(Basic Completion): Minor clarification.

[emacs.git] / lispref / minibuf.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998, 1999, 2001, 2002,
@c   2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@setfilename ../info/minibuf
@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 Emacs 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
(@pxref{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.
* Initial Input::             Specifying initial contents for the minibuffer.
* 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 Commands::       Commands used as key bindings in minibuffers.
* Minibuffer Contents::       How such commands access the minibuffer text.
* Minibuffer Windows::        Operating on the special minibuffer windows.
* Recursive Mini::            Whether recursive entry to minibuffer is allowed.
* 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 Emacs 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 text in the minibuffer always starts with the @dfn{prompt string},
the text that was specified by the program that is using the minibuffer
to tell the user what sort of input to type.  This text is marked
read-only so you won't accidentally delete or change it.  It is also
marked as a field (@pxref{Fields}), so that certain motion functions,
including @code{beginning-of-line}, @code{forward-word},
@code{forward-sentence}, and @code{forward-paragraph}, stop at the
boundary between the prompt and the actual text.  (In older Emacs
versions, the prompt was displayed using a special mechanism and was not
part of the buffer contents.)

  The minibuffer's window is normally a single line; it grows
automatically if necessary if the contents require more space.  You can
explicitly 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.

  Use of the minibuffer reads input events, and that alters the values
of variables such as @code{this-command} and @code{last-command}
(@pxref{Command Loop Info}).  Your program should bind them around the
code that uses the minibuffer, if you do not want that to change them.

  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} or by putting properties of that
name on command symbols (@pxref{Recursive Mini}).

  Like other buffers, a minibuffer uses a local keymap
(@pxref{Keymaps}) to specify special key bindings.  The function that
invokes the minibuffer also sets up its local map according to the job
to be done.  @xref{Text from Minibuffer}, for the non-completion
minibuffer local maps.  @xref{Completion Commands}, for the minibuffer
local maps for completion.

  When Emacs is running in batch mode, any request to read from the
minibuffer actually reads a line from the standard input descriptor that
was supplied when Emacs was started.

@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.  There are also specialized commands for reading
commands, variables, file names, etc. (@pxref{Completion}).

  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}
specification.  @xref{Defining Commands}.

@defun read-from-minibuffer prompt-string &optional initial-contents keymap read hist default inherit-input-method
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 the user can edit text in the minibuffer.

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 non-@code{nil}, the user can access it using
@code{next-history-element}, usually bound in the minibuffer to
@kbd{M-n}.  If @var{read} is non-@code{nil}, then @var{default} is
also used as the input to @code{read}, if the user enters empty input.
(If @var{read} is non-@code{nil} and @var{default} is @code{nil}, empty
input results in an @code{end-of-file} error.)  However, in the usual
case (where @var{read} is @code{nil}), @code{read-from-minibuffer}
ignores @var{default} when the user enters empty input and returns an
empty string, @code{""}.  In this respect, it is different from all
the other minibuffer input functions in this chapter.

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{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}.

If the variable @code{minibuffer-allow-text-properties} is
non-@code{nil}, then the string which is returned includes whatever text
properties were present in the minibuffer.  Otherwise all the text
properties are stripped when the value is returned.

If the argument @var{inherit-input-method} is non-@code{nil}, then the
minibuffer inherits the current input method (@pxref{Input Methods}) and
the setting of @code{enable-multibyte-characters} (@pxref{Text
Representations}) from whichever buffer was current before entering the
minibuffer.

