting Macros
address@hidden Indenting Macros
+
+  You can use the @code{declare} form in the macro definition to
+specify how to @key{TAB} should indent indent calls to the macro.  You
+write it like this:
+
address@hidden
+(declare (indent @var{indent-spec}))
address@hidden example
+
address@hidden
+Here are the possibilities for @var{indent-spec}:
+
address@hidden @asis
address@hidden @code{nil}
+This is the same as no property---use the standard indentation pattern.
address@hidden @code{defun}
+Handle this function like a @samp{def} construct: treat the second
+line as the start of a @dfn{body}.
address@hidden an integer, @var{number}
+The first @var{number} arguments of the function are
address@hidden arguments; the rest are considered the body
+of the expression.  A line in the expression is indented according to
+whether the first argument on it is distinguished or not.  If the
+argument is part of the body, the line is indented @code{lisp-body-indent}
+more columns than the open-parenthesis starting the containing
+expression.  If the argument is distinguished and is either the first
+or second argument, it is indented @emph{twice} that many extra columns.
+If the argument is distinguished and not the first or second argument,
+the line uses the standard pattern.
address@hidden a symbol, @var{symbol}
address@hidden should be a function name; that function is called to
+calculate the indentation of a line within this expression.  The
+function receives two arguments:
address@hidden @asis
address@hidden @var{state}
+The value returned by @code{parse-partial-sexp} (a Lisp primitive for
+indentation and nesting computation) when it parses up to the
+beginning of this line.
address@hidden @var{pos}
+The position at which the line being indented begins.
address@hidden table
address@hidden
+It should return either a number, which is the number of columns of
+indentation for that line, or a list whose car is such a number.  The
+difference between returning a number and returning a list is that a
+number says that all following lines at the same nesting level should
+be indented just like this one; a list says that following lines might
+call for different indentations.  This makes a difference when the
+indentation is being computed by @kbd{C-M-q}; if the value is a
+number, @kbd{C-M-q} need not recalculate indentation for the following
+lines until the end of the list.
address@hidden table
+
address@hidden
+   arch-tag: d4cce66d-1047-45c3-bfde-db6719d6e82b
address@hidden ignore