+++ /dev/null
-@c -*-texinfo-*-
-@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1998-1999, 2001-2014 Free Software Foundation, Inc.
-@c See the file elisp.texi for copying conditions.
-@node Advising Functions
-@chapter Advising Emacs Lisp Functions
-@cindex advising functions
-
-@cindex piece of advice
- The @dfn{advice} feature lets you add to the existing definition of
-a function, by @dfn{advising the function}. A function can have
-multiple @dfn{pieces of advice}, each of which can be separately
-defined, and separately enabled or disabled (@pxref{Activation of
-Advice}). Each piece of advice can alter almost anything about the
-function, including its argument list, what the function does when it
-runs, and the value it returns.
-
- Advice can be useful for altering the behavior of an existing
-function without having to redefine the whole function. However, it
-can be a source of bugs, since existing callers to the function may
-assume the old behavior, and work incorrectly when the behavior is
-changed by advice. Advice can also cause confusion in debugging, if
-the person doing the debugging does not notice or remember that the
-function has been modified by advice.
-
- For these reasons, advice should be reserved for the cases where you
-cannot modify a function's behavior in any other way. If it is
-possible to do the same thing via a hook, that is preferable
-(@pxref{Hooks}). If you simply want to change what a particular key
-does, it may be better to write a new command, and remap the old
-command's key bindings to the new one (@pxref{Remapping Commands}).
-In particular, Emacs's own source files should not put advice on
-functions in Emacs. (There are currently a few exceptions to this
-convention, but we aim to correct them.)
-
- Macros can also be advised, in much the same way as functions.
-However, special forms (@pxref{Special Forms}) cannot be advised.
-
- It is possible to advise a primitive (@pxref{What Is a Function}),
-but one should typically @emph{not} do so, for two reasons. Firstly,
-some primitives are used by the advice mechanism, and advising them
-could cause an infinite recursion. Secondly, many primitives are
-called directly from C, and such calls ignore advice; hence, one ends
-up in a confusing situation where some calls (occurring from Lisp
-code) obey the advice and other calls (from C code) do not.
-
-@menu
-* Simple Advice:: A simple example to explain the basics of advice.
-* Defining Advice:: Detailed description of @code{defadvice}.
-* Around-Advice:: Wrapping advice around a function's definition.
-* Computed Advice:: ...is to @code{defadvice} as @code{fset} is to @code{defun}.
-* Activation of Advice:: Advice doesn't do anything until you activate it.
-* Enabling Advice:: You can enable or disable each piece of advice.
-* Preactivation:: Preactivation is a way of speeding up the
- loading of compiled advice.
-* Argument Access in Advice:: How advice can access the function's arguments.
-* Combined Definition:: How advice is implemented.
-@end menu
-
-@node Simple Advice
-@section A Simple Advice Example
-
- The command @code{next-line} moves point down vertically one or more
-lines; it is the standard binding of @kbd{C-n}. When used on the last
-line of the buffer, this command inserts a newline to create a line to
-move to if @code{next-line-add-newlines} is non-@code{nil} (its default
-is @code{nil}.)
-
- Suppose you wanted to add a similar feature to @code{previous-line},
-which would insert a new line at the beginning of the buffer for the
-command to move to (when @code{next-line-add-newlines} is
-non-@code{nil}). How could you do this?
-
- You could do it by redefining the whole function, but that is not
-modular. The advice feature provides a cleaner alternative: you can
-effectively add your code to the existing function definition, without
-actually changing or even seeing that definition. Here is how to do
-this:
-
-@example
-(defadvice previous-line (before next-line-at-end
- (&optional arg try-vscroll))
- "Insert an empty line when moving up from the top line."
- (if (and next-line-add-newlines (= arg 1)
- (save-excursion (beginning-of-line) (bobp)))
- (progn
- (beginning-of-line)
- (newline))))
-@end example
-
- This expression defines a @dfn{piece of advice} for the function
-@code{previous-line}. This piece of advice is named
-@code{next-line-at-end}, and the symbol @code{before} says that it is
-@dfn{before-advice} which should run before the regular definition of
-@code{previous-line}. @code{(&optional arg try-vscroll)} specifies
-how the advice code can refer to the function's arguments.
-
- When this piece of advice runs, it creates an additional line, in the
-situation where that is appropriate, but does not move point to that
-line. This is the correct way to write the advice, because the normal
-definition will run afterward and will move back to the newly inserted
-line.
-
- Defining the advice doesn't immediately change the function
-@code{previous-line}. That happens when you @dfn{activate} the advice,
-like this:
-
-@example
-(ad-activate 'previous-line)
-@end example
-
-@noindent
-This is what actually begins to use the advice that has been defined so
-far for the function @code{previous-line}. Henceforth, whenever that
-function is run, whether invoked by the user with @kbd{C-p} or
-@kbd{M-x}, or called from Lisp, it runs the advice first, and its
-regular definition second.
-
- This example illustrates before-advice, which is one @dfn{class} of
-advice: it runs before the function's base definition. There are two
-other advice classes: @dfn{after-advice}, which runs after the base
-definition, and @dfn{around-advice}, which lets you specify an
-expression to wrap around the invocation of the base definition.
-
-@node Defining Advice
-@section Defining Advice
-@cindex defining advice
-@cindex advice, defining
-
- To define a piece of advice, use the macro @code{defadvice}. A call
-to @code{defadvice} has the following syntax, which is based on the
-syntax of @code{defun} and @code{defmacro}, but adds more:
-
-@findex defadvice
-@example
-(defadvice @var{function} (@var{class} @var{name}
- @r{[}@var{position}@r{]} @r{[}@var{arglist}@r{]}
- @var{flags}...)
- @r{[}@var{documentation-string}@r{]}
- @r{[}@var{interactive-form}@r{]}
- @var{body-forms}...)
-@end example
-
-@noindent
-Here, @var{function} is the name of the function (or macro) to be
-advised. From now on, we will write just ``function'' when describing
-the entity being advised, but this always includes macros.
