path: root/elpa/transient-0.8.6/transient.org

#+title: Transient User and Developer Manual
:PREAMBLE:
#+author: Jonas Bernoulli
#+email: emacs.transient@jonas.bernoulli.dev
#+date: 2018-{{{year}}}

#+texinfo_dir_category: Emacs misc features
#+texinfo_dir_title: Transient: (transient).
#+texinfo_dir_desc: Transient Commands
#+subtitle: for version 0.8.6

#+setupfile: .orgconfig

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 https://github.com/positron-solutions/transient-showcase.

#+texinfo: @noindent
This manual is for Transient version 0.8.6.

#+texinfo: @insertcopying
:END:
* 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 https://github.com/positron-solutions/transient-showcase.

** Some things that Transient can do
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

** Complexity in CLI programs
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

** Using Transient for composing interactive commands
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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: =delete-other-windows= vs.
=delete-other-windows-vertically= (among many similar examples).

Some Emacs commands will simply prompt you for the next "argument"
(=M-x switch-to-buffer=).  Another common solution is to use prefix
arguments which usually start with =C-u=.  Sometimes these are sensibly
numerical in nature (=C-u 4 M-x forward-paragraph= to move forward 4
paragraphs).  But sometimes they function instead as boolean
"switches" (=C-u C-SPACE= to jump to the last mark instead of just
setting it, =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 =C-u=,
/raised to the power of 10/.  Like =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 /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 =ls=, there is only one way to "execute" (hit
=Return=!), transients can "execute" using multiple different keys tied
to one of many self-documenting /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
=C-u what can I do here again?= way.

* Usage
** 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 displayed in a transient
menu, displayed in a popup buffer.  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 popup 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 *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.

** 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 popup 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)}}}.

#+attr_texinfo: :compact t
- Key: C-g (transient-quit-seq) ::
- Key: C-g (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.

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

- Key: C-q (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.

- Key: C-z (transient-suspend) ::

  Like ~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.

- Key: M-x transient-resume ::

  This command resumes the previously suspended stack of transients,
  if any.

** Common Suffix Commands
#+cindex: common suffix commands

A few shared suffix commands are available in all transients.  These
suffix commands are not shown in the popup buffer by default.

This includes the aborting commands mentioned in the previous section,
as well as some other commands that are all bound to {{{kbdvar(C-x <KEY>)}}}.  After
{{{kbd(C-x)}}} is pressed, a section featuring all these common commands is
temporarily shown in the popup buffer.  After invoking one of them,
the section disappears again.  Note, however, that one of these
commands is described as “Show common permanently”; invoke that if you
want the common commands to always be shown for all transients.

- Key: C-x t (transient-toggle-common) ::

  This command toggles whether the generic commands that are common to
  all transients are always displayed or only after typing the
  incomplete prefix key sequence {{{kbd(C-x)}}}.  This only affects the current
  Emacs session.

- User Option: transient-show-common-commands ::

  This option controls whether shared suffix commands are shown
  alongside the transient-specific infix and suffix commands.  By
  default, the shared commands are not shown to avoid overwhelming
  the user with too many options.

  While a transient is active, pressing {{{kbd(C-x)}}} always shows the common
  commands.  The value of this option can be changed for the current
  Emacs session by typing {{{kbd(C-x t)}}} while a transient is active.

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

** 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. [fn:1]

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 [[*Using History]]).

- Key: C-x s (transient-set) ::

  This command saves the value of the active transient for this Emacs
  session.

- Key: C-x C-s (transient-save) ::

  This command saves the value of the active transient persistently
  across Emacs sessions.

- Key: C-x C-k (transient-reset) ::

  This command clears the set and saved values of the active transient.

- User Option: transient-values-file ::

  This option names the file that is used to persist the values of
  transients between Emacs sessions.

[fn:1] ~magit-diff~ and ~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.

** 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.

#+attr_texinfo: :compact t
- Key: C-M-p (transient-history-prev) ::
- Key: C-x p ::

  This command switches to the previous value used for the active
  transient.

- Key: C-M-n (transient-history-next) ::
- Key: C-x n ::

  This command switches to the next value used for the active
  transient.

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
[[*Suffix Slots]]).

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

- User Option: transient-save-history ::

  This option controls whether the history of transient commands is
  saved when exiting Emacs.

- User Option: transient-history-file ::

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

- User Option: transient-history-limit ::

  This option controls how many history elements are kept at the time
  the history is saved in ~transient-history-file~.

** 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.

- Key: C-h (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 /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.

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.  ~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.

** 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...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 transient and then
pressing {{{kbd(C-x l)}}} to enter the “edit” mode, see below.

The default level for both transients and their suffixes is 4.  The
~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.

- User Option: 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.

- User Option: transient-levels-file ::

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

- Key: C-x l (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.  ~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.

- Key: C-x a (transient-toggle-level-limit) ::

  This command toggle whether suffixes that are on levels higher than
  the level specified by ~transient-default-level~ are temporarily
  available anyway.

- Function: 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 ~--gpg-sign~ argument (which is implemented using the
  command ~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 ~--gpg-sign~ available, use:

  #+begin_src emacs-lisp
    (transient-set-default-level 'magit:--gpg-sign 1)
  #+end_src

  To never make ~--gpg-sign~ available, use:

  #+begin_src emacs-lisp
    (transient-set-default-level 'magit:--gpg-sign 0)
  #+end_src

  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 ~transient-define-argument~, ~transient-define-infix~ and
  ~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
  ~transient-define-prefix,~ or later using ~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.)

