XEmacs 21.2.32 "Kastor & Polydeukes".

[chise/xemacs-chise.git.1] / man / lispref / minibuf.texi

@c -*-texinfo-*-
@c This is part of the XEmacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1997 Free Software Foundation, Inc. 
@c See the file lispref.texi for copying conditions.
@setfilename ../../info/minibuf.info
@node Minibuffers, Command Loop, Read and Print, Top
@chapter Minibuffers
@cindex arguments, reading
@cindex complex arguments
@cindex minibuffer

  A @dfn{minibuffer} is a special buffer that XEmacs commands use to read
arguments more complicated than the single numeric prefix argument.
These arguments include file names, buffer names, and command names (as
in @kbd{M-x}).  The minibuffer is displayed on the bottom line of the
frame, in the same place as the echo area, but only while it is in
use for reading an argument.

@menu
* Intro to Minibuffers::      Basic information about minibuffers.
* Text from Minibuffer::      How to read a straight text string.
* Object from Minibuffer::    How to read a Lisp object or expression.
* Minibuffer History::        Recording previous minibuffer inputs
                                so the user can reuse them.
* Completion::                How to invoke and customize completion.
* Yes-or-No Queries::         Asking a question with a simple answer.
* Multiple Queries::          Asking a series of similar questions.
* Reading a Password::        Reading a password from the terminal.
* Minibuffer Misc::           Various customization hooks and variables.
@end menu

@node Intro to Minibuffers
@section Introduction to Minibuffers

  In most ways, a minibuffer is a normal XEmacs buffer.  Most operations
@emph{within} a buffer, such as editing commands, work normally in a
minibuffer.  However, many operations for managing buffers do not apply
to minibuffers.  The name of a minibuffer always has the form @w{@samp{
*Minibuf-@var{number}}}, and it cannot be changed.  Minibuffers are
displayed only in special windows used only for minibuffers; these
windows always appear at the bottom of a frame.  (Sometimes frames have
no minibuffer window, and sometimes a special kind of frame contains
nothing but a minibuffer window; see @ref{Minibuffers and Frames}.)

  The minibuffer's window is normally a single line.  You can resize it
temporarily with the window sizing commands; it reverts to its normal
size when the minibuffer is exited.  You can resize it permanently by
using the window sizing commands in the frame's other window, when the
minibuffer is not active.  If the frame contains just a minibuffer, you
can change the minibuffer's size by changing the frame's size.

  If a command uses a minibuffer while there is an active minibuffer,
this is called a @dfn{recursive minibuffer}.  The first minibuffer is
named @w{@samp{ *Minibuf-0*}}.  Recursive minibuffers are named by
incrementing the number at the end of the name.  (The names begin with a
space so that they won't show up in normal buffer lists.)  Of several
recursive minibuffers, the innermost (or most recently entered) is the
active minibuffer.  We usually call this ``the'' minibuffer.  You can
permit or forbid recursive minibuffers by setting the variable
@code{enable-recursive-minibuffers}.

  Like other buffers, a minibuffer may use any of several local keymaps
(@pxref{Keymaps}); these contain various exit commands and in some cases
completion commands (@pxref{Completion}).

@itemize @bullet
@item
@code{minibuffer-local-map} is for ordinary input (no completion).

@item
@code{minibuffer-local-completion-map} is for permissive completion.

@item
@code{minibuffer-local-must-match-map} is for strict completion and
for cautious completion.
@end itemize

@node Text from Minibuffer
@section Reading Text Strings with the Minibuffer

  Most often, the minibuffer is used to read text as a string.  It can
also be used to read a Lisp object in textual form.  The most basic
primitive for minibuffer input is @code{read-from-minibuffer}; it can do
either one.

  In most cases, you should not call minibuffer input functions in the
middle of a Lisp function.  Instead, do all minibuffer input as part of
reading the arguments for a command, in the @code{interactive} spec.
@xref{Defining Commands}.

@defun read-from-minibuffer prompt-string &optional initial-contents keymap read hist abbrev-table default
This function is the most general way to get input through the
minibuffer.  By default, it accepts arbitrary text and returns it as a
string; however, if @var{read} is non-@code{nil}, then it uses
@code{read} to convert the text into a Lisp object (@pxref{Input
Functions}).

The first thing this function does is to activate a minibuffer and 
display it with @var{prompt-string} as the prompt.  This value must be a
string.

Then, if @var{initial-contents} is a string, @code{read-from-minibuffer}
inserts it into the minibuffer, leaving point at the end.  The
minibuffer appears with this text as its contents.

@c Emacs 19 feature
The value of @var{initial-contents} may also be a cons cell of the form
@code{(@var{string} . @var{position})}.  This means to insert
@var{string} in the minibuffer but put point @var{position} characters
from the beginning, rather than at the end.

When the user types a command to exit the minibuffer,
@code{read-from-minibuffer} constructs the return value from the text in
the minibuffer.  Normally it returns a string containing that text.
However, if @var{read} is non-@code{nil}, @code{read-from-minibuffer}
reads the text and returns the resulting Lisp object, unevaluated.
(@xref{Input Functions}, for information about reading.)

The argument @var{default} specifies a default value to make available
through the history commands.  It should be a string, or @code{nil}.

If @var{keymap} is non-@code{nil}, that keymap is the local keymap to
use in the minibuffer.  If @var{keymap} is omitted or @code{nil}, the
value of @code{minibuffer-local-map} is used as the keymap.  Specifying
a keymap is the most important way to customize the minibuffer for
various applications such as completion.

The argument @var{abbrev-table} specifies @code{local-abbrev-table} in
the minibuffer (@pxref{Standard Abbrev Tables}).

The argument @var{hist} specifies which history list variable to use
for saving the input and for history commands used in the minibuffer.
It defaults to @code{minibuffer-history}.  @xref{Minibuffer History}.

When the user types a command to exit the minibuffer,
@code{read-from-minibuffer} uses the text in the minibuffer to produce
its return value.  Normally it simply makes a string containing that
text.  However, if @var{read} is non-@code{nil},
@code{read-from-minibuffer} reads the text and returns the resulting
Lisp object, unevaluated.  (@xref{Input Functions}, for information
about reading.)