-
- In place of the argument list in an ordinary definition, an advice
-definition calls for several different pieces of information.
-
-@cindex class of advice
-@cindex before-advice
-@cindex after-advice
-@cindex around-advice
-@var{class} specifies the @dfn{class} of the advice---one of @code{before},
-@code{after}, or @code{around}. Before-advice runs before the function
-itself; after-advice runs after the function itself; around-advice is
-wrapped around the execution of the function itself. After-advice and
-around-advice can override the return value by setting
-@code{ad-return-value}.
-
-@defvar ad-return-value
-While advice is executing, after the function's original definition has
-been executed, this variable holds its return value, which will
-ultimately be returned to the caller after finishing all the advice.
-After-advice and around-advice can arrange to return some other value
-by storing it in this variable.
-@end defvar
-
-The argument @var{name} is the name of the advice, a non-@code{nil}
-symbol. The advice name uniquely identifies one piece of advice, within all
-the pieces of advice in a particular class for a particular
-@var{function}. The name allows you to refer to the piece of
-advice---to redefine it, or to enable or disable it.
-
-The optional @var{position} specifies where, in the current list of
-advice of the specified @var{class}, this new advice should be placed.
-It should be either @code{first}, @code{last} or a number that specifies
-a zero-based position (@code{first} is equivalent to 0). If no position
-is specified, the default is @code{first}. Position values outside the
-range of existing positions in this class are mapped to the beginning or
-the end of the range, whichever is closer. The @var{position} value is
-ignored when redefining an existing piece of advice.
-
-The optional @var{arglist} can be used to define the argument list for
-the sake of advice. This becomes the argument list of the combined
-definition that is generated in order to run the advice (@pxref{Combined
-Definition}). Therefore, the advice expressions can use the argument
-variables in this list to access argument values.
-
-The argument list used in advice need not be the same as the argument
-list used in the original function, but must be compatible with it, so
-that it can handle the ways the function is actually called. If two
-pieces of advice for a function both specify an argument list, they must
-specify the same argument list.
-
-@xref{Argument Access in Advice}, for more information about argument
-lists and advice, and a more flexible way for advice to access the
-arguments.
-
-The remaining elements, @var{flags}, are symbols that specify further
-information about how to use this piece of advice. Here are the valid
-symbols and their meanings:
-
-@table @code
-@item activate
-Activate the advice for @var{function} now. Changes in a function's
-advice always take effect the next time you activate advice for the
-function; this flag says to do so, for @var{function}, immediately after
-defining this piece of advice.
-
-@cindex forward advice
-This flag has no immediate effect if @var{function} itself is not defined yet (a
-situation known as @dfn{forward advice}), because it is impossible to
-activate an undefined function's advice. However, defining
-@var{function} will automatically activate its advice.
-
-@item protect
-Protect this piece of advice against non-local exits and errors in
-preceding code and advice. Protecting advice places it as a cleanup in
-an @code{unwind-protect} form, so that it will execute even if the
-previous code gets an error or uses @code{throw}. @xref{Cleanups}.
-
-@item compile
-Compile the combined definition that is used to run the advice. This
-flag is ignored unless @code{activate} is also specified.
-@xref{Combined Definition}.
-
-@item disable
-Initially disable this piece of advice, so that it will not be used
-unless subsequently explicitly enabled. @xref{Enabling Advice}.
-
-@item preactivate
-Activate advice for @var{function} when this @code{defadvice} is
-compiled or macroexpanded. This generates a compiled advised definition
-according to the current advice state, which will be used during
-activation if appropriate. @xref{Preactivation}.
-
-This is useful only if this @code{defadvice} is byte-compiled.
-@end table
-
-The optional @var{documentation-string} serves to document this piece of
-advice. When advice is active for @var{function}, the documentation for
-@var{function} (as returned by @code{documentation}) combines the
-documentation strings of all the advice for @var{function} with the
-documentation string of its original function definition.
-
-The optional @var{interactive-form} form can be supplied to change the
-interactive behavior of the original function. If more than one piece
-of advice has an @var{interactive-form}, then the first one (the one
-with the smallest position) found among all the advice takes precedence.
-
-The possibly empty list of @var{body-forms} specifies the body of the
-advice. The body of an advice can access or change the arguments, the
-return value, the binding environment, and perform any other kind of
-side effect.
-
-@strong{Warning:} When you advise a macro, keep in mind that macros are
-expanded when a program is compiled, not when a compiled program is run.
-All subroutines used by the advice need to be available when the byte
-compiler expands the macro.
-
-@deffn Command ad-unadvise function
-This command deletes all pieces of advice from @var{function}.
-@end deffn
-
-@deffn Command ad-unadvise-all
-This command deletes all pieces of advice from all functions.
-@end deffn
-
-@node Around-Advice
-@section Around-Advice
-
- Around-advice lets you ``wrap'' a Lisp expression ``around'' the
-original function definition. You specify where the original function
-definition should go by means of the special symbol @code{ad-do-it}.
-Where this symbol occurs inside the around-advice body, it is replaced
-with a @code{progn} containing the forms of the surrounded code. Here
-is an example:
-
-@example
-(defadvice foo (around foo-around)
- "Ignore case in `foo'."
- (let ((case-fold-search t))
- ad-do-it))
-@end example
-
-@noindent
-Its effect is to make sure that case is ignored in
-searches when the original definition of @code{foo} is run.
-
-@defvar ad-do-it
-This is not really a variable, rather a place-holder that looks like a
-variable. You use it in around-advice to specify the place to run the
-function's original definition and other ``earlier'' around-advice.
-@end defvar
-
-If the around-advice does not use @code{ad-do-it}, then it does not run
-the original function definition. This provides a way to override the
-original definition completely. (It also overrides lower-positioned
-pieces of around-advice).