** 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 ~scroll-up-command~ and
~scroll-down-command~ in other buffers.

- Command: transient-scroll-up arg ::

  This command scrolls text of transient popup window upward {{{var(ARG)}}}
  lines.  If {{{var(ARG)}}} is ~nil~, then it scrolls near full screen.  This
  is a wrapper around ~scroll-up-command~ (which see).

- Command: transient-scroll-down arg ::

  This command scrolls text of transient popup window down {{{var(ARG)}}}
  lines.  If {{{var(ARG)}}} is ~nil~, then it scrolls near full screen.  This
  is a wrapper around ~scroll-down-command~ (which see).

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

- Command: transient-copy-menu-text ::

  This command copies the contents of the menu buffer to the kill
  ring.

- 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.

** Configuration

More options are described in [[* Common Suffix Commands]], in [[* Saving
Values]], in [[* Using History]] and in [[* Enabling and Disabling Suffixes]].

*** Essential Options
:PROPERTIES:
:UNNUMBERED: notoc
:END:

Also see [[* Common Suffix Commands]].

- User Option: transient-show-popup ::

  This option controls whether the current transient's infix and
  suffix commands are shown in the popup buffer.

  - If ~t~ (the default) then the popup buffer is shown as soon as a
    transient prefix command is invoked.

  - If ~nil~, then the popup buffer is not shown unless the user
    explicitly requests it, by pressing an incomplete prefix key
    sequence.

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

    The popup is shown when the user explicitly requests it by
    pressing an incomplete prefix key sequence.  Unless this is zero,
    the popup is shown after that many seconds of inactivity (using
    the absolute value).

- User Option: transient-show-common-commands ::

  This option controls whether shared suffix commands are shown
  alongside the transient-specific infix and suffix commands.  By
  default, the shared commands are not shown to avoid overwhelming
  the user with too many options.

  While a transient is active, pressing {{{kbd(C-x)}}} always shows the common
  commands.  The value of this option can be changed for the current
  Emacs session by typing {{{kbd(C-x t)}}} while a transient is active.

- User Option: 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 ~nil~ (the default), hide the menu while the minibuffer is in use.
  When ~t~, keep showing the menu, but allow for the menu window to be
  resized, to ensure that completion candidates can be displayed.

  When ~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 ~t~ or an integer, as described
  below.

  If the value is ~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 ~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 ~balance-windows~ on ~post-command-hook~), then
  ~fixed~ (or ~nil~) is likely a better choice than ~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.

- User Option: 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 ~nil~, there is no initial input and the first
  element has to be accessed the same way as the older elements.

- User Option: transient-enable-popup-navigation ::

  This option controls whether navigation commands are enabled in the
  transient popup buffer.  If the value is ~verbose~ (the default),
  brief documentation about the command under point is additionally
  show in the echo area.

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

  - {{{kbd(UP)}}} moves the cursor to the previous suffix.
  - {{{kbd(DOWN)}}} moves the cursor to the next suffix.
  - {{{kbd(M-RET)}}} invokes the suffix the cursor is on.
  - {{{kbd(mouse-1)}}} invokes the clicked on suffix.
  - {{{kbd(C-s)}}} and {{{kbd(C-r)}}} start isearch in the popup buffer.

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

- User Option: transient-display-buffer-action ::

  This option specifies the action used to display the transient popup
  buffer.  The transient popup buffer is displayed in a window using
  {{{codevar((display-buffer BUFFER transient-display-buffer-action))}}}.

  The value of this option has the form {{{codevar((FUNCTION . 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 [[info:elisp#Choosing Window]],
  for details.

  The default is:

  #+BEGIN_SRC emacs-lisp
    (display-buffer-in-side-window
     (side . bottom)
     (dedicated . t)
     (inhibit-same-window . t))
  #+END_SRC

  This displays the window at the bottom of the selected frame.
  For alternatives see [[info:elisp#Buffer Display Action Functions]],
  and [[info:elisp#Buffer Display Action Alists]].

  When you switch to a different ACTION, you should keep the ALIST
  entries for ~dedicated~ and ~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 ~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 ~transient-mode-line-format~.

  This user option may be overridden if ~:display-action~ is passed
  when creating a new prefix with ~transient-define-prefix~.

*** Accessibility Options
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- User Option: 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.

*** Auxiliary Options
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- User Option: transient-mode-line-format ::

  This option controls whether the transient popup buffer has a
  mode-line, separator line, or neither.

  If ~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 ~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 ~line~ and numbers are synonyms for ~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 ~transient-key-noop~ (if non-suffixes are disallowed),
  ~transient-key-stay~ (if allowed and transient stays active), or
  ~transient-key-exit~ (if allowed and they exit the transient) is
  used to draw the line.

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

  Otherwise this can be any mode-line format.  See [[info:elisp#Mode
  Line Format]], for details.

- User Option: transient-semantic-coloring ::

  This option controls whether colors are used to indicate the
  transient behavior of commands.

  If non-~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 ~transient-mode-line-format~ is ~line~.

