yasnippet/doc/define_snippet.rst

=========================
How to define a snippet ?
=========================

:Author: pluskid
:Contact: pluskid@gmail.com
:Date: 2008-03-20

.. contents::

The most convenient way to define snippets for YASnippet is to put
them in a directory arranged by the mode and use
``yas/load-directory`` to load them. 

However, this might slow down the Emacs startup speed if you have many
snippets. You can use ``yas/define-snippets`` to define a bunch of
snippets for a perticular mode. But this is hard to maintain! So,
there's a better way: define your snippets in directory and use
``yas/compile-bundle`` to compile it into a bundle file when you
modified your snippets.

The release bundle of YASnippet is produced by
``yas/compile-bundle``. The bundle use ``yas/define-snippets`` to
define snippets. This avoid the IO and parsing overhead when loading
snippets.

Finally, you can use ``yas/define`` to define a single snippet at your
convenience. I ofthen use this to do some testing.

Define snippets in files
========================

Directory hierarchy
-------------------

Here's the directory hierarchy of the ``snippets`` directory comes
with YASnippet:

.. sourcecode:: text

  snippets
  `-- text-mode/
      |-- cc-mode/
      |   |-- c++-mode/
      |   |   |-- beginend
      |   |   |-- class
      |   |   `-- using
      |   |-- c-mode/
      |   |   `-- fopen
      |   |-- do
      |   |-- for
      |   |-- if
      |   |-- inc
      |   |-- inc.1
      |   |-- main
      |   |-- once
      |   `-- struct
      |-- css-mode/
      |   |-- background
      |   |-- background.1
      |   `-- border
      |-- email
      |-- html-mode/
      |   |-- div
      |   |-- doctype
      |   |-- doctype.xhml1
      |   |-- doctype.xhtml1_1
      |   |-- doctype.xhtml1_strict
      |   `-- doctype.xhtml1_transitional
      |-- objc-mode/
      |   `-- prop
      |-- perl-mode/
      |   |-- cperl-mode/
      |   |-- eval
      |   |-- for
      |   |-- fore
      |   |-- if
      |   |-- ife
      |   |-- ifee
      |   |-- sub
      |   |-- unless
      |   |-- while
      |   |-- xfore
      |   |-- xif
      |   |-- xunless
      |   `-- xwhile
      |-- python-mode/
      |   |-- __
      |   |-- class
      |   |-- def
      |   |-- for
      |   |-- ifmain
      |   `-- while
      |-- rst-mode/
      |   |-- chapter
      |   |-- section
      |   `-- title
      |-- ruby-mode/
      |   |-- #
      |   |-- =b
      |   |-- Comp
      |   |-- all
      |   |-- am
      |   |-- any
      |   |-- app
      |   |-- bm
      |   |-- case
      |   |-- cla
      |   |-- classify
      |   |-- cls
      |   |-- collect
      |   |-- dee
      |   |-- deli
      |   |-- det
      |   |-- ea
      |   |-- eac
      |   |-- eai
      |   |-- eav
      |   |-- eawi
      |   |-- forin
      |   |-- if
      |   |-- ife
      |   |-- inject
      |   |-- mm
      |   |-- r
      |   |-- rb
      |   |-- reject
      |   |-- req
      |   |-- rreq
      |   |-- rw
      |   |-- select
      |   |-- w
      |   |-- y
      |   `-- zip
      `-- time

Snippet definitions are put in plain text files. They are arranged by
subdirectories. For example, snippets for ``c-mode`` are put in the
``c-mode`` directory.

The parent directory acts as the *parent mode*. This is the way of
YASnippet to share snippet definitions among different modes. As you
can see above, ``c-mode`` and ``c++-mode`` share the same parents
``cc-mode``, while all modes are derived from ``text-mode``. This can
be also used to as an *alias* -- ``cperl-mode`` is an empty directory
whose parent is ``perl-mode``.

File names act as the snippet trigger key. Note files starting with a
dot (``.``) are ignored.

File content
------------

A file defining a snippet may just contain the template for the
snippet. Optionally it can also contains some meta data for the
snippet as well as comments.

Generally speaking, if the file contains a line of ``# --``, then all
contents above that line are considered as meta data and comments;
below are template. Or else the whole file content is considered as
the template.