-
-If the around-advice uses @code{ad-do-it} more than once, the original
-definition is run at each place. In this way, around-advice can execute
-the original definition (and lower-positioned pieces of around-advice)
-several times. Another way to do that is by using @code{ad-do-it}
-inside of a loop.
-
-@node Computed Advice
-@section Computed Advice
-
-The macro @code{defadvice} resembles @code{defun} in that the code for
-the advice, and all other information about it, are explicitly stated in
-the source code. You can also create advice whose details are computed,
-using the function @code{ad-add-advice}.
-
-@defun ad-add-advice function advice class position
-Calling @code{ad-add-advice} adds @var{advice} as a piece of advice to
-@var{function} in class @var{class}. The argument @var{advice} has
-this form:
-
-@example
-(@var{name} @var{protected} @var{enabled} @var{definition})
-@end example
-
-@noindent
-Here, @var{protected} and @var{enabled} are flags; if @var{protected}
-is non-@code{nil}, the advice is protected against non-local exits
-(@pxref{Defining Advice}), and if @var{enabled} is @code{nil} the
-advice is initially disabled (@pxref{Enabling Advice}).
-@var{definition} should have the form
-
-@example
-(advice . @var{lambda})
-@end example
-
-@noindent
-where @var{lambda} is a lambda expression; this lambda expression is
-called in order to perform the advice. @xref{Lambda Expressions}.
-
-If the @var{function} argument to @code{ad-add-advice} already has one
-or more pieces of advice in the specified @var{class}, then
-@var{position} specifies where in the list to put the new piece of
-advice. The value of @var{position} can either be @code{first},
-@code{last}, or a number (counting from 0 at the beginning of the
-list). Numbers outside the range are mapped to the beginning or the
-end of the range, whichever is closer. The @var{position} value is
-ignored when redefining an existing piece of advice.
-
-If @var{function} already has a piece of @var{advice} with the same
-name, then the position argument is ignored and the old advice is
-replaced with the new one.
-@end defun
-
-@node Activation of Advice
-@section Activation of Advice
-@cindex activating advice
-@cindex advice, activating
-
-By default, advice does not take effect when you define it---only when
-you @dfn{activate} advice for the function. However, the advice will
-be activated automatically if you define or redefine the function
-later. You can request the activation of advice for a function when
-you define the advice, by specifying the @code{activate} flag in the
-@code{defadvice}; or you can activate the advice separately by calling
-the function @code{ad-activate} or one of the other activation
-commands listed below.
-
-Separating the activation of advice from the act of defining it permits
-you to add several pieces of advice to one function efficiently, without
-redefining the function over and over as each advice is added. More
-importantly, it permits defining advice for a function before that
-function is actually defined.
-
-When a function's advice is first activated, the function's original
-definition is saved, and all enabled pieces of advice for that function
-are combined with the original definition to make a new definition.
-(Pieces of advice that are currently disabled are not used; see
-@ref{Enabling Advice}.) This definition is installed, and optionally
-byte-compiled as well, depending on conditions described below.
-
-In all of the commands to activate advice, if @var{compile} is
-@code{t} (or anything but @code{nil} or a negative number), the
-command also compiles the combined definition which implements the
-advice. If it is @code{nil} or a negative number, what happens
-depends on @code{ad-default-compilation-action} as described below.
-
-@deffn Command ad-activate function &optional compile
-This command activates all the advice defined for @var{function}.
-@end deffn
-
- Activating advice does nothing if @var{function}'s advice is already
-active. But if there is new advice, added since the previous time you
-activated advice for @var{function}, it activates the new advice.
-
-@deffn Command ad-deactivate function
-This command deactivates the advice for @var{function}.
-@cindex deactivating advice
-@c @cindex advice, deactivating "advice, activating" is just above
-@end deffn
-
-@deffn Command ad-update function &optional compile
-This command activates the advice for @var{function}
-if its advice is already activated. This is useful
-if you change the advice.
-@end deffn
-
-@deffn Command ad-activate-all &optional compile
-This command activates the advice for all functions.
-@end deffn
-
-@deffn Command ad-deactivate-all
-This command deactivates the advice for all functions.
-@end deffn
-
-@deffn Command ad-update-all &optional compile
-This command activates the advice for all functions
-whose advice is already activated. This is useful
-if you change the advice of some functions.
-@end deffn
-
-@deffn Command ad-activate-regexp regexp &optional compile
-This command activates all pieces of advice whose names match
-@var{regexp}. More precisely, it activates all advice for any function
-which has at least one piece of advice that matches @var{regexp}.
-@end deffn
-
-@deffn Command ad-deactivate-regexp regexp
-This command deactivates all pieces of advice whose names match
-@var{regexp}. More precisely, it deactivates all advice for any
-function which has at least one piece of advice that matches
-@var{regexp}.
-@end deffn
-
-@deffn Command ad-update-regexp regexp &optional compile
-This command activates pieces of advice whose names match @var{regexp},
-but only those for functions whose advice is already activated.
-@cindex reactivating advice
-
-Reactivating a function's advice is useful for putting into effect all
-the changes that have been made in its advice (including enabling and
-disabling specific pieces of advice; @pxref{Enabling Advice}) since the
-last time it was activated.
-@end deffn
-
-@deffn Command ad-start-advice
-Turn on automatic advice activation when a function is defined or
-redefined. This is the default mode.
-@end deffn
-
-@deffn Command ad-stop-advice
-Turn off automatic advice activation when a function is defined or
-redefined.
-@end deffn
-
-@defopt ad-default-compilation-action
-This variable controls whether to compile the combined definition
-that results from activating advice for a function.
-
-A value of @code{always} specifies to compile unconditionally.
-A value of @code{never} specifies never compile the advice.
-
-A value of @code{maybe} specifies to compile if the byte compiler is
-already loaded. A value of @code{like-original} specifies to compile
-the advice if the original definition of the advised function is
-compiled or a built-in function.
-
-This variable takes effect only if the @var{compile} argument of
-@code{ad-activate} (or any of the above functions) did not force
-compilation.
-@end defopt
-
- If the advised definition was constructed during ``preactivation''
-(@pxref{Preactivation}), then that definition must already be compiled,
-because it was constructed during byte-compilation of the file that
-contained the @code{defadvice} with the @code{preactivate} flag.
-
-@node Enabling Advice
-@section Enabling and Disabling Advice
-@cindex enabling advice
-@cindex advice, enabling and disabling
-@cindex disabling advice
-
- Each piece of advice has a flag that says whether it is enabled or
-not. By enabling or disabling a piece of advice, you can turn it on
-and off without having to undefine and redefine it. For example, here is
-how to disable a particular piece of advice named @code{my-advice} for
-the function @code{foo}:
-
-@example
-(ad-disable-advice 'foo 'before 'my-advice)
-@end example
-
- This function by itself only changes the enable flag for a piece of
-advice. To make the change take effect in the advised definition, you
-must activate the advice for @code{foo} again:
-
-@example
-(ad-activate 'foo)
-@end example
-
-@deffn Command ad-disable-advice function class name
-This command disables the piece of advice named @var{name} in class
-@var{class} on @var{function}.
-@end deffn
-
-@deffn Command ad-enable-advice function class name
-This command enables the piece of advice named @var{name} in class
-@var{class} on @var{function}.
-@end deffn
-
- You can also disable many pieces of advice at once, for various
-functions, using a regular expression. As always, the changes take real
-effect only when you next reactivate advice for the functions in
-question.
-
-@deffn Command ad-disable-regexp regexp
-This command disables all pieces of advice whose names match
-@var{regexp}, in all classes, on all functions.
-@end deffn
-
-@deffn Command ad-enable-regexp regexp
-This command enables all pieces of advice whose names match
-@var{regexp}, in all classes, on all functions.
-@end deffn
-
-@node Preactivation
-@section Preactivation
-@cindex preactivating advice
-@cindex advice, preactivating
-
- Constructing a combined definition to execute advice is moderately
-expensive. When a library advises many functions, this can make loading
-the library slow. In that case, you can use @dfn{preactivation} to
-construct suitable combined definitions in advance.
-
- To use preactivation, specify the @code{preactivate} flag when you
-define the advice with @code{defadvice}. This @code{defadvice} call
-creates a combined definition which embodies this piece of advice
-(whether enabled or not) plus any other currently enabled advice for the
-same function, and the function's own definition. If the
-@code{defadvice} is compiled, that compiles the combined definition
-also.
-
- When the function's advice is subsequently activated, if the enabled
-advice for the function matches what was used to make this combined
-definition, then the existing combined definition is used, thus avoiding
-the need to construct one. Thus, preactivation never causes wrong
-results---but it may fail to do any good, if the enabled advice at the
-time of activation doesn't match what was used for preactivation.
-
- Here are some symptoms that can indicate that a preactivation did not
-work properly, because of a mismatch.
-
-@itemize @bullet
-@item
-Activation of the advised
-function takes longer than usual.
-@item
-The byte compiler gets
-loaded while an advised function gets activated.
-@item
-@code{byte-compile} is included in the value of @code{features} even
-though you did not ever explicitly use the byte compiler.
-@end itemize
-
-Compiled preactivated advice works properly even if the function itself
-is not defined until later; however, the function needs to be defined
-when you @emph{compile} the preactivated advice.
-
-There is no elegant way to find out why preactivated advice is not being
-used. What you can do is to trace the function
-@code{ad-cache-id-verification-code} (with the function
-@code{trace-function-background}) before the advised function's advice
-is activated. After activation, check the value returned by
-@code{ad-cache-id-verification-code} for that function: @code{verified}
-means that the preactivated advice was used, while other values give
-some information about why they were considered inappropriate.
-
- @strong{Warning:} There is one known case that can make preactivation
-fail, in that a preconstructed combined definition is used even though
-it fails to match the current state of advice. This can happen when two
-packages define different pieces of advice with the same name, in the
-same class, for the same function. But you should avoid that anyway.
-
-@node Argument Access in Advice
-@section Argument Access in Advice
-
- The simplest way to access the arguments of an advised function in the
-body of a piece of advice is to use the same names that the function
-definition uses. To do this, you need to know the names of the argument
-variables of the original function.
-
- While this simple method is sufficient in many cases, it has a
-disadvantage: it is not robust, because it hard-codes the argument names
-into the advice. If the definition of the original function changes,
-the advice might break.
-
- Another method is to specify an argument list in the advice itself.
-This avoids the need to know the original function definition's argument
-names, but it has a limitation: all the advice on any particular
-function must use the same argument list, because the argument list
-actually used for all the advice comes from the first piece of advice
-for that function.
-
- A more robust method is to use macros that are translated into the
-proper access forms at activation time, i.e., when constructing the
-advised definition. Access macros access actual arguments by their
-(zero-based) position, regardless of how these actual arguments get
-distributed onto the argument variables of a function. This is robust
-because in Emacs Lisp the meaning of an argument is strictly
-determined by its position in the argument list.
-
-@defmac ad-get-arg position
-This returns the actual argument that was supplied at @var{position}.
-@end defmac
-
-@defmac ad-get-args position
-This returns the list of actual arguments supplied starting at
-@var{position}.
-@end defmac
-
-@defmac ad-set-arg position value
-This sets the value of the actual argument at @var{position} to
-@var{value}
-@end defmac
-
-@defmac ad-set-args position value-list
-This sets the list of actual arguments starting at @var{position} to
-@var{value-list}.
-@end defmac
-
- Now an example. Suppose the function @code{foo} is defined as
-
-@example
-(defun foo (x y &optional z &rest r) ...)
-@end example
-
-@noindent
-and is then called with
-
-@example
-(foo 0 1 2 3 4 5 6)
-@end example
-
-@noindent
-which means that @var{x} is 0, @var{y} is 1, @var{z} is 2 and @var{r} is
-@code{(3 4 5 6)} within the body of @code{foo}. Here is what
-@code{ad-get-arg} and @code{ad-get-args} return in this case:
-
-@example
-(ad-get-arg 0) @result{} 0
-(ad-get-arg 1) @result{} 1
-(ad-get-arg 2) @result{} 2
-(ad-get-arg 3) @result{} 3
-(ad-get-args 2) @result{} (2 3 4 5 6)
-(ad-get-args 4) @result{} (4 5 6)
-@end example
-
- Setting arguments also makes sense in this example:
-
-@example
-(ad-set-arg 5 "five")
-@end example
-
-@noindent
-has the effect of changing the sixth argument to @code{"five"}. If this
-happens in advice executed before the body of @code{foo} is run, then
-@var{r} will be @code{(3 4 "five" 6)} within that body.
-
- Here is an example of setting a tail of the argument list:
-
-@example
-(ad-set-args 0 '(5 4 3 2 1 0))
-@end example
-
-@noindent
-If this happens in advice executed before the body of @code{foo} is run,
-then within that body, @var{x} will be 5, @var{y} will be 4, @var{z}
-will be 3, and @var{r} will be @code{(2 1 0)} inside the body of
-@code{foo}.
-
- These argument constructs are not really implemented as Lisp macros.
-Instead they are implemented specially by the advice mechanism.
-
-@node Combined Definition
-@section The Combined Definition
-
- Suppose that a function has @var{n} pieces of before-advice
-(numbered from 0 through @var{n}@minus{}1), @var{m} pieces of
-around-advice and @var{k} pieces of after-advice. Assuming no piece
-of advice is protected, the combined definition produced to implement
-the advice for a function looks like this:
-
-@example
-(lambda @var{arglist}
- @r{[} @r{[}@var{advised-docstring}@r{]} @r{[}(interactive ...)@r{]} @r{]}
- (let (ad-return-value)
- @r{before-0-body-form}...
- ....
- @r{before-@var{n}@minus{}1-body-form}...
- @r{around-0-body-form}...
- @r{around-1-body-form}...
- ....
- @r{around-@var{m}@minus{}1-body-form}...
- (setq ad-return-value
- @r{apply original definition to @var{arglist}})
- @r{end-of-around-@var{m}@minus{}1-body-form}...
- ....
- @r{end-of-around-1-body-form}...
- @r{end-of-around-0-body-form}...
- @r{after-0-body-form}...
- ....
- @r{after-@var{k}@minus{}1-body-form}...
- ad-return-value))
-@end example
-
-Macros are redefined as macros, which means adding @code{macro} to
-the beginning of the combined definition.
-
-The interactive form is present if the original function or some piece
-of advice specifies one. When an interactive primitive function is
-advised, advice uses a special method: it calls the primitive with
-@code{call-interactively} so that it will read its own arguments.
-In this case, the advice cannot access the arguments.
-
-The body forms of the various advice in each class are assembled
-according to their specified order. The forms of around-advice @var{l}
-are included in one of the forms of around-advice @var{l} @minus{} 1.
-
-The innermost part of the around advice onion is
-
-@display
-apply original definition to @var{arglist}
-@end display
-
-@noindent
-whose form depends on the type of the original function. The variable
-@code{ad-return-value} is set to whatever this returns. The variable is
-visible to all pieces of advice, which can access and modify it before
-it is actually returned from the advised function.
-
-The semantic structure of advised functions that contain protected
-pieces of advice is the same. The only difference is that
-@code{unwind-protect} forms ensure that the protected advice gets
-executed even if some previous piece of advice had an error or a
-non-local exit. If any around-advice is protected, then the whole
-around-advice onion is protected as a result.
* Function Cells:: Accessing or setting the function definition
of a symbol.
* Closures:: Functions that enclose a lexical environment.
+* Advising Functions:: Adding to the definition of a function.
* Obsolete Functions:: Declaring functions obsolete.
* Inline Functions:: Functions that the compiler will expand inline.
* Declare Form:: Adding additional information about a function.
this is not checked. The argument @var{symbol} is an ordinary evaluated
argument.
-The primary use of this function is as a subroutine by constructs that
-define or alter functions, like @code{defadvice} (@pxref{Advising
-Functions}). (If @code{defun} were not a primitive, it could be
-written as a Lisp macro using @code{fset}.) You can also use it to
-give a symbol a function definition that is not a list, e.g., a
-keyboard macro (@pxref{Keyboard Macros}):
+The primary use of this function is as a subroutine by constructs that define
+or alter functions, like @code{defun} or @code{advice-add} (@pxref{Advising
+Functions}). You can also use it to give a symbol a function definition that
+is not a function, e.g., a keyboard macro (@pxref{Keyboard Macros}):
@example
;; @r{Define a named keyboard macro.}
implementation detail. For this reason, we recommend against directly
examining or altering the structure of closure objects.
+@node Advising Functions
+@section Advising Emacs Lisp Functions
+@cindex advising functions
+@cindex piece of advice
+
+Any variable or object field which holds a function can be modified with the
+appropriate setter function, such as @code{set-process-filter}, @code{fset}, or
+@code{setq}, but those can be too blunt, completely throwing away the
+previous value.
+
+In order to modify such hooks in a more controlled way, Emacs provides the
+macros @code{add-function} and @code{remove-function}, which let you modify the
+existing function value by composing it with another function.
+
+For example, in order to trace the calls to a process filter, you can use:
+
+@example
+(add-function :before (process-filter proc) #'my-tracing-function)
+@end example
+
+This will cause the process's output to be passed first to
+@code{my-tracing-function} and then to the original process filter.
+When you're done with it, you can revert to the untraced behavior with:
+
+@example
+(remove-function (process-filter proc) #'my-tracing-function)
+@end example
+
+The argument @code{:before} specifies how the two functions are composed, since
+there are many different ways to do it. The added function is also called an
+@emph{advice}.
+
+The function cell of a symbol can be manipulated similarly, but since it can
+contain other things than a plain function, you have to use @var{advice-add}
+and @var{advice-remove} instead, which
+@c use @var{add-function} and @var{remove-function} internally, but
+know how to handle cases such as when the function cell holds a macro rather
+than function, or when the function is autoloaded so the advice's activation
+needs to be postponed.
+
+@menu
+* Advising Primitives:: Primitives to Manipulate Advices
+* Advising Named Functions:: Advising Named Functions
+@end menu
+
+@node Advising Primitives
+@subsection Primitives to manipulate advice
+
+@defmac add-function where place function &optional props
+This macro is the handy way to add the advice @var{function} to the function
+stored in @var{place} (@pxref{Generalized Variables}).
+
+@var{where} determines how @var{function} is composed with the
+existing function. It can be one of the following:
+
+@table @code
+@item :before
+Call @var{function} before the old function. Both functions receive the
+same arguments, and the return value of the composition is the return value of
+the old function. More specifically, the composition of the two functions
+behaves like:
+@example
+(lambda (&rest r) (apply @var{function} r) (apply @var{oldfun} r))
+@end example
+This is similar to @code{(add-hook @var{hook} @var{function})}, except that it
+applies to single-function hooks rather than normal hooks.
+
+@item :after
+Call @var{function} after the old function. Both functions receive the
+same arguments, and the return value of the composition is the return value of
+the old function. More specifically, the composition of the two functions
+behaves like:
+@example
+(lambda (&rest r) (prog1 (apply @var{oldfun} r) (apply @var{function} r)))
+@end example
+This is similar to @code{(add-hook @var{hook} @var{function} nil 'append)},
+except that it applies to single-function hooks rather than normal hooks.
+
+@item :override
+This completely replaces the old function with the new one. The old function
+can of course be recovered if you later call @code{remove-function}.
+
+@item :around
+Call @var{function} instead of the old function, but provide the old function
+as an extra argument to @var{function}. This is the most flexible composition.
+For example, it lets you call the old function with different arguments, or
+within a let-binding, or you can sometimes delegate the work to the old
+function and sometimes override it completely. More specifically, the
+composition of the two functions behaves like:
+@example
+(lambda (&rest r) (apply @var{function} @var{oldfun} r))
+@end example
+
+@item :before-while
+Call @var{function} before the old function and don't call the old
+function if @var{function} returns @code{nil}. Both functions receive the
+same arguments, and the return value of the composition is the return value of
+the old function. More specifically, the composition of the two functions
+behaves like:
+@example
+(lambda (&rest r) (and (apply @var{function} r) (apply @var{oldfun} r)))
+@end example
+This is reminiscent of @code{(add-hook @var{hook} @var{function})}, when
+@var{hook} is run via @code{run-hook-with-args-until-failure}.
+
+@item :before-until
+Call @var{function} before the old function and only call the old function if
+@var{function} returns @code{nil}. More specifically, the composition of the
+two functions behaves like:
+@example
+(lambda (&rest r) (or (apply @var{function} r) (apply @var{oldfun} r)))
+@end example
+This is reminiscent of @code{(add-hook @var{hook} @var{function})}, when
+@var{hook} is run via @code{run-hook-with-args-until-success}.
+
+@item :after-while
+Call @var{function} after the old function and only if the old function
+returned non-@code{nil}. Both functions receive the same arguments, and the
+return value of the composition is the return value of @var{function}.
+More specifically, the composition of the two functions behaves like:
+@example
+(lambda (&rest r) (and (apply @var{oldfun} r) (apply @var{function} r)))
+@end example
+This is reminiscent of @code{(add-hook @var{hook} @var{function} nil 'append)},
+when @var{hook} is run via @code{run-hook-with-args-until-failure}.
+
+@item :after-until
+Call @var{function} after the old function and only if the old function
+returned @code{nil}. More specifically, the composition of the two functions
+behaves like:
+@example
+(lambda (&rest r) (or (apply @var{oldfun} r) (apply @var{function} r)))
+@end example
+This is reminiscent of @code{(add-hook @var{hook} @var{function} nil 'append)},
+when @var{hook} is run via @code{run-hook-with-args-until-success}.
+
+@item :filter-args
+Call @var{function} first and use the result (which should be a list) as the
+new arguments to pass to the old function. More specifically, the composition
+of the two functions behaves like:
+@example
+(lambda (&rest r) (apply @var{oldfun} (funcall @var{function} r)))
+@end example
+
+@item :filter-return
+Call the old function first and pass the result to @var{function}.
+More specifically, the composition of the two functions behaves like:
+@example
+(lambda (&rest r) (funcall @var{function} (apply @var{oldfun} r)))
+@end example
+@end table
+
+When modifying a variable (whose name will usually end with @code{-function}),
+you can choose whether @var{function} is used globally or only in the current
+buffer: if @var{place} is just a symbol, then @var{function} is added to the
+global value of @var{place}. Whereas if @var{place} is of the form
+@code{(local @var{symbol})}, where @var{symbol} is an expression which returns
+the variable name, then @var{function} will only be added in the
+current buffer.
+
+Every function added with @code{add-function} can be accompanied by an
+association list of properties @var{props}. Currently only two of those
+properties have a special meaning:
+
+@table @code
+@item name
+This gives a name to the advice, which @code{remove-function} can use to
+identify which function to remove. Typically used when @var{function} is an
+anonymous function.
+
+@item depth
+This specifies where to place the advice, in case several advices are present.
+By default, the depth is 0. A depth of 100 indicates that this advice should
+be kept as deep as possible, whereas a depth of -100 indicates that it
+should stay as the outermost advice. When two advices specify the same depth,
+the most recently added advice will be outermost.
+@end table
+@end defmac
+
+@defmac remove-function place function
+This macro removes @var{function} from the function stored in
+@var{place}. This only works if @var{function} was added to @var{place}
+using @code{add-function}.
+
+@var{function} is compared with functions added to @var{place} using
+@code{equal}, to try and make it work also with lambda expressions. It is
+additionally compared also with the @code{name} property of the functions added
+to @var{place}, which can be more reliable than comparing lambda expressions
+using @code{equal}.
+@end defmac
+
+@defun advice-function-member-p advice function-def
+Return non-@code{nil} if @var{advice} is already in @var{function-def}.
+Like for @code{remove-function} above, instead of @var{advice} being the actual
+function, it can also be the @code{name} of the piece of advice.
+@end defun
+
+@defun advice-function-mapc f function-def
+Call the function @var{f} for every advice that was added to
+@var{function-def}. @var{f} is called with two arguments: the advice function
+and its properties.
+@end defun
+
+@node Advising Named Functions
+@subsection Advising Named Functions
+
+A common use of advice is for named functions and macros.
+Since @var{add-function} does not know how to deal with macros and autoloaded
+functions, Emacs provides a separate set of functions to manipulate pieces of
+advice applied to named functions.
+
+ Advice can be useful for altering the behavior of an existing
+function without having to redefine the whole function. However, it
+can be a source of bugs, since existing callers to the function may
+assume the old behavior, and work incorrectly when the behavior is
+changed by advice. Advice can also cause confusion in debugging, if
+the person doing the debugging does not notice or remember that the
+function has been modified by advice.
+
+ For these reasons, advice should be reserved for the cases where you
+cannot modify a function's behavior in any other way. If it is
+possible to do the same thing via a hook, that is preferable
+(@pxref{Hooks}). If you simply want to change what a particular key
+does, it may be better to write a new command, and remap the old
+command's key bindings to the new one (@pxref{Remapping Commands}).
+In particular, Emacs's own source files should not put advice on
+functions in Emacs. (There are currently a few exceptions to this
+convention, but we aim to correct them.)
+
+ Macros can also be advised, in much the same way as functions.
+However, special forms (@pxref{Special Forms}) cannot be advised.
+
+ It is possible to advise a primitive (@pxref{What Is a Function}),
+but one should typically @emph{not} do so, for two reasons. Firstly,
+some primitives are used by the advice mechanism, and advising them
+could cause an infinite recursion. Secondly, many primitives are
+called directly from C, and such calls ignore advice; hence, one ends
+up in a confusing situation where some calls (occurring from Lisp
+code) obey the advice and other calls (from C code) do not.
+
+@defun advice-add symbol where function &optional props
+Add the advice @var{function} to the named function @var{symbol}.
+@var{where} and @var{props} have the same meaning as for @code{add-function}
+(@pxref{Advising Primitives}).
+@end defun
+
+@defun advice-remove symbol function
+Remove the advice @var{function} from the named function @var{symbol}.
+@var{function} can also be the @code{name} of an advice.
+@end defun
+
+@defun advice-member-p function symbol
+Return non-@code{nil} if the advice @var{function} is already in the named
+function @var{symbol}. @var{function} can also be the @code{name} of
+an advice.
+@end defun
+
+@defun advice-mapc function symbol
+Call @var{function} for every advice that was added to the named function
+@var{symbol}. @var{function} is called with two arguments: the advice function
+and its properties.
+@end defun
+
@node Obsolete Functions
@section Declaring Functions Obsolete
@cindex obsolete functions
+2014-01-10 Stefan Monnier <monnier@iro.umontreal.ca>
+
+ * cl.texi (Function Bindings): Fix incorrect description of cl-let.
+
2014-01-09 Rüdiger Sonderfeld <ruediger@c-plusplus.de>
* Makefile.in: Add eww.texi.
(Advanced configuration)
(Header arguments in Org mode properties): Spelling fixes.
(Special blocks): Add #+BEGIN_ABSTRACT as another example.
- (@LaTeX{} specific attributes): New index entries. Use
- #+BEGIN_ABSTRACT in the example.
+ (@LaTeX{} specific attributes): New index entries.
+ Use #+BEGIN_ABSTRACT in the example.
2013-01-07 Nicolas Goaziou <n.goaziou@gmail.com>
2014-01-02 Aidan Gauland <aidalgol@amuri.net>
- * eshell.texi (Command Basics): Removed `Command basics' chapter.
+ * eshell.texi (Command Basics): Remove `Command basics' chapter.
2014-01-02 Aidan Gauland <aidalgol@amuri.net>
* org.texi (Orgstruct mode): Fix suggested setting of
`orgstruct-heading-prefix-regexp'.
- * org.texi (Export settings): Document
- `org-export-allow-bind-keywords'.
+ * org.texi (Export settings):
+ Document `org-export-allow-bind-keywords'.
* org.texi (History and Acknowledgments): Small rephrasing.
in a year datetree.
* org.texi (Beamer export, @LaTeX{} and PDF export)
- (Header and sectioning, @LaTeX{} specific attributes): Enhance
- style.
+ (Header and sectioning, @LaTeX{} specific attributes):
+ Enhance style.
* org.texi (Agenda commands): Add a footnote about dragging agenda
lines: it does not persist and it does not change the .org files.
* org.texi (Other built-in back-ends): New section.
- * org.texi (Editing source code): Document
- `org-edit-src-auto-save-idle-delay' and
+ * org.texi (Editing source code):
+ Document `org-edit-src-auto-save-idle-delay' and
`org-edit-src-turn-on-auto-save'.
* org.texi (External links): Document contributed link types
separately.
- * org.texi (Closing items): Document
- `org-closed-keep-when-no-todo'.
+ * org.texi (Closing items):
+ Document `org-closed-keep-when-no-todo'.
* org.texi (Export back-ends): Rename from "Export formats".
(The Export Dispatcher): Remove reference to
(Agenda commands): Move details about filtering commands to
the new section, only include a summary here.
(Customizing tables in ODT export)
- (System-wide header arguments, Conflicts, Dynamic blocks): Use
- spaces for indentation.
+ (System-wide header arguments, Conflicts, Dynamic blocks):
+ Use spaces for indentation.
