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

#+SETUPFILE: org-setup.inc

#+TITLE: Writing snippets

* Snippet development

** Quickly finding snippets

There are some ways you can quickly find a snippet file:

-  =M-x yas-new-snippet=

   Prompts you for a snippet name, then tries to guess a suitable
   directory to store it, prompting you for creation if it does not
   exist. Finally, places you in a new buffer set to =snippet-mode= so
   you can write your snippet.

-  =M-x yas-find-snippets=

   Lets you find the snippet file in the directory the snippet was
   loaded from (if it exists) like =find-file-other-window=. The
   directory searching logic is similar to =M-x yas-new-snippet=.

-  =M-x yas-visit-snippet-file=

   Prompts you for possible snippet expansions like
   [[sym:yas-insert-snippet][=yas-insert-snippet=]], but instead of expanding it, takes you directly
   to the snippet definition's file, if it exists.

Once you find this file it will be set to =snippet-mode= (see ahead) and
you can start editing your snippet.

** Using the =snippet-mode= major mode

There is a major mode =snippet-mode= to edit snippets. You can set the
buffer to this mode with =M-x snippet-mode=. It provides reasonably
useful syntax highlighting.

Two commands are defined in this mode:

-  =M-x yas-load-snippet-buffer=

     When editing a snippet, this loads the snippet into the correct
     mode and menu. Bound to =C-c C-c= by default while in
     =snippet-mode=.

-  =M-x yas-tryout-snippet=

     When editing a snippet, this opens a new empty buffer, sets it to
     the appropriate major mode and inserts the snippet there, so you
     can see what it looks like. This is bound to =C-c C-t= while in
     =snippet-mode=.

There are also /snippets for writing snippets/: =vars=, =$f= and =$m=
:-).

* File content

A file defining a snippet generally contains the template to be
expanded.

Optionally, if the file contains a line of =# --=, the lines above it
count as comments, some of which can be /directives/ (or meta data).
Snippet directives look like =# property: value= and tweak certain
snippets properties described below. If no =# --= is found, the whole
file is considered the snippet template.

Here's a typical example:

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

Here's a list of currently supported directives:

** =# key:= snippet abbrev

This is the probably the most important directive, it's the
abbreviation you type to expand a snippet just before hitting the key
that runs [[sym:yas-expand][=yas-expand=]]. If you don't specify this
the snippet will not be expandable through the trigger mechanism.

** =# name:= snippet name

This is a one-line description of the snippet. It will be displayed in
the menu. It's a good idea to select a descriptive name for a snippet --
especially distinguishable among similar snippets.

If you omit this name it will default to the file name the snippet was
loaded from.

** =# condition:= snippet condition

This is a piece of Emacs-lisp code. If a snippet has a condition, then
it will only be expanded when the condition code evaluate to some
non-nil value.

See also [[sym:yas-buffer-local-condition][=yas-buffer-local-condition=]] in
[[./snippet-expansion.org][Expanding snippets]]

** =# group:= snippet menu grouping

When expanding/visiting snippets from the menu-bar menu, snippets for a
given mode can be grouped into sub-menus . This is useful if one has too
many snippets for a mode which will make the menu too long.

The =# group:= property only affect menu construction (See
[[./snippet-menu.org][the YASnippet menu]]) and the same effect can be
achieved by grouping snippets into sub-directories and using the
=.yas-make-groups= special file (for this see
[[./snippet-organization.org][Organizing Snippets]]

Refer to the bundled snippets for =ruby-mode= for examples on the
=# group:= directive. Group can also be nested, e.g.
=control structure.loops= tells that the snippet is under the =loops=
group which is under the =control structure= group.

** =# expand-env:= expand environment

This is another piece of Emacs-lisp code in the form of a =let= /varlist
form/, i.e. a list of lists assigning values to variables. It can be
used to override variable values while the snippet is being expanded.

Interesting variables to override are [[sym:yas-wrap-around-region][=yas-wrap-around-region=]] and
[[sym:yas-indent-line][=yas-indent-line=]] (see [[./snippet-expansion.org][Expanding Snippets]]).

