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17. Advising Emacs Lisp Functions

The advice feature lets you add to the existing definition of a function, by advising the function. This is a clean method for a library to customize functions defined by other parts of Emacs--cleaner than redefining the whole function.

Each function can have multiple pieces of advice, separately defined. Each defined piece of advice can be enabled or disabled explicitly. All the enabled pieces of advice for any given function actually take effect when you activate advice for that function, or when you define or redefine the function. Note that enabling a piece of advice and activating advice for a function are not the same thing.

Usage Note: Advice is useful for altering the behavior of existing calls to an existing function. If you want the new behavior for new calls, or for key bindings, it is cleaner to define a new function (or a new command) which uses the existing function.

17.1 A Simple Advice Example  A simple example to explain the basics of advice.
17.2 Defining Advice  Detailed description of defadvice.
17.3 Around-Advice  Wrapping advice around a function's definition.
17.4 Computed Advice  ...is to defadvice as fset is to defun.
17.5 Activation of Advice  Advice doesn't do anything until you activate it.
17.6 Enabling and Disabling Advice  You can enable or disable each piece of advice.
17.7 Preactivation  Preactivation is a way of speeding up the loading of compiled advice.
17.8 Argument Access in Advice  How advice can access the function's arguments.
17.9 Definition of Subr Argument Lists  Accessing arguments when advising a primitive.
17.10 The Combined Definition  How advice is implemented.

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17.1 A Simple Advice Example

The command next-line moves point down vertically one or more lines; it is the standard binding of C-n. When used on the last line of the buffer, this command inserts a newline to create a line to move to if next-line-add-newlines is non-nil (its default is nil.)

Suppose you wanted to add a similar feature to previous-line, which would insert a new line at the beginning of the buffer for the command to move to. 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:

(defadvice previous-line (before next-line-at-end (arg))
  "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)))

This expression defines a piece of advice for the function previous-line. This piece of advice is named next-line-at-end, and the symbol before says that it is before-advice which should run before the regular definition of previous-line. (arg) 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 previous-line. That happens when you activate the advice, like this:

(ad-activate 'previous-line)

This is what actually begins to use the advice that has been defined so far for the function previous-line. Henceforth, whenever that function is run, whether invoked by the user with C-p or M-x, or called from Lisp, it runs the advice first, and its regular definition second.

This example illustrates before-advice, which is one class of advice: it runs before the function's base definition. There are two other advice classes: after-advice, which runs after the base definition, and around-advice, which lets you specify an expression to wrap around the invocation of the base definition.

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17.2 Defining Advice

To define a piece of advice, use the macro defadvice. A call to defadvice has the following syntax, which is based on the syntax of defun and defmacro, but adds more:

(defadvice function (class name
                         [position] [arglist]

Here, function is the name of the function (or macro or special form) to be advised. From now on, we will write just "function" when describing the entity being advised, but this always includes macros and special forms.

class specifies the class of the advice--one of before, after, or 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 ad-return-value.

Variable: 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.

The argument name is the name of the advice, a non-nil symbol. The advice name uniquely identifies one piece of advice, within all the pieces of advice in a particular class for a particular function. The name allows you to refer to the piece of advice--to redefine it, or to enable or disable it.

In place of the argument list in an ordinary definition, an advice definition calls for several different pieces of information.

The optional position specifies where, in the current list of advice of the specified class, this new advice should be placed. It should be either first, last or a number that specifies a zero-based position (first is equivalent to 0). If no position is specified, the default is 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 position value is ignored when redefining an existing piece of advice.

The optional 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 (see section 17.10 The 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.

See section 17.8 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, flags, are symbols that specify further information about how to use this piece of advice. Here are the valid symbols and their meanings:

Activate the advice for 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 function, immediately after defining this piece of advice.

This flag has no immediate effect if function itself is not defined yet (a situation known as forward advice), because it is impossible to activate an undefined function's advice. However, defining function will automatically activate its advice.

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 unwind-protect form, so that it will execute even if the previous code gets an error or uses throw. See section 10.5.4 Cleaning Up from Nonlocal Exits.