- User Option: 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-~nil~, the key binding for an infix argument
  is highlighted when only a long argument (e.g., ~--verbose~) is
  specified but no shorthand (e.g., ~-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
  ~transient-mismatched-key~ and ~transient-nonstandard-key~.

- User Option: transient-substitute-key-function ::

  This function is used to modify key bindings.  If the value of this
  option is ~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 ~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.

  #+BEGIN_SRC emacs-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_SRC

- User Option: transient-align-variable-pitch ::

  This option controls whether columns are aligned pixel-wise in the
  popup buffer.

  If this is non-~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 ~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 ~default~ face.

- User Option: transient-force-fixed-pitch ::

  This option controls whether to force the use of a monospaced font
  in popup buffer.  Even if you use a proportional font for the
  ~default~ face, you might still want to use a monospaced font in
  transient's popup buffer.  Setting this option to ~t~ causes ~default~
  to be remapped to ~fixed-pitch~ in that buffer.

*** Developer Options
:PROPERTIES:
:UNNUMBERED: notoc
:END:

These options are mainly intended for developers.

- User Option: 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.

- User Option: transient-highlight-higher-levels ::

  This option controls whether suffixes that would not be available by
  default are highlighted.

  When non-~nil~ then the descriptions of suffixes are highlighted if
  their level is above 4, the default of ~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.

*** Hook Variables
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- Variable: transient-exit-hook ::

  This hook is run after a transient is exited.

- Variable: 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.

* Modifying Existing Transients
#+cindex: modifying existing transients

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

#+begin_src emacs-lisp
  (transient-append-suffix 'magit-patch-apply "-3"
    '("-R" "Apply in reverse" "--reverse"))
#+end_src

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

The following functions share a few arguments:

- {{{var(PREFIX)}}} is a transient prefix command, a symbol.

- {{{var(SUFFIX)}}} is a transient infix or suffix specification in the same form
  as expected by ~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 [[*Suffix Specifications]].

  {{{var(SUFFIX)}}} may also be a group in the same form as expected by
  ~transient-define-prefix~.  See [[*Group Specifications]].

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

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

- Function: transient-insert-suffix prefix loc suffix &optional keep-other ::
- Function: 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 [[*Predicate Slots]].

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

- Function: 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)}}}.

- Function: transient-remove-suffix prefix loc ::

  This function removes the suffix or group at {{{var(LOC)}}} in {{{var(PREFIX)}}}.

- Function: 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.

- Function: 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)}}}.

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 ~transient-get-suffix~ and ~transient-suffix-put~ may
signal an error.

* Defining New Commands
** 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 ~pre-command-hook~ and ~post-command-hook~.  The available suffix
and infix commands and their state are shown in a popup 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 popup 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 ~transient-current-suffixes~ and the function ~transient-args~
serve about the same purpose as the variables ~prefix-arg~ and
~current-prefix-arg~ do for any command that was called after the prefix
arguments have been set using a command such as ~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 popup 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 ~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 [[*Using History]].

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

** 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 *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.

- Macro: transient-define-prefix name arglist [docstring] [keyword value]... group... [body...] ::

  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 ~:class~ or a keyword
  argument supported by the constructor of that class.  The
  ~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 popup buffer.  At least one
  {{{var(GROUP)}}} has to be specified.  See [[*Binding Suffix and Infix Commands]].

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

  #+BEGIN_SRC emacs-lisp
    (lambda ()
      (interactive)
      (transient-setup 'NAME))
  #+END_SRC

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

  #+cindex: scope of a transient
  All transients have a (possibly ~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 ~interactive~ form and has to be passed to the
  setup function:

  #+BEGIN_SRC emacs-lisp
    (transient-setup 'NAME nil nil :scope SCOPE)
  #+END_SRC

  For example, the scope of the ~magit-branch-configure~ transient is
  the branch whose variables are being configured.

** Binding Suffix and Infix Commands

The macro ~transient-define-prefix~ is used to define a transient.
This defines the actual transient prefix command (see [[*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 ~transient-insert-suffix~ (see [[*Modifying Existing
Transients]]).  These functions take a “suffix specification” as one of
their arguments, which has the same form as the specifications used in
~transient-define-prefix~.

*** 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 [[*Group Classes]].

Groups are specified in the call to ~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:

#+begin_src emacs-lisp
  [{LEVEL} {DESCRIPTION} {KEYWORD VALUE}... ELEMENT...]
#+end_src

The {{{var(LEVEL)}}} is optional and defaults to 4.  See [[*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 ~:class~ or a keyword argument supported by the
constructor of that class.

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

- Likewise ~:level~ is equivalent to {{{var(LEVEL)}}}.

- Other important keywords include the ~:if...~ and ~:inapt-if...~
  keywords.  These keywords control whether the group is available
  in a certain situation.

  For example, one group of the ~magit-rebase~ transient uses ~:if
  magit-rebase-in-progress-p~, which contains the suffixes that are
  useful while rebase is already in progress; and another that uses
  ~: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 [[*Predicate Slots]].

- The value of ~:hide~, if non-~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:

  #+BEGIN_SRC emacs-lisp
    (lambda ()
      (eq (car transient--redisplay-key)
          ?\C-c)) ; the prefix key shared by all bindings
  #+END_SRC

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

  If you explicitly specify children and then transform them using
  ~: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
  ~:setup-children~, then you must specify the class using ~:class~.
  For backward compatibility, if you fail to do so, ~transient-column~
  is used and a warning is displayed.  This warning will eventually
  be replaced with an error.