Use of @var{initial-contents} is mostly deprecated; we recommend using
a non-@code{nil} value only in conjunction with specifying a cons cell
for @var{hist}.  @xref{Initial Input}.
@end defun

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

The optional argument @var{default} is used as in
@code{read-from-minibuffer}, except that, if non-@code{nil}, it also
specifies a default value to return if the user enters null input.  As
in @code{read-from-minibuffer} it should be a string, or @code{nil},
which is equivalent to an empty 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} @var{inherit})
@equiv{}
(let ((value
       (read-from-minibuffer @var{prompt} @var{initial} nil nil
                             @var{history} @var{default} @var{inherit})))
  (if (and (equal value "") @var{default})
      @var{default}
    value))
@end group
@end smallexample
@end defun

@defvar minibuffer-allow-text-properties
If this variable is @code{nil}, then @code{read-from-minibuffer} strips
all text properties from the minibuffer input before returning it.
This variable also affects @code{read-string}.  However,
@code{read-no-blanks-input} (see below), as well as
@code{read-minibuffer} and related functions (@pxref{Object from
Minibuffer,, Reading Lisp Objects With the Minibuffer}), and all
functions that do minibuffer input with completion, discard text
properties unconditionally, regardless of the value of this variable.
@end defvar

@defvar minibuffer-local-map
This
@anchor{Definition of minibuffer-local-map}
@c avoid page break at anchor; work around Texinfo deficiency
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}
@itemx @key{DOWN}
@code{next-history-element}

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

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

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

@c In version 18, initial is required
@c Emacs 19 feature
@defun read-no-blanks-input prompt &optional initial inherit-input-method
This function reads a string from the minibuffer, but does not allow
whitespace characters as part of the input: instead, those characters
terminate the input.  The arguments @var{prompt}, @var{initial}, and
@var{inherit-input-method} are used as in @code{read-from-minibuffer}.

This is a simplified interface to the @code{read-from-minibuffer}
function, and passes the value of the @code{minibuffer-local-ns-map}
keymap as the @var{keymap} argument for that function.  Since the keymap
@code{minibuffer-local-ns-map} does not rebind @kbd{C-q}, it @emph{is}
possible to put a space into the string, by quoting it.

This function discards text properties, regardless of the value of
@code{minibuffer-allow-text-properties}.

@smallexample
@group
(read-no-blanks-input @var{prompt} @var{initial})
@equiv{}
(let (minibuffer-allow-text-properties)
  (read-from-minibuffer @var{prompt} @var{initial} minibuffer-local-ns-map))
@end group
@end smallexample
@end defun

@defvar minibuffer-local-ns-map
This built-in variable is the keymap used as the minibuffer local keymap
in the function @code{read-no-blanks-input}.  By default, it makes the
following bindings, in addition to those of @code{minibuffer-local-map}:

@table @asis
@item @key{SPC}
@cindex @key{SPC} in minibuffer
@code{exit-minibuffer}

@item @key{TAB}
@cindex @key{TAB} in minibuffer
@code{exit-minibuffer}

@item @kbd{?}
@cindex @kbd{?} in minibuffer
@code{self-insert-and-exit}
@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-minibuffer prompt &optional initial
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}.

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

@smallexample
@group
(read-minibuffer @var{prompt} @var{initial})
@equiv{}
(let (minibuffer-allow-text-properties)
  (read-from-minibuffer @var{prompt} @var{initial} nil t))
@end group
@end smallexample

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

@smallexample
@group
(read-minibuffer
 "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 eval-minibuffer prompt &optional initial
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}.

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

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

@defun edit-and-eval-command prompt form
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{form} 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{nil} 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.

  You specify the history list with the optional @var{hist} argument
to either @code{read-from-minibuffer} or @code{completing-read}.  Here
are the possible values for it:

@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} (a nonnegative integer).

Specifying 0 for @var{startpos} is equivalent to just specifying the
symbol @var{variable}.  @code{previous-history-element} will display
the most recent element of the history list in the minibuffer.  If you
specify a positive @var{startpos}, the minibuffer history functions
behave as if @code{(elt @var{variable} (1- @var{STARTPOS}))} were the
history element currently shown in the minibuffer.

For consistency, you should also specify that element of the history
as the initial minibuffer contents, using the @var{initial} argument
to the minibuffer input function (@pxref{Initial Input}).
@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.

