add code examples for snippet-expansion.rst

[gnu-emacs-elpa] / doc / snippet-expansion.org

#+SETUPFILE: org-setup.inc
#+OPTIONS: H:4

* Expanding snippets

  This section describes how YASnippet chooses snippets for expansion at point.

  Maybe, you'll want some snippets to be expanded in a particular
  mode, or only under certain conditions, or be prompted using

** Triggering expansion

   You can use YASnippet to expand snippets in different ways:

   - When [[sym:yas-minor-mode][=yas-minor-mode=]] is active:
     - Type the snippet's *trigger key* then calling [[sym:yas-expand][=yas-expand=]]
       (bound to =TAB= by default).

     - Use the snippet's *keybinding*.

     - By expanding directly from the "YASnippet" menu in the menu-bar

     - Using hippie-expand

   - Call [[sym:yas-insert-snippet][=yas-insert-snippet=]] (use =M-x yas-insert-snippet== or its
     keybinding =C-c & C-s=).

   - Use m2m's excellent auto-complete
     TODO: example for this

   - Expanding from emacs-lisp code

*** Trigger key

[[sym:yas-expand][=yas-expand=]] tries to expand a /snippet abbrev/ (also known as
/snippet key/) before point.

When [[sym:yas-minor-mode][=yas-minor-mode=]] is enabled, it binds [[sym:yas-expand][=yas-expand=]] to =TAB= and
=<tab>= by default, however, you can freely set it to some other key:

#+begin_src emacs-lisp :exports code
   (define-key yas-minor-mode-map (kbd "<tab>") nil)
   (define-key yas-minor-mode-map (kbd "TAB") nil)
   (define-key yas-minor-mode-map (kbd "<the new key>") 'yas-expand)
#+end_src

To enable the YASnippet minor mode in all buffers globally use the
command [[sym:yas-global-mode][=yas-global-mode=]]. This will enable a modeline indicator,
=yas=:

[[./images/minor-mode-indicator.png]]

When you use [[sym:yas-global-mode][=yas-global-mode=]] you can also selectively disable
YASnippet in some buffers by setting the buffer-local variable
[[sym:yas-dont-active][=yas-dont-active=]] in the buffer's mode hook.

**** Fallback bahaviour

[[sym:yas-fallback-behaviour][=yas-fallback-behaviour=]] is a customization variable bound to
'=call-other-command= by default. If [[sym:yas-expand][=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 bound to the same 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.

However, you can change this behavior by customizing the
[[sym:yas-fallback-behavior][=yas-fallback-behavior=]] variable. If you set this variable to
'=return-nil=, it will return =nil= instead of trying to call the
/original/ command when no snippet is found.

*** Insert at point

The command =M-x yas-insert-snippet= lets you insert snippets at point
/for you current major mode/. It prompts you for the snippet key first,
and then for a snippet template if more than one template exists for the
same key.

The list presented contains the snippets that can be inserted at point,
according to the condition system. If you want to see all applicable
snippets for the major mode, prefix this command with =C-u=.

The prompting methods used are again controlled by
[[sym:yas-prompt-functions][=yas-prompt-functions=]].

*** Snippet keybinding

See the section of the =# binding:= directive in
[[./snippet-development.org][Writing Snippets]].

*** Expanding from the menu

See [[./snippet-menu.org][the YASnippet Menu]].

*** Expanding with =hippie-expand=

To integrate with =hippie-expand=, just put [[sym:yas-hippie-try-expand][=yas-hippie-try-expand=]] in
=hippie-expand-try-functions-list=. This probably makes more sense when
placed at the top of the list, but it can be put anywhere you prefer.

*** Expanding from emacs-lisp code

Sometimes you might want to expand a snippet directly from you own elisp
code. You should call [[sym:yas-expand-snippet][=yas-expand-snippet=]] instead of [[sym:yas-expand][=yas-expand=]] in
this case.

As with expanding from the menubar, the condition system and multiple
candidates doesn't affect expansion. In fact, expanding from the
YASnippet menu has the same effect of evaluating the follow code:

#+BEGIN_SRC emacs-lisp
  (yas-expand-snippet template)
#+END_SRC

See the internal documentation on [[sym:yas-expand-snippet][=yas-expand-snippet=]] for more
information.

** Controlling expansion

*** Eligible snippets

YASnippet does quite a bit of filtering to find out which snippets are
eligible for expanding at the current cursor position.