  #+BEGIN_SRC emacs-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_SRC

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

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

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 ~,~.  This feature is experimental and should be avoided.

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

*** Suffix Specifications
#+cindex: suffix specifications

A transient's suffix and infix commands are bound when the transient
prefix command is defined using ~transient-define-prefix~, see
[[*Defining Transients]].  The commands are organized into groups, see
[[*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 ~transient-insert-suffix~, see [[*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:

#+begin_src emacs-lisp
  ([LEVEL] [KEY [DESCRIPTION]] COMMAND|ARGUMENT [KEYWORD VALUE]...)
#+end_src

{{{var(LEVEL)}}}, {{{var(KEY)}}} and {{{var(DESCRIPTION)}}} can also be specified using the {{{var(KEYWORD)}}}s
~:level~, ~:key~ and ~: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.

- {{{var(LEVEL)}}} is the suffix level, an integer between 1 and 7.  See
  [[*Enabling and Disabling Suffixes]].

- {{{var(KEY)}}} is the key binding, either a vector or key description string.

- {{{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 ~:description~ in that case.

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

- {{{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 ~transient-define-suffix~ or
  ~transient-define-infix~ were used to define it).

  COMMAND can also be a ~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:

  #+BEGIN_SRC emacs-lisp
    (transient-insert-suffix 'some-prefix "k"
      '("!" "Ceci n'est pas une commande" no-command
        :if (lambda () (featurep 'no-library))))
  #+END_SRC

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

- 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 ~:shortarg~) and the second as the long argument
  (which can also be specified using ~:argument~).

  Only the long argument is displayed in the popup buffer.  See
  ~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 ===
  (e.g., =--format==) then ~transient-option~ is used, otherwise
  ~transient-switch~.

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

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

** 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”.

- Macro: transient-define-suffix name arglist [docstring] [keyword value]... body... ::

  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 ~:class~ or a keyword
  argument supported by the constructor of that class.  The
  ~transient-suffix~ class is used if the class is not specified
  explicitly.

  The {{{var(BODY)}}} must begin with an ~interactive~ form that matches {{{var(ARGLIST)}}}.
  The infix arguments are usually accessed by using ~transient-args~
  inside ~interactive~.

- Macro: transient-define-infix name arglist [docstring] [keyword value]... ::

  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 ~equal~ to each other (but not ~eq~).  It is meaningless
  to define an infix command, without providing at least one keyword
  argument (usually ~:argument~ or ~:variable~, depending on the class).
  The suffix class defaults to ~transient-switch~ and can be set using
  the ~:class~ keyword.

  The function definition is always:

  #+BEGIN_SRC emacs-lisp
    (lambda ()
      (interactive)
      (let ((obj (transient-suffix-object)))
        (transient-infix-set obj (transient-infix-read obj)))
      (transient--show))
  #+END_SRC

  ~transient-infix-read~ and ~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
  ~transient-define-suffix~ to define the infix command and use ~t~ as the
  value of the ~:transient~ keyword.

- Macro: transient-define-argument name arglist [docstring] [keyword value]... ::

  This macro defines {{{var(NAME)}}} as a transient infix command.

  This is an alias for ~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
  ~transient-define-infix~ instead.

** 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 ~nil~ (which causes the function to return
~nil~ too).

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

- Function: 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)}}}.

- Function: 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 ~transient-set-value~ and ~transient-save-value~ methods.  Unlike
  ~transient-args~, this does not include the values of suffixes whose
  ~unsavable~ slot is non-~nil~.

- Function: 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 ~nil~ if the option does not appear in {{{var(ARGS)}}}.

- Function: 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)}}}.

** Using Prefix Scope

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

- Function: 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 ~: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
  ~interactive~ form of a suffix command, PREFIXES and/or CLASSES should
  be non-nil.  If either is non-nil, try the following in order:

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

  - 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.

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

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

  - 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.

  If no prefix matches, return nil.

** Current Suffix Command

- Function: 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 ~transient--default-infix-command~, which
  is defined like this:

  #+begin_src emacs-lisp
    (defun transient--default-infix-command ()
      (interactive)
      (let ((obj (transient-suffix-object)))
        (transient-infix-set obj (transient-infix-read obj)))
      (transient--show))
  #+end_src

  (User input is read outside of ~interactive~ to prevent the
  command from being added to ~command-history~.)

  Such commands need to be able to access their associated object
  to guide how ~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 ~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:

  #+begin_src emacs-lisp
    (get COMMAND 'transient--suffix)
  #+end_src

- Variable: 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 ~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.

** Current Prefix Command

- Function: 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 ~transient--prefix~ can be
  used to access the prefix object.  Thus this is what has to be
  used in suffix methods such as ~transient-format-description~,
  and in object-specific functions that are stored in suffix slots
  such as ~description~.

  When a suffix command is invoked (i.e., in its ~interactive~ form
  and function body) then the variable ~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.

- Variable: transient-current-prefix ::

  The transient from which this suffix command was invoked.  The value
  is a ~transient-prefix~ object, which holds information associated
  with the transient prefix command.

- Variable: transient-current-command ::

  The transient from which this suffix command was invoked.  The value
  is a symbol, the transient prefix command.