Here's a typical example:

.. sourcecode:: text

  #contributor : pluskid <pluskid@gmail.com>
  #name : __...__
  # --
  __${init}__

Meta data are specified in the syntax of

.. sourcecode:: text

  #data-name : data value

Any other text above ``# --`` is considered as comment and
ignored. Here's a list of currently supported meta data:

* ``name``: The name of the snippet. This is a one-line description of
  the snippet. It will be displayed in the menu. So it's a good idea
  to select a descriptive name fo a snippet -- especially
  distinguishable among similar snippets.
* ``contributor``: The contributor of the snippet.
* ``condition``: The condition of the snippet. This is a piece of
  elisp code. If a snippet has a condition, then it will only be
  expanded when the condition code evaluate to some non-nil value.



Define snippets using elisp code
--------------------------------

As I mentioned above, you can define snippets directly by writing
elisp code.

yas/define-snippets
~~~~~~~~~~~~~~~~~~~

The basic syntax of ``yas/define-snippets`` is

.. sourcecode:: common-lisp

  (yas/define-snippets MODE SNIPPETS &optional PARENT)

The parameters are self-descriptive. If you specify a ``PARENT``, then
the snippets of the parents may be shared by ``MODE``. Note if you use
this function several times, the later specified ``PARENT`` will
overwrite the original one. However, not specifying a ``PARENT`` won't
erase the original parent.

The ``SNIPPETS`` parameter is a list of snippet definitions. Each
element should have the following form:

.. sourcecode:: common-lisp

  (KEY TEMPLATE NAME CONDITION)

The ``NAME`` and ``CONDITION`` can be omitted if you don't want to
provide one. Here's an example:

.. sourcecode:: common-lisp

  (yas/define-snippets 'c++-mode
  '(
    ("using" "using namespace ${std};
  $0" "using namespace ... " nil)
    ("class" "class ${1:Name}
  {
  public:
      $1($2);
      virtual ~$1();
  };" "class ... { ... }" nil)
    ("beginend" "${1:v}.begin(), $1.end" "v.begin(), v.end()" nil)
    )
  'cc-mode)

The example above is auto-generated code by ``yas/compile-bundle``.

yas/compile-bundle
~~~~~~~~~~~~~~~~~~

``yas/compile-bundle`` can be used to parse the snippets from a
directory hierarchy and translate them into the elisp form. The
translated code is faster to load. Further more, the generated bundle
is a stand-alone file not depending on ``yasnippet.el``. The released
bundles of YASnippet are all generated this way.

The basic syntax of ``yas/compile-bundle`` is

.. sourcecode:: common-lisp

  (yas/compile-bundle &optional yasnippet yasnippet-bundle snippet-roots code)

As you can see, all the parameters are optional. The default values
for those parameters are convenient for me to produce the default
release bundle:

.. sourcecode:: common-lisp

  (yas/compile-bundle "yasnippet.el"
                      "./yasnippet-bundle.el"
                      '("snippets")
                      "(yas/initialize)")

The ``snippet-roots`` can be a list of root directories. This is
useful when you have multiple snippet directories (maybe from other
users). The ``code`` parameter can be used to specify your own
customization code instead of the default ``(yas/initialize)``. For
example, you can set ``yas/trigger-key`` to ``(kbd "SPC")`` here if
you like.

