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Definition Commands

The @deffn command and the other definition commands enable you to describe functions, variables, macros, commands, user options, special forms and other such artifacts in a uniform format.

In the Info file, a definition causes the entity category---`Function', `Variable', or whatever--to appear at the beginning of the first line of the definition, followed by the entity's name and arguments. In the printed manual, the command causes TeX to print the entity's name and its arguments on the left margin and print the category next to the right margin. In both output formats, the body of the definition is indented. Also, the name of the entity is entered into the appropriate index: @deffn enters the name into the index of functions, @defvr enters it into the index of variables, and so on.

A manual need not and should not contain more than one definition for a given name. An appendix containing a summary should use @table rather than the definition commands.

The Template for a Definition

The @deffn command is used for definitions of entities that resemble functions. To write a definition using the @deffn command, write the @deffn command at the beginning of a line and follow it on the same line by the category of the entity, the name of the entity itself, and its arguments (if any). Then write the body of the definition on succeeding lines. (You may embed examples in the body.) Finally, end the definition with an @end deffn command written on a line of its own. (The other definition commands follow the same format.)

The template for a definition looks like this: 

@deffn category name arguments...
body-of-definition
@end deffn

For example, 

@deffn Command forward-word count
This command moves point forward @var{count} words
(or backward if @var{count} is negative). ...
@end deffn

produces

Command: forward-word count
This function moves point forward count words (or backward if count is negative). ...

Capitalize the category name like a title. If the name of the category contains spaces, as in the phrase `Interactive Command', write braces around it. For example: 

@deffn {Interactive Command} isearch-forward
...
@end deffn

Otherwise, the second word will be mistaken for the name of the entity.

Some of the definition commands are more general than others. The @deffn command, for example, is the general definition command for functions and the like--for entities that may take arguments. When you use this command, you specify the category to which the entity belongs. The @deffn command possesses three predefined, specialized variations, @defun, @defmac, and @defspec, that specify the category for you: "Function", "Macro", and "Special Form" respectively. (In Lisp, a special form is an entity much like a function.) The @defvr command also is accompanied by several predefined, specialized variations for describing particular kinds of variables.

The template for a specialized definition, such as @defun, is similar to the template for a generalized definition, except that you do not need to specify the category: 

@defun name arguments...
body-of-definition
@end defun

Thus, 

@defun buffer-end flag
This function returns @code{(point-min)} if @var{flag}
is less than 1, @code{(point-max)} otherwise.
...
@end defun

produces

Function: buffer-end flag
This function returns (point-min) if flag is less than 1, (point-max) otherwise. ...

See section A Sample Function Definition, for a more detailed example of a function definition, including the use of @example inside the definition.

The other specialized commands work like @defun.

Optional and Repeated Arguments

Some entities take optional or repeated arguments, which may be specified by a distinctive glyph that uses square brackets and ellipses. For example, a special form often breaks its argument list into separate arguments in more complicated ways than a straightforward function.

An argument enclosed within square brackets is optional. Thus, the phrase `[optional-arg]' means that optional-arg is optional. An argument followed by an ellipsis is optional and may be repeated more than once. Thus, `repeated-args...' stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure in Lisp.

Here is the @defspec line of an example of an imaginary special form:

Special Form: foobar (var [from to [inc]]) body...

In this example, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.

In a Texinfo source file, this @defspec line is written like this (except it would not be split over two lines, as it is in this example). 

@defspec foobar (@var{var} [@var{from} @var{to}
     [@var{inc}]]) @var{body}@dots{}

The function is listed in the Command and Variable Index under `foobar'.

Two or More `First' Lines

To create two or more `first' or header lines for a definition, follow the first @deffn line by a line beginning with @deffnx. The @deffnx command works exactly like @deffn except that it does not generate extra vertical white space between it and the preceding line.

For example, 

@deffn {Interactive Command} isearch-forward
@deffnx {Interactive Command} isearch-backward
These two search commands are similar except ...
@end deffn

produces

Interactive Command: isearch-forward
Interactive Command: isearch-backward
These two search commands are similar except ...

Each definition command has an `x' form: @defunx, @defvrx, @deftypefunx, etc.

The `x' forms work just like @itemx; see section @itemx.

The Definition Commands

Texinfo provides more than a dozen definition commands, all of which are described in this section.

The definition commands automatically enter the name of the entity in the appropriate index: for example, @deffn, @defun, and @defmac enter function names in the index of functions; @defvr and @defvar enter variable names in the index of variables.

Although the examples that follow mostly illustrate Lisp, the commands can be used for other programming languages.

