= Expansions

While parsing a command (see <<command-parsing#,`:doc command-parsing`>>),
Kakoune recognises certain patterns and will replace them with their
associated value before executing the command. These patterns are called
expansions.

Every expansion consists of a `%`, followed by the expansion _type_ (one
or more alphabetic characters), a nestable punctuation character (`(`, `[`,
`{`, or `<`), and then all the text up to and including its matching opposite
(`)`, `]`, `}`, or `>`). It doesn't matter which character is used, but
`{}` are most common. For example, the command `echo %val{session}` will
echo the ID of the current Kakoune session.

Expansions are processed when unquoted and anywhere inside double-quoted or
%-strings, but not inside unquoted words, inside single-quoted strings, or
inside other expansions. So:

* `echo %val{session}` echoes the current session ID

* `echo x%val{session}x` echoes the literal text `x%val{session}x`

* `echo '%val{session}'` echoes the literal text `%val{session}`

* `echo "x%val{session}x"` echoes the current session ID, surrounded by `x`

* `echo %{x%val{session}x}"` echoes the current session ID, surrounded by `x`

* `echo %sh{ echo %val{session} }"` echoes the literal text `%val{session}`

When processing an expansion, Kakoune ensures the result is properly quoted so
that any special characters do not interfere with the rest of the command. If
an expansion expands to multiple values, each value is individually quoted.

== Argument expansions

Expansions with the type `arg` can only be used inside the "commands" parameter
of the `define-command` command (See <<commands#declaring-new-commands,`:doc
commands declaring-new-commands`>>).

The following expansions are available:

*%arg{n}*::
     (where _n_ is a decimal number) +
     expands to argument number _n_ of the current command

*%arg{@}*::
    expands to all the arguments of the current command, individually quoted

== Option expansions

Expansions with the type `opt` expand to the value associated with the named
option in the current scope (See <<options#,`:doc options`>>).

For example, `%opt{BOM}` expands to `utf8` or to `none`, according to the
current state of the `BOM` option.

== Register expansions

Expansions with the type `reg` expand to the contents of the named
register. For registers named after symbols (like the search register
`/`), the expansion can use either the symbol or the alphabetic name (See
<<registers#,`:doc registers`>>).

For example, `%reg{/}` expands to the content of the `/` register, and so does
`%reg{slash}`.

== Shell expansions

Expansions with the type `sh` are executed as shell-scripts, and whatever
the script prints to standard output replaces the expansion. For example,
the command `echo %sh{date}` will echo the output of the `date` command.

TIP: If a shell expansion writes to standard error, that output is appended to
Kakoune's `\*debug*` buffer. If you're trying to debug a shell expansion,
check the debug buffer with `:buffer \*debug*` to see if anything shows up.

Because Kakoune does not expand expansions inside the text of an expansion,
you can't use normal expansions inside `%sh{}`. Instead, Kakoune can export
expansions as environment variables to make them available to the shell.
Here's how expansion patterns map to variable names:

*%arg{n}*::
    (where _n_ is a decimal number) +
    becomes `$_n_`. For example, `%arg{3}` becomes `$3`.

*%arg{@}*::
    becomes `$@`

*%opt{x}*::
    becomes `$kak_opt_x`

*%reg{x}*::
    (where _x_ is the alphabetic name of a register) +
    `$kak_reg_x` contains all the selections in register _x_ +
    `$kak_main_reg_x` contains only the main selection

*%val{x}*::
    becomes `$kak_x`

When turned into environment variables, list-type options, `$kak_reg_x`, and
"quoted list" values will be shell-quoted so the shell doesn't get confused
about how many items the list contains. You will need to apply `eval` to get
back the original values. For example, if you want to process the contents
of each selection, you can do something like:

----
eval set -- $kak_selections
while [ $# -gt 0 ]; do
    # ...do a thing with $1...
    shift
done
----

The `eval` command will take the expanded `$kak_selections` and unquote them,
then execute the resulting `set` command, which sets the shell's argument
variables to the items from `$kak_selections`. The `while` loop with `shift`
iterates through the arguments one by one.

Only variables actually mentioned in the body of the shell expansion will
be exported into the shell's environment. For example:

----
echo %sh{ env | grep ^kak_ }
----

...will find none of Kakoune's special environment variables, but:

----
echo %sh{ env | grep ^kak_ # $kak_session }
----

...will find the `$kak_session` variable because it was mentioned inside the
shell script, even just in a comment.