- Function: transient-active-prefix &optional prefixes ::

  This function returns the active transient object.  It returns ~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 ~transient-current-prefix~, which is only ever non-~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-~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.

** 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:

- Invoking an infix command does not affect the transient state; the
  transient remains active.

- Invoking a (non-infix) suffix command “deactivates” the transient
  state by removing the transient keymap and performing some
  additional cleanup.

- 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.

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 ~transient--do-VERB~, and the symbol
~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?"  ~t~ means that
the transient stays active, while ~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 ~t~ means that when you exit the
inner prefix, then the outer prefix becomes active again, while ~nil~
means that all outer prefixes are exited at once.

- The behavior for non-suffixes can be set for a particular prefix,
  by the prefix's ~transient-non-suffix~ slot to a boolean, a suitable
  pre-command function, or a shorthand for such a function.  See
  [[*Pre-commands for Non-Suffixes]].

- The common behavior for the suffixes of a particular prefix can be
  set using the prefix's ~transient-suffixes~ slot.

  The value specified in this slot does *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.

- The behavior of an individual suffix can be changed using its
  ~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
  ~transient-define-suffix~ and/or in the definition of the prefix.  If
  set in both places, then the latter takes precedence, as usual.

The available pre-command functions are documented in the following
sub-sections.  They are called by ~transient--pre-command~, a function
on ~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
~transient--exit~ or ~transient--stay~ is used (that way we don't have to
remember if ~t~ means “exit” or “stay”).

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

For completeness sake, some notes about complications:

- The transient-ness of certain built-in suffix commands is specified
  using ~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 ~transient-suffix~ slot, but not
  the suffix's ~transient~ slot.

- 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 ~replace~ can be used as the value of the sub-prefix's ~transient~
  slot.

*** Pre-commands for Infixes
:PROPERTIES:
:UNNUMBERED: notoc
:END:

The default for infixes is ~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 ~transient-suffix~ slot is ~t~.  In
extremely rare cases, one might want to use something else, which can
be done by setting the infix's ~transient~ slot directly.

- Function: transient--do-stay ::

  Call the command without exporting variables and stay transient.

*** Pre-commands for Suffixes
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

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

- Function: transient--do-exit ::

  Call the command after exporting variables and exit the transient.

- Function: transient--do-return ::

  Call the command after exporting variables and return to the parent
  prefix.  If there is no parent prefix, then call ~transient--do-exit~.

- Function: transient--do-call ::

  Call the command after exporting variables and stay transient.

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 ~transient~
slot for the sub-prefix itself.

- Function: 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 ~transient--do-stack~ is that it changes the
  value of the ~transient-suffix~ slot to ~t~.

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

- Function: transient--do-stack ::

  Call the transient prefix command, stacking the active transient.
  Push the active transient to the transient stack.

  Unless ~transient--do-recurse~ is explicitly used, this pre-command
  is automatically used for suffixes that are prefixes themselves,
  i.e., for sub-prefixes.

- Function: transient--do-replace ::

  Call the transient prefix command, replacing the active transient.
  Do not push the active transient to the transient stack.

  Unless ~transient--do-recurse~ is explicitly used, this pre-command
  is automatically used for suffixes that are prefixes themselves,
  i.e., for sub-prefixes.

- Function: transient--do-suspend ::

  Suspend the active transient, saving the transient stack.

  This is used by the command ~transient-suspend~ and optionally also by
  “external events” such as ~handle-switch-frame~.  Such bindings should
  be added to ~transient-predicate-map~.

*** Pre-commands for Non-Suffixes
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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 ~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 ~t~, then non-suffixes can be invoked, when it is ~nil~
(the default) then they cannot be invoked.

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

- Function: transient--do-warn ::

  Call ~transient-undefined~ and stay transient.

- Function: transient--do-stay ::

  Call the command without exporting variables and stay transient.

- Function: transient--do-leave ::

  Call the command without exporting variables and exit the transient.

# transient--do-noop intentionally left undocumented.

*** Special Pre-Commands
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- Function: 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)}}}.

- Function: transient--do-quit-all ::

  Exit all transients without saving the transient stack.

  This is used when the user pressed {{{kbd(C-q)}}}.

- Function: transient--do-suspend ::

  Suspend the active transient, saving the transient stack.

  This is used when the user pressed {{{kbd(C-z)}}}.

* 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.

- 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.

- 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 ~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.

** Group Classes

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

- The abstract ~transient-child~ class is the base class of both
  ~transient-group~ (and therefore all groups) as well as of
  ~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.

- The abstract ~transient-group~ class is the superclass of all other
  group classes.

- The ~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.

- The ~transient-row~ class displays all elements on a single line.

- The ~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.

- The ~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.

** Group Methods

- Function: 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 ~setup-children~ slot of {{{var(GROUP)}}} is non-~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.

- Function: 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 popup 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 ~transient--shadowed-buffer~ the current buffer.

** Prefix Classes

Transient itself provides a single class for prefix commands,
~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 [[* Prefix Methods]]).

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

** Suffix Classes

- All suffix and infix classes derive from ~transient-suffix~, which in
  turn derives from ~transient-child~, from which ~transient-group~ also
  derives (see [[*Group Classes]]).

- All infix classes derive from the abstract ~transient-infix~ class,
  which in turn derives from the ~transient-suffix~ class.