Functions and Similar Entities

This section describes the commands for describing functions and similar entities:

@deffn category name arguments...
The @deffn command is the general definition command for functions, interactive commands, and similar entities that may take arguments. You must choose a term to describe the category of entity being defined; for example, "Function" could be used if the entity is a function. The @deffn command is written at the beginning of a line and is followed on the same line by the category of entity being described, the name of this particular entity, and its arguments, if any. Terminate the definition with @end deffn on a line of its own. For example, here is a definition: 
@deffn Command forward-char nchars
Move point forward @var{nchars} characters.
@end deffn
This shows a rather terse definition for a "command" named forward-char with one argument, nchars. @deffn prints argument names such as nchars in italics or upper case, as if @var had been used, because we think of these names as metasyntactic variables--they stand for the actual argument values. Within the text of the description, write an argument name explicitly with @var to refer to the value of the argument. In the example above, we used `@var{nchars}' in this way. The template for @deffn is: 
@deffn category name arguments...
body-of-definition
@end deffn
@defun name arguments...
The @defun command is the definition command for functions. @defun is equivalent to `@deffn Function ...'. For example, 
@defun set symbol new-value
Change the value of the symbol @var{symbol}
to @var{new-value}.
@end defun
shows a rather terse definition for a function set whose arguments are symbol and new-value. The argument names on the @defun line automatically appear in italics or upper case as if they were enclosed in @var. Terminate the definition with @end defun on a line of its own. The template is: 
@defun function-name arguments...
body-of-definition
@end defun
@defun creates an entry in the index of functions.
@defmac name arguments...
The @defmac command is the definition command for macros. @defmac is equivalent to `@deffn Macro ...' and works like @defun.
@defspec name arguments...
The @defspec command is the definition command for special forms. (In Lisp, a special form is an entity much like a function, see section `Special Forms' in GNU Emacs Lisp Reference Manual.) @defspec is equivalent to `@deffn {Special Form} ...' and works like @defun.

Variables and Similar Entities

Here are the commands for defining variables and similar entities:

@defvr category name
The @defvr command is a general definition command for something like a variable--an entity that records a value. You must choose a term to describe the category of entity being defined; for example, "Variable" could be used if the entity is a variable. Write the @defvr command at the beginning of a line and follow it on the same line by the category of the entity and the name of the entity. Capitalize the category name like a title. If the name of the category contains spaces, as in the name "User Option", enclose it in braces. Otherwise, the second word will be mistaken for the name of the entity. For example, 
@defvr {User Option} fill-column
This buffer-local variable specifies
the maximum width of filled lines.
...
@end defvr
Terminate the definition with @end defvr on a line of its own. The template is: 
@defvr category name
body-of-definition
@end defvr
@defvr creates an entry in the index of variables for name.
@defvar name
The @defvar command is the definition command for variables. @defvar is equivalent to `@defvr Variable ...'. For example: 
@defvar kill-ring
...
@end defvar
The template is: 
@defvar name
body-of-definition
@end defvar
@defvar creates an entry in the index of variables for name.
@defopt name
The @defopt command is the definition command for user options, i.e., variables intended for users to change according to taste; Emacs has many such (see section `Variables' in The GNU Emacs Manual). @defopt is equivalent to `@defvr {User Option} ...' and works like @defvar.

Functions in Typed Languages

The @deftypefn command and its variations are for describing functions in languages in which you must declare types of variables and functions, such as C and C++.

@deftypefn category data-type name arguments...
The @deftypefn command is the general definition command for functions and similar entities that may take arguments and that are typed. The @deftypefn command is written at the beginning of a line and is followed on the same line by the category of entity being described, the type of the returned value, the name of this particular entity, and its arguments, if any. For example, 
@deftypefn {Library Function} int foobar
   (int @var{foo}, float @var{bar})
...
@end deftypefn
(where the text before the "...", shown above as two lines, would actually be a single line in a real Texinfo file) produces the following in Info: 
-- Library Function: int foobar (int FOO, float BAR)
...
In a printed manual, it produces:

Library Function: int foobar (int foo, float bar)
...
This means that foobar is a "library function" that returns an int, and its arguments are foo (an int) and bar (a float). The argument names that you write in @deftypefn are not subject to an implicit @var---since the actual names of the arguments in @deftypefn are typically scattered among data type names and keywords, Texinfo cannot find them without help. Instead, you must write @var explicitly around the argument names. In the example above, the argument names are `foo' and `bar'. The template for @deftypefn is: 
@deftypefn category data-type name arguments ...
body-of-description
@end deftypefn
Note that if the category or data type is more than one word then it must be enclosed in braces to make it a single argument. If you are describing a procedure in a language that has packages, such as Ada, you might consider using @deftypefn in a manner somewhat contrary to the convention described in the preceding paragraphs. For example: 
@deftypefn stacks private push
        (@var{s}:in out stack;
        @var{n}:in integer)
...
@end deftypefn
(The @deftypefn arguments are shown split into three lines, but would be a single line in a real Texinfo file.) In this instance, the procedure is classified as belonging to the package stacks rather than classified as a `procedure' and its data type is described as private. (The name of the procedure is push, and its arguments are s and n.) @deftypefn creates an entry in the index of functions for name.
@deftypefun data-type name arguments...
The @deftypefun command is the specialized definition command for functions in typed languages. The command is equivalent to `@deftypefn Function ...'. Thus, 
@deftypefun int foobar (int @var{foo}, float @var{bar})
...
@end deftypefun
produces the following in Info: 
-- Function: int foobar (int FOO, float BAR)
...
and the following in a printed manual:

Function: int foobar (int foo, float bar)
...
The template is: 
@deftypefun type name arguments...
body-of-description
@end deftypefun
@deftypefun creates an entry in the index of functions for name.

Variables in Typed Languages

Variables in typed languages are handled in a manner similar to functions in typed languages. See section Functions in Typed Languages. The general definition command @deftypevr corresponds to @deftypefn and the specialized definition command @deftypevar corresponds to @deftypefun.

@deftypevr category data-type name
The @deftypevr command is the general definition command for something like a variable in a typed language--an entity that records a value. You must choose a term to describe the category of the entity being defined; for example, "Variable" could be used if the entity is a variable. The @deftypevr command is written at the beginning of a line and is followed on the same line by the category of the entity being described, the data type, and the name of this particular entity. For example: 
@deftypevr {Global Flag} int enable
...
@end deftypevr
produces the following in Info: 
-- Global Flag: int enable
...
and the following in a printed manual:

Global Flag: int enable
...
The template is: 
@deftypevr category data-type name
body-of-description
@end deftypevr
@deftypevr creates an entry in the index of variables for name.
@deftypevar data-type name
The @deftypevar command is the specialized definition command for variables in typed languages. @deftypevar is equivalent to `@deftypevr Variable ...'. For example: 
@deftypevar int fubar
...
@end deftypevar
produces the following in Info: 
-- Variable: int fubar
...
and the following in a printed manual:

Variable: int fubar
...
The template is: 
@deftypevar data-type name
body-of-description
@end deftypevar
@deftypevar creates an entry in the index of variables for name.

Object-Oriented Programming

Here are the commands for formatting descriptions about abstract objects, such as are used in object-oriented programming. A class is a defined type of abstract object. An instance of a class is a particular object that has the type of the class. An instance variable is a variable that belongs to the class but for which each instance has its own value.

In a definition, if the name of a class is truly a name defined in the programming system for a class, then you should write an @code around it. Otherwise, it is printed in the usual text font.

@defcv category class name
The @defcv command is the general definition command for variables associated with classes in object-oriented programming. The @defcv command is followed by three arguments: the category of thing being defined, the class to which it belongs, and its name. Thus, 
@defcv {Class Option} Window border-pattern
...
@end defcv
illustrates how you would write the first line of a definition of the border-pattern class option of the class Window. The template is: 
@defcv category class name
...
@end defcv
@defcv creates an entry in the index of variables.
@defivar class name
The @defivar command is the definition command for instance variables in object-oriented programming. @defivar is equivalent to `@defcv {Instance Variable} ...' The template is: 
@defivar class instance-variable-name
body-of-definition
@end defivar
@defivar creates an entry in the index of variables.
@deftypeivar class data-type name
The @deftypeivar command is the definition command for typed instance variables in object-oriented programming. It is similar to @defivar with the addition of the data-type parameter to specify the type of the instance variable. @deftypeivar creates an entry in the index of variables.
@defop category class name arguments...
The @defop command is the general definition command for entities that may resemble methods in object-oriented programming. These entities take arguments, as functions do, but are associated with particular classes of objects. For example, some systems have constructs called wrappers that are associated with classes as methods are, but that act more like macros than like functions. You could use @defop Wrapper to describe one of these. Sometimes it is useful to distinguish methods and operations. You can think of an operation as the specification for a method. Thus, a window system might specify that all window classes have a method named expose; we would say that this window system defines an expose operation on windows in general. Typically, the operation has a name and also specifies the pattern of arguments; all methods that implement the operation must accept the same arguments, since applications that use the operation do so without knowing which method will implement it. Often it makes more sense to document operations than methods. For example, window application developers need to know about the expose operation, but need not be concerned with whether a given class of windows has its own method to implement this operation. To describe this operation, you would write: 
@defop Operation windows expose
The @defop command is written at the beginning of a line and is followed on the same line by the overall name of the category of operation, the name of the class of the operation, the name of the operation, and its arguments, if any. The template is: 
@defop category class name arguments...
body-of-definition
@end defop
@defop creates an entry, such as `expose on windows', in the index of functions.
@deftypeop category class data-type name arguments...
The @deftypeop command is the definition command for typed operations in object-oriented programming. It is similar to @defop with the addition of the data-type parameter to specify the return type of the method. @deftypeop creates an entry in the index of functions.
@defmethod class name arguments...
The @defmethod command is the definition command for methods in object-oriented programming. A method is a kind of function that implements an operation for a particular class of objects and its subclasses. @defmethod is equivalent to `@defop Method ...'. The command is written at the beginning of a line and is followed by the name of the class of the method, the name of the method, and its arguments, if any. For example: 
@defmethod bar-class bar-method argument
...
@end defmethod
illustrates the definition for a method called bar-method of the class bar-class. The method takes an argument. The template is: 
@defmethod class method-name arguments...
body-of-definition
@end defmethod
@defmethod creates an entry, such as `bar-method on bar-class', in the index of functions.
@deftypemethod class data-type name arguments...
The @deftypemethod command is the definition command for methods in object-oriented typed languages, such as C++ and Java. It is similar to the @defmethod command with the addition of the data-type parameter to specify the return type of the method.

Data Types

Here is the command for data types:

@deftp category name attributes...
The @deftp command is the generic definition command for data types. The command is written at the beginning of a line and is followed on the same line by the category, by the name of the type (which is a word like int or float), and then by names of attributes of objects of that type. Thus, you could use this command for describing int or float, in which case you could use data type as the category. (A data type is a category of certain objects for purposes of deciding which operations can be performed on them.) In Lisp, for example, pair names a particular data type, and an object of that type has two slots called the CAR and the CDR. Here is how you would write the first line of a definition of pair. 
@deftp {Data type} pair car cdr
...
@end deftp
The template is: 
@deftp category name-of-type attributes...
body-of-definition
@end deftp
@deftp creates an entry in the index of data types.

Conventions for Writing Definitions

When you write a definition using @deffn, @defun, or one of the other definition commands, please take care to use arguments that indicate the meaning, as with the count argument to the forward-word function. Also, if the name of an argument contains the name of a type, such as integer, take care that the argument actually is of that type.

A Sample Function Definition

A function definition uses the @defun and @end defun commands. The name of the function follows immediately after the @defun command and it is followed, on the same line, by the parameter list.

Here is a definition from section `Calling Functions' in The GNU Emacs Lisp Reference Manual.

Function: apply function &rest arguments
apply calls function with arguments, just like funcall but with one difference: the last of arguments is a list of arguments to give to function, rather than a single argument. We also say that this list is appended to the other arguments.

apply returns the result of calling function. As with funcall, function must either be a Lisp function or a primitive function; special forms and macros do not make sense in apply. 

(setq f 'list)
     => list
(apply f 'x 'y 'z)
error--> Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
     => 10
(apply '+ '(1 2 3 4))
     => 10

(apply 'append '((a b c) nil (x y z) nil))
     => (a b c x y z)

An interesting example of using apply is found in the description of mapcar.

In the Texinfo source file, this example looks like this: 

@defun apply function &rest arguments
@code{apply} calls @var{function} with
@var{arguments}, just like @code{funcall} but with one
difference: the last of @var{arguments} is a list of
arguments to give to @var{function}, rather than a single
argument.  We also say that this list is @dfn{appended}
to the other arguments.

@code{apply} returns the result of calling
@var{function}.  As with @code{funcall},
@var{function} must either be a Lisp function or a
primitive function; special forms and macros do not make
sense in @code{apply}.

@example
(setq f 'list)
     @result{} list
(apply f 'x 'y 'z)
@error{} Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
     @result{} 10
(apply '+ '(1 2 3 4))
     @result{} 10

(apply 'append '((a b c) nil (x y z) nil))
     @result{} (a b c x y z)
@end example

An interesting example of using @code{apply} is found
in the description of @code{mapcar}.
@end defun

In this manual, this function is listed in the Command and Variable Index under apply.

Ordinary variables and user options are described using a format like that for functions except that variables do not take arguments.

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