TIP: These environment variables are also available in other contexts where
Kakoune uses a shell command, such as the `|`, `!` or `$` normal mode commands
(See <<keys#,`:doc keys`>>).

== Value expansions

Expansions with the type `val` can be used in various places, depending on the
specific value.

The following expansions are supported:

*%val{buffile}*::
    _in buffer scope_ +
    full path of the file or same as `%val{bufname}` when there’s no
    associated file

*%val{buf_line_count}*::
    _in buffer scope_ +
    number of lines in the current buffer

*%val{buflist}*::
    quoted list of the names of currently-open buffers (as seen in
    `%val{bufname}`)

*%val{bufname}*::
    _in buffer scope_ +
    name of the current buffer

*%val{client_env_X}*::
    _in window scope_ +
    value of the `$X` environment variable in the client displaying the current
    window (e.g. `%val{client_env_SHELL}` is `$SHELL` in the client's
    environment)

*%val{client_list}*::
    unquoted list of the names of clients (as seen in `%val{client}`)
    connected to the current session

*%val{client}*::
    _in window scope_ +
    name of the client displaying the current window

*%val{client_pid}*::
    _in window scope_ +
    process id of the client displaying the current window

*%val{config}*::
    directory containing the user configuration

*%val{count}*::
    _in `map` command <keys> parameter_ +
    current count when the mapping was triggered, defaults to 0 if no
    count given

*%val{cursor_byte_offset}*::
    _in window scope_ +
    offset of the main cursor from the beginning of the buffer (in bytes)

*%val{cursor_char_column}*::
    _in window scope_ +
    column of the main cursor (in characters), the fourth component of
    `%val{selection_desc}`

*%val{cursor_char_value}*::
    _in window scope_ +
    unicode value of the codepoint under the main cursor

*%val{cursor_column}*::
    _in window scope_ +
    column of the main cursor (in bytes)

*%val{cursor_line}*::
    _in window scope_ +
    line of the main cursor, the third component of `%val{selection_desc}`

*%val{history_id}*::
    _in buffer scope_ +
    history id of the current buffer, an integer value which refers to a
    specific buffer version in the undo tree (see also `%val{timestamp}`)

*%val{hook_param_capture_n}*::
    _in `hook` command <command> parameter_ +
    text captured by capture group _n_, if the executing hook's filter regex
    used capture groups

*%val{hook_param}*::
    _in `hook` command <command> parameter_ +
    the complete parameter string of the executing hook

*%val{modified}*::
    _in buffer scope_ +
    `true` if the buffer has modifications not saved, otherwise `false`

*%val{register}*::
    _in `map` command <keys> parameter_ +
    current register when the mapping was triggered

*%val{runtime}*::
    directory containing the kak support files, determined from Kakoune's
    binary location

*%val{selection}*::
    _in window scope_ +
    content of the main selection

*%val{selections}*::
    _in window scope_ +
    quoted list of the contents of all selections

*%val{selection_desc}*::
    _in window scope_ +
    range of the main selection, represented as `a.b,c.d` where _a_
    is the anchor line, _b_ is the anchor column, _c_ is the cursor
    line (like `%val{cursor_line}`), _d_ is the cursor column (like
    `%val{cursor_char_column}`), and all are 1-based decimal integers

*%val{selections_desc}*::
    _in window scope_ +
    unquoted list of the ranges of all selections, in the same format as
    `%val{selection_desc}`

*%val{session}*::
    name of the current session

*%val{source}*::
    _in `.kak` file_ +
    path of the file currently getting executed (through the source command)

*%val{text}*::
    _in `prompt` command <command> parameter_ +
    the text entered by the user in response to the `prompt` command

*%val{timestamp}*::
    _in buffer scope_ +
    timestamp of the current buffer, an integer that increments each time the
    buffer is modified, including undoing and redoing previous modifications
    (see also `%val{history_id}`)

*%val{user_modes}*::
    unquoted list of user modes.

*%val{version}*::
    version of the current Kakoune server (git hash or release name)

*%val{window_height}*::
    _in window scope_ +
    height of the current Kakoune window

*%val{window_width}*::
    _in window scope_ +
    width of the current Kakoune window

Values in the above list that do not mention a context are available
everywhere. Values that mention "buffer scope" are also available in window
scope.

A value described as a "quoted list" will follow the rules of Kakoune string
quoting (See <<command-parsing#,`:doc command-parsing`>>). An "unquoted list"
cannot contain any special characters that would require quoting.