Compile the combined definition that is used to run the advice. This flag is ignored unless activate is also specified. See section 17.10 The Combined Definition.

Initially disable this piece of advice, so that it will not be used unless subsequently explicitly enabled. See section 17.6 Enabling and Disabling Advice.

Activate advice for function when this 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. See section 17.7 Preactivation.

This is useful only if this defadvice is byte-compiled.

The optional documentation-string serves to document this piece of advice. When advice is active for function, the documentation for function (as returned by documentation) combines the documentation strings of all the advice for function with the documentation string of its original function definition.

The optional interactive-form form can be supplied to change the interactive behavior of the original function. If more than one piece of advice has an interactive-form, then the first one (the one with the smallest position) found among all the advice takes precedence.

The possibly empty list of 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.

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.

Command: ad-unadvise function
This command deletes the advice from function.

Command: ad-unadvise-all
This command deletes all pieces of advice from all functions.

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17.3 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 ad-do-it. Where this symbol occurs inside the around-advice body, it is replaced with a progn containing the forms of the surrounded code. Here is an example:

(defadvice foo (around foo-around)
  "Ignore case in `foo'."
  (let ((case-fold-search t))

Its effect is to make sure that case is ignored in searches when the original definition of foo is run.

Variable: ad-do-it
This is not really a variable, but it is somewhat used like one in around-advice. It specifies the place to run the function's original definition and other "earlier" around-advice.

If the around-advice does not use 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 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 ad-do-it inside of a loop.

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17.4 Computed Advice

The macro defadvice resembles 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 ad-add-advice.

Function: ad-add-advice function advice class position
Calling ad-add-advice adds advice as a piece of advice to function in class class. The argument advice has this form:

(name protected enabled definition)

Here protected and enabled are flags, and definition is the expression that says what the advice should do. If enabled is nil, this piece of advice is initially disabled (see section 17.6 Enabling and Disabling Advice).

If function already has one or more pieces of advice in the specified class, then position specifies where in the list to put the new piece of advice. The value of position can either be first, 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 position value is ignored when redefining an existing piece of advice.

If function already has a piece of advice with the same name, then the position argument is ignored and the old advice is replaced with the new one.

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17.5 Activation of Advice

By default, advice does not take effect when you define it--only when you activate advice for the function that was advised. You can request the activation of advice for a function when you define the advice, by specifying the activate flag in the defadvice. But normally you activate the advice for a function by calling the function 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 17.6 Enabling and Disabling 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 compile is t, the command also compiles the combined definition which implements the advice.

Command: ad-activate function &optional compile
This command activates all the advice defined for function.

To activate advice for a function whose advice is already active is not a no-op. It is a useful operation which puts into effect any changes in that function's advice since the previous activation of advice for that function.

Command: ad-deactivate function
This command deactivates the advice for function.

Command: ad-update function &optional compile
This command activates the advice for function if its advice is already activated. This is useful if you change the advice.

Command: ad-activate-all &optional compile
This command activates the advice for all functions.

Command: ad-deactivate-all
This command deactivates the advice for all functions.

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.

Command: ad-activate-regexp regexp &optional compile
This command activates all pieces of advice whose names match regexp. More precisely, it activates all advice for any function which has at least one piece of advice that matches regexp.

Command: ad-deactivate-regexp regexp
This command deactivates all pieces of advice whose names match regexp. More precisely, it deactivates all advice for any function which has at least one piece of advice that matches regexp.

Command: ad-update-regexp regexp &optional compile
This command activates pieces of advice whose names match regexp, but only those for functions whose advice is already activated.

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; see section 17.6 Enabling and Disabling Advice) since the last time it was activated.

Command: ad-start-advice
Turn on automatic advice activation when a function is defined or redefined. If you turn on this mode, then advice takes effect immediately when defined.

Command: ad-stop-advice
Turn off automatic advice activation when a function is defined or redefined.

User Option: ad-default-compilation-action
This variable controls whether to compile the combined definition that results from activating advice for a function.

A value of always specifies to compile unconditionally. A value of nil specifies never compile the advice.

