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#+TITLE: Yasnippet
[[http://joaotavora.github.io/yasnippet/][Yasnippet is a plugin]] that features interactive expansions of snippets
in Emacs. Snippets are defined with a /domain-specific language/ that
allows multiple input parameters and their transformation through
embedded elisp code.
* Installing Yasnippet
Either install through /package.el/ by hitting =M-x packages-install
yasnippet=, or use an alternative package manager such as
/straight.el/ and declare it like so:
#+BEGIN_SRC Emacs-Lisp
(straight-use-package 'yasnippet)
#+END_SRC
You'll need to define a key combo to expand snippet keys on
demand. My choice is =M-j=, configure like so:
#+BEGIN_SRC Emacs-Lisp
(setq yas-snippet-dirs '("~/.emacs.d/snippets"))
(defun my--yas-hook ()
(define-key yas-minor-mode-map (kbd "M-j") 'yas-expand))
(add-hook 'yas-minor-mode-hook #'my--yas-hook)
#+END_SRC
In order to use /yasnippet/, either enable it globally in your
configuration like so:
#+BEGIN_SRC Emacs-Lisp
(yas-global-mode)
#+END_SRC
Or define the use of the minor mode in each mode hook you want it
like so (for org-mode):
#+BEGIN_SRC Emacs-Lisp
(add-hook 'org-mode-hook #'yas-minor-mode)
#+END_SRC
* Writing snippets
Snippets are written in a specific syntax that defines a few options
like /key/ and /name/. Each placeholder is denoted with a =$= and a
number. The final spot where the /point/ will end is written with
=$0=.
#+BEGIN_SRC Snippet
# key: if
# --
if [ "$1" $2 "$3" ]; then
$0
fi
#+END_SRC
** Snippet directory structure
Snippets live under the =yas-snippet-dirs= directory. The folder
structure and file naming they follow is specific. Each sub-folder
of =yas-snippets-dir= must be a mode name and each snippet file
within must be named after its key. For example this =if= snippet
for =Shell= would live in =(yas-snippet-dirs)/Shell/if=:
#+BEGIN_SRC Snippet
# key: if
# --
if [ "$1" $2 "$3" ]; then
$0
fi
#+END_SRC
** Placeholders
Snippet placeholders are filled in by the user by order of
number; =$1=, then =$2= and so on, ending with =$0=. They can be
repeated in the snippet to fill multiple places at once. This is
convenient for languages like XML like so:
#+BEGIN_SRC Snippet
# key: tag
# --
<$1>
$0
</$1>
#+END_SRC
** Transformations
Placeholders can have transformations applied, for example case
changes by invoking lisp code in their definition. Like so:
#+BEGIN_SRC Snippet
# key: tag
# --
<${1:$$(upcase yas-text)}>
$0
</$1>
#+END_SRC
Note that the tranformation applied to =$1= also applied to its
mirror without having to redefine the transformation.
** Choices
Placeholders can also come with a list of choices. This is also
accomplished through lisp code like so:
#+BEGIN_SRC Snippet
# key: em
# --
<${1:$$(yas-choose-value '("em" "b" "i")}>
$0
</$1>
#+END_SRC
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