As an example, you might normally have [[sym:yas-indent-line][=yas-indent-line=]] set to '=auto=
and [[sym:yas-wrap-around-region][=yas-wrap-around-region=]] set to =t=, but for this particularly
brilliant piece of ASCII art these values would mess up your hard work.
You can then use:

#+BEGIN_SRC snippet
  # name: ASCII home
  # expand-env: ((yas-indent-line 'fixed) (yas-wrap-around-region 'nil))
  # --
                  welcome to my
              X      humble
             / \      home,
            /   \      $0
           /     \
          /-------\
          |       |
          |  +-+  |
          |  | |  |
          +--+-+--+
#+END_SRC

** =# binding:= direct keybinding

You can use this directive to expand a snippet directly from a normal
Emacs keybinding. The keybinding will be registered in the Emacs keymap
named after the major mode the snippet is active for.

Additionally a variable [[sym:yas-prefix][=yas-prefix=]] is set to to the prefix argument
you normally use for a command. This allows for small variations on the
same snippet, for example in this "html-mode" snippet.

#+BEGIN_SRC snippet
  # name: <p>...</p>
  # binding: C-c C-c C-m
  # --
  <p>`(when yas-prefix "\n")`$0`(when yas-prefix "\n")`</p>
#+END_SRC

This binding will be recorded in the keymap =html-mode-map=. To expand a
paragraph tag newlines, just press =C-u C-c C-c C-m=. Omitting the =C-u=
will expand the paragraph tag without newlines.

** =# type:= =snippet= or =command=

If the =type= directive is set to =command=, the body of the snippet
is interpreted as lisp code to be evaluated when the snippet is
triggered.

If it's =snippet= (the default when there is no =type= directive), the
snippet body will be parsed according to the [[Template Syntax]],
described below.

** =# uuid:= unique identifier

This provides to a way to identify a snippet, independent of its name.
Loading a second snippet file with the same uuid would replace the
previous snippet.

** =# contributor:= snippet author

This is optional and has no effect whatsoever on snippet functionality,
but it looks nice.

* <<Template syntax>>

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 Emacs-lisp code

Emacs-Lisp code can be embedded inside the template, written inside
back-quotes (=`=). The lisp forms 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:

#+BEGIN_SRC snippet
  #ifndef ${1:_`(upcase (file-name-nondirectory (file-name-sans-extension (buffer-file-name))))`_H_}
  #define $1

  $0

  #endif /* $1 */
#+END_SRC

From version 0.6, snippets expansions are run with some special
Emacs-lisp variables bound. One of this is [[sym:yas-selected-text][=yas-selected-text=]]. You can
therefore define a snippet like:

#+BEGIN_SRC snippet
  for ($1;$2;$3) {
    `yas-selected-text`$0
  }
#+END_SRC

to "wrap" the selected region inside your recently inserted snippet.
Alternatively, you can also customize the variable
[[sym:yas-wrap-around-region][=yas-wrap-around-region=]] to =t= which will do this automatically.

** Tab stop fields

Tab stops are fields that you can navigate back and forth by =TAB= and
=S-TAB=. 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:

#+BEGIN_SRC snippet
  <div$1>
      $0
  </div>
#+END_SRC
** Placeholder fields

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

#+BEGIN_SRC snippet
  ${N:default value}
#+END_SRC

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>>

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:

#+BEGIN_SRC snippet
  \begin{${1:enumerate}}
      $0
  \end{$1}
#+END_SRC

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([[http://www.youtube.com/watch?v=vOj7btx3ATg][YouTube]] or [[http://yasnippet.googlecode.com/files/yasnippet.avi][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.

