\input texinfo    @c -*- texinfo -*-
@c %**start of header
@setfilename transient.info
@settitle Transient User and Developer Manual
@documentencoding UTF-8
@documentlanguage en
@c %**end of header

@copying
@quotation
Copyright (C) 2018--2025 Free Software Foundation, Inc.

You can redistribute this document and/or modify it under the terms
of the GNU General Public License as published by the Free Software
Foundation, either version 3 of the License, or (at your option) any
later version.

This document is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE@.  See the GNU
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@end quotation
@end copying

@dircategory Emacs misc features
@direntry
* Transient: (transient). Transient Commands.
@end direntry

@finalout
@titlepage
@title Transient User and Developer Manual
@subtitle for version 0.8.8
@author Jonas Bernoulli
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top Transient User and Developer Manual

Transient is the library used to implement the keyboard-driven ``menus''
in Magit.  It is distributed as a separate package, so that it can be
used to implement similar menus in other packages.

This manual can be bit hard to digest when getting started.  A useful
resource to get over that hurdle is Psionic K's interactive tutorial,
available at @uref{https://github.com/positron-solutions/transient-showcase}.

@noindent
This manual is for Transient version 0.8.8.

@insertcopying
@end ifnottex

@menu
* Introduction::
* Usage::
* Modifying Existing Transients::
* Defining New Commands::
* Classes and Methods::
* FAQ::
* Keystroke Index::
* Command and Function Index::
* Variable Index::
* Concept Index::
* GNU General Public License::

@detailmenu
--- The Detailed Node Listing ---

Usage

* Invoking Transients::
* Aborting and Resuming Transients::
* Common Suffix Commands::
* Saving Values::
* Using History::
* Getting Help for Suffix Commands::
* Enabling and Disabling Suffixes::
* Other Commands::
* Configuration::

Defining New Commands

* Technical Introduction::
* Defining Transients::
* Binding Suffix and Infix Commands::
* Defining Suffix and Infix Commands::
* Using Infix Arguments::
* Using Prefix Scope::
* Current Suffix Command::
* Current Prefix Command::
* Transient State::

Classes and Methods

* Group Classes::
* Group Methods::
* Prefix Classes::
* Suffix Classes::
* Prefix Methods::
* Suffix Methods::
* Prefix Slots::
* Suffix Slots::
* Predicate Slots::

@end detailmenu
@end menu

@node Introduction
@chapter Introduction

Transient is the library used to implement the keyboard-driven @dfn{menus}
in Magit.  It is distributed as a separate package, so that it can be
used to implement similar menus in other packages.

This manual can be bit hard to digest when getting started.  A useful
resource to get over that hurdle is Psionic K's interactive tutorial,
available at @uref{https://github.com/positron-solutions/transient-showcase}.

@anchor{Some things that Transient can do}
@heading Some things that Transient can do

@itemize
@item
Display current state of arguments
@item
Display and manage lifecycle of modal bindings
@item
Contextual user interface
@item
Flow control for wizard-like composition of interactive forms
@item
History & persistence
@item
Rendering arguments for controlling CLI programs
@end itemize

@anchor{Complexity in CLI programs}
@heading Complexity in CLI programs

Complexity tends to grow with time.  How do you manage the complexity
of commands?  Consider the humble shell command @samp{ls}.  It now has over
@emph{fifty} command line options.  Some of these are boolean flags (@samp{ls -l}).
Some take arguments (@samp{ls --sort=s}).  Some have no effect unless paired
with other flags (@samp{ls -lh}).  Some are mutually exclusive.  Some shell
commands even have so many options that they introduce @emph{subcommands}
(@samp{git branch}, @samp{git commit}), each with their own rich set of options
(@samp{git branch -f}).

@anchor{Using Transient for composing interactive commands}
@heading Using Transient for composing interactive commands

What about Emacs commands used interactively?  How do these handle
options?  One solution is to make many versions of the same command,
so you don't need to!  Consider: @samp{delete-other-windows} vs.
@samp{delete-other-windows-vertically} (among many similar examples).

Some Emacs commands will simply prompt you for the next "argument"
(@samp{M-x switch-to-buffer}).  Another common solution is to use prefix
arguments which usually start with @samp{C-u}.  Sometimes these are sensibly
numerical in nature (@samp{C-u 4 M-x forward-paragraph} to move forward 4
paragraphs).  But sometimes they function instead as boolean
"switches" (@samp{C-u C-SPACE} to jump to the last mark instead of just
setting it, @samp{C-u C-u C-SPACE} to unconditionally set the mark).  Since
there aren't many standards for the use of prefix options, you have to
read the command's documentation to find out what the possibilities
are.

But when an Emacs command grows to have a truly large set of options
and arguments, with dependencies between them, lots of option values,
etc., these simple approaches just don't scale.  Transient is designed
to solve this issue.  Think of it as the humble prefix argument @samp{C-u},
@emph{raised to the power of 10}.  Like @samp{C-u}, it is key driven.  Like the
shell, it supports boolean "flag" options, options that take
arguments, and even "sub-commands", with their own options.  But
instead of searching through a man page or command documentation,
well-designed transients @emph{guide} their users to the relevant set of
options (and even their possible values!) directly, taking into
account any important pre-existing Emacs settings.  And while for
shell commands like @samp{ls}, there is only one way to "execute" (hit
@samp{Return}!), transients can "execute" using multiple different keys tied
to one of many self-documenting @emph{actions} (imagine having 5 different
colored return keys on your keyboard!).  Transients make navigating
and setting large, complex groups of command options and arguments
easy.  Fun even.  Once you've tried it, it's hard to go back to the
@samp{C-u what can I do here again?} way.

@node Usage
@chapter Usage

@menu
* Invoking Transients::
* Aborting and Resuming Transients::
* Common Suffix Commands::
* Saving Values::
* Using History::
* Getting Help for Suffix Commands::
* Enabling and Disabling Suffixes::
* Other Commands::
* Configuration::
@end menu

@node Invoking Transients
@section Invoking Transients

@cindex invoking transients

A transient prefix command is invoked like any other command by
pressing the key that is bound to that command.  The main difference
to other commands is that a transient prefix command activates a
transient keymap, which temporarily binds the transient's infix and
suffix commands, and that those bindings are shown in menu buffer,
which is displayed in a new window, until the menu is exited.
Bindings from other keymaps may, or may not, be disabled while the
transient state is in effect.

There are two kinds of commands that are available after invoking a
transient prefix command; infix and suffix commands.  Infix commands
set some value (which is then shown in the menu buffer), without
leaving the transient.  Suffix commands, on the other hand, usually
quit the transient and they may use the values set by the infix
commands, i.e., the infix @strong{arguments}.

Instead of setting arguments to be used by a suffix command, infix
commands may also set some value by side-effect, e.g., by setting the
value of some variable.

@node Aborting and Resuming Transients
@section Aborting and Resuming Transients

@cindex aborting transients
@cindex resuming transients

@cindex quit transient
To quit the transient without invoking a suffix command press @kbd{C-g}.

Key bindings in transient keymaps may be longer than a single event.
After pressing a valid prefix key, all commands whose bindings do not
begin with that prefix key are temporarily unavailable and grayed out.
To abort the prefix key press @kbd{C-g} (which in this case only quits the
prefix key, but not the complete transient).

A transient prefix command can be bound as a suffix of another
transient.  Invoking such a suffix replaces the current transient
state with a new transient state, i.e., the available bindings change
and the information displayed in the menu buffer is updated
accordingly.  Pressing @kbd{C-g} while a nested transient is active only
quits the innermost transient, causing a return to the previous
transient.

@kbd{C-q} or @kbd{C-z} on the other hand always exits all transients.  If you use
the latter, then you can later resume the stack of transients using
@kbd{M-x transient-resume}.

@table @asis
@item @kbd{C-g} (@code{transient-quit-seq})
@itemx @kbd{C-g} (@code{transient-quit-one})
@kindex C-g
@kindex C-g
@findex transient-quit-seq
@findex transient-quit-one
This key quits the currently active incomplete key sequence, if any,
or else the current transient.  When quitting the current transient,
it returns to the previous transient, if any.
@end table

Transient's predecessor bound @kbd{q} instead of @kbd{C-g} to the quit command.
To learn how to get that binding back see @code{transient-bind-q-to-quit}'s
documentation string.

@table @asis
@item @kbd{C-q} (@code{transient-quit-all})
@kindex C-q
@findex transient-quit-all
This command quits the currently active incomplete key sequence, if
any, and all transients, including the active transient and all
suspended transients, if any.

@item @kbd{C-z} (@code{transient-suspend})
@kindex C-z
@findex transient-suspend
Like @code{transient-quit-all}, this command quits an incomplete key
sequence, if any, and all transients.  Additionally, it saves the
stack of transients so that it can easily be resumed (which is
particularly useful if you quickly need to do ``something else'' and
the stack is deeper than a single transient, and/or you have already
changed the values of some infix arguments).

Note that only a single stack of transients can be saved at a time.
If another stack is already saved, then saving a new stack discards
the previous stack.

@item @kbd{M-x transient-resume}
@findex transient-resume
This command resumes the previously suspended stack of transients,
if any.
@end table

@node Common Suffix Commands
@section Common Suffix Commands

@cindex common suffix commands

A few shared suffix commands are available in all transients.  These
suffix commands are not shown permanently in every menu by default.
Most of these commands share a common prefix key and pressing that key
causes the common commands to be temporarily shown in the active menu.

@defopt transient-show-common-commands
This option controls whether shared suffix commands are permanently
shown alongside the menu-specific infix and suffix commands.  By
default, the shared commands are not permanently shown to avoid
wasting precious space and overwhelming the user with too many
choices.

If you prefer to always see these commands, then set this option to
a non-@code{nil} value.  Alternatively the value can be toggled for the
current Emacs session only, using @code{transient-toggle-common}, described
below.
@end defopt

@defopt transient-common-command-prefix
This option specifies the prefix key used in all transient menus
to invoke most of the shared commands, which are available in all
transient menus.  By default these bindings are only shown after
pressing that prefix key and before following that up with a valid
key binding (but see the previous option).

For historic reasons @kbd{C-x} is used by default, but users are
encouraged to pick another key, preferably one that is not commonly
used in Emacs but is still convenient to them.

Usually, while a transient menu is active, the user cannot invoke
commands that are not bound in the menu itself.  For those menus it
does not matter, if @kbd{C-x} or another commonly used prefix key is used
for common menu commands.  However, certain other, newer menus do
not suppress key bindings established outside the menu itself, and
in those cases a binding for a common menu command could shadow an
external binding.  For example, @kbd{C-x C-s} could not be used to invoke
@code{save-buffer}, if that binding is shadowed by the menu binding for
@code{transient-save}.

Which key is most suitable depends on the user's preferences, but
good choices may include function keys and @kbd{C-z} (for many keyboard
layouts @kbd{z} is right next to @kbd{x}, and invoking @code{suspend-frame}, while a
transient menu is active, would not be a good idea anyway).
@end defopt

@table @asis
@item @kbd{C-x t} (@code{transient-toggle-common})
@kindex C-x t
@findex transient-toggle-common
This command toggles whether the generic commands, that are common
to all transients, are permanently displayed or only after typing
the incomplete prefix key sequence@kbd{}.  This only affects the current
Emacs session.
@end table

The other common commands are described in either the previous or in
one of the following sections.

@node Saving Values
@section Saving Values

@cindex saving values of arguments

After setting the infix arguments in a transient, the user can save
those arguments for future invocations.

Most transients will start out with the saved arguments when they are
invoked.  There are a few exceptions, though.  Some transients are
designed so that the value that they use is stored externally as the
buffer-local value of some variable.  Invoking such a transient again
uses the buffer-local value. @footnote{@code{magit-diff} and @code{magit-log} are two prominent examples, and their
handling of buffer-local values is actually a bit more complicated
than outlined above and even customizable.}

If the user does not save the value and just exits using a regular
suffix command, then the value is merely saved to the transient's
history.  That value won't be used when the transient is next invoked,
but it is easily accessible (see @ref{Using History}).

Option @code{transient-common-command-prefix} controls the prefix key used
in the following bindings.  For simplicity's sake the default, @kbd{C-x},
is shown below.

@table @asis
@item @kbd{C-x s} (@code{transient-set})
@kindex C-x s
@findex transient-set
This command saves the value of the active transient for this Emacs
session.