@strong{Usage note:} The @var{initial-contents} argument and the
@var{default} argument are two alternative features for more or less the
same job.  It does not make sense to use both features in a single call
to @code{read-from-minibuffer}.  In general, we recommend using
@var{default}, since this permits the user to insert the default value
when it is wanted, but does not burden the user with deleting it from
the minibuffer on other occasions.  However, if user is supposed to edit
default value, @var{initial-contents} may be preferred.
@end defun

@defun read-string prompt &optional initial history default-value
This function reads a string from the minibuffer and returns it.  The
arguments @var{prompt} and @var{initial} are used as in
@code{read-from-minibuffer}.  The keymap used is
@code{minibuffer-local-map}.

The optional argument @var{history}, if non-nil, specifies a history
list and optionally the initial position in the list.  The optional
argument @var{default} specifies a default value to return if the user
enters null input; it should be a string.

This function is a simplified interface to the
@code{read-from-minibuffer} function:

@smallexample
@group
(read-string @var{prompt} @var{initial} @var{history} @var{default})
@equiv{}
(read-from-minibuffer @var{prompt} @var{initial} nil nil
                      @var{history} nil @var{default})))
@end group
@end smallexample
@end defun

@defvar minibuffer-local-map
This is the default local keymap for reading from the minibuffer.  By
default, it makes the following bindings:

@table @asis
@item @kbd{C-j}
@code{exit-minibuffer}

@item @key{RET}
@code{exit-minibuffer}

@item @kbd{C-g}
@code{abort-recursive-edit}

@item @kbd{M-n}
@code{next-history-element}

@item @kbd{M-p}
@code{previous-history-element}

@item @kbd{M-r}
@code{next-matching-history-element}

@item @kbd{M-s}
@code{previous-matching-history-element}
@end table
@end defvar

@node Object from Minibuffer
@section Reading Lisp Objects with the Minibuffer

  This section describes functions for reading Lisp objects with the
minibuffer.

@defun read-expression prompt &optional initial history default-value
This function reads a Lisp object using the minibuffer, and returns it
without evaluating it.  The arguments @var{prompt} and @var{initial} are
used as in @code{read-from-minibuffer}.

The optional argument @var{history}, if non-nil, specifies a history
list and optionally the initial position in the list.  The optional
argument @var{default-value} specifies a default value to return if the
user enters null input; it should be a string.

This is a simplified interface to the
@code{read-from-minibuffer} function:

@smallexample
@group
(read-expression @var{prompt} @var{initial} @var{history} @var{default-value})
@equiv{}
(read-from-minibuffer @var{prompt} @var{initial} nil t
                      @var{history} nil @var{default-value})
@end group
@end smallexample

Here is an example in which we supply the string @code{"(testing)"} as
initial input:

@smallexample
@group
(read-expression
 "Enter an expression: " (format "%s" '(testing)))

;; @r{Here is how the minibuffer is displayed:}
@end group

@group
---------- Buffer: Minibuffer ----------
Enter an expression: (testing)@point{}
---------- Buffer: Minibuffer ----------
@end group
@end smallexample

@noindent
The user can type @key{RET} immediately to use the initial input as a
default, or can edit the input.
@end defun

@defun read-minibuffer prompt &optional initial history default-value

This is a FSF Emacs compatible function.  Use @code{read-expression}
instead.
@end defun

@defun eval-minibuffer prompt &optional initial history default-value
This function reads a Lisp expression using the minibuffer, evaluates
it, then returns the result.  The arguments @var{prompt} and
@var{initial} are used as in @code{read-from-minibuffer}.

The optional argument @var{history}, if non-nil, specifies a history
list and optionally the initial position in the list.  The optional
argument @var{default-value} specifies a default value to return if the
user enters null input; it should be a string.

This function simply evaluates the result of a call to
@code{read-expression}:

@smallexample
@group
(eval-minibuffer @var{prompt} @var{initial})
@equiv{}
(eval (read-expression @var{prompt} @var{initial}))
@end group
@end smallexample
@end defun

@defun edit-and-eval-command prompt command &optional history
This function reads a Lisp expression in the minibuffer, and then
evaluates it.  The difference between this command and
@code{eval-minibuffer} is that here the initial @var{command} is not
optional and it is treated as a Lisp object to be converted to printed
representation rather than as a string of text.  It is printed with
@code{prin1}, so if it is a string, double-quote characters (@samp{"})
appear in the initial text.  @xref{Output Functions}.

The first thing @code{edit-and-eval-command} does is to activate the
minibuffer with @var{prompt} as the prompt.  Then it inserts the printed
representation of @var{form} in the minibuffer, and lets the user edit it.
When the user exits the minibuffer, the edited text is read with
@code{read} and then evaluated.  The resulting value becomes the value
of @code{edit-and-eval-command}.

In the following example, we offer the user an expression with initial
text which is a valid form already:

@smallexample
@group
(edit-and-eval-command "Please edit: " '(forward-word 1))

;; @r{After evaluation of the preceding expression,} 
;;   @r{the following appears in the minibuffer:}
@end group

@group
---------- Buffer: Minibuffer ----------
Please edit: (forward-word 1)@point{}
---------- Buffer: Minibuffer ----------
@end group
@end smallexample

@noindent
Typing @key{RET} right away would exit the minibuffer and evaluate the
expression, thus moving point forward one word.
@code{edit-and-eval-command} returns @code{t} in this example.
@end defun

@node Minibuffer History
@section Minibuffer History
@cindex minibuffer history
@cindex history list

A @dfn{minibuffer history list} records previous minibuffer inputs so
the user can reuse them conveniently.  A history list is actually a
symbol, not a list; it is a variable whose value is a list of strings
(previous inputs), most recent first.

There are many separate history lists, used for different kinds of
inputs.  It's the Lisp programmer's job to specify the right history
list for each use of the minibuffer.

The basic minibuffer input functions @code{read-from-minibuffer} and
@code{completing-read} both accept an optional argument named @var{hist}
which is how you specify the history list.  Here are the possible
values:

@table @asis
@item @var{variable}
Use @var{variable} (a symbol) as the history list.

@item (@var{variable} . @var{startpos})
Use @var{variable} (a symbol) as the history list, and assume that the
initial history position is @var{startpos} (an integer, counting from
zero which specifies the most recent element of the history).

If you specify @var{startpos}, then you should also specify that element
of the history as the initial minibuffer contents, for consistency.
@end table

If you don't specify @var{hist}, then the default history list
@code{minibuffer-history} is used.  For other standard history lists,
see below.  You can also create your own history list variable; just
initialize it to @code{nil} before the first use.