yas/define
~~~~~~~~~~

The basic syntax for ``yas/define`` is

.. sourcecode:: common-lisp

  (yas/define mode key template &optional name condition)

This is only a syntax sugar for

.. sourcecode:: common-lisp

  (yas/define-snippets mode
                       (list (list key template name condition)))

The strategy to select a snippet
================================

When user press the ``yas/trigger-key``, YASnippet try to find a
proper snippet to expand. The strategy to find such a snippet is
explained here.

Finding the key
---------------

YASnippet search from current point backward trying to find the
snippet to be expanded. The default searching strategy is quite
powerful. For example, in ``c-mode``, ``"bar"``, ``"foo_bar"``,
``"#foo_bar"`` can all be recognized as a template key. Further more,
the searching is in that order. In other words, if ``"bar"`` is found
to be a key to some *valid* snippet, then ``"foo_bar"`` and
``"#foobar"`` won't be searched.

However, this strategy can also be customized easily from the
``yas/key-syntaxes`` variable. It is a list of syntax rules, the
default value is ``("w" "w_" "w_." "^ ")``. Which means search the
following thing until found one:

* a word.
* a symbol. In lisp, ``-`` and ``?`` can all be part of a symbol.
* a sequence of characters of either word, symbol or punctuation.
* a sequence of characters of non-whitespace characters.

But you'd better keep the default value unless you understand what
Emacs's syntax rule mean.

The condition system
--------------------

I write forked snippet.el to make the smart-snippet.el. I call it
*smart*-snippet because a condition can be attached to a snippet. This
is really a good idea. However, writing condition for a snippet
usually needs good elisp and Emacs knowledge, so it is strange to many
user.

Later I write YASnippet and persuade people to use it instead of
smart-snippet.el. However, some user still love smart-snippet because
it is smart. So I make YASnippet smart. Even smarter than
smart-snippet.el. :p

Consider this scenario: you are an old Emacs hacker. You like the
abbrev-way and set ``yas/trigger-key`` to ``(kbd "SPC")``. However,
you don't want ``if`` to be expanded as a snippet when you are typing
in a comment block or a string (e.g. in ``python-mode``). 

It's OK, just specify the condition for ``if`` to be ``(not
(python-in-string/comment))``. But how about ``while``, ``for``,
etc. ? Writing the same condition for all the snippets is just
boring. So YASnippet introduce a buffer local variable
``yas/buffer-local-condition``. You can set this variable to ``(not
(python-in-string/comment))`` in ``python-mode-hook``. There's no way
to do this in smart-snippet.el!

Then, what if you really want some snippet even in comment? This is
also possible! But let's stop telling the story and look at the rules:

* If ``yas/buffer-local-condition`` evaluate to nil, snippet won't be
  expanded.
* If it evaluate to the a cons cell where the ``car`` is the symbol
  ``require-snippet-condition`` and the ``cdr`` is a symbol (let's
  call it ``requirement``):

  * If the snippet has no condition, then it won't be expanded.
  * If the snippet has a condition but evaluate to nil or error
    occured during evaluation, it won't be expanded.
  * If the snippet has a condition that evaluate to non-nil (let's
    call it ``result``):

    * If ``requirement`` is ``t``, the snippet is ready to be
      expanded.
    * If ``requirement`` is ``eq`` to ``result``, the snippet is ready
      to be expanded.
    * Otherwise the snippet won't be expanded.

* If it evaluate to other non-nil value:

  * If the snippet has no condition, or has a condition that evaluate
    to non-nil, it is ready to be expanded.
  * Otherwise, it won't be expanded.

So set ``yas/buffer-local-condition`` like this

.. sourcecode:: common-lisp

  (add-hook 'python-mode-hook
            '(lambda ()
               (setq yas/buffer-local-condition
                     '(if (python-in-string/comment)
                          '(require-snippet-condition . force-in-comment)
                        t))))

And specify the condition for a snippet that you're going to expand in
comment to be evaluated to the symbol ``force-in-comment``. Then it
can be expanded as you expected, while other snippets like ``if``
still can't expanded in comment. 

Multiple snippet with the same key
----------------------------------

There can be multiple snippet bind to the same key. If you define a
snippet with a key that is already used, you'll overwrite the original
snippet definition. However, you can add a different *postfix* to the
key.

In general, the *extension* (consider a file name) is *ignored* when
defining a snippet. So ``def``, ``def.1`` and ``def.mine`` will all be
valid candidates when the key is ``def``.

When there are multiple candidates, YASnippet will let you select
one. The UI for selecting multiple candidate can be
customized. There're two variable related:

* ``yas/window-system-popup-function``: the function used when you
  have a window system.
* ``yas/text-popup-function``: the function used when you don't have a
  window system, i.e. when you are working in a terminal.

 Currently there're three solution come with YASnippet.

Popup Menu
~~~~~~~~~~

The function ``yas/x-popup-menu-for-template`` can be used to show a
popup menu for you to select. This menu will be part of you native
window system widget, which means:

* It usually looks beautiful. E.g. when you compile Emacs with gtk
  support, this menu will be rendered with your gtk theme.
* Emacs have little control over it. E.g. you can't use ``C-n``,
  ``C-p`` to navigate.
* This function can't be used when in a terminal.

Just select the first one
~~~~~~~~~~~~~~~~~~~~~~~~~

This one is originally used in terminal mode. It doesn't let you to
choose anything, it just select the first one on behalf of you. So I
bet you never want to use this. :p

Use a dropdown-menu.el
~~~~~~~~~~~~~~~~~~~~~~

Originally, only the above two function is available in
YASnippet. They are difficult to use -- especially in a
terminal. Until later Jaeyoun Chung show me his
``dropdown-menu.el``, I say wow! It's wonderful!