@item @kbd{C-x C-s} (@code{transient-save})
@kindex C-x C-s
@findex transient-save
This command saves the value of the active transient persistently
across Emacs sessions.

@item @kbd{C-x C-k} (@code{transient-reset})
@kindex C-x C-k
@findex transient-reset
This command clears the set and saved values of the active transient.
@end table

@defopt transient-values-file
This option names the file that is used to persist the values of
transients between Emacs sessions.
@end defopt

@node Using History
@section Using History

@cindex value history

Every time the user invokes a suffix command the transient's current
value is saved to its history.  These values can be cycled through,
the same way one can cycle through the history of commands that read
user-input in the minibuffer.

Option @code{transient-common-command-prefix} controls the prefix key used
in the following bindings.  For simplicity's sake the default, @kbd{C-x},
is shown below.

@table @asis
@item @kbd{C-M-p} (@code{transient-history-prev})
@itemx @kbd{C-x p}
@kindex C-M-p
@kindex C-x p
@findex transient-history-prev
This command switches to the previous value used for the active
transient.

@item @kbd{C-M-n} (@code{transient-history-next})
@itemx @kbd{C-x n}
@kindex C-M-n
@kindex C-x n
@findex transient-history-next
This command switches to the next value used for the active
transient.
@end table

In addition to the transient-wide history, infixes can have their own
history.  When an infix reads user-input using the minibuffer, the
user can use the regular minibuffer history commands to cycle through
previously used values.  Usually the same keys as those mentioned
above are bound to those commands.

Authors of transients should arrange for different infix commands that
read the same kind of value to also use the same history key (see
@ref{Suffix Slots}).

Both kinds of history are saved to a file when Emacs is exited.

@defopt transient-save-history
This option controls whether the history of transient commands is
saved when exiting Emacs.
@end defopt

@defopt transient-history-file
This option names the file that is used to persist the history of
transients and their infixes between Emacs sessions.
@end defopt

@defopt transient-history-limit
This option controls how many history elements are kept at the time
the history is saved in @code{transient-history-file}.
@end defopt

@node Getting Help for Suffix Commands
@section Getting Help for Suffix Commands

@cindex getting help

Transients can have many suffixes and infixes that the user might not
be familiar with.  To make it trivial to get help for these, Transient
provides access to the documentation directly from the active
transient.

@table @asis
@item @kbd{C-h} (@code{transient-help})
@kindex C-h
@findex transient-help
This command enters help mode.  When help mode is active, typing a
key shows information about the suffix command that the key normally
is bound to (instead of invoking it).  Pressing @kbd{C-h} a second time
shows information about the @emph{prefix} command.

After typing a key, the stack of transient states is suspended and
information about the suffix command is shown instead.  Typing @kbd{q} in
the help buffer buries that buffer and resumes the transient state.
@end table

What sort of documentation is shown depends on how the transient was
defined.  For infix commands that represent command-line arguments
this ideally shows the appropriate manpage.  @code{transient-help} then tries
to jump to the correct location within that.  Info manuals are also
supported.  The fallback is to show the command's documentation
string, for non-infix suffixes this is usually appropriate.

@node Enabling and Disabling Suffixes
@section Enabling and Disabling Suffixes

@cindex enabling suffixes
@cindex disabling suffixes

The user base of a package that uses transients can be very diverse.
This is certainly the case for Magit; some users have been using it and
Git for a decade, while others are just getting started now.

@cindex levels
For that reason a mechanism is needed that authors can use to classify a
transient's infixes and suffixes along the essentials@dots{}everything
spectrum.  We use the term @dfn{levels} to describe that mechanism.

@cindex transient-level
Each suffix command is placed on a level and each transient has a
level (called @dfn{transient-level}), which controls which suffix commands
are available.  Integers between 1 and 7 (inclusive) are valid levels.
For suffixes, 0 is also valid; it means that the suffix is not
displayed at any level.

The levels of individual transients and/or their individual suffixes
can be changed interactively, by invoking the menu and entering its
``edit'' mode using the command @code{transient-set-level}, as described below.

The default level for both transients and their suffixes is 4.  The
@code{transient-default-level} option only controls the default for
transients.  The default suffix level is always 4.  The authors of
transients should place certain suffixes on a higher level, if they
expect that it won't be of use to most users, and they should place
very important suffixes on a lower level, so that they remain
available even if the user lowers the transient level.

@defopt transient-default-level
This option controls which suffix levels are made available by
default.  It sets the transient-level for transients for which the
user has not set that individually.
@end defopt

@defopt transient-levels-file
This option names the file that is used to persist the levels of
transients and their suffixes between Emacs sessions.
@end defopt

Option @code{transient-common-command-prefix} controls the prefix key used
in the following bindings.  For simplicity's sake the default, @kbd{C-x},
is shown below.

@table @asis
@item @kbd{C-x l} (@code{transient-set-level})
@kindex C-x l
@findex transient-set-level
This command enters edit mode.  When edit mode is active, then all
infixes and suffixes that are currently usable are displayed along
with their levels.  The colors of the levels indicate whether they
are enabled or not.  The level of the transient is also displayed
along with some usage information.

In edit mode, pressing the key that would usually invoke a certain
suffix instead prompts the user for the level that suffix should be
placed on.

Help mode is available in edit mode.

To change the transient level press @kbd{C-x l} again.

To exit edit mode press @kbd{C-g}.

Note that edit mode does not display any suffixes that are not
currently usable.  @code{magit-rebase}, for example, shows different
suffixes depending on whether a rebase is already in progress or
not.  The predicates also apply in edit mode.

Therefore, to control which suffixes are available given a certain
state, you have to make sure that that state is currently active.

@item @kbd{C-x a} (@code{transient-toggle-level-limit})
@kindex C-x a
@findex transient-toggle-level-limit
This command toggle whether suffixes that are on levels higher than
the level specified by @code{transient-default-level} are temporarily
available anyway.
@end table

@defun transient-set-default-level suffix level
This function sets the default level of the suffix COMMAND to LEVEL@.

If a suffix command appears in multiple menus, it may make sense to
consistently change its level in all those menus at once.  For
example, the @code{--gpg-sign} argument (which is implemented using the
command @code{magit:--gpg-sign}), is bound in all of Magit's menu which
create commits.  Users who sometimes sign their commits would want
that argument to be available in all of these menus, while for users
who never sign it is just unnecessary noise in any menus.

To always make @code{--gpg-sign} available, use:

@lisp
(transient-set-default-level 'magit:--gpg-sign 1)
@end lisp

To never make @code{--gpg-sign} available, use:

@lisp
(transient-set-default-level 'magit:--gpg-sign 0)
@end lisp

This sets the level in the suffix prototype object for this command.
Commands only have a suffix prototype if they were defined using one
of @code{transient-define-argument}, @code{transient-define-infix} and
@code{transient-define-suffix}.  For all other commands this would signal
an error.  (This is one of the reasons why package authors should
use one of these functions to define shared suffix commands, and
especially shared arguments.)

If the user changes the level of a suffix in a particular menu,
using @kbd{C-x l} as shown above, then that obviously shadows the default.

It is also possible to set the level of a suffix binding in a
particular menu, either when defining the menu using
@code{transient-define-prefix,} or later using @code{transient-insert-suffix}.  If
such bindings specify a level, then that also overrides the default.
(Per-suffix default levels is a new feature, so you might encounter
this quite often.)
@end defun

@node Other Commands
@section Other Commands

When invoking a transient in a small frame, the transient window may
not show the complete buffer, making it necessary to scroll, using the
following commands.  These commands are never shown in the transient
window, and the key bindings are the same as for @code{scroll-up-command} and
@code{scroll-down-command} in other buffers.

@deffn Command transient-scroll-up arg
This command scrolls text of transient's menu window upward @var{ARG}
lines.  If @var{ARG} is @code{nil}, then it scrolls near full screen.  This
is a wrapper around @code{scroll-up-command} (which see).
@end deffn

@deffn Command transient-scroll-down arg
This command scrolls text of transient's menu window down @var{ARG}
lines.  If @var{ARG} is @code{nil}, then it scrolls near full screen.  This
is a wrapper around @code{scroll-down-command} (which see).
@end deffn

The following commands are not available by default.  If you would
like to use them for all menus, bind them in @code{transient-map}.

@deffn Command transient-copy-menu-text
This command copies the contents of the menu buffer to the kill
ring.
@end deffn

@deffn Command transient-toggle-docstrings
This command toggle between showing suffix descriptions in the menu
(as usual) or showing the first lines of the respective docstrings
in their place.  For commands that do not have a docstring, always
display the suffix description.  Because there likely isn't enough
room to display multiple docstrings side-by-side, a single column
is used when displaying docstrings.
@end deffn

@node Configuration
@section Configuration

More options are described in @ref{Common Suffix Commands}, in @ref{Saving Values}, in @ref{Using History} and in @ref{Enabling and Disabling Suffixes}.

@anchor{Essential Options}
@subheading Essential Options

Two more essential options are documented in @ref{Common Suffix Commands}.

@defopt transient-show-popup
This option controls whether and when transient's menu buffer is
shown.

@itemize
@item
If @code{t} (the default), then the buffer is shown as soon as a
transient prefix command is invoked.

@item
If @code{nil}, then the buffer is not shown unless the user explicitly
requests it, by pressing an incomplete prefix key sequence.

@item
If a number, then the a brief one-line summary is shown instead of
the menu buffer.  If zero or negative, then not even that summary
is shown; only the pressed key itself is shown.

The buffer is shown once the user explicitly requests it by
pressing an incomplete prefix key sequence.  Unless this is zero,
the menu is shown after that many seconds of inactivity (using the
absolute value).
@end itemize
@end defopt

@defopt transient-show-during-minibuffer-read
This option controls whether the transient menu continues to be
displayed while the minibuffer is used to read user input.

This is only relevant to commands that do not close the menu, such as
commands that set infix arguments.  If a command exits the menu, and
uses the minibuffer, then the menu is always closed before the
minibuffer is entered, irrespective of the value of this option.

When @code{nil} (the default), hide the menu while the minibuffer is in use.
When @code{t}, keep showing the menu, but allow for the menu window to be
resized, to ensure that completion candidates can be displayed.

When @code{fixed}, keep showing the menu and prevent it from being resized,
which may make it impossible to display the completion candidates.  If
that ever happens for you, consider using @code{t} or an integer, as described
below.

If the value is @code{fixed} and the menu window uses the full height of its
frame, then the former is ignored and resizing is allowed anyway.  This
is necessary because individual menus may use unusual display actions
different from what @code{transient-display-buffer-action} specifies (likely
to display that menu in a side-window).

When using a third-party mode, which automatically resizes windows
(e.g., by calling @code{balance-windows} on @code{post-command-hook}), then
@code{fixed} (or @code{nil}) is likely a better choice than @code{t}.

The value can also be an integer, in which case the behavior depends on
whether at least that many lines are left to display windows other than
the menu window.  If that is the case, display the menu and preserve the
size of that window.  Otherwise, allow resizing the menu window if the
number is positive, or hide the menu if it is negative.
@end defopt

@defopt transient-read-with-initial-input
This option controls whether the last history element is used as the
initial minibuffer input when reading the value of an infix argument
from the user.  If @code{nil}, there is no initial input and the first
element has to be accessed the same way as the older elements.
@end defopt

@defopt transient-enable-popup-navigation
This option controls whether navigation commands are enabled in
transient's menu buffer.  If the value is @code{verbose} (the default),
brief documentation about the command under point is additionally
show in the echo area.

While a transient is active the menu buffer is not the current
buffer, making it necessary to use dedicated commands to act on that
buffer itself.  If this option is non-@code{nil}, then the following
features are available:

@itemize
@item
@kbd{@key{UP}} moves the cursor to the previous suffix.
@item
@kbd{@key{DOWN}} moves the cursor to the next suffix.
@item
@kbd{M-@key{RET}} invokes the suffix the cursor is on.
@item
@kbd{mouse-1} invokes the clicked on suffix.
@item
@kbd{C-s} and @kbd{C-r} start isearch in the menu buffer.
@end itemize

By default @kbd{M-@key{RET}} is bound to @code{transient-push-button}, instead of
@kbd{@key{RET}}, because if a transient allows the invocation of non-suffixes,
then it is likely, that you would want @kbd{@key{RET}} to do what it would do
if no transient were active."
@end defopt

@defopt transient-display-buffer-action
This option specifies the action used to display the transient's
menu buffer.  The menu buffer is displayed in a window using
@code{(display-buffer @var{BUFFER} transient-display-buffer-action)}.