Both @code{read-from-minibuffer} and @code{completing-read} add new
elements to the history list automatically, and provide commands to
allow the user to reuse items on the list.  The only thing your program
needs to do to use a history list is to initialize it and to pass its
name to the input functions when you wish.  But it is safe to modify the
list by hand when the minibuffer input functions are not using it.

  Here are some of the standard minibuffer history list variables:

@defvar minibuffer-history
The default history list for minibuffer history input.
@end defvar

@defvar query-replace-history
A history list for arguments to @code{query-replace} (and similar
arguments to other commands).
@end defvar

@defvar file-name-history
A history list for file name arguments.
@end defvar

@defvar regexp-history
A history list for regular expression arguments.
@end defvar

@defvar extended-command-history
A history list for arguments that are names of extended commands.
@end defvar

@defvar shell-command-history
A history list for arguments that are shell commands.
@end defvar

@defvar read-expression-history
A history list for arguments that are Lisp expressions to evaluate.
@end defvar

@defvar Info-minibuffer-history
A history list for Info mode's minibuffer.
@end defvar

@defvar Manual-page-minibuffer-history
A history list for @code{manual-entry}.
@end defvar

  There are many other minibuffer history lists, defined by various
libraries.  An @kbd{M-x apropos} search for @samp{history} should prove
fruitful in discovering them.

@node Completion
@section Completion
@cindex completion

  @dfn{Completion} is a feature that fills in the rest of a name
starting from an abbreviation for it.  Completion works by comparing the
user's input against a list of valid names and determining how much of
the name is determined uniquely by what the user has typed.  For
example, when you type @kbd{C-x b} (@code{switch-to-buffer}) and then
type the first few letters of the name of the buffer to which you wish
to switch, and then type @key{TAB} (@code{minibuffer-complete}), Emacs
extends the name as far as it can.

  Standard XEmacs commands offer completion for names of symbols, files,
buffers, and processes; with the functions in this section, you can
implement completion for other kinds of names.

  The @code{try-completion} function is the basic primitive for
completion: it returns the longest determined completion of a given
initial string, with a given set of strings to match against.

  The function @code{completing-read} provides a higher-level interface
for completion.  A call to @code{completing-read} specifies how to
determine the list of valid names.  The function then activates the
minibuffer with a local keymap that binds a few keys to commands useful
for completion.  Other functions provide convenient simple interfaces
for reading certain kinds of names with completion.

@menu
* Basic Completion::       Low-level functions for completing strings.
                             (These are too low level to use the minibuffer.)
* Minibuffer Completion::  Invoking the minibuffer with completion.
* Completion Commands::    Minibuffer commands that do completion.
* High-Level Completion::  Convenient special cases of completion
                             (reading buffer name, file name, etc.)
* Reading File Names::     Using completion to read file names.
* Programmed Completion::  Finding the completions for a given file name.
@end menu

@node Basic Completion
@subsection Basic Completion Functions

  The two functions @code{try-completion} and @code{all-completions}
have nothing in themselves to do with minibuffers.  We describe them in
this chapter so as to keep them near the higher-level completion
features that do use the minibuffer.

@defun try-completion string collection &optional predicate
This function returns the longest common substring of all possible
completions of @var{string} in @var{collection}.  The value of
@var{collection} must be an alist, an obarray, or a function that
implements a virtual set of strings (see below).

Completion compares @var{string} against each of the permissible
completions specified by @var{collection}; if the beginning of the
permissible completion equals @var{string}, it matches.  If no permissible
completions match, @code{try-completion} returns @code{nil}.  If only
one permissible completion matches, and the match is exact, then
@code{try-completion} returns @code{t}.  Otherwise, the value is the
longest initial sequence common to all the permissible completions that
match.

If @var{collection} is an alist (@pxref{Association Lists}), the
@sc{car}s of the alist elements form the set of permissible completions.

@cindex obarray in completion
If @var{collection} is an obarray (@pxref{Creating Symbols}), the names
of all symbols in the obarray form the set of permissible completions.  The
global variable @code{obarray} holds an obarray containing the names of
all interned Lisp symbols.

Note that the only valid way to make a new obarray is to create it
empty and then add symbols to it one by one using @code{intern}.
Also, you cannot intern a given symbol in more than one obarray.

If the argument @var{predicate} is non-@code{nil}, then it must be a
function of one argument.  It is used to test each possible match, and
the match is accepted only if @var{predicate} returns non-@code{nil}.
The argument given to @var{predicate} is either a cons cell from the alist
(the @sc{car} of which is a string) or else it is a symbol (@emph{not} a
symbol name) from the obarray.

You can also use a symbol that is a function as @var{collection}.  Then
the function is solely responsible for performing completion;
@code{try-completion} returns whatever this function returns.  The
function is called with three arguments: @var{string}, @var{predicate}
and @code{nil}.  (The reason for the third argument is so that the same
function can be used in @code{all-completions} and do the appropriate
thing in either case.)  @xref{Programmed Completion}.

In the first of the following examples, the string @samp{foo} is
matched by three of the alist @sc{car}s.  All of the matches begin with
the characters @samp{fooba}, so that is the result.  In the second
example, there is only one possible match, and it is exact, so the value
is @code{t}.

@smallexample
@group
(try-completion 
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)))
     @result{} "fooba"
@end group

@group
(try-completion "foo" '(("barfoo" 2) ("foo" 3)))
     @result{} t
@end group
@end smallexample

In the following example, numerous symbols begin with the characters
@samp{forw}, and all of them begin with the word @samp{forward}.  In
most of the symbols, this is followed with a @samp{-}, but not in all,
so no more than @samp{forward} can be completed.

@smallexample
@group
(try-completion "forw" obarray)
     @result{} "forward"
@end group
@end smallexample

Finally, in the following example, only two of the three possible
matches pass the predicate @code{test} (the string @samp{foobaz} is
too short).  Both of those begin with the string @samp{foobar}.

@smallexample
@group
(defun test (s) 
  (> (length (car s)) 6))
     @result{} test
@end group
@group
(try-completion 
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4)) 
 'test)
     @result{} "foobar"
@end group
@end smallexample
@end defun

@defun all-completions string collection &optional predicate nospace
This function returns a list of all possible completions of
@var{string}.  The arguments to this function are the same as those of
@code{try-completion}.