  Emacs functions that add a new element to a history list can also
delete old elements if the list gets too long.  The variable
@code{history-length} specifies the maximum length for most history
lists.  To specify a different maximum length for a particular history
list, put the length in the @code{history-length} property of the
history list symbol.  The variable @code{history-delete-duplicates}
specifies whether to delete duplicates in history.

@defun add-to-history history-var newelt &optional maxelt keep-all
This function adds a new element @var{newelt}, if it isn't the empty
string, to the history list stored in the variable @var{history-var},
and returns the updated history list.  It limits the list length to
the value of @var{maxelt} (if non-@code{nil}) or @code{history-length}
(described below).  The possible values of @var{maxelt} have the same
meaning as the values of @code{history-length}.

Normally, @code{add-to-history} removes duplicate members from the
history list if @code{history-delete-duplicates} is non-@code{nil}.
However, if @var{keep-all} is non-@code{nil}, that says not to remove
duplicates, and to add @var{newelt} to the list even if it is empty.
@end defun

@defvar history-add-new-input
If the value of this variable is @code{nil}, standard functions that
read from the minibuffer don't add new elements to the history list.
This lets Lisp programs explicitly manage input history by using
@code{add-to-history}.  By default, @code{history-add-new-input} is
set to a non-@code{nil} value.
@end defvar

@defvar history-length
The value of this variable specifies the maximum length for all
history lists that don't specify their own maximum lengths.  If the
value is @code{t}, that means there no maximum (don't delete old
elements).  The value of @code{history-length} property of the history
list variable's symbol, if set, overrides this variable for that
particular history list.
@end defvar

@defvar history-delete-duplicates
If the value of this variable is @code{t}, that means when adding a
new history element, all previous identical elements are deleted.
@end defvar

  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 buffer-name-history
A history list for buffer-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

@node Initial Input
@section Initial Input

Several of the functions for minibuffer input have an argument called
@var{initial} or @var{initial-contents}.  This is a mostly-deprecated
feature for specifying that the minibuffer should start out with
certain text, instead of empty as usual.

If @var{initial} is a string, the minibuffer starts out containing the
text of the string, with point at the end, when the user starts to
edit the text.  If the user simply types @key{RET} to exit the
minibuffer, it will use the initial input string to determine the
value to return.

@strong{We discourage use of a non-@code{nil} value for
@var{initial}}, because initial input is an intrusive interface.
History lists and default values provide a much more convenient method
to offer useful default inputs to the user.

There is just one situation where you should specify a string for an
@var{initial} argument.  This is when you specify a cons cell for the
@var{hist} or @var{history} argument.  @xref{Minibuffer History}.

@var{initial} can 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 at @var{position} within the string's text.

As a historical accident, @var{position} was implemented
inconsistently in different functions.  In @code{completing-read},
@var{position}'s value is interpreted as origin-zero; that is, a value
of 0 means the beginning of the string, 1 means after the first
character, etc.  In @code{read-minibuffer}, and the other
non-completion minibuffer input functions that support this argument,
1 means the beginning of the string 2 means after the first character,
etc.

Use of a cons cell as the value for @var{initial} arguments is
deprecated in user code.

@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 Emacs 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::  Writing your own completion-function.
@end menu

@node Basic Completion
@subsection Basic Completion Functions

  The completion functions @code{try-completion},
@code{all-completions} and @code{test-completion} 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.

  If you store a completion alist in a variable, you should mark the
variable as ``risky'' with a non-@code{nil}
@code{risky-local-variable} property.

@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 a list of strings or symbols, an alist, an
obarray, a hash table, 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
permissible completions are the elements of the alist that are either
strings, symbols, or conses whose @sc{car} is a string or symbol.
Symbols are converted to strings using @code{symbol-name}.  Other
elements of the alist are ignored. (Remember that in Emacs Lisp, the
elements of alists do not @emph{have} to be conses.)  In particular, a
list of strings or symbols is allowed, even though we usually do not
think of such lists as alists.