In particular, the following things matter:

-  Currently loaded snippets tables

   These are loaded from a directory hierarchy in your file system. See
   [[./snippet-organization.org][Organizing Snippets]]. They are named
   after major modes like =html-mode=, =ruby-mode=, etc...

-  Major mode of the current buffer

   If the currrent major mode matches one of the loaded snippet tables,
   then all that table's snippets are considered for expansion. Use
   =M-x describe-variable RET major-mode RET= to find out which major
   mode you are in currently.

-  Parent tables

   Snippet tables defined as the parent of some other eligible table are
   also considered. This works recursively, i.e. parents of parents of
   eligible tables are also considered.

-  Buffer-local list of extra modes

   Use [[#yas-activate-extra-mode][=yas-activate-extra-mode=]] to consider snippet tables whose name
   does not correspond to a major mode. Typically, you call this from
   a minor mode hook, for example:

#+BEGIN_SRC emacs-lisp
  ;; When entering rinari-minor-mode, consider also the snippets in the
  ;; snippet table "rails-mode"
  (add-hook 'rinari-minor-mode-hook
            #'(lambda ()
                (yas-activate-extra-mode 'rails-mode)))
#+END_SRC

-  Buffer-local [[sym:yas-buffer-local-condition][=yas-buffer-local-condition=]] variable

   This variable provides finer grained control over what snippets can
   be expanded in the current buffer. The default value won't let you
   expand snippets inside comments or string literals for example. See
   The condition system\_ for more info.

*** The condition system

Consider this scenario: you are an old Emacs hacker. You like the
abbrev-way and bind [[sym:yas-expand][=yas-expand=]] to =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=).

If you use the =# condition := directive (see
[[./snippet-development.org][Writing Snippets]]) you could 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 has a buffer local variable
[[sym:yas-buffer-local-condition][=yas-buffer-local-condition=]]. You can set this variable to
=(not (python-in-string/comment))= in =python-mode-hook=.

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

-  If [[sym:yas-buffer-local-condition][=yas-buffer-local-condition=]] evaluate to nil, no snippets will be
   considered for expansion.

-  If it evaluates 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=), then:

   -  Snippets having no =# condition:= directive won't be considered;

   -  Snippets with conditions that evaluate to nil (or produce an
      error) won't be considered;

   -  If the snippet has a condition that evaluates 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 considered.

-  If it evaluates to the symbol =always=, all snippets are considered
   for expansion, regardless of any conditions.

-  If it evaluate to =t= or some 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 considered.

In the mentioned scenario, set [[sym:yas-buffer-local-condition][=yas-buffer-local-condition=]] like this

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

... 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.

*** Multiples snippet with the same key

The rules outlined [[Eligible%20snippets][above]] can return more than
one snippet to be expanded at point.

When there are multiple candidates, YASnippet will let you select one.
The UI for selecting multiple candidate can be customized through
[[sym:yas-prompt-functions][=yas-prompt-functions=]] , which defines your preferred methods of being
prompted for snippets.

You can customize it with
=M-x customize-variable RET yas-prompt-functions RET=. Alternatively you
can put in your emacs-file:

#+BEGIN_SRC emacs-lisp
  (setq yas-prompt-functions '(yas-x-prompt yas-dropdown-prompt))
#+END_SRC

Currently there are some alternatives solution with YASnippet.

**** Use the X window system

[[./images/x-menu.png]]

The function [[sym:yas-x-prompt][=yas-x-prompt=]] 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.
-  Your window system may or may not allow to you use =C-n=, =C-p= to
   navigate this menu.
-  This function can't be used when in a terminal.

**** Minibuffer prompting

[[./images/ido-menu.png]]

You can use functions [[sym:yas-completing-prompt][=yas-completing-prompt=]] for the classic emacs
completion method or [[sym:yas-ido-prompt][=yas-ido-prompt=]] for a much nicer looking method.
The best way is to try it. This works in a terminal.

**** Use =dropdown-menu.el=

[[./images/dropdown-menu.png]]

The function [[sym:yas-dropdown-prompt][=yas-dropdown-prompt=]] can also be placed in the
[[sym:yas-prompt-functions][=yas-prompt-functions=]] list.

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

**** Roll your own

See the documentation on variable [[sym:yas-prompt-functions][=yas-prompt-functions=]]