If @var{collection} is a function, it is called with three arguments:
@var{string}, @var{predicate} and @code{t}; then @code{all-completions}
returns whatever the function returns.  @xref{Programmed Completion}.

If @var{nospace} is non-@code{nil}, completions that start with a space
are ignored unless @var{string} also starts with a space.

Here is an example, using the function @code{test} shown in the
example for @code{try-completion}:

@smallexample
@group
(defun test (s) 
  (> (length (car s)) 6))
     @result{} test
@end group

@group
(all-completions  
 "foo"
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
 'test)
     @result{} ("foobar1" "foobar2")
@end group
@end smallexample
@end defun

@defvar completion-ignore-case
If the value of this variable is 
non-@code{nil}, XEmacs does not consider case significant in completion.
@end defvar

@node Minibuffer Completion
@subsection Completion and the Minibuffer

  This section describes the basic interface for reading from the
minibuffer with completion.

@defun completing-read prompt collection &optional predicate require-match initial hist default
This function reads a string in the minibuffer, assisting the user by
providing completion.  It activates the minibuffer with prompt
@var{prompt}, which must be a string.  If @var{initial} is
non-@code{nil}, @code{completing-read} inserts it into the minibuffer as
part of the input.  Then it allows the user to edit the input, providing
several commands to attempt completion.

The actual completion is done by passing @var{collection} and
@var{predicate} to the function @code{try-completion}.  This happens in
certain commands bound in the local keymaps used for completion.

If @var{require-match} is @code{t}, the usual minibuffer exit commands
won't exit unless the input completes to an element of @var{collection}.
If @var{require-match} is neither @code{nil} nor @code{t}, then the exit
commands won't exit unless the input typed is itself an element of
@var{collection}.  If @var{require-match} is @code{nil}, the exit
commands work regardless of the input in the minibuffer.

However, empty input is always permitted, regardless of the value of
@var{require-match}; in that case, @code{completing-read} returns
@var{default}.  The value of @var{default} (if non-@code{nil}) is also
available to the user through the history commands.

The user can exit with null input by typing @key{RET} with an empty
minibuffer.  Then @code{completing-read} returns @code{""}.  This is how
the user requests whatever default the command uses for the value being
read.  The user can return using @key{RET} in this way regardless of the
value of @var{require-match}, and regardless of whether the empty string
is included in @var{collection}.

The function @code{completing-read} works by calling
@code{read-expression}.  It uses @code{minibuffer-local-completion-map}
as the keymap if @var{require-match} is @code{nil}, and uses
@code{minibuffer-local-must-match-map} if @var{require-match} is
non-@code{nil}.  @xref{Completion Commands}.

The argument @var{hist} specifies which history list variable to use for
saving the input and for minibuffer history commands.  It defaults to
@code{minibuffer-history}.  @xref{Minibuffer History}.

Completion ignores case when comparing the input against the possible
matches, if the built-in variable @code{completion-ignore-case} is
non-@code{nil}.  @xref{Basic Completion}.

Here's an example of using @code{completing-read}:

@smallexample
@group
(completing-read
 "Complete a foo: "
 '(("foobar1" 1) ("barfoo" 2) ("foobaz" 3) ("foobar2" 4))
 nil t "fo")
@end group

@group
;; @r{After evaluation of the preceding expression,} 
;;   @r{the following appears in the minibuffer:}

---------- Buffer: Minibuffer ----------
Complete a foo: fo@point{}
---------- Buffer: Minibuffer ----------
@end group
@end smallexample

@noindent
If the user then types @kbd{@key{DEL} @key{DEL} b @key{RET}},
@code{completing-read} returns @code{barfoo}.

The @code{completing-read} function binds three variables to pass
information to the commands that actually do completion.  These
variables are @code{minibuffer-completion-table},
@code{minibuffer-completion-predicate} and
@code{minibuffer-completion-confirm}.  For more information about them,
see @ref{Completion Commands}.
@end defun

@node Completion Commands
@subsection Minibuffer Commands That Do Completion

  This section describes the keymaps, commands and user options used in
the minibuffer to do completion.

@defvar minibuffer-local-completion-map
@code{completing-read} uses this value as the local keymap when an
exact match of one of the completions is not required.  By default, this
keymap makes the following bindings:

@table @asis
@item @kbd{?}
@code{minibuffer-completion-help}

@item @key{SPC}
@code{minibuffer-complete-word}

@item @key{TAB}
@code{minibuffer-complete}
@end table

@noindent
with other characters bound as in @code{minibuffer-local-map}
(@pxref{Text from Minibuffer}).
@end defvar

@defvar minibuffer-local-must-match-map
@code{completing-read} uses this value as the local keymap when an
exact match of one of the completions is required.  Therefore, no keys
are bound to @code{exit-minibuffer}, the command that exits the
minibuffer unconditionally.  By default, this keymap makes the following
bindings:

@table @asis
@item @kbd{?}
@code{minibuffer-completion-help}

@item @key{SPC}
@code{minibuffer-complete-word}

@item @key{TAB}
@code{minibuffer-complete}

@item @kbd{C-j}
@code{minibuffer-complete-and-exit}

@item @key{RET}
@code{minibuffer-complete-and-exit}
@end table

@noindent
with other characters bound as in @code{minibuffer-local-map}.
@end defvar

@defvar minibuffer-completion-table
The value of this variable is the alist or obarray used for completion
in the minibuffer.  This is the global variable that contains what
@code{completing-read} passes to @code{try-completion}.  It is used by
minibuffer completion commands such as @code{minibuffer-complete-word}.
@end defvar

@defvar minibuffer-completion-predicate
This variable's value is the predicate that @code{completing-read}
passes to @code{try-completion}.  The variable is also used by the other
minibuffer completion functions.
@end defvar

@deffn Command minibuffer-complete-word
This function completes the minibuffer contents by at most a single
word.  Even if the minibuffer contents have only one completion,
@code{minibuffer-complete-word} does not add any characters beyond the
first character that is not a word constituent.  @xref{Syntax Tables}.
@end deffn

@deffn Command minibuffer-complete
This function completes the minibuffer contents as far as possible.
@end deffn

@deffn Command minibuffer-complete-and-exit
This function completes the minibuffer contents, and exits if
confirmation is not required, i.e., if
@code{minibuffer-completion-confirm} is @code{nil}.  If confirmation
@emph{is} required, it is given by repeating this command
immediately---the command is programmed to work without confirmation
when run twice in succession.
@end deffn