@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 @var{collection} is a hash table, then the keys that are strings
are the possible completions.  Other keys are ignored.

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}.

If the argument @var{predicate} is non-@code{nil}, then it must be a
function of one argument, unless @var{collection} is a hash table, in
which case it should be a function of two arguments.  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 string or a cons cell (the @sc{car} of
which is a string) from the alist, or a symbol (@emph{not} a symbol
name) from the obarray.  If @var{collection} is a hash table,
@var{predicate} is called with two arguments, the string key and the
associated value.

In addition, to be acceptable, a completion must also match all the
regular expressions in @code{completion-regexp-list}.  (Unless
@var{collection} is a function, in which case that function has to
handle @code{completion-regexp-list} itself.)

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 (aside from
@var{nospace}) are the same as those of @code{try-completion}.  Also,
this function uses @code{completion-regexp-list} in the same way that
@code{try-completion} does.  The optional argument @var{nospace} only
matters if @var{string} is the empty string.  In that case, if
@var{nospace} is non-@code{nil}, completions that start with a space
are ignored.

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}.

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

@defun test-completion string collection &optional predicate
@anchor{Definition of test-completion}
This function returns non-@code{nil} if @var{string} is a valid
completion possibility specified by @var{collection} and
@var{predicate}.  The arguments are the same as in
@code{try-completion}.  For instance, if @var{collection} is a list of
strings, this is true if @var{string} appears in the list and
@var{predicate} is satisfied.

@code{test-completion} uses @code{completion-regexp-list} in the same
way that @code{try-completion} does.

If @var{predicate} is non-@code{nil} and if @var{collection} contains
several strings that are equal to each other, as determined by
@code{compare-strings} according to @code{completion-ignore-case},
then @var{predicate} should accept either all or none of them.
Otherwise, the return value of @code{test-completion} is essentially
unpredictable.

If @var{collection} is a function, it is called with three arguments,
the values @var{string}, @var{predicate} and @code{lambda}; whatever
it returns, @code{test-completion} returns in turn.
@end defun

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

@defvar completion-regexp-list
This is a list of regular expressions.  The completion functions only
consider a completion acceptable if it matches all regular expressions
in this list, with @code{case-fold-search} (@pxref{Searching and Case})
bound to the value of @code{completion-ignore-case}.
@end defvar

@defmac lazy-completion-table var fun
This macro provides a way to initialize the variable @var{var} as a
collection for completion in a lazy way, not computing its actual
contents until they are first needed.  You use this macro to produce a
value that you store in @var{var}.  The actual computation of the
proper value is done the first time you do completion using @var{var}.
It is done by calling @var{fun} with no arguments.  The
value @var{fun} returns becomes the permanent value of @var{var}.

Here is an example of use:

@smallexample
(defvar foo (lazy-completion-table foo make-my-alist))
@end smallexample
@end defmac

@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 inherit-input-method
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.

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.
Some of these commands also call @code{test-completion}.  Thus, if
@var{predicate} is non-@code{nil}, it should be compatible with
@var{collection} and @code{completion-ignore-case}.  @xref{Definition
of test-completion}.

If @var{require-match} is @code{nil}, the exit commands work regardless
of the input in the minibuffer.  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 already in the buffer matches an element of @var{collection}.

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

The function @code{completing-read} 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}.

The argument @var{initial} is mostly deprecated; we recommend using a
non-@code{nil} value only in conjunction with specifying a cons cell
for @var{hist}.  @xref{Initial Input}.  For default input, use
@var{default} instead.

If the argument @var{inherit-input-method} is non-@code{nil}, then the
minibuffer inherits the current input method (@pxref{Input
Methods}) and the setting of @code{enable-multibyte-characters}
(@pxref{Text Representations}) from whichever buffer was current before
entering the minibuffer.