The value of this option has the form @code{(@var{FUNCTION} . @var{ALIST})},
where @var{FUNCTION} is a function or a list of functions.  Each such
function should accept two arguments: a buffer to display and an
alist of the same form as @var{ALIST}.  See @ref{Choosing Window,,,elisp,},
for details.

The default is:

@lisp
(display-buffer-in-side-window
 (side . bottom)
 (dedicated . t)
 (inhibit-same-window . t))
@end lisp

This displays the window at the bottom of the selected frame.
For alternatives see @ref{Buffer Display Action Functions,,,elisp,},
and @ref{Buffer Display Action Alists,,,elisp,}.

When you switch to a different ACTION, you should keep the ALIST
entries for @code{dedicated} and @code{inhibit-same-window} in most cases.
Do not drop them because you are unsure whether they are needed;
if you are unsure, then keep them.

Note that the buffer that was current before the transient buffer
is shown should remain the current buffer.  Many suffix commands
act on the thing at point, if appropriate, and if the transient
buffer became the current buffer, then that would change what is
at point.  To that effect @code{inhibit-same-window} ensures that the
selected window is not used to show the transient buffer.

It may be possible to display the window in another frame, but
whether that works in practice depends on the window-manager.
If the window manager selects the new window (Emacs frame),
then that unfortunately changes which buffer is current.

If you change the value of this option, then you might also
want to change the value of @code{transient-mode-line-format}.

This user option may be overridden if @code{:display-action} is passed
when creating a new prefix with @code{transient-define-prefix}.
@end defopt

@anchor{Accessibility Options}
@subheading Accessibility Options

@defopt transient-force-single-column
This option controls whether the use of a single column to display
suffixes is enforced.  This might be useful for users with low
vision who use large text and might otherwise have to scroll in two
dimensions.
@end defopt

@anchor{Auxiliary Options}
@subheading Auxiliary Options

@defopt transient-mode-line-format
This option controls whether transient's menu buffer has a
mode-line, separator line, or neither.

If @code{nil}, then the buffer has no mode-line.  If the buffer is not
displayed right above the echo area, then this probably is not a
good value.

If @code{line} (the default) or a natural number, then the buffer has no
mode-line, but a line is drawn in its place.  If a number is used,
that specifies the thickness of the line.  On termcap frames we
cannot draw lines, so there @code{line} and numbers are synonyms for @code{nil}.

The color of the line is used to indicate if non-suffixes are
allowed and whether they exit the transient.  The foreground
color of @code{transient-key-noop} (if non-suffixes are disallowed),
@code{transient-key-stay} (if allowed and transient stays active), or
@code{transient-key-exit} (if allowed and they exit the transient) is
used to draw the line.

This user option may be overridden if @code{:mode-line-format} is passed
when creating a new prefix with @code{transient-define-prefix}.

Otherwise this can be any mode-line format.  See @ref{Mode Line Format,,,elisp,}, for details.
@end defopt

@defopt transient-semantic-coloring
This option controls whether colors are used to indicate the
transient behavior of commands.

If non-@code{nil}, then the key binding of each suffix is colorized to
indicate whether it exits the transient state or not.  The color of
the prefix is indicated using the line that is drawn when the value
of @code{transient-mode-line-format} is @code{line}.
@end defopt

@defopt transient-highlight-mismatched-keys
This option controls whether key bindings of infix commands that do
not match the respective command-line argument should be highlighted.
For other infix commands this option has no effect.

When this option is non-@code{nil}, the key binding for an infix argument
is highlighted when only a long argument (e.g., @code{--verbose}) is
specified but no shorthand (e.g., @code{-v}).  In the rare case that a
shorthand is specified but the key binding does not match, then it
is highlighted differently.

Highlighting mismatched key bindings is useful when learning the
arguments of the underlying command-line tool; you wouldn't want to
learn any short-hands that do not actually exist.

The highlighting is done using one of the faces
@code{transient-mismatched-key} and @code{transient-nonstandard-key}.
@end defopt

@defopt transient-substitute-key-function
This function is used to modify key bindings.  If the value of this
option is @code{nil} (the default), then no substitution is performed.

This function is called with one argument, the prefix object, and
must return a key binding description, either the existing key
description it finds in the @code{key} slot, or the key description that
replaces the prefix key.  It could be used to make other
substitutions, but that is discouraged.

For example, @kbd{=} is hard to reach using my custom keyboard layout,
so I substitute @kbd{(} for that, which is easy to reach using a layout
optimized for lisp.

@lisp
(setq transient-substitute-key-function
      (lambda (obj)
        (let ((key (oref obj key)))
          (if (string-match "\\`\\(=\\)[a-zA-Z]" key)
              (replace-match "(" t t key 1)
            key))))
@end lisp
@end defopt

@defopt transient-align-variable-pitch
This option controls whether columns are aligned pixel-wise in the
menu buffer.

If this is non-@code{nil}, then columns are aligned pixel-wise to support
variable-pitch fonts.  Keys are not aligned, so you should use a
fixed-pitch font for the @code{transient-key} face.  Other key faces
inherit from that face unless a theme is used that breaks that
relationship.

This option is intended for users who use a variable-pitch font for
the @code{default} face.
@end defopt

@defopt transient-force-fixed-pitch
This option controls whether to force the use of a monospaced font
in menu buffer.  Even if you use a proportional font for the @code{default}
face, you might still want to use a monospaced font in the menu
buffer.  Setting this option to @code{t} causes @code{default} to be remapped to
@code{fixed-pitch} in that buffer.
@end defopt

@anchor{Developer Options}
@subheading Developer Options

These options are mainly intended for developers.

@defopt transient-detect-key-conflicts
This option controls whether key binding conflicts should be
detected at the time the transient is invoked.  If so, this results
in an error, which prevents the transient from being used.  Because
of that, conflicts are ignored by default.

Conflicts cannot be determined earlier, i.e., when the transient is
being defined and when new suffixes are being added, because at that
time there can be false-positives.  It is actually valid for
multiple suffixes to share a common key binding, provided the
predicates of those suffixes prevent that more than one of them is
enabled at a time.
@end defopt

@defopt transient-error-on-insert-failure
This option controls whether to signal an error when
@code{transient-insert-suffix} or @code{transient-append-suffix} failed to insert
a suffix into an existing prefix.  By default a warning is shown
instead.
@end defopt

@defopt transient-highlight-higher-levels
This option controls whether suffixes that would not be available by
default are highlighted.

When non-@code{nil} then the descriptions of suffixes are highlighted if
their level is above 4, the default of @code{transient-default-level}.
Assuming you have set that variable to 7, this highlights all
suffixes that won't be available to users without them making the
same customization.
@end defopt

@anchor{Hook Variables}
@subheading Hook Variables

@defvar transient-exit-hook
This hook is run after a transient menu is exited, even if another
transient menu becomes active at the same time.
@end defvar

@defvar transient-post-exit-hook
This hook is run after a transient menu is exited, provided no other
transient menu becomes active at the same time.
@end defvar

@defvar transient-setup-buffer-hook
This hook is run when the transient buffer is being setup.
That buffer is current and empty when this hook is runs.
@end defvar

@node Modifying Existing Transients
@chapter Modifying Existing Transients

@cindex modifying existing transients

To an extent, transients can be customized interactively, see
@ref{Enabling and Disabling Suffixes}.  This section explains how existing
transients can be further modified non-interactively.  Let's begin
with an example:

@lisp
(transient-append-suffix 'magit-patch-apply "-3"
  '("-R" "Apply in reverse" "--reverse"))
@end lisp

This inserts a new infix argument to toggle the @code{--reverse} argument
after the infix argument that toggles @code{-3} in @code{magit-patch-apply}.

The following functions share a few arguments:

@itemize
@item
@var{PREFIX} is a transient prefix command, a symbol.

@item
@var{SUFFIX} is a transient infix or suffix specification in the same form
as expected by @code{transient-define-prefix}.  Note that an infix is a
special kind of suffix.  Depending on context ``suffixes'' means
``suffixes (including infixes)'' or ``non-infix suffixes''.  Here it
means the former.  See @ref{Suffix Specifications}.

@var{SUFFIX} may also be a group in the same form as expected by
@code{transient-define-prefix}.  See @ref{Group Specifications}.

@item
@var{LOC} is a command, a key vector, a key description (a string as
returned by @code{key-description}), or a list specifying coordinates (the
last element may also be a command or key).  For example @code{(1 0 -1)}
identifies the last suffix (@code{-1}) of the first subgroup (@code{0}) of the
second group (@code{1}).

If @var{LOC} is a list of coordinates, then it can be used to identify a
group, not just an individual suffix command.

The function @code{transient-get-suffix} can be useful to determine whether
a certain coordination list identifies the suffix or group that you
expect it to identify.  In hairy cases it may be necessary to look
at the definition of the transient prefix command.
@end itemize

These functions operate on the information stored in the
@code{transient--layout} property of the @var{PREFIX} symbol.  Suffix entries in
that tree are not objects but have the form @code{(@var{LEVEL} @var{CLASS} @var{PLIST})}, where
@var{PLIST} should set at least @code{:key}, @code{:description} and @code{:command}.

@defun transient-insert-suffix prefix loc suffix &optional keep-other
@end defun
@defun transient-append-suffix prefix loc suffix &optional keep-other
These functions insert the suffix or group @var{SUFFIX} into @var{PREFIX} before
or after @var{LOC}.

Conceptually adding a binding to a transient prefix is similar to
adding a binding to a keymap, but this is complicated by the fact
that multiple suffix commands can be bound to the same key, provided
they are never active at the same time, see @ref{Predicate Slots}.

Unfortunately both false-positives and false-negatives are possible.
To deal with the former use non-@code{nil} @var{KEEP-OTHER@.}  The symbol @code{always}
prevents the removal of a false-positive in some cases where other
non-@code{nil} values would fail.  To deal with false-negatives remove the
conflicting binding separately, using @code{transient-remove-suffix}.
@end defun

@defun transient-replace-suffix prefix loc suffix
This function replaces the suffix or group at @var{LOC} in @var{PREFIX} with
suffix or group @var{SUFFIX}.
@end defun

@defun transient-remove-suffix prefix loc
This function removes the suffix or group at @var{LOC} in @var{PREFIX}.
@end defun

@defun transient-get-suffix prefix loc
This function returns the suffix or group at @var{LOC} in @var{PREFIX}.  The
returned value has the form mentioned above.
@end defun

@defun transient-suffix-put prefix loc prop value
This function edits the suffix or group at @var{LOC} in @var{PREFIX}, by setting
the @var{PROP} of its plist to @var{VALUE}.
@end defun

Most of these functions do not signal an error if they cannot perform
the requested modification.  The functions that insert new suffixes
show a warning if @var{LOC} cannot be found in @var{PREFIX} without signaling an
error.  The reason for doing it like this is that establishing a key
binding (and that is what we essentially are trying to do here) should
not prevent the rest of the configuration from loading.  Among these
functions only @code{transient-get-suffix} and @code{transient-suffix-put} signal
an error by default.  If you really want the insert functions to also
signal an error, set @code{transient-error-on-insert-failure} to @code{t}.

@node Defining New Commands
@chapter Defining New Commands

@menu
* Technical Introduction::
* Defining Transients::
* Binding Suffix and Infix Commands::
* Defining Suffix and Infix Commands::
* Using Infix Arguments::
* Using Prefix Scope::
* Current Suffix Command::
* Current Prefix Command::
* Transient State::
@end menu

@node Technical Introduction
@section Technical Introduction

Taking inspiration from prefix keys and prefix arguments, Transient
implements a similar abstraction involving a prefix command, infix
arguments and suffix commands.

When the user calls a transient prefix command, a transient
(temporary) keymap is activated, which binds the transient's infix and
suffix commands, and functions that control the transient state are
added to @code{pre-command-hook} and @code{post-command-hook}.  The available suffix
and infix commands and their state are shown in a menu buffer until
the transient state is exited by invoking a suffix command.