@defvar minibuffer-completion-confirm
When the value of this variable is non-@code{nil}, XEmacs asks for
confirmation of a completion before exiting the minibuffer.  The
function @code{minibuffer-complete-and-exit} checks the value of this
variable before it exits.
@end defvar

@deffn Command minibuffer-completion-help
This function creates a list of the possible completions of the
current minibuffer contents.  It works by calling @code{all-completions}
using the value of the variable @code{minibuffer-completion-table} as
the @var{collection} argument, and the value of
@code{minibuffer-completion-predicate} as the @var{predicate} argument.
The list of completions is displayed as text in a buffer named
@samp{*Completions*}.
@end deffn

@defun display-completion-list completions &rest cl-keys
This function displays @var{completions} to the stream in
@code{standard-output}, usually a buffer.  (@xref{Read and Print}, for more
information about streams.)  The argument @var{completions} is normally
a list of completions just returned by @code{all-completions}, but it
does not have to be.  Each element may be a symbol or a string, either
of which is simply printed, or a list of two strings, which is printed
as if the strings were concatenated.

This function is called by @code{minibuffer-completion-help}.  The
most common way to use it is together with
@code{with-output-to-temp-buffer}, like this:

@example
(with-output-to-temp-buffer "*Completions*"
  (display-completion-list
    (all-completions (buffer-string) my-alist)))
@end example
@end defun

@defopt completion-auto-help
If this variable is non-@code{nil}, the completion commands
automatically display a list of possible completions whenever nothing
can be completed because the next character is not uniquely determined.
@end defopt

@node High-Level Completion
@subsection High-Level Completion  Functions

  This section describes the higher-level convenient functions for
reading certain sorts of names with completion.

  In most cases, you should not call these functions in the middle of a
Lisp function.  When possible, do all minibuffer input as part of
reading the arguments for a command, in the @code{interactive} spec.
@xref{Defining Commands}.

@defun read-buffer prompt &optional default existing
This function reads the name of a buffer and returns it as a string.
The argument @var{default} is the default name to use, the value to
return if the user exits with an empty minibuffer.  If non-@code{nil},
it should be a string or a buffer.  It is mentioned in the prompt, but
is not inserted in the minibuffer as initial input.

If @var{existing} is non-@code{nil}, then the name specified must be
that of an existing buffer.  The usual commands to exit the minibuffer
do not exit if the text is not valid, and @key{RET} does completion to
attempt to find a valid name.  (However, @var{default} is not checked
for validity; it is returned, whatever it is, if the user exits with the
minibuffer empty.)

In the following example, the user enters @samp{minibuffer.t}, and
then types @key{RET}.  The argument @var{existing} is @code{t}, and the
only buffer name starting with the given input is
@samp{minibuffer.texi}, so that name is the value.

@example
(read-buffer "Buffer name? " "foo" t)
@group
;; @r{After evaluation of the preceding expression,} 
;;   @r{the following prompt appears,}
;;   @r{with an empty minibuffer:}
@end group

@group
---------- Buffer: Minibuffer ----------
Buffer name? (default foo) @point{}
---------- Buffer: Minibuffer ----------
@end group

@group
;; @r{The user types @kbd{minibuffer.t @key{RET}}.}
     @result{} "minibuffer.texi"
@end group
@end example
@end defun

@defun read-command prompt &optinal default-value
This function reads the name of a command and returns it as a Lisp
symbol.  The argument @var{prompt} is used as in
@code{read-from-minibuffer}.  Recall that a command is anything for
which @code{commandp} returns @code{t}, and a command name is a symbol
for which @code{commandp} returns @code{t}.  @xref{Interactive Call}.

The argument @var{default-value} specifies what to return if the user
enters null input.  It can be a symbol or a string; if it is a string,
@code{read-command} interns it before returning it.  If @var{default} is
@code{nil}, that means no default has been specified; then if the user
enters null input, the return value is @code{nil}.

@example
(read-command "Command name? ")

@group
;; @r{After evaluation of the preceding expression,} 
;;   @r{the following prompt appears with an empty minibuffer:}
@end group

@group
---------- Buffer: Minibuffer ---------- 
Command name?  
---------- Buffer: Minibuffer ----------
@end group
@end example

@noindent
If the user types @kbd{forward-c @key{RET}}, then this function returns
@code{forward-char}.

The @code{read-command} function is a simplified interface to the
function @code{completing-read}.  It uses the variable @code{obarray} so
as to complete in the set of extant Lisp symbols, and it uses the
@code{commandp} predicate so as to accept only command names:

@cindex @code{commandp} example
@example
@group
(read-command @var{prompt})
@equiv{}
(intern (completing-read @var{prompt} obarray 
                         'commandp t nil))
@end group
@end example
@end defun

@defun read-variable prompt &optional default-value
This function reads the name of a user variable and returns it as a
symbol.

The argument @var{default-value} specifies what to return if the user
enters null input.  It can be a symbol or a string; if it is a string,
@code{read-variable} interns it before returning it.  If @var{default}
is @code{nil}, that means no default has been specified; then if the
user enters null input, the return value is @code{nil}.

@example
@group
(read-variable "Variable name? ")

;; @r{After evaluation of the preceding expression,} 
;;   @r{the following prompt appears,} 
;;   @r{with an empty minibuffer:}
@end group

@group
---------- Buffer: Minibuffer ----------
Variable name? @point{}
---------- Buffer: Minibuffer ----------
@end group
@end example

@noindent
If the user then types @kbd{fill-p @key{RET}}, @code{read-variable}
returns @code{fill-prefix}.

This function is similar to @code{read-command}, but uses the
predicate @code{user-variable-p} instead of @code{commandp}:

@cindex @code{user-variable-p} example
@example
@group
(read-variable @var{prompt})
@equiv{}
(intern
 (completing-read @var{prompt} obarray
                  'user-variable-p t nil))
@end group
@end example
@end defun

@node Reading File Names
@subsection Reading File Names

  Here is another high-level completion function, designed for reading a
file name.  It provides special features including automatic insertion
of the default directory.

@defun read-file-name prompt &optional directory default existing initial history
This function reads a file name in the minibuffer, prompting with
@var{prompt} and providing completion.  If @var{default} is
non-@code{nil}, then the function returns @var{default} if the user just
types @key{RET}.  @var{default} is not checked for validity; it is
returned, whatever it is, if the user exits with the minibuffer empty.