If the built-in variable @code{completion-ignore-case} is
non-@code{nil}, completion ignores case when comparing the input
against the possible matches.  @xref{Basic Completion}.  In this mode
of operation, @var{predicate} must also ignore case, or you will get
surprising results.

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 variables to pass
information to the commands that actually do completion.
They are described in the following section.
@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.  The description refers to the
situation when Partial Completion mode is disabled (as it is by
default).  When enabled, this minor mode uses its own alternatives to
some of the commands described below.  @xref{Completion Options,,,
emacs, The GNU Emacs Manual}, for a short description of Partial
Completion mode.

@defvar minibuffer-completion-table
The value of this variable is the collection 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

@defvar minibuffer-completion-confirm
When the value of this variable is non-@code{nil}, Emacs asks for
confirmation of a completion before exiting the minibuffer.
@code{completing-read} binds this variable, and the function
@code{minibuffer-complete-and-exit} checks the value before exiting.
@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

@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 &optional common-substring
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.  It can also be a list of two strings,
which is printed as if the strings were concatenated.  The first of
the two strings is the actual completion, the second string serves as
annotation.

The argument @var{common-substring} is the prefix that is common to
all the completions.  With normal Emacs completion, it is usually the
same as the string that was completed.  @code{display-completion-list}
uses this to highlight text in the completion list for better visual
feedback.  This is not needed in the minibuffer; for minibuffer
completion, you can pass @code{nil}.

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)
    (buffer-string)))
@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

@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{Definition of minibuffer-local-map}).
@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-local-filename-completion-map
This is like @code{minibuffer-local-completion-map}
except that it does not bind @key{SPC}.  This keymap is used by the
function @code{read-file-name}.
@end defvar

@defvar minibuffer-local-must-match-filename-map
This is like @code{minibuffer-local-must-match-map}
except that it does not bind @key{SPC}.  This keymap is used by the
function @code{read-file-name}.
@end defvar

@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}
specification.  @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.

The argument @var{prompt} should be a string ending with a colon and a
space.  If @var{default} is non-@code{nil}, the function inserts it in
@var{prompt} before the colon to follow the convention for reading from
the minibuffer with a default value (@pxref{Programming Tips}).

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.  If @var{existing} is neither @code{nil}
nor @code{t}, confirmation is required after completion.  (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

@defvar read-buffer-function
This variable specifies how to read buffer names.  For example, if you
set this variable to @code{iswitchb-read-buffer}, all Emacs commands
that call @code{read-buffer} to read a buffer name will actually use the
@code{iswitchb} package to read it.
@end defvar

@defun read-command prompt &optional default
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} 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{(intern "")}, that is, a
symbol whose name is an empty string.

@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
@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
@anchor{Definition of read-variable}
This function reads the name of a user variable and returns it as a
symbol.

The argument @var{default} 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{(intern "")}.

@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}.

In general, @code{read-variable} 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

  See also the functions @code{read-coding-system} and
@code{read-non-nil-coding-system}, in @ref{User-Chosen Coding Systems},
and @code{read-input-method-name}, in @ref{Input Methods}.

@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 predicate
This function reads a file name in the minibuffer, prompting with
@var{prompt} and providing completion.

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.

@code{read-file-name} uses
@code{minibuffer-local-filename-completion-map} as the keymap if
@var{existing} is @code{nil}, and uses
@code{minibuffer-local-must-match-filename-map} if @var{existing} is
non-@code{nil}.  @xref{Completion Commands}.

The argument @var{directory} specifies the directory to use for
completion of relative file names.  It should be an absolute directory
name.  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}.  @strong{Please
note:} we recommend using @var{default} rather than @var{initial} in
most cases.