Calling an infix command causes its value to be changed.  How that is
done depends on the type of the infix command.  The simplest case is
an infix command that represents a command-line argument that does not
take a value.  Invoking such an infix command causes the switch to be
toggled on or off.  More complex infix commands may read a value from
the user, using the minibuffer.

Calling a suffix command usually causes the transient to be exited;
the transient keymaps and hook functions are removed, the menu buffer
no longer shows information about the (no longer bound) suffix
commands, the values of some public global variables are set, while
some internal global variables are unset, and finally the command is
actually called.  Suffix commands can also be configured to not exit
the transient.

A suffix command can, but does not have to, use the infix arguments in
much the same way any command can choose to use or ignore the prefix
arguments.  For a suffix command that was invoked from a transient, the
variable @code{transient-current-suffixes} and the function @code{transient-args}
serve about the same purpose as the variables @code{prefix-arg} and
@code{current-prefix-arg} do for any command that was called after the prefix
arguments have been set using a command such as @code{universal-argument}.

@cindex command dispatchers
Transient can be used to implement simple ``command dispatchers''.  The
main benefit then is that the user can see all the available commands
in a temporarily shown buffer, which can be thought of as a ``menu''.
That is useful by itself because it frees the user from having to
remember all the keys that are valid after a certain prefix key or
command.  Magit's @code{magit-dispatch} (on @kbd{C-x M-g}) command is an example of
using Transient to merely implement a command dispatcher.

In addition to that, Transient also allows users to interactively pass
arguments to commands.  These arguments can be much more complex than
what is reasonable when using prefix arguments.  There is a limit to
how many aspects of a command can be controlled using prefix
arguments.  Furthermore, what a certain prefix argument means for
different commands can be completely different, and users have to read
documentation to learn and then commit to memory what a certain prefix
argument means to a certain command.

Transient suffix commands, on the other hand, can accept dozens of
different arguments without the user having to remember anything.
When using Transient, one can call a command with arguments that are
just as complex as when calling the same function non-interactively
from Lisp.

Invoking a transient suffix command with arguments is similar to
invoking a command in a shell with command-line completion and history
enabled.  One benefit of the Transient interface is that it remembers
history not only on a global level (``this command was invoked using
these arguments, and previously it was invoked using those other
arguments''), but also remembers the values of individual arguments
independently.  See @ref{Using History}.

After a transient prefix command is invoked, @kbd{C-h @var{KEY}} can be used to
show the documentation for the infix or suffix command that @kbd{@var{KEY}} is
bound to (see @ref{Getting Help for Suffix Commands}), and infixes and
suffixes can be removed from the transient using @kbd{C-x l @var{KEY}}.  Infixes
and suffixes that are disabled by default can be enabled the same way.
See @ref{Enabling and Disabling Suffixes}.

Transient ships with support for a few different types of specialized
infix commands.  A command that sets a command line option, for example,
has different needs than a command that merely toggles a boolean flag.
Additionally, Transient provides abstractions for defining new types,
which the author of Transient did not anticipate (or didn't get around
to implementing yet).

Note that suffix commands also support regular prefix arguments.  A
suffix command may even be called with both infix and prefix arguments
at the same time.  If you invoke a command as a suffix of a transient
prefix command, but also want to pass prefix arguments to it, then
first invoke the prefix command, and only after doing that invoke the
prefix arguments, before finally invoking the suffix command.  If you
instead began by providing the prefix arguments, then those would
apply to the prefix command, not the suffix command.  Likewise, if you
want to change infix arguments before invoking a suffix command with
prefix arguments, then change the infix arguments before invoking the
prefix arguments.  In other words, regular prefix arguments always
apply to the next command, and since transient prefix, infix and
suffix commands are just regular commands, the same applies to them.
(Regular prefix keys behave differently because they are not commands
at all, instead they are just incomplete key sequences, and those
cannot be interrupted with prefix commands.)

@node Defining Transients
@section Defining Transients

A transient consists of a prefix command and at least one suffix
command, though usually a transient has several infix and suffix
commands.  The below macro defines the transient prefix command @strong{and}
binds the transient's infix and suffix commands.  In other words, it
defines the complete transient, not just the transient prefix command
that is used to invoke that transient.

@defmac transient-define-prefix name arglist [docstring] [keyword value]@dots{} group@dots{} [body@dots{}]
This macro defines @var{NAME} as a transient prefix command and binds the
transient's infix and suffix commands.

@var{ARGLIST} are the arguments that the prefix command takes.
@var{DOCSTRING} is the documentation string and is optional.

These arguments can optionally be followed by keyword-value pairs.
Each key has to be a keyword symbol, either @code{:class} or a keyword
argument supported by the constructor of that class.  The
@code{transient-prefix} class is used if the class is not specified
explicitly.

@var{GROUP}s add key bindings for infix and suffix commands and specify
how these bindings are presented in the menu buffer.  At least one
@var{GROUP} has to be specified.  See @ref{Binding Suffix and Infix Commands}.

The @var{BODY} is optional.  If it is omitted, then @var{ARGLIST} is ignored and
the function definition becomes:

@lisp
(lambda ()
  (interactive)
  (transient-setup 'NAME))
@end lisp

If @var{BODY} is specified, then it must begin with an @code{interactive} form
that matches @var{ARGLIST}, and it must call @code{transient-setup}.  It may,
however, call that function only when some condition is satisfied.

@cindex scope of a transient
All transients have a (possibly @code{nil}) value, which is exported when
suffix commands are called, so that they can consume that value.
For some transients it might be necessary to have a sort of
secondary value, called a ``scope''.  Such a scope would usually be
set in the command's @code{interactive} form and has to be passed to the
setup function:

@lisp
(transient-setup 'NAME nil nil :scope SCOPE)
@end lisp

For example, the scope of the @code{magit-branch-configure} transient is
the branch whose variables are being configured.
@end defmac

@node Binding Suffix and Infix Commands
@section Binding Suffix and Infix Commands

The macro @code{transient-define-prefix} is used to define a transient.
This defines the actual transient prefix command (see @ref{Defining Transients}) and adds the transient's infix and suffix bindings, as
described below.

Users and third-party packages can add additional bindings using
functions such as @code{transient-insert-suffix} (see @ref{Modifying Existing Transients}).  These functions take a ``suffix specification'' as one of
their arguments, which has the same form as the specifications used in
@code{transient-define-prefix}.

@anchor{Group Specifications}
@subsection Group Specifications

@cindex group specifications

The suffix and infix commands of a transient are organized in groups.
The grouping controls how the descriptions of the suffixes are
outlined visually but also makes it possible to set certain properties
for a set of suffixes.

Several group classes exist, some of which organize suffixes in
subgroups.  In most cases the class does not have to be specified
explicitly, but see @ref{Group Classes}.

Groups are specified in the call to @code{transient-define-prefix}, using
vectors.  Because groups are represented using vectors, we cannot use
square brackets to indicate an optional element and instead use curly
brackets to do the latter.

Group specifications then have this form:

@lisp
[@{LEVEL@} @{DESCRIPTION@} @{KEYWORD VALUE@}... ELEMENT...]
@end lisp

The @var{LEVEL} is optional and defaults to 4.  See @ref{Enabling and Disabling Suffixes}.

The @var{DESCRIPTION} is optional.  If present, it is used as the heading of
the group.

The @var{KEYWORD}-@var{VALUE} pairs are optional.  Each keyword has to be a
keyword symbol, either @code{:class} or a keyword argument supported by the
constructor of that class.

@itemize
@item
One of these keywords, @code{:description}, is equivalent to specifying
@var{DESCRIPTION} at the very beginning of the vector.  The recommendation
is to use @code{:description} if some other keyword is also used, for
consistency, or @var{DESCRIPTION} otherwise, because it looks better.

@item
Likewise @code{:level} is equivalent to @var{LEVEL}.

@item
Other important keywords include the @code{:if...} and @code{:inapt-if...}
keywords.  These keywords control whether the group is available
in a certain situation.

For example, one group of the @code{magit-rebase} transient uses @code{:if
  magit-rebase-in-progress-p}, which contains the suffixes that are
useful while rebase is already in progress; and another that uses
@code{:if-not magit-rebase-in-progress-p}, which contains the suffixes that
initiate a rebase.

These predicates can also be used on individual suffixes and are
only documented once, see @ref{Predicate Slots}.

@item
The value of @code{:hide}, if non-@code{nil}, is a predicate that controls
whether the group is hidden by default.  The key bindings for
suffixes of a hidden group should all use the same prefix key.
Pressing that prefix key should temporarily show the group and its
suffixes, which assumes that a predicate like this is used:

@lisp
(lambda ()
  (eq (car transient--redisplay-key)
      ?\C-c)) ; the prefix key shared by all bindings
@end lisp

@item
The value of @code{:setup-children}, if non-@code{nil}, is a function that takes
one argument, a potentially list of children, and must return a list
of children or an empty list.  This can either be used to somehow
transform the group's children that were defined the normal way, or
to dynamically create the children from scratch.

The returned children must have the same form as stored in the
prefix's @code{transient--layout} property, but it is often more convenient
to use the same form as understood by @code{transient-define-prefix},
described below.  If you use the latter approach, you can use the
@code{transient-parse-suffixes} and @code{transient-parse-suffix} functions to
transform them from the convenient to the expected form.  Depending
on the used group class, @code{transient-parse-suffixes}'s SUFFIXES must be
a list of group vectors (for @code{transient-columns}) or a list of suffix
lists (for all other group classes).

If you explicitly specify children and then transform them using
@code{:setup-children}, then the class of the group is determined as usual,
based on explicitly specified children.

If you do not explicitly specify children and thus rely solely on
@code{:setup-children}, then you must specify the class using @code{:class}.
For backward compatibility, if you fail to do so, @code{transient-column}
is used and a warning is displayed.  This warning will eventually
be replaced with an error.

@lisp
(transient-define-prefix my-finder-by-keyword ()
  "Select a keyword and list matching packages."
  ;; The real `finder-by-keyword' is more convenient
  ;; of course, but that is not the point here.
  [:class transient-columns
   :setup-children
   (lambda (_)
     (transient-parse-suffixes
      'my-finder-by-keyword
      (let ((char (1- ?A)))
        (mapcar                  ; a list ...
         (lambda (partition)
           (vconcat              ; of group vectors ...
            (mapcar (lambda (elt)
                      (let ((keyword (symbol-name (car elt))))
                                 ; ... where each suffix is a list
                        (list (format "%c" (cl-incf char))
                              keyword
                              (lambda ()
                                (interactive)
                                (finder-list-matches keyword)))))
                    partition)))
         (seq-partition finder-known-keywords 7)))))])
@end lisp

@item
The boolean @code{:pad-keys} argument controls whether keys of all suffixes
contained in a group are right padded, effectively aligning the
descriptions.

@item
If a keyword argument accepts a function as value, you an use a
@code{lambda} expression.  As a special case, the @code{##} macro (which returns a
@code{lambda} expression and is implemented in the @code{llama} package) is also
supported.  Inside group specifications, the use of @code{##} is not
supported anywhere but directly following a keyword symbol.
@end itemize

The @var{ELEMENT}s are either all subgroups, or all suffixes and strings.
(At least currently no group type exists that would allow mixing
subgroups with commands at the same level, though in principle there
is nothing that prevents that.)

If the @var{ELEMENT}s are not subgroups, then they can be a mixture of
lists, which specify commands, and strings.  Strings are inserted
verbatim into the buffer.  The empty string can be used to insert gaps
between suffixes, which is particularly useful if the suffixes are
outlined as a table.

Inside group specifications, including inside contained suffix
specifications, nothing has to be quoted and quoting anyway is
invalid.  The value following a keyword, can be explicitly unquoted
using @code{,}.  This feature is experimental and should be avoided.

The form of suffix specifications is documented in the next node.

@anchor{Suffix Specifications}
@subsection Suffix Specifications

@cindex suffix specifications

A transient's suffix and infix commands are bound when the transient
prefix command is defined using @code{transient-define-prefix}, see
@ref{Defining Transients}.  The commands are organized into groups, see
@ref{Group Specifications}.  Here we describe the form used to bind an
individual suffix command.

The same form is also used when later binding additional commands
using functions such as @code{transient-insert-suffix}, see @ref{Modifying Existing Transients}.

Note that an infix is a special kind of suffix.  Depending on context
``suffixes'' means ``suffixes (including infixes)'' or ``non-infix
suffixes''.  Here it means the former.