If @var{existing} is non-@code{nil}, then the user must specify the name
of an existing file; @key{RET} performs completion to make the name
valid if possible, and then refuses to exit if it is not valid.  If the
value of @var{existing} is neither @code{nil} nor @code{t}, then
@key{RET} also requires confirmation after completion.  If
@var{existing} is @code{nil}, then the name of a nonexistent file is
acceptable.

The argument @var{directory} specifies the directory to use for
completion of relative file names.  If @code{insert-default-directory}
is non-@code{nil}, @var{directory} is also inserted in the minibuffer as
initial input.  It defaults to the current buffer's value of
@code{default-directory}.

@c Emacs 19 feature
If you specify @var{initial}, that is an initial file name to insert in
the buffer (after @var{directory}, if that is inserted).  In this
case, point goes at the beginning of @var{initial}.  The default for
@var{initial} is @code{nil}---don't insert any file name.  To see what
@var{initial} does, try the command @kbd{C-x C-v}.

Here is an example: 

@example
@group
(read-file-name "The file is ")

;; @r{After evaluation of the preceding expression,} 
;;   @r{the following appears in the minibuffer:}
@end group

@group
---------- Buffer: Minibuffer ----------
The file is /gp/gnu/elisp/@point{}
---------- Buffer: Minibuffer ----------
@end group
@end example

@noindent
Typing @kbd{manual @key{TAB}} results in the following:

@example
@group
---------- Buffer: Minibuffer ----------
The file is /gp/gnu/elisp/manual.texi@point{}
---------- Buffer: Minibuffer ----------
@end group
@end example

@c Wordy to avoid overfull hbox in smallbook mode.
@noindent
If the user types @key{RET}, @code{read-file-name} returns the file name
as the string @code{"/gp/gnu/elisp/manual.texi"}.
@end defun

@defopt insert-default-directory
This variable is used by @code{read-file-name}.  Its value controls
whether @code{read-file-name} starts by placing the name of the default
directory in the minibuffer, plus the initial file name if any.  If the
value of this variable is @code{nil}, then @code{read-file-name} does
not place any initial input in the minibuffer (unless you specify
initial input with the @var{initial} argument).  In that case, the
default directory is still used for completion of relative file names,
but is not displayed.

For example:

@example
@group
;; @r{Here the minibuffer starts out with the default directory.}
(let ((insert-default-directory t))
  (read-file-name "The file is "))
@end group

@group
---------- Buffer: Minibuffer ----------
The file is ~lewis/manual/@point{}
---------- Buffer: Minibuffer ----------
@end group

@group
;; @r{Here the minibuffer is empty and only the prompt}
;;   @r{appears on its line.}
(let ((insert-default-directory nil))
  (read-file-name "The file is "))
@end group

@group
---------- Buffer: Minibuffer ----------
The file is @point{}
---------- Buffer: Minibuffer ----------
@end group
@end example
@end defopt

@node Programmed Completion
@subsection Programmed Completion
@cindex programmed completion

  Sometimes it is not possible to create an alist or an obarray
containing all the intended possible completions.  In such a case, you
can supply your own function to compute the completion of a given string.
This is called @dfn{programmed completion}.

  To use this feature, pass a symbol with a function definition as the
@var{collection} argument to @code{completing-read}.  The function
@code{completing-read} arranges to pass your completion function along
to @code{try-completion} and @code{all-completions}, which will then let
your function do all the work.

  The completion function should accept three arguments:

@itemize @bullet
@item
The string to be completed.

@item
The predicate function to filter possible matches, or @code{nil} if
none.  Your function should call the predicate for each possible match,
and ignore the possible match if the predicate returns @code{nil}.

@item
A flag specifying the type of operation.
@end itemize

  There are three flag values for three operations:

@itemize @bullet
@item
@code{nil} specifies @code{try-completion}.  The completion function
should return the completion of the specified string, or @code{t} if the
string is a unique and exact match already, or @code{nil} if the string
matches no possibility.

If the string is an exact match for one possibility, but also matches
other longer possibilities, the function should return the string, not
@code{t}.

@item
@code{t} specifies @code{all-completions}.  The completion function
should return a list of all possible completions of the specified
string.

@item
@code{lambda} specifies a test for an exact match.  The completion
function should return @code{t} if the specified string is an exact
match for some possibility; @code{nil} otherwise.
@end itemize

  It would be consistent and clean for completion functions to allow
lambda expressions (lists that are functions) as well as function
symbols as @var{collection}, but this is impossible.  Lists as
completion tables are already assigned another meaning---as alists.  It
would be unreliable to fail to handle an alist normally because it is
also a possible function.  So you must arrange for any function you wish
to use for completion to be encapsulated in a symbol.

  Emacs uses programmed completion when completing file names.
@xref{File Name Completion}.

@node Yes-or-No Queries
@section Yes-or-No Queries
@cindex asking the user questions
@cindex querying the user
@cindex yes-or-no questions

  This section describes functions used to ask the user a yes-or-no
question.  The function @code{y-or-n-p} can be answered with a single
character; it is useful for questions where an inadvertent wrong answer
will not have serious consequences.  @code{yes-or-no-p} is suitable for
more momentous questions, since it requires three or four characters to
answer.  Variations of these functions can be used to ask a yes-or-no
question using a dialog box, or optionally using one.

   If either of these functions is called in a command that was invoked
using the mouse, then it uses a dialog box or pop-up menu to ask the
question.  Otherwise, it uses keyboard input.

  Strictly speaking, @code{yes-or-no-p} uses the minibuffer and
@code{y-or-n-p} does not; but it seems best to describe them together.

@defun y-or-n-p prompt
This function asks the user a question, expecting input in the echo
area.  It returns @code{t} if the user types @kbd{y}, @code{nil} if the
user types @kbd{n}.  This function also accepts @key{SPC} to mean yes
and @key{DEL} to mean no.  It accepts @kbd{C-]} to mean ``quit'', like
@kbd{C-g}, because the question might look like a minibuffer and for
that reason the user might try to use @kbd{C-]} to get out.  The answer
is a single character, with no @key{RET} needed to terminate it.  Upper
and lower case are equivalent.

``Asking the question'' means printing @var{prompt} in the echo area,
followed by the string @w{@samp{(y or n) }}.  If the input is not one of
the expected answers (@kbd{y}, @kbd{n}, @kbd{@key{SPC}},
@kbd{@key{DEL}}, or something that quits), the function responds
@samp{Please answer y or n.}, and repeats the request.