* It works in both window system and terminal.
* It is customizable, you can use ``C-n``, ``C-p`` to navigate, ``q``
  to quite and even press ``6`` as a shortcut to select the 6th
  candidate.

So I added ``yas/dropdown-list-popup-for-template`` to support
``dropdown-list.el``. And upload ``dropdown-list.el`` to YASnippet
hompage for an optional download (since Jaeyoun didn't provide a URL).

Then finally, in 0.4.0, I included a copy of the content of
``dropdown-list.el`` [1]_ in ``yasnippet.el`` and made it the default
way for selecting multiple candidates.

However, the original functions are still there, you can still use this

.. sourcecode:: common-lisp

  (setq yas/window-system-popup-function
        'yas/x-popup-menu-for-template)

if you prefer a *modern* UI. :)

The Trigger Key
---------------

YASnippet is implemented as a minor-mode (``yas/minor-mode``). The
trigger key ``yas/trigger-key`` is defined in ``yas/minor-mode-map``
to call ``yas/expand`` to try to expand a snippet.

The Minor Mode
~~~~~~~~~~~~~~

When ``yas/minor-mode`` is enabled, the trigger key will take
effect. The default key is ``(kbd "TAB")``, however, you can freely
set it to some other key. By default, YASnippet add a hook to
``after-change-major-mode-hook`` to enable ``yas/minor-mode`` [2]_ in
every buffer. This works fine for most modes, however, some mode
doesn't follow the Emacs convention and doens't call this hook. You
can either explicitly hook for those mode or just add it to
``yas/extra-mode-hooks`` to let YASnippet do it for you:

.. sourcecode:: common-lisp

  (require 'yasnippet)
  (add-to-list 'yas/extra-mode-hooks
               'ruby-mode-hook)
  (yas/initialize)

Note that **should** be put after ``(require 'yasnippet)`` and before
``(yas/initialize)``. Further more, you may report it to me, I'll add
that to the default value.

The Fallback
~~~~~~~~~~~~

If ``yas/expand`` failed to find any suitable snippet to expand, it
will disable the minor mode temporarily and find if there's any other
command bind the ``yas/trigger-key``. If found, the command will be
called. Usually this works very well -- when there's a snippet, expand
it, otherwise, call whatever command originally bind to the trigger
key.

Other way to select a snippet
-----------------------------

When you use the trigger key (so ``yas/expand``) to expand a snippet,
the key for the snippet is deleted before the template for the snippet
is inserted. 

However, there're other ways to insert a snippet.