Suffix specifications have this form:

@lisp
([LEVEL] [KEY [DESCRIPTION]] COMMAND|ARGUMENT [KEYWORD VALUE]...)
@end lisp

@var{LEVEL}, @var{KEY} and @var{DESCRIPTION} can also be specified using the @var{KEYWORD}s
@code{:level}, @code{:key} and @code{:description}.  If the object that is associated with
@var{COMMAND} sets these properties, then they do not have to be specified
here.  You can however specify them here anyway, possibly overriding
the object's values just for the binding inside this transient.

@itemize
@item
@var{LEVEL} is the suffix level, an integer between 1 and 7.  See
@ref{Enabling and Disabling Suffixes}.

@item
@var{KEY} is the key binding, either a vector or key description string.

@item
@var{DESCRIPTION} is the description, either a string or a function that
takes zero or one arguments (the suffix object) and returns a string.
The function should be a lambda expression to avoid ambiguity.  In
some cases a symbol that is bound as a function would also work but
to be safe you should use @code{:description} in that case.
@end itemize

The next element is either a command or an argument.  This is the only
argument that is mandatory in all cases.

@itemize
@item
@var{COMMAND} should be a symbol that is bound as a function, which has
to be defined or at least autoloaded as a command by the time the
containing prefix command is invoked.

Any command will do; it does not need to have an object associated
with it (as would be the case if @code{transient-define-suffix} or
@code{transient-define-infix} were used to define it).

COMMAND can also be a @code{lambda} expression.

As mentioned above, the object that is associated with a command can
be used to set the default for certain values that otherwise have to
be set in the suffix specification.  Therefore if there is no object,
then you have to make sure to specify the @var{KEY} and the @var{DESCRIPTION}.

As a special case, if you want to add a command that might be neither
defined nor autoloaded, you can use a workaround like:

@lisp
(transient-insert-suffix 'some-prefix "k"
  '("!" "Ceci n'est pas une commande" no-command
    :if (lambda () (featurep 'no-library))))
@end lisp

Instead of @code{featurep} you could also use @code{require} with a non-@code{nil} value
for @var{NOERROR}.

@item
The mandatory argument can also be a command-line argument, a
string.  In that case an anonymous command is defined and bound.

Instead of a string, this can also be a list of two strings, in
which case the first string is used as the short argument (which can
also be specified using @code{:shortarg}) and the second as the long argument
(which can also be specified using @code{:argument}).

Only the long argument is displayed in the menu buffer.  See
@code{transient-detect-key-conflicts} for how the short argument may be
used.

Unless the class is specified explicitly, the appropriate class is
guessed based on the long argument.  If the argument ends with @samp{=}
(e.g., @samp{--format=}) then @code{transient-option} is used, otherwise
@code{transient-switch}.
@end itemize

Finally, details can be specified using optional @var{KEYWORD}-@var{VALUE} pairs.
Each keyword has to be a keyword symbol, either @code{:class} or a keyword
argument supported by the constructor of that class.  See @ref{Suffix Slots}.

If a keyword argument accepts a function as value, you an use a @code{lambda}
expression.  As a special case, the @code{##} macro (which returns a @code{lambda}
expression and is implemented in the @code{llama} package) is also supported.
Inside suffix bindings, the use of @code{##} is not supported anywhere but
directly following a keyword symbol.

@node Defining Suffix and Infix Commands
@section Defining Suffix and Infix Commands

@cindex defining suffix commands
@cindex defining infix commands

Note that an infix is a special kind of suffix.  Depending on context
``suffixes'' means ``suffixes (including infixes)'' or ``non-infix
suffixes''.

@defmac transient-define-suffix name arglist [docstring] [keyword value]@dots{} body@dots{}
This macro defines @var{NAME} as a transient suffix command.

@var{ARGLIST} are the arguments that the command takes.
@var{DOCSTRING} is the documentation string and is optional.

These arguments can optionally be followed by keyword-value pairs.
Each keyword has to be a keyword symbol, either @code{:class} or a keyword
argument supported by the constructor of that class.  The
@code{transient-suffix} class is used if the class is not specified
explicitly.

The @var{BODY} must begin with an @code{interactive} form that matches @var{ARGLIST}.
The infix arguments are usually accessed by using @code{transient-args}
inside @code{interactive}.
@end defmac

@defmac transient-define-infix name arglist [docstring] [keyword value]@dots{}
This macro defines @var{NAME} as a transient infix command.

@var{ARGLIST} is always ignored (but mandatory never-the-less) and
reserved for future use.  @var{DOCSTRING} is the documentation string and
is optional.

At least one key-value pair is required.  All transient infix
commands are @code{equal} to each other (but not @code{eq}).  It is meaningless
to define an infix command, without providing at least one keyword
argument (usually @code{:argument} or @code{:variable}, depending on the class).
The suffix class defaults to @code{transient-switch} and can be set using
the @code{:class} keyword.

The function definition is always:

@lisp
(lambda ()
  (interactive)
  (let ((obj (transient-suffix-object)))
    (transient-infix-set obj (transient-infix-read obj)))
  (transient--show))
@end lisp

@code{transient-infix-read} and @code{transient-infix-set} are generic functions.
Different infix commands behave differently because the concrete
methods are different for different infix command classes.  In rare
cases the above command function might not be suitable, even if you
define your own infix command class.  In that case you have to use
@code{transient-define-suffix} to define the infix command and use @code{t} as the
value of the @code{:transient} keyword.
@end defmac

@defmac transient-define-argument name arglist [docstring] [keyword value]@dots{}
This macro defines @var{NAME} as a transient infix command.

This is an alias for @code{transient-define-infix}.  Only use this alias
to define an infix command that actually sets an infix argument.
To define an infix command that, for example, sets a variable, use
@code{transient-define-infix} instead.
@end defmac

@node Using Infix Arguments
@section Using Infix Arguments

The functions and the variables described below allow suffix commands
to access the value of the transient from which they were invoked;
which is the value of its infix arguments.  These variables are set
when the user invokes a suffix command that exits the transient, but
before actually calling the command.

When returning to the command-loop after calling the suffix command,
the arguments are reset to @code{nil} (which causes the function to return
@code{nil} too).

Like for Emacs's prefix arguments, it is advisable, but not mandatory,
to access the infix arguments inside the command's @code{interactive} form.
The preferred way of doing that is to call the @code{transient-args}
function, which for infix arguments serves about the same purpose as
@code{prefix-arg} serves for prefix arguments.

@defun transient-args prefix
This function returns the value of the transient prefix command
@var{PREFIX}.

If the current command was invoked from the transient prefix command
@var{PREFIX}, then it returns the active infix arguments.  If the current
command was not invoked from @var{PREFIX}, then it returns the set, saved
or default value for @var{PREFIX}.
@end defun

@defun transient-get-value
This function returns the value of the current prefix.

This is mostly intended for internal use, but may also be of use
in @code{transient-set-value} and @code{transient-save-value} methods.  Unlike
@code{transient-args}, this does not include the values of suffixes whose
@code{unsavable} slot is non-@code{nil}.
@end defun

@defun transient-arg-value arg args
This function returns the value of @var{ARG} as it appears in @var{ARGS}.

For a switch a boolean is returned.  For an option the value is
returned as a string, using the empty string for the empty value,
or @code{nil} if the option does not appear in @var{ARGS}.
@end defun

@defun transient-suffixes prefix
This function returns the suffixes of the transient prefix command
@var{PREFIX}.  This is a list of objects.  This function should only be
used if you need the objects (as opposed to just their values) and
if the current command is not being invoked from @var{PREFIX}.
@end defun

@node Using Prefix Scope
@section Using Prefix Scope

Some transients have a sort of secondary value, called a scope.  A
prefix's scope can be accessed using @code{transient-scope}; similar to how
its value can be accessed using @code{transient-args}.

@defun transient-scope prefixes classes
This function returns the scope of the active or current transient
prefix command.

If optional PREFIXES and CLASSES are both nil, return the scope of
the prefix currently being setup, making this variation useful, e.g.,
in @code{:if*} predicates.  If no prefix is being setup, but the current
command was invoked from some prefix, then return the scope of that.

If PREFIXES is non-nil, it must be a prefix command or a list of such
commands.  If CLASSES is non-nil, it must be a prefix class or a list
of such classes.  When this function is called from the body or the
@code{interactive} form of a suffix command, PREFIXES and/or CLASSES should
be non-nil.  If either is non-nil, try the following in order:

@itemize
@item
If the current suffix command was invoked from a prefix, which
appears in PREFIXES, return the scope of that prefix.

@item
If the current suffix command was invoked from a prefix, and its
class derives from one of the CLASSES, return the scope of that
prefix.

@item
If a prefix is being setup and it appears in PREFIXES, return its
scope.

@item
If a prefix is being setup and its class derives from one of the
CLASSES, return its scope.

@item
Finally try to return the default scope of the first command in
PREFIXES@.  This only works if that slot is set in the respective
class definition or using its `transient-init-scope' method.
@end itemize

If no prefix matches, return nil.
@end defun

@node Current Suffix Command
@section Current Suffix Command

@defun transient-suffix-object command
This function returns the object associated with the current suffix
command.

Each suffix commands is associated with an object, which holds
additional information about the suffix, such as its value (in
the case of an infix command, which is a kind of suffix command).

This function is intended to be called by infix commands, which
are usually aliases of @code{transient--default-infix-command}, which
is defined like this:

@lisp
(defun transient--default-infix-command ()
  (interactive)
  (let ((obj (transient-suffix-object)))
    (transient-infix-set obj (transient-infix-read obj)))
  (transient--show))
@end lisp

(User input is read outside of @code{interactive} to prevent the
command from being added to @code{command-history}.)

Such commands need to be able to access their associated object
to guide how @code{transient-infix-read} reads the new value and to
store the read value.  Other suffix commands (including non-infix
commands) may also need the object to guide their behavior.

This function attempts to return the object associated with the
current suffix command even if the suffix command was not invoked
from a transient.  (For some suffix command that is a valid thing
to do, for others it is not.)  In that case @code{nil} may be returned,
if the command was not defined using one of the macros intended
to define such commands.

The optional argument COMMAND is intended for internal use.  If
you are contemplating using it in your own code, then you should
probably use this instead:

@lisp
(get COMMAND 'transient--suffix)
@end lisp
@end defun

@defvar transient-current-suffixes
The suffixes of the transient from which this suffix command was
invoked.  This is a list of objects.  Usually it is sufficient to
instead use the function @code{transient-args}, which returns a list of
values.  In complex cases it might be necessary to use this variable
instead, i.e., if you need access to information beside the value.
@end defvar

@node Current Prefix Command
@section Current Prefix Command

@defun transient-prefix-object
This function returns the current prefix as an object.

While a transient is being setup or refreshed (which involves
preparing its suffixes) the variable @code{transient--prefix} can be
used to access the prefix object.  Thus this is what has to be
used in suffix methods such as @code{transient-format-description},
and in object-specific functions that are stored in suffix slots
such as @code{description}.

When a suffix command is invoked (i.e., in its @code{interactive} form
and function body) then the variable @code{transient-current-prefix}
has to be used instead.

Two distinct variables are needed, because any prefix may itself
be used as a suffix of another prefix, and such sub-prefixes have
to be able to tell themselves apart from the prefix they were
invoked from.

Regular suffix commands, which are not prefixes, do not have to
concern themselves with this distinction, so they can use this
function instead.  In the context of a plain suffix, it always
returns the value of the appropriate variable.
@end defun

@defvar transient-current-prefix
The transient from which this suffix command was invoked.  The value
is a @code{transient-prefix} object, which holds information associated
with the transient prefix command.
@end defvar

@defvar transient-current-command
The transient from which this suffix command was invoked.  The value
is a symbol, the transient prefix command.
@end defvar

@defun transient-active-prefix &optional prefixes
This function returns the active transient object.  It returns @code{nil}
if there is no active transient, if the transient buffer isn't
shown, and while the active transient is suspended (e.g., while the
minibuffer is in use).

Unlike @code{transient-current-prefix}, which is only ever non-@code{nil} in code
that is run directly by a command that is invoked while a transient
is current, this function is also suitable for use in asynchronous
code, such as timers and callbacks (this function's main use-case).