  Infixes are a special type of suffixes.  The primary difference is
  that infixes always use the ~transient--do-stay~ pre-command, while
  non-infix suffixes use a variety of pre-commands (see [[*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 [[*Defining Suffix and Infix Commands]]).

- Classes used for infix commands that represent arguments should
  be derived from the abstract ~transient-argument~ class.

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

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

- The ~transient-switches~ class can be used for a set of mutually
  exclusive command-line switches.

- The ~transient-files~ class can be used for a =--= argument that
  indicates that all remaining arguments are files.

- Classes used for infix commands that represent variables should
  derived from the abstract ~transient-variable~ class.

- The ~transient-information~ and ~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:

  #+begin_src emacs-lisp
    ([LEVEL] :info DESCRIPTION [KEYWORD VALUE]...)
    ([LEVEL] :info* DESCRIPTION [KEYWORD VALUE]...)
  #+end_src

  The ~:info~ and ~:info*~ keyword arguments replaces the ~:description~
  keyword used for other suffix classes.  Other keyword arguments that
  you might want to set, include ~:face~, predicate keywords (such as
  ~:if~ and ~:inapt-if~), and ~:format~.  By default the value of ~:format~
  includes ~%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 ~:info*~ aligns
  its description with the descriptions of suffix commands, while for
  ~:info~ the description bleeds into the area where suffixes display
  their key bindings.

- The ~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.

- The ~transient-describe-target~ class is used by the command
  ~transient-describe~.

- The ~transient-value-preset~ class is used to implement the command
  ~transient-preset~, which activates a value preset.

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.

** Prefix Methods

To get information about the methods implementing these generic
functions use ~describe-function~.

- Function: 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 ~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
  ~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.

- Function: 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
  ~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,
  ~nil~ is returned.

- Function: 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 ~transient-infix-value~.

- Function: 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.

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

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

** Suffix Methods

To get information about the methods implementing these generic
functions use ~describe-function~.

*** Suffix Value Methods

- Function: 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 ~transient-argument~ objects this function handles setting the
  value by itself.

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

  If you implement a class that derives from ~transient-infix~ directly,
  then you must implement a dedicated method for this function and/or
  ~transient-default-value~.

- Function: transient-default-value obj ::

  This generic function returns the default value of the object {{{var(OBJ)}}}.
  Methods exist for both prefix and suffix objects.

- Function: transient-infix-read obj ::

  This generic function determines the new value of the infix object
  {{{var(OBJ)}}}.

  This function merely determines the value; ~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 ~reader~ slot (using the
  ~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.

- Function: transient-prompt obj ::

  This generic function returns the prompt to be used to read infix
  object {{{var(OBJ)}}}'s value.

- Function: transient-infix-set obj value ::

  This generic function sets the value of infix object {{{var(OBJ)}}} to {{{var(VALUE)}}}.

- Function: transient-infix-value obj ::

  This generic function returns the value of the suffix object {{{var(OBJ)}}}.

  This function is called by ~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.
  ~nil~ is not a value, it means “no value”.

  Usually only infixes have a value, but see the method for
  ~transient-suffix~.

- Function: 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 ~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.

*** Suffix Format Methods

# Low-level transient--init-suffix-key intentionally left out.

- Function: 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 ~transient--source-buffer~ current.

- Function: transient-format-key obj ::

  This generic function formats {{{var(OBJ)}}}'s ~key~ for display and returns the
  result.

- Function: transient-format-description obj ::

  This generic function formats {{{var(OBJ)}}}'s ~description~ for display and
  returns the result.

- Function: transient-format-value obj ::

  This generic function formats {{{var(OBJ)}}}'s value for display and returns
  the result.

- Function: transient-show-help obj ::

  Show help for the prefix, infix or suffix command represented by
  {{{var(OBJ)}}}.

  Regardless of OBJ's type, if its ~show-help~ slot is non-~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 ~transient-show-help-window~ macro is intended for
  use in such functions.

  For prefixes, show the info manual, if that is specified using the
  ~info-manual~ slot.  Otherwise, show the manpage if that is specified
  using the ~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.

- Macro: transient-with-help-window &rest body ::

  Evaluate BODY, send output to ~*Help*~ buffer, and display it in a
  window.  Select the help window, and make the help buffer current
  and return it.

- Function: transient-show-summary obj &optional return ::

  This generic function shows or, if optional RETURN is non-~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 ~transient-enable-popup-navigation~ is ~verbose~) when
  the user navigates to a command using the keyboard.

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

  If RETURN is non-~nil~, this function returns the summary instead of
  showing it.  This is used when a tooltip is needed.

** Prefix Slots
*** Value and Scope
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~default-value~ The default value of the prefix.  Use the keyword
  argument ~:value~ (sic) to set this slot in the definition of a
  prefix.

- ~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 ~transient-init-value~ method is called instead.

- ~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.

- ~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 ~argument~ or ~argument-format~ slot of the respective
  suffix objects, usually something like ~--switch~ or ~--option=%s~.  For
  options and ~transient-switches~ suffixes it is also possible to match
  against a specific value, as returned by ~transient-infix-value~,
  for example, ~--option=one~.

- ~scope~ For some transients it might be necessary to have a sort of
  secondary value, called a “scope”.  See ~transient-define-prefix~.