The Menu
~~~~~~~~

YASnippet will setup a menu just after the *Buffers* Menu in the
menubar. The snippets for all *real* modes are listed there under the
menu. You can select a snippet from the menu to expand it. Since you
select manually from the menu, you can expand any snippet. For
example, you can expand a snippet defined for ``python-mode`` in a
``c-mode`` buffer by selecting it from the menu:

* Condition system is ignored since you select to expand it
  explicitly.
* There will be no muliple candidates since they are listed in the
  menu as different items.

This can be convenient sometimes. However, if you don't like the
menubar of Emacs and never use it. You can tell YASnippet don't boring
to build a menu by setting ``yas/use-menu`` to nil.

Another thing to note is that only *real* modes are listed under the
menu. As you know, common snippets can be shared by making up a
*virtual* parent mode. It's too bad if the menu is floored by those
*virtual* modes. So YASnippet only show menus for those *real*
modes. But the snippets fo the *virtual* modes can still be accessed
through the ``parent`` submenu of some *real* mode.

YASnippet use a simple way to check whether a mode is *real* or
*virtual*: ``(fboundp mode)``. For example, the symbol ``c-mode`` is
bound to a function while ``cc-mode`` is not. But this is not enough,
some modes aren't part of Emacs, and maybe when initializing
YASnippet, those modes haven't been initialized. So YASnippet also
maintain a list of known modes (``yas/known-modes``). You can add item
to that list if you need.

Expanding From Elisp Code
~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes you might want to expand a snippet directly by calling a
functin from elisp code. You should call ``yas/expand-snippet``
instead of ``yas/expand`` in this case.

As with expanding from the menubar, condition system and multiple
candidates won't exists here. In fact, expanding from menubar has the
same effect of evaluating the follow code:

.. sourcecode:: common-lisp

  (yas/expand-snippet (point) (point) template)

Where ``template`` is the template of a snippet. It is never required
to belong to any snippet -- you can even make up it on the fly. The
1st and 2nd parameter defines the region to be deleted after YASnippet
inserted the template. It is used by ``yas/expand`` to indicate the
region of the key. There's usually no need to delete any region when
we are expanding a snippet from elisp code, so passing two ``(point)``
is fine. Note only ``(point)`` will be fine because the 1st parameter
also indicate where to insert and expand the ``template``.

The Syntax of the Template
==========================

The syntax of the snippet template is simple but powerful, very
similar to TextMate's.

Plain Text
----------

Arbitrary text can be included as the content of a template. They are
usually interpreted as plain text, except ``$`` and `````. You need to
use ``\`` to escape them: ``\$`` and ``\```. The ``\`` itself may also
needed to be escaped as ``\\`` sometimes.

Embedded elisp code
-------------------

Elisp code can be embedded inside the template. They are written
inside back-quotes (`````):

They are evaluated when the snippet is being expanded. The evaluation
is done in the same buffer as the snippet being expanded. Here's an
example for ``c-mode`` to calculate the header file guard dynamically:

.. sourcecode:: text

  #ifndef ${1:_`(upcase (file-name-nondirectory (file-name-sans-extension (buffer-file-name))))`_H_}
  #define $1
  
  $0
  
  #endif /* $1 */

Tab Stops
---------

Tab stops are fields that you can navigate back and forth by ``TAB``
and ``S-TAB`` [3]_. They are written by ``$`` followed with a
number. ``$0`` has the special meaning of the *exit point* of a
snippet. That is the last place to go when you've traveled all the
fields. Here's a typical example:

.. sourcecode:: text

  <div$1>
      $0
  </div>

Placeholders
------------

Tab stops can have default values -- a.k.a placeholders. The syntax is
like this:

.. sourcecode:: text

  ${N:default value}

They acts as the default value for a tab stop. But when you firstly
type at a tab stop, the default value will be replaced by your
typing. The number can be omitted if you don't want to create
`mirrors`_ or `transformations`_ for this field.

.. _mirrors:

Mirrors
-------

We refer the tab stops with placeholders as a *field*. A field can have
mirrors. Its mirrors will get updated when you change the text of a
field. Here's an example:

.. sourcecode:: text

  \begin{${1:enumerate}}
      $0
  \end{$1}

When you type ``"document"`` at ``${1:enumerate}``, the word
``"document"`` will also be inserted at ``\end{$1}``. The best
explanation is to see the screencast(`YouTube
<http://www.youtube.com/watch?v=vOj7btx3ATg>`_ or `avi video
<http://yasnippet.googlecode.com/files/yasnippet.avi>`_).

The tab stops with the same number to the field act as its mirrors. If
none of the tab stops has an initial value, the first one is selected
as the field and others mirrors.

.. _transformations:

Transformations
---------------

If the default value of a field starts with ``$``, then it is interpreted
as the transformation code instead of default value. A transformation
is some arbitrary elisp code that will get evaluated in an environment
when the variable text is bind to the inputted text of the
field. Here's an example for Objective-C:

.. sourcecode:: text

  - (${1:id})${2:foo}
  {
      return $2;
  }
  
  - (void)set${2:$(capitalize text)}:($1)aValue
  {
      [$2 autorelease];
      $2 = [aValue retain];
  }
  $0

Look at ``${2:$(capitalize text)}``, it is a transformation instead of
a placeholder. The actual placeholder is at the first line:
``${2:foo}``. When you type text in ``${2:foo}``, the transformation
will be evaluated and the result will be placed there as the
transformated text. So in this example, if you type baz in the field,
the transformed text will be Baz. This example is also available in
the screencast.

Another example is for ``rst-mode``. In reStructuredText, the document
title can be some text surrounded by "===" below and above. The "==="
should be at least as long as the text. So

.. sourcecode:: text

  =====
  Title
  =====

is a valid title but

.. sourcecode:: text

  ===
  Title
  ===

is not. Here's an snippet for rst title: 

.. sourcecode:: text

  ${1:$(make-string (string-width text) ?\=)}
  ${1:Title}
  ${1:$(make-string (string-width text) ?\=)}
  
  $0

.. [1] With some minor change, mainly for fixing some trivial bugs.
.. [2] This is done when you call ``yas/initialize``.
.. [3] Of course, this can be customized.