If @var{default} is non-@code{nil}, then the function returns
@var{default} if the user exits the minibuffer with the same non-empty
contents that @code{read-file-name} inserted initially.  The initial
minibuffer contents are always non-empty if
@code{insert-default-directory} is non-@code{nil}, as it is by
default.  @var{default} is not checked for validity, regardless of the
value of @var{existing}.  However, if @var{existing} is
non-@code{nil}, the initial minibuffer contents should be a valid file
(or directory) name.  Otherwise @code{read-file-name} attempts
completion if the user exits without any editing, and does not return
@var{default}.  @var{default} is also available through the history
commands.

If @var{default} is @code{nil}, @code{read-file-name} tries to find a
substitute default to use in its place, which it treats in exactly the
same way as if it had been specified explicitly.  If @var{default} is
@code{nil}, but @var{initial} is non-@code{nil}, then the default is
the absolute file name obtained from @var{directory} and
@var{initial}.  If both @var{default} and @var{initial} are @code{nil}
and the buffer is visiting a file, @code{read-file-name} uses the
absolute file name of that file as default.  If the buffer is not
visiting a file, then there is no default.  In that case, if the user
types @key{RET} without any editing, @code{read-file-name} simply
returns the pre-inserted contents of the minibuffer.

If the user types @key{RET} in an empty minibuffer, this function
returns an empty string, regardless of the value of @var{existing}.
This is, for instance, how the user can make the current buffer visit
no file using @code{M-x set-visited-file-name}.

If @var{predicate} is non-@code{nil}, it specifies a function of one
argument that decides which file names are acceptable completion
possibilities.  A file name is an acceptable value if @var{predicate}
returns non-@code{nil} for it.

@code{read-file-name} does not automatically expand file names.  You
must call @code{expand-file-name} yourself if an absolute file name is
required.

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

@defvar read-file-name-function
If non-@code{nil}, this should be a function that accepts the same
arguments as @code{read-file-name}.  When @code{read-file-name} is
called, it calls this function with the supplied arguments instead of
doing its usual work.
@end defvar

@defvar read-file-name-completion-ignore-case
If this variable is non-@code{nil}, @code{read-file-name} ignores case
when performing completion.
@end defvar

@defun read-directory-name prompt &optional directory default existing initial
This function is like @code{read-file-name} but allows only directory
names as completion possibilities.

If @var{default} is @code{nil} and @var{initial} is non-@code{nil},
@code{read-directory-name} constructs a substitute default by
combining @var{directory} (or the current buffer's default directory
if @var{directory} is @code{nil}) and @var{initial}.  If both
@var{default} and @var{initial} are @code{nil}, this function uses
@var{directory} as substitute default, or the current buffer's default
directory if @var{directory} is @code{nil}.
@end defun

@defopt insert-default-directory
This variable is used by @code{read-file-name}, and thus, indirectly,
by most commands reading file names.  (This includes all commands that
use the code letters @samp{f} or @samp{F} in their interactive form.
@xref{Interactive Codes,, Code Characters for interactive}.)  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.

If this variable is @code{nil} and the initial minibuffer contents are
empty, the user may have to explicitly fetch the next history element
to access a default value.  If the variable is non-@code{nil}, the
initial minibuffer contents are always non-empty and the user can
always request a default value by immediately typing @key{RET} in an
unedited minibuffer.  (See above.)

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 @code{test-completion}.  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 already have other meanings, and it would be
unreliable to treat one differently just 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}.

@defmac dynamic-completion-table function
This macro is a convenient way to write a function that can act as
programmed completion function.  The argument @var{function} should be
a function that takes one argument, a string, and returns an alist of
possible completions of it.  You can think of
@code{dynamic-completion-table} as a transducer between that interface
and the interface for programmed completion functions.
@end defmac

@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.

   If either of these functions 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.  Otherwise, it
uses keyboard input.  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.

  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 y-or-n-p-with-timeout prompt seconds default-value
Like @code{y-or-n-p}, except that if the user fails to answer within
@var{seconds} seconds, this function stops waiting and returns
@var{default-value}.  It works by setting up a timer; see @ref{Timers}.
The argument @var{seconds} may be an integer or a floating point number.
@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

@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 no-cursor-in-echo-area
This function 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.