*** Behavior
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~transient-suffix~, ~transient-non-suffix~ and ~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 [[*Transient State]].

- ~refresh-suffixes~ Normally suffix objects and keymaps are only setup
  once, when the prefix is invoked.  Setting this to ~t~, causes them to
  be recreated after every command.  This is useful when using ~: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.

- ~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 ~funcall~ that function with no arguments.
  During initialization the second argument is the prefix object
  being initialized.  This slot is still experimental.

*** Appearance
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~display-action~ determines how this prefix is displayed, overriding
  ~transient-display-buffer-action~.  It should have the same type.

- ~mode-line-format~ is this prefix's mode line format, overriding
  ~transient-mode-line-format~.  It should have the same type.

- ~column-widths~ is only respected inside ~transient-columns~ groups and
  allows aligning columns across separate instances of that.  A list
  of integers.

- ~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.

*** Documentation
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

- ~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 ~transient-command-summary-or-name~ is a useful
  value.

*** Internal
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

- ~prototype~ When a transient prefix command is invoked, then a clone
  of that object is stored in the global variable ~transient--prefix~
  and the prototype is stored in the clone's ~prototype~ slot.

- ~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 ~transient--prefix~ property
  of the same symbol.

- ~level~ The level of the prefix commands.  The suffix commands whose
  layer is equal or lower are displayed.  See [[*Enabling and Disabling
  Suffixes]].

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

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

- ~unwind-suffix~ is used internally to ensure transient state is
  properly exited, even in case of an error.

** 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 [[*Predicate Slots]].

Also see [[*Suffix Classes]].

*** Slots of ~transient-child~
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

- ~parent~ The object for the parent group.

*** Slots of ~transient-suffix~
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~key~ The key, a key vector or a key description string.

- ~command~ The command, a symbol.

- ~transient~ Whether to stay transient.  See [[*Transient State]].

- ~format~ The format used to display the suffix in the popup buffer.
  It must contain the following %-placeholders:

  - ~%k~ For the key.
  - ~%d~ For the description.
  - ~%v~ For the infix value.  Non-infix suffixes don't have a value.

- ~description~ The description, either a string or a function, which is
  called with zero or one argument (the suffix object), and returns a
  string.

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

- ~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 ~transient-show-help~.

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

- ~definition~ A command, which is used if the body is omitted when
  defining a command using ~transient-define-suffix~.

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.

- ~advice~ A function used to advise the command.  The advise is called
  using ~(apply advice command args)~, i.e., it behaves like an "around"
  advice.

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

*** Slots of ~transient-infix~
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

- ~argument~ The long argument, e.g., ~--verbose~.

- ~shortarg~ The short argument, e.g., ~-v~.

- ~value~ The value.  Should not be accessed directly.

- ~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 ~transient-init-value~ method is called instead.

- ~unsavable~ Whether the value of the suffix is not saved as part of
  the prefixes.

- ~multi-value~ For options, whether the option can have multiple
  values.  If this is non-~nil~, then the values are read using
  ~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-~nil~ values are:

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

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

  - Use ~repeat~ for an option that can be specified multiple times.

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

  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
  ~transient-args~, e.g., ~("--other" "--o=1" "--o=2" ("--" "f1" "f2"))~.

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

- ~allow-empty~ For options, whether the empty string is a valid value.

- ~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.

- ~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.

- ~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.

- ~choices~ A list of valid values, or a function that returns such a
  list.  The latter is not implemented for ~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.

*** Slots of ~transient-variable~
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~variable~ The variable.

*** Slots of ~transient-switches~
:PROPERTIES:
:UNNUMBERED: notoc
:END:

- ~argument-format~ The display format.  Must contain ~%s~, one of the
  ~choices~ is substituted for that.  E.g., ~--%s-order~.

- ~argument-regexp~ The regexp used to match any one of the switches.
  E.g., ~\\(--\\(topo\\|author-date\\|date\\)-order\\)~.

** 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.

- ~if~ Enable if predicate returns non-~nil~.
- ~if-not~ Enable if predicate returns ~nil~.
- ~if-non-nil~ Enable if variable's value is non-~nil~.
- ~if-nil~ Enable if variable's value is ~nil~.
- ~if-mode~ Enable if major-mode matches value.
- ~if-not-mode~ Enable if major-mode does not match value.
- ~if-derived~ Enable if major-mode derives from value.
- ~if-not-derived~ Enable if major-mode does not derive from value.

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.

- ~inapt-if~ Inapt if predicate returns non-~nil~.
- ~inapt-if-not~ Inapt if predicate returns ~nil~.
- ~inapt-if-non-nil~ Inapt if variable's value is non-~nil~.
- ~inapt-if-nil~ Inapt if variable's value is ~nil~.
- ~inapt-if-mode~ Inapt if major-mode matches value.
- ~inapt-if-not-mode~ Inapt if major-mode does not match value.
- ~inapt-if-derived~ Inapt if major-mode derives from value.
- ~inapt-if-not-derived~ Inapt if major-mode does not derive from value.

By default these predicates run when the prefix command is invoked,
but this can be changes, using the ~refresh-suffixes~ prefix slot.
See [[*Prefix Slots]].

One more slot is shared between group and suffix classes, ~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.  ~level~ controls whether a suffix or a group should be
available depending on user preference.
See [[*Enabling and Disabling Suffixes]].