** Mirrors with <<transformations>>

If the value of an =${n:=-construct starts with and contains =$(=, then
it is interpreted as a mirror for field =n= with a transformation. The
mirror's text content is calculated according to this transformation,
which is Emacs-lisp code that gets evaluated in an environment where the
variable =text= (or [[sym:yas-text][=yas-text=]]) is bound to the text content (string)
contained in the field =n=.Here's an example for Objective-C:

#+BEGIN_SRC snippet
  - (${1:id})${2:foo}
  {
      return $2;
  }

  - (void)set${2:$(capitalize text)}:($1)aValue
  {
      [$2 autorelease];
      $2 = [aValue retain];
  }
  $0
#+END_SRC

Look at =${2:$(capitalize text)}=, it is a mirror with transformation
instead of a field. The actual field 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 transformed 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

#+BEGIN_SRC rst
  =====
  Title
  =====
#+END_SRC

is a valid title but

#+BEGIN_SRC rst
  ===
  Title
  ===
#+END_SRC

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

#+BEGIN_SRC snippet
  ${1:$(make-string (string-width text) ?\=)}
  ${1:Title}
  ${1:$(make-string (string-width text) ?\=)}

  $0
#+END_SRC

** Fields with transformations

From version 0.6 on, you can also have lisp transformation inside
fields. These work mostly mirror transformations but are evaluated when
you first enter the field, after each change you make to the field and
also just before you exit the field.

The syntax is also a tiny bit different, so that the parser can
distinguish between fields and mirrors. In the following example

:  #define "${1:mydefine$(upcase yas-text)}"

=mydefine= gets automatically upcased to =MYDEFINE= once you enter the
field. As you type text, it gets filtered through the transformation
every time.

Note that to tell this kind of expression from a mirror with a
transformation, YASnippet needs extra text between the =:= and the
transformation's =$=. If you don't want this extra-text, you can use two
=$='s instead.

:  #define "${1:$$(upcase yas-text)}"

Please note that as soon as a transformation takes place, it changes the
value of the field and sets it its internal modification state to
=true=. As a consequence, the auto-deletion behaviour of normal fields
does not take place. This is by design.

** Choosing fields value from a list and other tricks

As mentioned, the field transformation is invoked just after you enter
the field, and with some useful variables bound, notably
[[sym:yas-modified-p][=yas-modified-p=]] and [[sym:yas-moving-away-p][=yas-moving-away-p=]]. Because of this feature you
can place a transformation in the primary field that lets you select
default values for it.

The [[sym:yas-choose-value][=yas-choose-value=]] does this work for you. For example:

#+BEGIN_SRC snippet
  <div align="${2:$$(yas-choose-value '("right" "center" "left"))}">
    $0
  </div>
#+END_SRC

See the definition of [[sym:yas-choose-value][=yas-choose-value=]] to see how it was written using
the two variables.

Here's another use, for LaTeX-mode, which calls reftex-label just as you
enter snippet field 2. This one makes use of [[sym:yas-modified-p][=yas-modified-p=]] directly.

#+BEGIN_SRC snippet
  \section{${1:"Titel der Tour"}}%
  \index{$1}%
  \label{{2:"waiting for reftex-label call..."$(unless yas-modified-p (reftex-label nil 'dont-
  insert))}}%
#+END_SRC

The function [[sym:yas-verify-value][=yas-verify-value=]] has another neat trick, and makes use
of [[sym:yas-moving-away-p][=yas-moving-away-p=]]. Try it and see! Also, check out this [[http://groups.google.com/group/smart-snippet/browse_thread/thread/282a90a118e1b662][thread]]

** Nested placeholder fields

From version 0.6 on, you can also have nested placeholders of the type:

#+BEGIN_SRC snippet
  <div${1: id="${2:some_id}"}>$0</div>
#+END_SRC

This allows you to choose if you want to give this =div= an =id=
attribute. If you tab forward after expanding it will let you change
"some\_id" to whatever you like. Alternatively, you can just press =C-d=
(which executes [[sym:yas-skip-and-clear-or-delete-char][=yas-skip-and-clear-or-delete-char=]]) and go straight to
the exit marker.

By the way, =C-d= will only clear the field if you cursor is at the
beginning of the field /and/ it hasn't been changed yet. Otherwise, it
performs the normal Emacs =delete-char= command.