Normally, @code{map-y-or-n-p} binds @code{cursor-in-echo-area} while
prompting.  But if @var{no-cursor-in-echo-area} is non-@code{nil}, it
does not do that.

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.  If @var{default} is @code{nil},
then @code{read-passwd} returns the null string in that case.
@end defun

@node Minibuffer Commands
@section Minibuffer Commands

  This section describes some commands meant for use in the
minibuffer.

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

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

@node Minibuffer Windows
@section Minibuffer Windows

  These functions access and select minibuffer windows
and test whether they are active.

@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
@anchor{Definition of minibuffer-window}
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

@defun set-minibuffer-window window
This function specifies @var{window} as the minibuffer window to use.
This affects where the minibuffer is displayed if you put text in it
without invoking the usual minibuffer commands.  It has no effect on
the usual minibuffer input functions because they all start by
choosing the minibuffer window according to the current frame.
@end defun

@c Emacs 19 feature
@defun window-minibuffer-p &optional window
This function returns non-@code{nil} if @var{window} is a minibuffer
window.
@var{window} defaults to the selected 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

@node Minibuffer Contents
@section Minibuffer Contents

  These functions access the minibuffer prompt and contents.

@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-end
This function returns the current
position of the end of the minibuffer prompt, if a minibuffer is
current.  Otherwise, it returns the minimum valid buffer position.
@end defun

@defun minibuffer-prompt-width
This function returns the current display-width of the minibuffer
prompt, if a minibuffer is current.  Otherwise, it returns zero.
@end defun

@defun minibuffer-contents
This function returns the editable
contents of the minibuffer (that is, everything except the prompt) as
a string, if a minibuffer is current.  Otherwise, it returns the
entire contents of the current buffer.
@end defun

@defun minibuffer-contents-no-properties
This is like @code{minibuffer-contents}, except that it does not copy text
properties, just the characters themselves.  @xref{Text Properties}.
@end defun

@defun minibuffer-completion-contents
This is like @code{minibuffer-contents}, except that it returns only
the contents before point.  That is the part that completion commands
operate on.  @xref{Minibuffer Completion}.
@end defun

@defun delete-minibuffer-contents
This function erases the editable contents of the minibuffer (that is,
everything except the prompt), if a minibuffer is current.  Otherwise,
it erases the entire current buffer.
@end defun

@node Recursive Mini
@section Recursive Minibuffers

  These 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.

If this variable is @code{nil}, you cannot invoke minibuffer
commands when the minibuffer window is active, not even if you switch to
another window to do it.
@end defopt

@c Emacs 19 feature
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.  A command can
also achieve this by binding @code{enable-recursive-minibuffers}
to @code{t} in the interactive declaration (@pxref{Using Interactive}).
The minibuffer command @code{next-matching-history-element} (normally
@kbd{M-s} in the minibuffer) does the latter.

@node Minibuffer Misc
@section Minibuffer Miscellany

@defun minibufferp &optional buffer-or-name
This function returns non-@code{nil} if @var{buffer-or-name} is a
minibuffer.  If @var{buffer-or-name} is omitted, it tests the current
buffer.
@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
@anchor{Definition of 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

@defvar minibuffer-scroll-window
@anchor{Definition of 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

@defun minibuffer-selected-window
This function returns the window which was selected when the
minibuffer was entered.  If selected window is not a minibuffer
window, it returns @code{nil}.
@end defun

@defopt max-mini-window-height
This variable specifies the maximum height for resizing minibuffer
windows.  If a float, it specifies a fraction of the height of the
frame.  If an integer, it specifies a number of lines.
@end defopt

@defun minibuffer-message string
This function displays @var{string} temporarily at the end of the
minibuffer text, for two seconds, or until the next input event
arrives, whichever comes first.
@end defun

@ignore
   arch-tag: bba7f945-9078-477f-a2ce-18818a6e1218
@end ignore