If optional PREFIXES is non-@code{nil}, it must be a prefix command symbol
or a list of symbols, in which case the active transient object is
only returned if it matches one of the PREFIXES@.
@end defun

@node Transient State
@section Transient State

@cindex transient state

Invoking a transient prefix command ``activates'' the respective
transient, i.e., it puts a transient keymap into effect, which binds
the transient's infix and suffix commands.

The default behavior while a transient is active is as follows:

@itemize
@item
Invoking an infix command does not affect the transient state; the
transient remains active.

@item
Invoking a (non-infix) suffix command ``deactivates'' the transient
state by removing the transient keymap and performing some
additional cleanup.

@item
Invoking a command that is bound in a keymap other than the
transient keymap is disallowed and trying to do so results in a
warning.  This does not ``deactivate'' the transient.
@end itemize

The behavior can be changed for all suffixes of a particular prefix
and/or for individual suffixes.  The values should nearly always be
booleans, but certain functions, called ``pre-commands'', can also be
used.  These functions are named @code{transient--do-VERB}, and the symbol
@code{VERB} can be used as a shorthand.

A boolean is interpreted as answering the question "does the
transient stay active, when this command is invoked?"  @code{t} means that
the transient stays active, while @code{nil} means that invoking the command
exits the transient.

Note that when the suffix is a ``sub-prefix'', invoking that command
always activates that sub-prefix, causing the outer prefix to no
longer be active and displayed.  Here @code{t} means that when you exit the
inner prefix, then the outer prefix becomes active again, while @code{nil}
means that all outer prefixes are exited at once.

@itemize
@item
The behavior for non-suffixes can be set for a particular prefix,
by the prefix's @code{transient-non-suffix} slot to a boolean, a suitable
pre-command function, or a shorthand for such a function.  See
@ref{Pre-commands for Non-Suffixes}.

@item
The common behavior for the suffixes of a particular prefix can be
set using the prefix's @code{transient-suffixes} slot.

The value specified in this slot does @strong{not} affect infixes.  Because
it affects both regular suffixes as well as sub-prefixes, which
have different needs, it is best to avoid explicitly specifying a
function.

@item
The behavior of an individual suffix can be changed using its
@code{transient} slot.  While it is usually best to use a boolean, for this
slot it can occasionally make sense to specify a function explicitly.

Note that this slot can be set when defining a suffix command using
@code{transient-define-suffix} and/or in the definition of the prefix.  If
set in both places, then the latter takes precedence, as usual.
@end itemize

The available pre-command functions are documented in the following
sub-sections.  They are called by @code{transient--pre-command}, a function
on @code{pre-command-hook}, and the value that they return determines whether
the transient is exited.  To do so the value of one of the constants
@code{transient--exit} or @code{transient--stay} is used (that way we don't have to
remember if @code{t} means ``exit'' or ``stay'').

Additionally, these functions may change the value of @code{this-command}
(which explains why they have to be called using @code{pre-command-hook}),
call @code{transient-export}, @code{transient--stack-zap} or @code{transient--stack-push};
and set the values of @code{transient--exitp}, @code{transient--helpp} or
@code{transient--editp}.

For completeness sake, some notes about complications:

@itemize
@item
The transient-ness of certain built-in suffix commands is specified
using @code{transient-predicate-map}.  This is a special keymap, which
binds commands to pre-commands (as opposed to keys to commands) and
takes precedence over the prefix's @code{transient-suffix} slot, but not
the suffix's @code{transient} slot.

@item
While a sub-prefix is active we nearly always want @kbd{C-g} to take the
user back to the ``super-prefix'', even when the other suffixes don't
do that.  However, in rare cases this may not be desirable, in which
case @code{replace} can be used as the value of the sub-prefix's @code{transient}
slot.
@end itemize

@anchor{Pre-commands for Infixes}
@subheading Pre-commands for Infixes

The default for infixes is @code{transient--do-stay}.  This is also the only
function that makes sense for infixes, which is why this predicate is
used even if the value of the prefix's @code{transient-suffix} slot is @code{t}.  In
extremely rare cases, one might want to use something else, which can
be done by setting the infix's @code{transient} slot directly.

@defun transient--do-stay
Call the command without exporting variables and stay transient.
@end defun

@anchor{Pre-commands for Suffixes}
@subheading Pre-commands for Suffixes

By default, invoking a suffix causes the transient to be exited.

The behavior for an individual suffix command can be changed by
setting its @code{transient} slot to a boolean (which is highly recommended),
or to one of the following pre-commands.

@defun transient--do-exit
Call the command after exporting variables and exit the transient.
@end defun

@defun transient--do-return
Call the command after exporting variables and return to the parent
prefix.  If there is no parent prefix, then call @code{transient--do-exit}.
@end defun

@defun transient--do-call
Call the command after exporting variables and stay transient.
@end defun

The following pre-commands are only suitable for sub-prefixes.  It is
not necessary to explicitly use these predicates because the correct
predicate is automatically picked based on the value of the @code{transient}
slot for the sub-prefix itself.

@defun transient--do-recurse
Call the transient prefix command, preparing for return to outer
transient.

Whether we actually return to the parent transient is ultimately
under the control of each invoked suffix.  The difference between
this pre-command and @code{transient--do-stack} is that it changes the
value of the @code{transient-suffix} slot to @code{t}.

If there is no parent transient, then only call this command and
skip the second step.
@end defun

@defun transient--do-stack
Call the transient prefix command, stacking the active transient.
Push the active transient to the transient stack.

Unless @code{transient--do-recurse} is explicitly used, this pre-command
is automatically used for suffixes that are prefixes themselves,
i.e., for sub-prefixes.
@end defun

@defun transient--do-replace
Call the transient prefix command, replacing the active transient.
Do not push the active transient to the transient stack.

Unless @code{transient--do-recurse} is explicitly used, this pre-command
is automatically used for suffixes that are prefixes themselves,
i.e., for sub-prefixes.
@end defun

@defun transient--do-suspend
Suspend the active transient, saving the transient stack.

This is used by the command @code{transient-suspend} and optionally also by
``external events'' such as @code{handle-switch-frame}.  Such bindings should
be added to @code{transient-predicate-map}.
@end defun

@anchor{Pre-commands for Non-Suffixes}
@subheading Pre-commands for Non-Suffixes

By default, non-suffixes (commands that are bound in other keymaps
beside the transient keymap) cannot be invoked.  Trying to invoke
such a command results in a warning and the transient stays active.

If you want a different behavior, then set the @code{transient-non-suffix}
slot of the transient prefix command.  The value should be a boolean,
answering the question, "is it allowed to invoke non-suffix commands?,
a pre-command function, or a shorthand for such a function.

If the value is @code{t}, then non-suffixes can be invoked, when it is @code{nil}
(the default) then they cannot be invoked.

The only other recommended value is @code{leave}.  If that is used, then
non-suffixes can be invoked, but if one is invoked, then that exits
the transient.

@defun transient--do-warn
Call @code{transient-undefined} and stay transient.
@end defun

@defun transient--do-stay
Call the command without exporting variables and stay transient.
@end defun

@defun transient--do-leave
Call the command without exporting variables and exit the transient.
@end defun

@anchor{Special Pre-Commands}
@subheading Special Pre-Commands

@defun transient--do-quit-one
If active, quit help or edit mode, else exit the active transient.

This is used when the user pressed @kbd{C-g}.
@end defun

@defun transient--do-quit-all
Exit all transients without saving the transient stack.

This is used when the user pressed @kbd{C-q}.
@end defun

@defun transient--do-suspend
Suspend the active transient, saving the transient stack.

This is used when the user pressed @kbd{C-z}.
@end defun

@node Classes and Methods
@chapter Classes and Methods

@cindex classes and methods

Transient uses classes and generic functions to make it possible to
define new types of suffix and prefix commands, which are similar to
existing types, but behave differently in some respects.

Every prefix, infix and suffix command is associated with an object,
which holds information, which controls certain aspects of its
behavior.  This happens in two ways.

@itemize
@item
Associating a command with a certain class gives the command a type.
This makes it possible to use generic functions to do certain things
that have to be done differently depending on what type of command
it acts on.

That in turn makes it possible for third-parties to add new types
without having to convince the maintainer of Transient, that that
new type is important enough to justify adding a special case to a
dozen or so functions.

@item
Associating a command with an object makes it possible to easily
store information that is specific to that particular command.

Two commands may have the same type, but obviously their key
bindings and descriptions still have to be different, for example.

The values of some slots are functions.  The @code{reader} slot for example
holds a function that is used to read a new value for an infix
command.  The values of such slots are regular functions.

Generic functions are used when a function should do something
different based on the type of the command, i.e., when all commands
of a certain type should behave the same way but different from the
behavior for other types.  Object slots that hold a regular function
as value are used when the task that they perform is likely to
differ even between different commands of the same type.
@end itemize

@menu
* Group Classes::
* Group Methods::
* Prefix Classes::
* Suffix Classes::
* Prefix Methods::
* Suffix Methods::
* Prefix Slots::
* Suffix Slots::
* Predicate Slots::
@end menu

@node Group Classes
@section Group Classes

The type of a group can be specified using the @code{:class} property at the
beginning of the class specification, e.g., @code{[:class transient-columns
...]} in a call to @code{transient-define-prefix}.

@itemize
@item
The abstract @code{transient-child} class is the base class of both
@code{transient-group} (and therefore all groups) as well as of
@code{transient-suffix} (and therefore all suffix and infix commands).

This class exists because the elements (or ``children'') of certain
groups can be other groups instead of suffix and infix commands.

@item
The abstract @code{transient-group} class is the superclass of all other
group classes.

@item
The @code{transient-column} class is the simplest group.

This is the default ``flat'' group.  If the class is not specified
explicitly and the first element is not a vector (i.e., not a group),
then this class is used.

This class displays each element on a separate line.

@item
The @code{transient-row} class displays all elements on a single line.

@item
The @code{transient-columns} class displays commands organized in columns.

Direct elements have to be groups whose elements have to be commands
or strings.  Each subgroup represents a column.  This class takes
care of inserting the subgroups' elements.

This is the default ``nested'' group.  If the class is not specified
explicitly and the first element is a vector (i.e., a group), then
this class is used.

@item
The @code{transient-subgroups} class wraps other groups.

Direct elements have to be groups whose elements have to be commands
or strings.  This group inserts an empty line between subgroups.
The subgroups themselves are responsible for displaying their
elements.
@end itemize

@node Group Methods
@section Group Methods

@defun transient-setup-children group children
This generic function can be used to setup the children or a group.

The default implementation usually just returns the children
unchanged, but if the @code{setup-children} slot of @var{GROUP} is non-@code{nil}, then
it calls that function with @var{CHILDREN} as the only argument and
returns the value.

The children are given as a (potentially empty) list consisting of
either group or suffix specifications.  These functions can make
arbitrary changes to the children including constructing new
children from scratch.
@end defun

@defun transient--insert-group group
This generic function formats the group and its elements and inserts
the result into the current buffer, which is a temporary buffer.
The contents of that buffer are later inserted into the menu buffer.

Functions that are called by this function may need to operate in
the buffer from which the transient was called.  To do so they can
temporarily make the @code{transient--shadowed-buffer} the current buffer.
@end defun

@node Prefix Classes
@section Prefix Classes

Transient itself provides a single class for prefix commands,
@code{transient-prefix}, but package authors may wish to define specialized
classes.  Doing so makes it possible to change the behavior of the set
of prefix commands that use that class, by implementing specialized
methods for certain generic functions (see @ref{Prefix Methods}).

A transient prefix command's object is stored in the @code{transient--prefix}
property of the command symbol.  While a transient is active, a clone
of that object is stored in the variable @code{transient--prefix}.  A clone
is used because some changes that are made to the active transient's
object should not affect later invocations.

@node Suffix Classes
@section Suffix Classes

@itemize
@item
All suffix and infix classes derive from @code{transient-suffix}, which in
turn derives from @code{transient-child}, from which @code{transient-group} also
derives (see @ref{Group Classes}).

@item
All infix classes derive from the abstract @code{transient-infix} class,
which in turn derives from the @code{transient-suffix} class.

Infixes are a special type of suffixes.  The primary difference is
that infixes always use the @code{transient--do-stay} pre-command, while
non-infix suffixes use a variety of pre-commands (see @ref{Transient State}).  Doing that is most easily achieved by using this class,
though theoretically it would be possible to define an infix class
that does not do so.  If you do that then you get to implement many
methods.