* FAQ
:PROPERTIES:
:APPENDIX:   t
:END:

** Can I control how the popup buffer is displayed?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

Yes, see ~transient-display-buffer-action~ in [[*Configuration]].  You can
also control how the popup buffer is displayed on a case-by-case basis
by passing ~:display-action~ to ~transient-define-prefix~.

** How can I copy text from the popup buffer?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

To be able to mark text in Transient's popup 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 popup, you will be able to yank it in another buffer.

#+begin_src emacs-lisp
  (keymap-set transient-predicate-map
              "<mouse-set-region>"
              #'transient--do-stay)
#+end_src

** How can I autoload prefix and suffix commands?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

If your package only supports Emacs 30, just prefix the definition
with ~;;;###autoload~.  If your package supports released versions of
Emacs, you unfortunately have to use a long form autoload comment
as described in [[info:elisp#Autoload]].

#+begin_src emacs-lisp
  ;;;###autoload (autoload 'magit-dispatch "magit" nil t)
  (transient-define-prefix magit-dispatch ()
    ...)
#+end_src

** How does Transient compare to prefix keys and universal arguments?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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

** How does Transient compare to Magit-Popup and Hydra?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

See https://github.com/magit/transient/wiki/Comparison-with-other-packages.

** Why did some of the key bindings change?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

You may have noticed that the bindings for some of the common commands
do *not* have the prefix {{{kbd(C-x)}}} and that furthermore some of these commands
are grayed out while others are not.  That unfortunately is a bit
confusing if the section of common commands is not shown permanently,
making the following explanation necessary.

The purpose of usually hiding that section but showing it after the
user pressed the respective prefix key is to conserve space and not
overwhelm users with too much noise, while allowing the user to
quickly list common bindings on demand.

That however should not keep us from using the best possible key
bindings.  The bindings that do use a prefix do so to avoid wasting
too many non-prefix bindings, keeping them available for use in
individual transients.  The bindings that do not use a prefix and that
are *not* grayed out are very important bindings that are *always*
available, even when invoking the “common command key prefix” or *any
other* transient-specific prefix.  The non-prefix keys that *are* grayed
out however, are not available when any incomplete prefix key sequence
is active.  They do not use the “common command key prefix” because it
is likely that users want to invoke them several times in a row and
e.g., {{{kbd(M-p M-p M-p)}}} is much more convenient than {{{kbd(C-x M-p C-x M-p C-x M-p)}}}.

You may also have noticed that the “Set” command is bound to {{{kbd(C-x s)}}},
while Magit-Popup used to bind {{{kbd(C-c C-c)}}} instead.  I have seen several
users praise the latter binding (sic), so I did not change it
willy-nilly.  The reason that I changed it is that using different
prefix keys for different common commands, would have made the
temporary display of the common commands even more confusing, i.e.,
after pressing {{{kbd(C-c)}}} all the bindings that begin with the {{{kbd(C-x)}}} prefix
would be grayed out.

Using a single prefix for common commands key means that all other
potential prefix keys can be used for transient-specific commands
*without* the section of common commands also popping up.  {{{kbd(C-c)}}} in
particular is a prefix that I want to (and already do) use for Magit, and
also using that for a common command would prevent me from doing so.

(See also the next question.)

** Why does {{{kbd(q)}}} not quit popups anymore?
:PROPERTIES:
:UNNUMBERED: notoc
:END:

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 ~magit-blame~ for example binds {{{kbd(q)}}} to the
command that turns ~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, ~transient-bind-q-to-quit~ that makes the
necessary changes.  See its documentation string for more information.

* Keystroke Index
:PROPERTIES:
:APPENDIX:   t
:INDEX:      ky
:COOKIE_DATA: recursive
:END:
* Command and Function Index
:PROPERTIES:
:APPENDIX:   t
:INDEX:      fn
:END:
* Variable Index
:PROPERTIES:
:APPENDIX:   t
:INDEX:      vr
:END:
* Concept Index
:PROPERTIES:
:APPENDIX:   t
:INDEX:      cp
:END:

* GNU General Public License
:PROPERTIES:
:APPENDIX:   t
:END:
#+texinfo: @include gpl.texi

* Copying
:PROPERTIES:
:COPYING:    t
:END:

#+begin_quote
Copyright (C) 2018--{{{year}}} 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
General Public License for more details.
#+end_quote

#  LocalWords:  ARGLIST ARGS CLI DOCSTRING ELEMENTs EVAL GROUPs Infixes
#  LocalWords:  Infixes KEYWORDs LOC LocalWords MERCHANTABILITY Magit
#  LocalWords:  Magit's Makefile OBJ OBJ's Pre alist arglist
#  LocalWords:  args autoloaded boolean booleans customizable docstring
#  LocalWords:  eval featurep infixes init isearch keymap keymaps loc
#  LocalWords:  magit manpage minibuffer monospaced ness nilly noop noops
#  LocalWords:  plist pre prev rebase redisplay src subclass subclasses
#  LocalWords:  subcommands subprocess superclass tooltip termcap utils

# IMPORTANT: Also update ORG_ARGS and ORG_EVAL in the Makefile.
# Local Variables:
# indent-tabs-mode: nil
# org-hide-macro-markers: t
# org-src-preserve-indentation: nil
# End: