1 files changed, 72 insertions, 1 deletions

diff --git a/lispref/customize.texi b/lispref/customize.texi
index 8621cb65662..90600f410b7 100644
--- a/lispref/customize.texi
+++ b/lispref/customize.texi
@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1997, 1998, 1999, 2000, 2002 Free Software Foundation, Inc.
+@c Copyright (C) 1997, 1998, 1999, 2000, 2002, 2003 Free Software Foundation, Inc.
 @c See the file elisp.texi for copying conditions.
 @setfilename ../info/customize
 @node Customization, Loading, Macros, Top
@@ -373,6 +373,7 @@ equivalent to @code{(string)}.
 * Composite Types::
 * Splicing into Lists::
 * Type Keywords::
+* Defining New Types::
 @end menu
 
 All customization types are implemented as widgets; see @ref{Top, ,
@@ -1056,6 +1057,76 @@ arguments, which will be used when creating the @code{radio-button} or
 @end ignore
 @end table
 
+@node Defining New Types
+@subsection Defining New Types
+
+In the previous sections we have described how to construct elaborate
+type specifications for @code{defcustom}.  In some cases you may want to
+give such a type specification a name.  The obvious case is when you are
+using the same type for many user options, rather than repeat the
+specification for each option, you can give the type specification a
+name once, and use that name each @code{defcustom}.  The other case is
+when a user option accept a recursive datastructure.  To make it
+possible for a datatype to refer to itself, it needs to have a name.
+
+Since custom types are implemented as widgets, the way to define a new
+customize type is to define a new widget.  We are not going to describe
+the widget interface here in details, see @ref{Top, , Introduction,
+widget, The Emacs Widget Library}, for that.  Instead we are going to
+demonstrate the minimal functionality needed for defining new customize
+types by a simple example.
+
+@example
+(define-widget 'binary-tree-of-string 'lazy
+  "A binary tree made of cons-cells and strings."
+  :offset 4
+  :tag "Node"
+  :type '(choice (string :tag "Leaf" :value "")
+                 (cons :tag "Interior"
+                       :value ("" . "") 
+                       binary-tree-of-string
+                       binary-tree-of-string)))
+
+(defcustom foo-bar ""
+  "Sample variable holding a binary tree of strings."
+  :type 'binary-tree-of-string)
+@end example
+
+The function to define a new widget is name @code{define-widget}.  The
+first argument is the symbol we want to make a new widget type.  The
+second argument is a symbol representing an existing widget, the new
+widget is going to be defined in terms of difference from the existing
+widget.  For the purpose of defining new customization types, the
+@code{lazy} widget is perfect, because it accept a @code{:type} keyword
+argument with the same syntax as the keyword argument to
+@code{defcustom} with the same name.  The third argument is a
+documentation string for the new widget.  You will be able to see that
+string with the @kbd{M-x widget-browse @key{ret} binary-tree-of-string
+@key{ret}} command.  
+
+After these mandatory arguments follows the keyword arguments.  The most
+important is @code{:type}, which describes the datatype we want to match
+with this widget.  Here a @code{binary-tree-of-string} is described as
+being either a string, or a cons-cell whose car and cdr are themselves
+both @code{binary-tree-of-string}.  Note the reference to the widget
+type we are currently in the process of defining.  The @code{:tag}
+attribute is a string to name the widget in the user interface, and the
+@code{:offset} argument are there to ensure that child nodes are
+indented four spaces relatively to the parent node, making the tree
+structure apparent in the customization buffer.
+
+The @code{defcustom} shows how the new widget can be used as an ordinary
+customization type. 
+
+If you wonder about the name @code{lazy}, know that the other composite
+widgets convert their inferior widgets to internal form when the widget
+is instantiated in a buffer.  This conversion is recursive, so the
+inferior widgets will convert @emph{their} inferior widgets.  If the
+datastructure is itself recursive, this conversion will go on forever,
+or at least until Emacs run out of stack space.  The @code{lazy} widget
+stop this recursion, it will only convert its @code{:type} argument when
+needed.
+
 @ignore
    arch-tag: d1b8fad3-f48c-4ce4-a402-f73b5ef19bd2
 @end ignore