= 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 quoting character, and then all the text
up to and including its matching character.

If a nestable punctuation character (`(`, `[`, `{`, or `<`) is used as the
opening quoting character, the expansion will end at its matching opposite
(`)`, `]`, `}`, or `>`). Nested pairs of the braces used in the expansion are
allowed, but they must be balanced. Braces other than the ones used in the
expansion need not be balanced, however. For example, `%{nest{ed} non[nested}`
is valid and expands to `nest{ed} non[nested`.

If any other character is used, the expansion will end at the next occurrence of
that character. The quoting character can be escaped inside the expansion if it
is doubled-up. For example, `%|abc||def|` expands to the text `abc|def`.

It doesn't matter which character is used, but `{}` are most common.

There are 2 types of quoting which can be used to group together words separated
by whitespace into a single argument or prevent expansions from expanding:

"double quoted strings"::
    Double quoted strings are mainly for grouping multiple `%` expansions or
    `%` expansions and regular text as a single argument. `%` and `"` can be
    escaped by doubling the characters (i.e. `%%` and `""`).

'single quoted strings'::
    Expansions are not processed inside single quoted strings. Single quotes can
    be escaped by doubling up (i.e. `''`).

Expansions are processed when unquoted and anywhere inside double-quoted
strings, but not inside unquoted words, inside single-quoted strings, or
inside %-strings or other expansions (see
<<command-parsing#typed-expansions, `:doc command-parsing typed-expansions`>>
for full details). For example:

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

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

Like "variable expansion" and "command substitution" in shell programming,
Kakoune expansions can expand to multiple "words" - that is, separate
arguments on the resulting command-line. However, unlike shell programming,
Kakoune expansions cannot _accidentally_ expand to multiple words because they
contain whitespace or other special characters. Only expansions which
semantically contain a list of values (list-type options, registers, selections,
etc.) expand to multiple arguments. While in shell-programming it's good
practice to always wrap expansions in double-quotes, in Kakoune it's perfectly
safe to leave expansions unquoted.

== 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, as individual words

== 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 by name
in a comment, even though it wasn't directly used.

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`>>).

== File expansions

Expansions with the type `file` will expand to the content of the filename
given in argument as read from the host filesystem.

== Value expansions

Expansions with the type `val` give access to Kakoune internal data that is
not stored in an option or a register. Some value expansions can only be used
in certain contexts, like `%val{hook_param}` that expands to the parameter
string of the currently-executing hook, and is not available outside a hook.

The following expansions are supported (with required context _in italics_):

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

*%val{buf_line_count}*::
    _in buffer, window 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, window 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 and `<a-;>` from object menu_ +
    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_ +
    1-based offset from the start of the line to the cursor position in
    Unicode codepoints, which may differ from visible columns if the document
    contains full-width codepoints (which occupy two columns) or zero-width
    codepoints

*%val{cursor_display_column}*::
    _in window scope_ +
    1-based offset from the start of the line to the cursor position in
    display column, taking into account tabs and character width.

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

*%val{cursor_column}*::
    _in window scope_ +
    1-based offset from the start of the line to the first byte of the
    character under the main cursor (in bytes), the fourth component of
    `%val{selection_desc}`

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

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

*%val{object_flags}*::
    _for commands executed from the object menu's `<a-;>` only_ +
    a pipe-separted list of words including `inner` if the user wants
    an inner selection, `to_begin` if the user wants to select to the
    beginning, and `to_end` if the user wants to select to the end

*%val{register}*::
    _in `map` command <keys> parameter and `<a-;>` from the object menu_ +
    current register when the mapping was triggered

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

*%val{select_mode}*::
    _for commands executed from the object menu's `<a-;>` only_ +
    `replace` if the new selection should replace the existing, `extend`
    otherwise

*%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 number of bytes from the start of the line to the
    anchor, _c_ is the cursor line (like `%val{cursor_line}`), _d_ is
    the number of bytes from the start of the line to the cursor (like
    `%val{cursor_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{selection_length}*::
    _in window scope_ +
    length (in codepoints) of the main selection

*%val{selections_length}*::
    _in window scope_ +
    unquoted list of the lengths (in codepoints) of the selections

*%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, window 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.

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.