* org.texi (Emphasis and monospace): Mention `org-emphasis-alist'.
(In-buffer settings): Update to reflect changes from the new
export engine.
- * org.texi (Matching tags and properties): More examples. Explain
- group tags expansion as regular expressions.
+ * org.texi (Matching tags and properties): More examples.
+ Explain group tags expansion as regular expressions.
* org.texi (Tag groups): New section.
* org.texi (Org syntax): New section.
- * org.texi (Orgstruct mode): Document
- `orgstruct-heading-prefix-regexp'.
+ * org.texi (Orgstruct mode):
+ Document `orgstruct-heading-prefix-regexp'.
* org.texi (Speeding up your agendas): New section.
* org.texi: Update the list contributions.
* org.texi (Agenda commands): Exporting the agenda to an .org file
- will not copy the subtrees and the inherited tags. Document
- `org-agenda-filter-by-regexp'.
+ will not copy the subtrees and the inherited tags.
+ Document `org-agenda-filter-by-regexp'.
* org.texi (Publishing action, Complex example): Fix names of
publishing functions.
* org.texi (Capture): Mention that org-remember.el is not
supported anymore.
- * org.texi (Top, Exporting, Beamer class export): Delete
- references to the TaskJuggler export.
+ * org.texi (Top, Exporting, Beamer class export):
+ Delete references to the TaskJuggler export.
(History and Acknowledgments): Mention that the TaskJuggler has
been rewritten by Nicolas and now lives in the contrib/ directory
of Org's distribution. Mention that Jambunathan rewrote the HTML
(@LaTeX{} and PDF export, Header and sectioning)
(Publishing options): Fix LaTeX options names.
- * org.texi (Export options, CSS support, In-buffer settings): Fix
- references to HTML_LINK_* and HTML_STYLE keywords.
+ * org.texi (Export options, CSS support, In-buffer settings):
+ Fix references to HTML_LINK_* and HTML_STYLE keywords.
* org.texi (Export options, In-buffer settings): Fix references to
#+SELECT_TAGS and #+EXCLUDE_TAGS and remove reference to #+XSLT.
* org.texi (Top, Markup, Initial text, Images and tables)
(@LaTeX{} fragments, @LaTeX{} fragments, Exporting)
- (Export options, JavaScript support, Beamer class export): Remove
- references to the DocBook export, which has been deleted.
+ (Export options, JavaScript support, Beamer class export):
+ Remove references to the DocBook export, which has been deleted.
(History and Acknowledgments): Mention that DocBook has been
deleted, suggest to use the Texinfo exporter instead, then to
convert the .texi to DocBook with makeinfo.
* org.texi (Deadlines and scheduling): Add a variable to the
index. Add documentation about delays for scheduled tasks.
- * org.texi (Emphasis and monospace): Mention
- `org-fontify-emphasized-text' and
+ * org.texi (Emphasis and monospace):
+ Mention `org-fontify-emphasized-text' and
`org-emphasis-regexp-components'.
* org.texi (References): Small enhancement.
* org.texi (Extracting source code): Mention the prefix argument
to org-babel-tangle.
- (noweb): Removed erroneous negative.
+ (noweb): Remove erroneous negative.
(Specific header arguments): Document new header arguments.
Documentation for new tangle-mode header argument.
(Top): Documentation for new tangle-mode header argument.
* org.texi (Header and sectioning): Add a footnote about the
different between LATEX_HEADER_EXTRA and LATEX_HEADER.
- * org.texi (The Export Dispatcher): Document
- `org-export-in-background'.
+ * org.texi (The Export Dispatcher):
+ Document `org-export-in-background'.
* org.texi (Footnotes): Export back-ends do not use
`org-footnote-normalize' anymore.
* org.texi (Include files): Remove reference to :prefix1
and :prefix. Give more details for :minlevel.
- * org.texi (Macro replacement): Fix macro name. Update
- documentation about possible locations and escaping mechanism.
+ * org.texi (Macro replacement): Fix macro name.
+ Update documentation about possible locations and escaping mechanism.
- * org.texi (Table of contents): Update documentation. Document
- lists of listings and lists of tables. Add documentation for
+ * org.texi (Table of contents): Update documentation.
+ Document lists of listings and lists of tables. Add documentation for
optional title and #+TOC: keyword.
2013-11-12 Rick Frankel <rick@rickster.com>
* org.texi (results): Add Format section, broken out of Type
section to match code.
- (hlines, colnames): Remove incorrect Emacs Lisp exception. Note
- that the actual default handling (at least for python and
+ (hlines, colnames): Remove incorrect Emacs Lisp exception.
+ Note that the actual default handling (at least for python and
emacs-lisp) does not seem to match the description.
2013-11-12 Sacha Chua <sacha@sachachua.com> (tiny change)
2013-11-12 Yasushi Shoji <yashi@atmark-techno.com>
- * org.texi (Resolving idle time): Document
- `org-clock-x11idle-program-name'.
+ * org.texi (Resolving idle time):
+ Document `org-clock-x11idle-program-name'.
2013-10-24 Michael Albinus <michael.albinus@gmx.de>
2013-07-29 Michael Albinus <michael.albinus@gmx.de>
- * tramp.texi (Frequently Asked Questions): Mention
- `tramp-use-ssh-controlmaster-options'.
+ * tramp.texi (Frequently Asked Questions):
+ Mention `tramp-use-ssh-controlmaster-options'.
2013-07-26 Tassilo Horn <tsdh@gnu.org>
2013-07-08 Tassilo Horn <tsdh@gnu.org>
* gnus.texi (lines): Correct description of
- `gnus-registry-track-extra's default value. Mention
- `gnus-registry-remove-extra-data'.
+ `gnus-registry-track-extra's default value.
+ Mention `gnus-registry-remove-extra-data'.
2013-07-06 Lars Ingebrigtsen <larsi@gnus.org>
projects / emacs.git / commitdiff