Also, infixes and non-infix suffixes are usually defined using
different macros (see @ref{Defining Suffix and Infix Commands}).

@item
Classes used for infix commands that represent arguments should
be derived from the abstract @code{transient-argument} class.

@item
The @code{transient-switch} class (or a derived class) is used for infix
arguments that represent command-line switches (arguments that do
not take a value).

@item
The @code{transient-option} class (or a derived class) is used for infix
arguments that represent command-line options (arguments that do
take a value).

@item
The @code{transient-switches} class can be used for a set of mutually
exclusive command-line switches.

@item
The @code{transient-files} class can be used for a @samp{--} argument that
indicates that all remaining arguments are files.

@item
Classes used for infix commands that represent variables should
derived from the abstract @code{transient-variable} class.

@item
The @code{transient-information} and @code{transient-information*} classes are
special in that suffixes that use these class are not associated
with a command and thus also not with any key binding.  Such
suffixes are only used to display arbitrary information, and that
anywhere a suffix can appear.  Display-only suffix specifications
take these form:

@lisp
([LEVEL] :info DESCRIPTION [KEYWORD VALUE]...)
([LEVEL] :info* DESCRIPTION [KEYWORD VALUE]...)
@end lisp

The @code{:info} and @code{:info*} keyword arguments replaces the @code{:description}
keyword used for other suffix classes.  Other keyword arguments that
you might want to set, include @code{:face}, predicate keywords (such as
@code{:if} and @code{:inapt-if}), and @code{:format}.  By default the value of @code{:format}
includes @code{%k}, which for this class is replaced with the empty string
or spaces, if keys are being padded in the containing group.

The only difference between these two classes is that @code{:info*} aligns
its description with the descriptions of suffix commands, while for
@code{:info} the description bleeds into the area where suffixes display
their key bindings.

@item
The @code{transient-lisp-variable} class can be used to show and change the
value of lisp variables.  This class is not fully featured yet and
it is somewhat likely that future improvements won't be fully
backward compatible.

@item
The @code{transient-describe-target} class is used by the command
@code{transient-describe}.

@item
The @code{transient-value-preset} class is used to implement the command
@code{transient-preset}, which activates a value preset.
@end itemize

Magit defines additional classes, which can serve as examples for the
fancy things you can do without modifying Transient.  Some of these
classes will likely get generalized and added to Transient.  For now
they are very much subject to change and not documented.

@node Prefix Methods
@section Prefix Methods

To get information about the methods implementing these generic
functions use @code{describe-function}.

@defun transient-init-value obj
This generic function sets the initial value of the object @var{OBJ}.
Methods exist for both prefix and suffix objects.

The default method for prefix objects sets the value of OBJ's @code{value}
slot to the set, saved or default value.  The value that is set for
the current session is preferred over the saved value, which is
preferred over the default value.

The default value is determined using the generic function
@code{transient-default-value}.  If you need to change how the value for a
prefix class is determined, its usually sufficient to implement a
method for that function.
@end defun

@defun transient-default-value obj
This generic function returns the default value of the object @var{OBJ}.
Methods exist for both prefix and suffix objects.

The default method for prefix objects returns the value of the
@code{default-value} slot if that is bound and not a function.  If it is a
function, that is called to get the value.  If the slot is unbound,
@code{nil} is returned.
@end defun

@defun transient-prefix-value obj
This generic function returns the value of the prefix object @var{OBJ}.
The respective generic function for infix and suffix objects is
named @code{transient-infix-value}.
@end defun

@defun transient-init-scope obj
This generic function sets the scope of the object @var{OBJ}.  Methods
exist for both prefix and suffix objects.

This function is called for all prefix and suffix commands, but
unless a concrete method is implemented this falls through to
the default implementation, which is a noop.
@end defun

@code{transient-set-value}, @code{transient-save-value}, @code{transient-reset-value},
@code{transient--history-key}, @code{transient--history-push} and
@code{transient--history-init} are other generic functions dealing with the
value of prefix objects.  See their doc-strings for more information.

@code{transient-show-help} is another generic function implemented for prefix
commands.  The default method effectively describes the command using
@code{describe-function}.

@node Suffix Methods
@section Suffix Methods

To get information about the methods implementing these generic
functions use @code{describe-function}.

@anchor{Suffix Value Methods}
@subsection Suffix Value Methods

@defun transient-init-value obj
This generic function sets the initial value of the object @var{OBJ}.
Methods exist for both prefix and suffix objects.

For @code{transient-argument} objects this function handles setting the
value by itself.

For other @code{transient-suffix} objects (including @code{transient-infix}
objects), this calls @code{transient-default-value} and uses the value
returned by that, unless it is the special value @code{eieio--unbound},
which indicates that there is no default value.  Since that is what
the default method for @code{transient-suffix} objects does, both of these
functions effectively are noops for these classes.

If you implement a class that derives from @code{transient-infix} directly,
then you must implement a dedicated method for this function and/or
@code{transient-default-value}.
@end defun

@defun transient-default-value obj
This generic function returns the default value of the object @var{OBJ}.
Methods exist for both prefix and suffix objects.
@end defun

@defun transient-infix-read obj
This generic function determines the new value of the infix object
@var{OBJ}.

This function merely determines the value; @code{transient-infix-set} is
used to actually store the new value in the object.

For most infix classes this is done by reading a value from the
user using the reader specified by the @code{reader} slot (using the
@code{transient-infix-value} method described below).

For some infix classes the value is changed without reading
anything in the minibuffer, i.e., the mere act of invoking the
infix command determines what the new value should be, based
on the previous value.
@end defun

@defun transient-prompt obj
This generic function returns the prompt to be used to read infix
object @var{OBJ}'s value.
@end defun

@defun transient-infix-set obj value
This generic function sets the value of infix object @var{OBJ} to @var{VALUE}.
@end defun

@defun transient-infix-value obj
This generic function returns the value of the suffix object @var{OBJ}.

This function is called by @code{transient-args} (which see), meaning this
function is how the value of a transient is determined so that the
invoked suffix command can use it.

Currently most values are strings, but that is not set in stone.
@code{nil} is not a value, it means ``no value''.

Usually only infixes have a value, but see the method for
@code{transient-suffix}.
@end defun

@defun transient-init-scope obj
This generic function sets the scope of the object @var{OBJ}.  Methods
exist for both prefix and suffix objects.

The scope is actually a property of the transient prefix, not of
individual suffixes.  However it is possible to invoke a suffix
command directly instead of from a transient.  In that case, if
the suffix expects a scope, then it has to determine that itself
and store it in its @code{scope} slot.

This function is called for all prefix and suffix commands, but
unless a concrete method is implemented, this falls through to
the default implementation, which is a noop.
@end defun

@anchor{Suffix Format Methods}
@subsection Suffix Format Methods

@defun transient-format obj
This generic function formats and returns @var{OBJ} for display.

When this function is called, then the current buffer is some
temporary buffer.  If you need the buffer from which the prefix
command was invoked to be current, then do so by temporarily
making @code{transient--source-buffer} current.
@end defun

@defun transient-format-key obj
This generic function formats @var{OBJ}'s @code{key} for display and returns the
result.
@end defun

@defun transient-format-description obj
This generic function formats @var{OBJ}'s @code{description} for display and
returns the result.
@end defun

@defun transient-format-value obj
This generic function formats @var{OBJ}'s value for display and returns
the result.
@end defun

@defun transient-show-help obj
Show help for the prefix, infix or suffix command represented by
@var{OBJ}.

Regardless of OBJ's type, if its @code{show-help} slot is non-@code{nil}, that
must be a function, which takes OBJ is its only argument.  It must
prepare, display and return a buffer, and select the window used to
display it.  The @code{transient-show-help-window} macro is intended for
use in such functions.

For prefixes, show the info manual, if that is specified using the
@code{info-manual} slot.  Otherwise, show the manpage if that is specified
using the @code{man-page} slot.  Otherwise, show the command's
documentation string.

For suffixes, show the command's documentation string.

For infixes, show the manpage if that is specified.  Otherwise show
the command's documentation string.
@end defun

@defmac transient-with-help-window &rest body
Evaluate BODY, send output to @code{*Help*} buffer, and display it in a
window.  Select the help window, and make the help buffer current
and return it.
@end defmac

@defun transient-show-summary obj &optional return
This generic function shows or, if optional RETURN is non-@code{nil},
returns a brief summary about the command at point or hovered with
the mouse.

This function is called when the mouse is moved over a command and
(if the value of @code{transient-enable-popup-navigation} is @code{verbose}) when
the user navigates to a command using the keyboard.

If OBJ's @code{summary} slot is a string, that is used.  If @code{summary} is a
function, that is called with OBJ as the only argument and the
returned string is used.  If @code{summary} is or returns something other
than a string or @code{nil}, no summary is shown.  If @code{summary} is or returns
@code{nil}, the first line of the documentation string is used, if any.

If RETURN is non-@code{nil}, this function returns the summary instead of
showing it.  This is used when a tooltip is needed.
@end defun

@node Prefix Slots
@section Prefix Slots

@anchor{Value and Scope}
@subheading Value and Scope

@itemize
@item
@code{default-value} The default value of the prefix.  Use the keyword
argument @code{:value} (sic) to set this slot in the definition of a
prefix.

@item
@code{init-value} A function that is responsible for setting the object's
value.  If bound, then this is called with the object as the only
argument.  Usually this is not bound, in which case the object's
primary @code{transient-init-value} method is called instead.

@item
@code{history-key} If multiple prefix commands should share a single value,
then this slot has to be set to the same value for all of them.  You
probably don't want that.

@item
@code{incompatible} A list of lists.  Each sub-list specifies a set of
mutually exclusive arguments.  Enabling one of these arguments
causes the others to be disabled.  An argument may appear in
multiple sub-lists.  Arguments must me given in the same form as
used in the @code{argument} or @code{argument-format} slot of the respective
suffix objects, usually something like @code{--switch} or @code{--option=%s}.  For
options and @code{transient-switches} suffixes it is also possible to match
against a specific value, as returned by @code{transient-infix-value},
for example, @code{--option=one}.

@item
@code{scope} For some transients it might be necessary to have a sort of
secondary value, called a ``scope''.  See @code{transient-define-prefix}.
@end itemize

@anchor{Behavior}
@subheading Behavior

@itemize
@item
@code{transient-suffix}, @code{transient-non-suffix} and @code{transient-switch-frame}
play a part when determining whether the currently active transient
prefix command remains active/transient when a suffix or arbitrary
non-suffix command is invoked.  See @ref{Transient State}.

@item
@code{refresh-suffixes} Normally suffix objects and keymaps are only setup
once, when the prefix is invoked.  Setting this to @code{t}, causes them to
be recreated after every command.  This is useful when using @code{:if...}
predicates, and those need to be rerun for some reason.  Doing this
is somewhat costly, and there is a risk of losing state, so this is
disabled by default and still considered experimental.

@item
@code{environment} A function used to establish an environment while
initializing, refreshing or redisplaying a transient prefix menu.
This is useful to establish a cache, in case multiple suffixes
require the same expensive work.  The provided function is called
with at least one argument, the function for which it establishes
the environment.  It must @code{funcall} that function with no arguments.
During initialization the second argument is the prefix object
being initialized.  This slot is still experimental.
@end itemize

@anchor{Appearance}
@subheading Appearance

@itemize
@item
@code{display-action} determines how this prefix is displayed, overriding
@code{transient-display-buffer-action}.  It should have the same type.

@item
@code{mode-line-format} is this prefix's mode line format, overriding
@code{transient-mode-line-format}.  It should have the same type.

@item
@code{column-widths} is only respected inside @code{transient-columns} groups and
allows aligning columns across separate instances of that.  A list
of integers.

@item
@code{variable-pitch} controls whether alignment is done pixel-wise to
account for use of variable-pitch characters, which is useful, e.g.,
when using emoji.
@end itemize

@anchor{Documentation}
@subheading Documentation

@itemize
@item
@code{show-help}, @code{man-page} or @code{info-manual} can be used to specify the
documentation for the prefix and its suffixes.  The command
@code{transient-help} uses the function @code{transient-show-help} (which see) to
lookup and use these values.

@item
@code{suffix-description} can be used to specify a function which provides
fallback descriptions for suffixes that lack a description.  This
is intended to be temporarily used when implementing of a new prefix
command, at which time @code{transient-command-summary-or-name} is a useful
value.
@end itemize

@anchor{Internal}
@subheading Internal

These slots are mostly intended for internal use.  They should not be
set in calls to @code{transient-define-prefix}.

@itemize
@item
@code{prototype} When a transient prefix command is invoked, then a clone
of that object is stored in the global variable @code{transient--prefix}
and the prototype is stored in the clone's @code{prototype} slot.

@item
@code{command} The command, a symbol.  Each transient prefix command
consists of a command, which is stored in a symbol's function slot
and an object, which is stored in the @code{transient--prefix} property
of the same symbol.

@item
@code{level} The level of the prefix commands.  The suffix commands whose
layer is equal or lower are displayed.  See @ref{Enabling and Disabling Suffixes}.

@item
@code{value} The likely outdated value of the prefix.  Instead of accessing
this slot directly you should use the function @code{transient-get-value},
which is guaranteed to return the up-to-date value.

@item
@code{history} and @code{history-pos} are used to keep track of historic values.
Unless you implement your own @code{transient-infix-read} method you should
not have to deal with these slots.

@item
@code{unwind-suffix} is used internally to ensure transient state is
properly exited, even in case of an error.
@end itemize

@node Suffix Slots
@section Suffix Slots

Here we document most of the slots that are only available for suffix
objects.  Some slots are shared by suffix and group objects, they are
documented in @ref{Predicate Slots}.

Also see @ref{Suffix Classes}.

@anchor{Slots of @code{transient-child}}
@subheading Slots of @code{transient-child}

This is the abstract superclass of @code{transient-suffix} and @code{transient-group}.
This is where the shared @code{if*} and @code{inapt-if*} slots (see @ref{Predicate Slots}),
the @code{level} slot (see @ref{Enabling and Disabling Suffixes}), and the @code{advice}
and @code{advice*} slots (see @ref{Slots of @code{transient-suffix}}) are defined.

@itemize
@item
@code{parent} The object for the parent group.
@end itemize

@anchor{Slots of @code{transient-suffix}}
@subheading Slots of @code{transient-suffix}

@itemize
@item
@code{key} The key, a key vector or a key description string.

@item
@code{command} The command, a symbol.

@item
@code{transient} Whether to stay transient.  See @ref{Transient State}.

@item
@code{format} The format used to display the suffix in the menu buffer.
It must contain the following %-placeholders:

@itemize
@item
@code{%k} For the key.
@item
@code{%d} For the description.
@item
@code{%v} For the infix value.  Non-infix suffixes don't have a value.
@end itemize

@item
@code{description} The description, either a string or a function, which is
called with zero or one argument (the suffix object), and returns a
string.

@item
@code{face} Face used for the description.  In simple cases it is easier
to use this instead of using a function as @code{description} and adding
the styling there.  @code{face} is appended using @code{add-face-text-property}.

@item
@code{show-help} A function used to display help for the suffix.  If
unspecified, the prefix controls how help is displayed for its
suffixes.  See also function @code{transient-show-help}.

@item
@code{summary} The summary displayed in the echo area, or as a tooltip.
If this is @code{nil}, which it usually should be, the first line of the
documentation string is used instead.  See @code{transient-show-summary}
for details.

@item
@code{definition} A command, which is used if the body is omitted when
defining a command using @code{transient-define-suffix}.
@end itemize

The following two slots are experimental.  They can also be set for a
group, in which case they apply to all suffixes in that group, except
for suffixes that set the same slot to a non-nil value.

@itemize
@item
@code{advice} A function used to advise the command.  The advise is called
using @code{(apply advice command args)}, i.e., it behaves like an "around"
advice.

@item
@code{advice*} A function used to advise the command.  Unlike @code{advice}, this
advises not only the command body but also its @code{interactive} spec.  If
both slots are non-nil, @code{advice} is used for the body and @code{advice*} is
used for the @code{interactive} form.  When advising the @code{interactive} spec,
called using @code{(funcall advice #'advice-eval-interactive-spec spec)}.
@end itemize

@anchor{Slots of @code{transient-infix}}
@subheading Slots of @code{transient-infix}

Some of these slots are only meaningful for some of the subclasses.
They are defined here anyway to allow sharing certain methods.

@itemize
@item
@code{argument} The long argument, e.g., @code{--verbose}.

@item
@code{shortarg} The short argument, e.g., @code{-v}.

@item
@code{value} The value.  Should not be accessed directly.

@item
@code{init-value} Function that is responsible for setting the object's
value.  If bound, then this is called with the object as the only
argument.  Usually this is not bound, in which case the object's
primary @code{transient-init-value} method is called instead.

@item
@code{unsavable} Whether the value of the suffix is not saved as part of
the prefixes.

@item
@code{multi-value} For options, whether the option can have multiple
values.  If this is non-@code{nil}, then the values are read using
@code{completing-read-multiple} by default and if you specify your own
reader, then it should read the values using that function or
similar.

Supported non-@code{nil} values are:

@itemize
@item
Use @code{rest} for an option that can have multiple values.  This is
useful e.g., for an @code{--} argument that indicates that all remaining
arguments are files (such as @code{git log -- file1 file2}).

In the list returned by @code{transient-args} such an option and its
values are represented by a single list of the form @code{(ARGUMENT
    . VALUES)}.

@item
Use @code{repeat} for an option that can be specified multiple times.

In the list returned by @code{transient-args} each instance of the option
and its value appears separately in the usual from, for example:
@code{("--another-argument" "--option=first" "--option=second")}.
@end itemize

In both cases the option's values have to be specified in the
default value of a prefix using the same format as returned by
@code{transient-args}, e.g., @code{("--other" "--o=1" "--o=2" ("--" "f1" "f2"))}.

@item
@code{always-read} For options, whether to read a value on every invocation.
If this is @code{nil}, then options that have a value are simply unset and
have to be invoked a second time to set a new value.

@item
@code{allow-empty} For options, whether the empty string is a valid value.

@item
@code{history-key} The key used to store the history.  This defaults to the
command name.  This is useful when multiple infixes should share the
same history because their values are of the same kind.

@item
@code{reader} The function used to read the value of an infix.  Not used
for switches.  The function takes three arguments, @var{PROMPT},
@var{INITIAL-INPUT} and @var{HISTORY}, and must return a string.

@item
@code{prompt} The prompt used when reading the value, either a string or a
function that takes the object as the only argument and which
returns a prompt string.

@item
@code{choices} A list of valid values, or a function that returns such a
list.  The latter is not implemented for @code{transient-switches}, because
I couldn't think of a use-case.  How exactly the choices are used
varies depending on the class of the suffix.
@end itemize

@anchor{Slots of @code{transient-variable}}
@subheading Slots of @code{transient-variable}

@itemize
@item
@code{variable} The variable.
@end itemize

@anchor{Slots of @code{transient-switches}}
@subheading Slots of @code{transient-switches}

@itemize
@item
@code{argument-format} The display format.  Must contain @code{%s}, one of the
@code{choices} is substituted for that.  E.g., @code{--%s-order}.

@item
@code{argument-regexp} The regexp used to match any one of the switches.
E.g., @code{\\(--\\(topo\\|author-date\\|date\\)-order\\)}.
@end itemize

@node Predicate Slots
@section Predicate Slots

Suffix and group objects share two sets of predicate slots that
control whether a group or suffix should be available depending on
some state.  Only one slot from each set can be used at the same
time.  It is undefined which slot is honored if you use more than
one.

Predicates from the first group control whether the suffix is present
in the menu at all.

@itemize
@item
@code{if} Enable if predicate returns non-@code{nil}.
@item
@code{if-not} Enable if predicate returns @code{nil}.
@item
@code{if-non-nil} Enable if variable's value is non-@code{nil}.
@item
@code{if-nil} Enable if variable's value is @code{nil}.
@item
@code{if-mode} Enable if major-mode matches value.
@item
@code{if-not-mode} Enable if major-mode does not match value.
@item
@code{if-derived} Enable if major-mode derives from value.
@item
@code{if-not-derived} Enable if major-mode does not derive from value.
@end itemize

Predicates from the second group control whether the suffix can be
invoked.  The suffix is shown in the menu regardless, but when it
is considered "inapt", then it is grayed out to indicated that it
currently cannot be invoked.

@itemize
@item
@code{inapt-if} Inapt if predicate returns non-@code{nil}.
@item
@code{inapt-if-not} Inapt if predicate returns @code{nil}.
@item
@code{inapt-if-non-nil} Inapt if variable's value is non-@code{nil}.
@item
@code{inapt-if-nil} Inapt if variable's value is @code{nil}.
@item
@code{inapt-if-mode} Inapt if major-mode matches value.
@item
@code{inapt-if-not-mode} Inapt if major-mode does not match value.
@item
@code{inapt-if-derived} Inapt if major-mode derives from value.
@item
@code{inapt-if-not-derived} Inapt if major-mode does not derive from value.
@end itemize

By default these predicates run when the prefix command is invoked,
but this can be changes, using the @code{refresh-suffixes} prefix slot.
See @ref{Prefix Slots}.

One more slot is shared between group and suffix classes, @code{level}.  Like
the slots documented above, it is a predicate, but it is used for a
different purpose.  The value has to be an integer between 1
and 7.  @code{level} controls whether a suffix or a group should be
available depending on user preference.
See @ref{Enabling and Disabling Suffixes}.

@node FAQ
@appendix FAQ

@anchor{Can I control how the menu buffer is displayed?}
@appendixsec Can I control how the menu buffer is displayed?

Yes, see @code{transient-display-buffer-action} in @ref{Configuration}.  You can
also control how the menu buffer is displayed on a case-by-case basis
by passing @code{:display-action} to @code{transient-define-prefix}.

@anchor{How can I copy text from the menu buffer?}
@appendixsec How can I copy text from the menu buffer?

To be able to mark text in Transient's menu buffer using the mouse,
you have to add the below binding.  Note that for technical reasons,
the region won't be visualized, while doing so.  After you have quit
the transient menu, you will be able to yank it in another buffer.

@lisp
(keymap-set transient-predicate-map
            "<mouse-set-region>"
            #'transient--do-stay)
@end lisp

@anchor{How can I autoload prefix and suffix commands?}
@appendixsec How can I autoload prefix and suffix commands?

If your package only supports Emacs 30, just prefix the definition
with @code{;;;###autoload}.  If your package supports released versions of
Emacs, you unfortunately have to use a long form autoload comment
as described in @ref{Autoload,,,elisp,}.

@lisp
;;;###autoload (autoload 'magit-dispatch "magit" nil t)
(transient-define-prefix magit-dispatch ()
  ...)
@end lisp

@anchor{How does Transient compare to prefix keys and universal arguments?}
@appendixsec How does Transient compare to prefix keys and universal arguments?

See @uref{https://github.com/magit/transient/wiki/Comparison-with-prefix-keys-and-universal-arguments}.

@anchor{How does Transient compare to Magit-Popup and Hydra?}
@appendixsec How does Transient compare to Magit-Popup and Hydra?

See @uref{https://github.com/magit/transient/wiki/Comparison-with-other-packages}.

@anchor{Why does @kbd{q} not quit popups anymore?}
@appendixsec Why does @kbd{q} not quit popups anymore?

I agree that @kbd{q} is a good binding for commands that quit something.
This includes quitting whatever transient is currently active, but it
also includes quitting whatever it is that some specific transient is
controlling.  The transient @code{magit-blame} for example binds @kbd{q} to the
command that turns @code{magit-blame-mode} off.

So I had to decide if @kbd{q} should quit the active transient (like
Magit-Popup used to) or whether @kbd{C-g} should do that instead, so that @kbd{q}
could be bound in individual transient to whatever commands make sense
for them.  Because all other letters are already reserved for use by
individual transients, I have decided to no longer make an exception
for @kbd{q}.

If you want to get @kbd{q}'s old binding back then you can do so.  Doing
that is a bit more complicated than changing a single key binding, so
I have implemented a function, @code{transient-bind-q-to-quit} that makes the
necessary changes.  See its documentation string for more information.

@node Keystroke Index
@appendix Keystroke Index

@printindex ky

@node Command and Function Index
@appendix Command and Function Index

@printindex fn

@node Variable Index
@appendix Variable Index

@printindex vr

@node Concept Index
@appendix Concept Index

@printindex cp

@node GNU General Public License
@appendix GNU General Public License

@include gpl.texi

@bye