A value of maybe specifies to compile if the byte-compiler is already loaded. A value of 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 compile argument of ad-activate (or any of the above functions) was supplied as nil. If that argument is non-nil, that means to compile the advice regardless.

If the advised definition was constructed during "preactivation" (see section 17.7 Preactivation), then that definition must already be compiled, because it was constructed during byte-compilation of the file that contained the defadvice with the preactivate flag.

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17.6 Enabling and 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 my-advice for the function foo:

(ad-disable-advice 'foo 'before 'my-advice)

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 foo again:

(ad-activate 'foo)

Command: ad-disable-advice function class name
This command disables the piece of advice named name in class class on function.

Command: ad-enable-advice function class name
This command enables the piece of advice named name in class class on function.

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.

Command: ad-disable-regexp regexp
This command disables all pieces of advice whose names match regexp, in all classes, on all functions.

Command: ad-enable-regexp regexp
This command enables all pieces of advice whose names match regexp, in all classes, on all functions.

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17.7 Preactivation

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 preactivation to construct suitable combined definitions in advance.

To use preactivation, specify the preactivate flag when you define the advice with defadvice. This 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 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.

Compiled preactivated advice works properly even if the function itself is not defined until later; however, the function needs to be defined when you 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 ad-cache-id-verification-code (with the function trace-function-background) before the advised function's advice is activated. After activation, check the value returned by ad-cache-id-verification-code for that function: verified means that the preactivated advice was used, while other values give some information about why they were considered inappropriate.

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.

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17.8 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 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.

Macro: ad-get-arg position
This returns the actual argument that was supplied at position.

Macro: ad-get-args position
This returns the list of actual arguments supplied starting at position.

Macro: ad-set-arg position value
This sets the value of the actual argument at position to value

Macro: ad-set-args position value-list
This sets the list of actual arguments starting at position to value-list.

Now an example. Suppose the function foo is defined as

(defun foo (x y &optional z &rest r) ...)

and is then called with

(foo 0 1 2 3 4 5 6)

which means that x is 0, y is 1, z is 2 and r is (3 4 5 6) within the body of foo. Here is what ad-get-arg and ad-get-args return in this case:

(ad-get-arg 0) => 0
(ad-get-arg 1) => 1
(ad-get-arg 2) => 2
(ad-get-arg 3) => 3
(ad-get-args 2) => (2 3 4 5 6)
(ad-get-args 4) => (4 5 6)

Setting arguments also makes sense in this example:

(ad-set-arg 5 "five")

has the effect of changing the sixth argument to "five". If this happens in advice executed before the body of foo is run, then r will be (3 4 "five" 6) within that body.

Here is an example of setting a tail of the argument list:

(ad-set-args 0 '(5 4 3 2 1 0))

If this happens in advice executed before the body of foo is run, then within that body, x will be 5, y will be 4, z will be 3, and r will be (2 1 0) inside the body of foo.

These argument constructs are not really implemented as Lisp macros. Instead they are implemented specially by the advice mechanism.

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17.9 Definition of Subr Argument Lists

When the advice facility constructs the combined definition, it needs to know the argument list of the original function. This is not always possible for primitive functions. When advice cannot determine the argument list, it uses (&rest ad-subr-args), which always works but is inefficient because it constructs a list of the argument values. You can use ad-define-subr-args to declare the proper argument names for a primitive function:

Function: ad-define-subr-args function arglist
This function specifies that arglist should be used as the argument list for function function.

For example,

(ad-define-subr-args 'fset '(sym newdef))

specifies the argument list for the function fset.

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17.10 The Combined Definition

Suppose that a function has n pieces of before-advice (numbered from 0 through n-1), m pieces of around-advice and 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:

(lambda arglist
  [ [advised-docstring] [(interactive ...)] ]
  (let (ad-return-value)
             (setq ad-return-value
                   apply original definition to arglist)

Macros are redefined as macros, which means adding 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 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 l are included in one of the forms of around-advice l - 1.

The innermost part of the around advice onion is

apply original definition to arglist

whose form depends on the type of the original function. The variable 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 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.

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