This function does not actually use the minibuffer, since it does not
allow editing of the answer.  It actually uses the echo area (@pxref{The
Echo Area}), which uses the same screen space as the minibuffer.  The
cursor moves to the echo area while the question is being asked.

The answers and their meanings, even @samp{y} and @samp{n}, are not
hardwired.  The keymap @code{query-replace-map} specifies them.
@xref{Search and Replace}.

In the following example, the user first types @kbd{q}, which is
invalid.  At the next prompt the user types @kbd{y}.

@smallexample
@group
(y-or-n-p "Do you need a lift? ")

;; @r{After evaluation of the preceding expression,} 
;;   @r{the following prompt appears in the echo area:}
@end group

@group
---------- Echo area ----------
Do you need a lift? (y or n) 
---------- Echo area ----------
@end group

;; @r{If the user then types @kbd{q}, the following appears:}

@group
---------- Echo area ----------
Please answer y or n.  Do you need a lift? (y or n) 
---------- Echo area ----------
@end group

;; @r{When the user types a valid answer,}
;;   @r{it is displayed after the question:}

@group
---------- Echo area ----------
Do you need a lift? (y or n) y
---------- Echo area ----------
@end group
@end smallexample

@noindent
We show successive lines of echo area messages, but only one actually
appears on the screen at a time.
@end defun

@defun yes-or-no-p prompt
This function asks the user a question, expecting input in the
minibuffer.  It returns @code{t} if the user enters @samp{yes},
@code{nil} if the user types @samp{no}.  The user must type @key{RET} to
finalize the response.  Upper and lower case are equivalent.

@code{yes-or-no-p} starts by displaying @var{prompt} in the echo area,
followed by @w{@samp{(yes or no) }}.  The user must type one of the
expected responses; otherwise, the function responds @samp{Please answer
yes or no.}, waits about two seconds and repeats the request.

@code{yes-or-no-p} requires more work from the user than
@code{y-or-n-p} and is appropriate for more crucial decisions.

Here is an example:

@smallexample
@group
(yes-or-no-p "Do you really want to remove everything? ")

;; @r{After evaluation of the preceding expression,} 
;;   @r{the following prompt appears,} 
;;   @r{with an empty minibuffer:}
@end group

@group
---------- Buffer: minibuffer ----------
Do you really want to remove everything? (yes or no) 
---------- Buffer: minibuffer ----------
@end group
@end smallexample

@noindent
If the user first types @kbd{y @key{RET}}, which is invalid because this
function demands the entire word @samp{yes}, it responds by displaying
these prompts, with a brief pause between them:

@smallexample
@group
---------- Buffer: minibuffer ----------
Please answer yes or no.
Do you really want to remove everything? (yes or no)
---------- Buffer: minibuffer ----------
@end group
@end smallexample
@end defun

@c The rest is XEmacs stuff
@defun yes-or-no-p-dialog-box prompt
This function asks the user a ``y or n'' question with a popup dialog
box.  It returns @code{t} if the answer is ``yes''.  @var{prompt} is the
string to display to ask the question.
@end defun

The following functions ask a question either in the minibuffer or a
dialog box, depending on whether the last user event (which presumably
invoked this command) was a keyboard or mouse event.  When XEmacs is
running on a window system, the functions @code{y-or-n-p} and
@code{yes-or-no-p} are replaced with the following functions, so that
menu items bring up dialog boxes instead of minibuffer questions.

@defun y-or-n-p-maybe-dialog-box prompt
This function asks user a ``y or n'' question, using either a dialog box
or the minibuffer, as appropriate.
@end defun

@defun yes-or-no-p-maybe-dialog-box prompt
This function asks user a ``yes or no'' question, using either a dialog
box or the minibuffer, as appropriate.
@end defun

@node Multiple Queries
@section Asking Multiple Y-or-N Questions

  When you have a series of similar questions to ask, such as ``Do you
want to save this buffer'' for each buffer in turn, you should use
@code{map-y-or-n-p} to ask the collection of questions, rather than
asking each question individually.  This gives the user certain
convenient facilities such as the ability to answer the whole series at
once.

@defun map-y-or-n-p prompter actor list &optional help action-alist
This function, new in Emacs 19, asks the user a series of questions,
reading a single-character answer in the echo area for each one.

The value of @var{list} specifies the objects to ask questions about.
It should be either a list of objects or a generator function.  If it is
a function, it should expect no arguments, and should return either the
next object to ask about, or @code{nil} meaning stop asking questions.

The argument @var{prompter} specifies how to ask each question.  If
@var{prompter} is a string, the question text is computed like this:

@example
(format @var{prompter} @var{object})
@end example

@noindent
where @var{object} is the next object to ask about (as obtained from
@var{list}).

If not a string, @var{prompter} should be a function of one argument
(the next object to ask about) and should return the question text.  If
the value is a string, that is the question to ask the user.  The
function can also return @code{t} meaning do act on this object (and
don't ask the user), or @code{nil} meaning ignore this object (and don't
ask the user).

The argument @var{actor} says how to act on the answers that the user
gives.  It should be a function of one argument, and it is called with
each object that the user says yes for.  Its argument is always an
object obtained from @var{list}.

If the argument @var{help} is given, it should be a list of this form:

@example
(@var{singular} @var{plural} @var{action})
@end example

@noindent
where @var{singular} is a string containing a singular noun that
describes the objects conceptually being acted on, @var{plural} is the
corresponding plural noun, and @var{action} is a transitive verb
describing what @var{actor} does.

If you don't specify @var{help}, the default is @code{("object"
"objects" "act on")}.

Each time a question is asked, the user may enter @kbd{y}, @kbd{Y}, or
@key{SPC} to act on that object; @kbd{n}, @kbd{N}, or @key{DEL} to skip
that object; @kbd{!} to act on all following objects; @key{ESC} or
@kbd{q} to exit (skip all following objects); @kbd{.} (period) to act on
the current object and then exit; or @kbd{C-h} to get help.  These are
the same answers that @code{query-replace} accepts.  The keymap
@code{query-replace-map} defines their meaning for @code{map-y-or-n-p}
as well as for @code{query-replace}; see @ref{Search and Replace}.

You can use @var{action-alist} to specify additional possible answers
and what they mean.  It is an alist of elements of the form
@code{(@var{char} @var{function} @var{help})}, each of which defines one
additional answer.  In this element, @var{char} is a character (the
answer); @var{function} is a function of one argument (an object from
@var{list}); @var{help} is a string.

