========================= 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 #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. .. image:: images/popup-menu.png :align: right 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 ~~~~~~~~~~~~~~~~~~~~~~ .. image:: images/dropdown-menu.png :align: right 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 ~~~~~~~~~~~~~~ .. image:: images/minor-mode-indicator.png :align: left 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: .. image:: images/menubar.png :align: right * 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 $0 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{$1}``. The best explanation is to see the screencast(`YouTube `_ or `avi video `_). The tab stops with the same number to the field act as its mirrors. If none of the tab stops has an initial value, the first one is selected as the field and others mirrors. .. _transformations: Transformations --------------- If the default value of a field starts with ``$``, 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.