When the user responds with @var{char}, @code{map-y-or-n-p} calls
@var{function}.  If it returns non-@code{nil}, the object is considered
``acted upon'', and @code{map-y-or-n-p} advances to the next object in
@var{list}.  If it returns @code{nil}, the prompt is repeated for the
same object.

If @code{map-y-or-n-p} is called in a command that was invoked using the
mouse---more precisely, if @code{last-nonmenu-event} (@pxref{Command
Loop Info}) is either @code{nil} or a list---then it uses a dialog box
or pop-up menu to ask the question.  In this case, it does not use
keyboard input or the echo area.  You can force use of the mouse or use
of keyboard input by binding @code{last-nonmenu-event} to a suitable
value around the call.

The return value of @code{map-y-or-n-p} is the number of objects acted on.
@end defun

@node Reading a Password
@section Reading a Password
@cindex passwords, reading

  To read a password to pass to another program, you can use the
function @code{read-passwd}.

@defun read-passwd prompt &optional confirm default
This function reads a password, prompting with @var{prompt}.  It does
not echo the password as the user types it; instead, it echoes @samp{.}
for each character in the password.

The optional argument @var{confirm}, if non-@code{nil}, says to read the
password twice and insist it must be the same both times.  If it isn't
the same, the user has to type it over and over until the last two
times match.

The optional argument @var{default} specifies the default password to
return if the user enters empty input.  It is translated to @samp{.}
and inserted in the minibuffer. If @var{default} is @code{nil}, then
@code{read-passwd} returns the null string in that case.
@end defun

@defopt passwd-invert-frame-when-keyboard-grabbed
If non-nil swap the foreground and background colors of all faces while
reading a password.  Default values is @code{t} unless feature
@code{infodock} is provided.
@end defopt

@defopt passwd-echo
This specifies the character echoed when typing a password.  When nil,
nothing is echoed.
@end defopt

@node Minibuffer Misc
@section Minibuffer Miscellany

  This section describes some basic functions and variables related to
minibuffers.

@deffn Command exit-minibuffer
This command exits the active minibuffer.  It is normally bound to
keys in minibuffer local keymaps.
@end deffn

@deffn Command self-insert-and-exit
This command exits the active minibuffer after inserting the last
character typed on the keyboard (found in @code{last-command-char};
@pxref{Command Loop Info}).
@end deffn

@deffn Command previous-history-element n
This command replaces the minibuffer contents with the value of the
@var{n}th previous (older) history element.
@end deffn

@deffn Command next-history-element n
This command replaces the minibuffer contents with the value of the
@var{n}th more recent history element.
@end deffn

@deffn Command previous-matching-history-element pattern
This command replaces the minibuffer contents with the value of the
previous (older) history element that matches @var{pattern} (a regular
expression).
@end deffn

@deffn Command next-matching-history-element pattern
This command replaces the minibuffer contents with the value of the next
(newer) history element that matches @var{pattern} (a regular
expression).
@end deffn

@defun minibuffer-prompt
This function returns the prompt string of the currently active
minibuffer.  If no minibuffer is active, it returns @code{nil}.
@end defun

@defun minibuffer-prompt-width
This function returns the display width of the prompt string of the
currently active minibuffer.  If no minibuffer is active, it returns 0.
@end defun

@defvar minibuffer-setup-hook
This is a normal hook that is run whenever the minibuffer is entered.
@xref{Hooks}.
@end defvar

@defvar minibuffer-exit-hook
This is a normal hook that is run whenever the minibuffer is exited.
@xref{Hooks}.
@end defvar

@defvar minibuffer-help-form
The current value of this variable is used to rebind @code{help-form}
locally inside the minibuffer (@pxref{Help Functions}).
@end defvar

@defun active-minibuffer-window
This function returns the currently active minibuffer window, or
@code{nil} if none is currently active.
@end defun

@defun minibuffer-window &optional frame
This function returns the minibuffer window used for frame @var{frame}.
If @var{frame} is @code{nil}, that stands for the current frame.  Note
that the minibuffer window used by a frame need not be part of that
frame---a frame that has no minibuffer of its own necessarily uses some
other frame's minibuffer window.
@end defun

@c Emacs 19 feature
@defun window-minibuffer-p window
This function returns non-@code{nil} if @var{window} is a minibuffer window.
@end defun

It is not correct to determine whether a given window is a minibuffer by
comparing it with the result of @code{(minibuffer-window)}, because
there can be more than one minibuffer window if there is more than one
frame.

@defun minibuffer-window-active-p window
This function returns non-@code{nil} if @var{window}, assumed to be
a minibuffer window, is currently active.
@end defun

@defvar minibuffer-scroll-window
If the value of this variable is non-@code{nil}, it should be a window
object.  When the function @code{scroll-other-window} is called in the
minibuffer, it scrolls this window.
@end defvar

Finally, some functions and variables deal with recursive minibuffers
(@pxref{Recursive Editing}):

@defun minibuffer-depth
This function returns the current depth of activations of the
minibuffer, a nonnegative integer.  If no minibuffers are active, it
returns zero.
@end defun

@defopt enable-recursive-minibuffers
If this variable is non-@code{nil}, you can invoke commands (such as
@code{find-file}) that use minibuffers even while the minibuffer window
is active.  Such invocation produces a recursive editing level for a new
minibuffer.  The outer-level minibuffer is invisible while you are
editing the inner one.

This variable only affects invoking the minibuffer while the
minibuffer window is selected.   If you switch windows while in the 
minibuffer, you can always invoke minibuffer commands while some other
window is selected.
@end defopt

@c Emacs 19 feature
In FSF Emacs 19, if a command name has a property
@code{enable-recursive-minibuffers} that is non-@code{nil}, then the
command can use the minibuffer to read arguments even if it is invoked
from the minibuffer.  The minibuffer command
@code{next-matching-history-element} (normally @kbd{M-s} in the
minibuffer) uses this feature.

This is not implemented in XEmacs because it is a kludge.  If you
want to explicitly set the value of @code{enable-recursive-minibuffers}
in this fashion, just use an evaluated interactive spec and bind
@code{enable-recursive-minibuffers} while reading from the minibuffer.
See the definition of @code{next-matching-history-element} in
@file{lisp/minibuf.el}.