# Vim documentation: usr_51

main help file

    *usr_51.txt*	For Vim version 9.0.  Last change: 2022 Jun 03

    		     VIM USER MANUAL - by Bram Moolenaar

    			      Write plugins

    Plugins can be used to define settings for a specific type of file, syntax
    highlighting and many other things.  This chapter explains how to write the
    most common Vim plugins.

    |51.1|	Writing a generic plugin
    |51.2|	Writing a filetype plugin
    |51.3|	Writing a compiler plugin
    |51.4|	Distributing Vim scripts

         Next chapter: |usr_52.txt|  Write large plugins
     Previous chapter: |usr_50.txt|  Advanced Vim script writing
    Table of contents: |usr_toc.txt|

    ==============================================================================

    *51.1*	Writing a generic plugin			*write-plugin*

    You can write a Vim script in such a way that many people can use it.  This is
    called a plugin.  Vim users can drop your script in their plugin directory and
    use its features right away |add-plugin|.

    There are actually two types of plugins:

      global plugins: For all types of files.
    filetype plugins: Only for files of a specific type.

    In this section the first type is explained.  Most items are also relevant for
    writing filetype plugins.  The specifics for filetype plugins are in the next
    section |write-filetype-plugin|.

    We will use |Vim9| syntax here, the recommended way to write new plugins.
    Make sure the file starts with the `vim9script` command.

    NAME

    First of all you must choose a name for your plugin.  The features provided
    by the plugin should be clear from its name.  And it should be unlikely that
    someone else writes a plugin with the same name but which does something
    different.

    A script that corrects typing mistakes could be called "typecorrect.vim".  We
    will use it here as an example.

    For the plugin to work for everybody, it should follow a few guidelines.  This
    will be explained step-by-step.  The complete example plugin is at the end.

    BODY

    Let's start with the body of the plugin, the lines that do the actual work:

     12	iabbrev teh the
     13	iabbrev otehr other
     14	iabbrev wnat want
     15	iabbrev synchronisation
     16		\ synchronization

    The actual list should be much longer, of course.

    The line numbers have only been added to explain a few things, don't put them
    in your plugin file!

    FIRST LINE

      1	vim9script noclear

    You need to use `vim9script` as the very first command.  Best is to put it in
    the very first line.

    The script we are writing will have a `finish` command to bail out when it is
    loaded a second time.  To avoid that the items defined in the script are lost
    the "noclear" argument is used.  More info about this at |vim9-reload|.

    HEADER

    You will probably add new corrections to the plugin and soon have several
    versions lying around.  And when distributing this file, people will want to
    know who wrote this wonderful plugin and where they can send remarks.
    Therefore, put a header at the top of your plugin:

      2	# Vim global plugin for correcting typing mistakes
      3	# Last Change:	2021 Dec 30
      4	# Maintainer:	Bram Moolenaar <Bram@vim.org>

    About copyright and licensing: Since plugins are very useful and it's hardly
    worth restricting their distribution, please consider making your plugin
    either public domain or use the Vim |license|.  A short note about this near
    the top of the plugin should be sufficient.  Example:

      5	# License:	This file is placed in the public domain.

    NOT LOADING

    It is possible that a user doesn't always want to load this plugin.  Or the
    system administrator has dropped it in the system-wide plugin directory, but a
    user has his own plugin he wants to use.  Then the user must have a chance to
    disable loading this specific plugin.  These lines will make it possible:

      7	if exists("g:loaded_typecorrect")
      8	  finish
      9	endif
     10	g:loaded_typecorrect = 1

    This also avoids that when the script is loaded twice it would pointlessly
    redefine functions and cause trouble for autocommands that are added twice.

    The name is recommended to start with "g:loaded_" and then the file name of
    the plugin, literally.  The "g:" is prepended to make the variable global, so
    that other places can check whether its functionality is available.  Without
    "g:" it would be local to the script.

    Using `finish` stops Vim from reading the rest of the file, it's much quicker
    than using if-endif around the whole file, since Vim would still need to parse
    the commands to find the `endif`.

    MAPPING

    Now let's make the plugin more interesting: We will add a mapping that adds a
    correction for the word under the cursor.  We could just pick a key sequence
    for this mapping, but the user might already use it for something else.  To
    allow the user to define which keys a mapping in a plugin uses, the <Leader>
    item can be used:

     20	  map <unique> <Leader>a  <Plug>TypecorrAdd;

    The "<Plug>TypecorrAdd;" thing will do the work, more about that further on.

    The user can set the "g:mapleader" variable to the key sequence that he wants
    plugin mappings to start with.  Thus if the user has done:

    	g:mapleader = "_"

    the mapping will define "_a".  If the user didn't do this, the default value
    will be used, which is a backslash.  Then a map for "\a" will be defined.

    Note that <unique> is used, this will cause an error message if the mapping
    already happened to exist. |:map-<unique>|

    But what if the user wants to define his own key sequence?  We can allow that
    with this mechanism:

     19	if !hasmapto('<Plug>TypecorrAdd;')
     20	  map <unique> <Leader>a  <Plug>TypecorrAdd;
     21	endif

    This checks if a mapping to "<Plug>TypecorrAdd;" already exists, and only
    defines the mapping from "<Leader>a" if it doesn't.  The user then has a
    chance of putting this in his vimrc file:

    	map ,c  <Plug>TypecorrAdd;

    Then the mapped key sequence will be ",c" instead of "_a" or "\a".

    PIECES

    If a script gets longer, you often want to break up the work in pieces.  You
    can use functions or mappings for this.  But you don't want these functions
    and mappings to interfere with the ones from other scripts.  For example, you
    could define a function Add(), but another script could try to define the same
    function.  To avoid this, we define the function local to the script.
    Fortunately, in |Vim9| script this is the default.  In a legacy script you
    would need to prefix the name with "s:".

    We will define a function that adds a new typing correction:

     28	def Add(from: string, correct: bool)
     29	  var to = input($"type the correction for {from}: ")
     30	  exe $":iabbrev {from} {to}"
     ...
     34	enddef

    Now we can call the function Add() from within this script.  If another
    script also defines Add(), it will be local to that script and can only
    be called from that script.  There can also be a global g:Add() function,
    which is again another function.

    <SID> can be used with mappings.  It generates a script ID, which identifies
    the current script.  In our typing correction plugin we use it like this:

     22	noremap <unique> <script> <Plug>TypecorrAdd;  <SID>Add
     ...
     26	noremap <SID>Add  :call <SID>Add(expand("<cword>"), true)<CR>

    Thus when a user types "\a", this sequence is invoked:

    	\a  ->  <Plug>TypecorrAdd;  ->  <SID>Add  ->  :call <SID>Add(...)

    If another script also maps <SID>Add, it will get another script ID and
    thus define another mapping.

    Note that instead of Add() we use <SID>Add() here.  That is because the
    mapping is typed by the user, thus outside of the script context.  The <SID>
    is translated to the script ID, so that Vim knows in which script to look for
    the Add() function.

    This is a bit complicated, but it's required for the plugin to work together
    with other plugins.  The basic rule is that you use <SID>Add() in mappings and
    Add() in other places (the script itself, autocommands, user commands).

    We can also add a menu entry to do the same as the mapping:

     24	noremenu <script> Plugin.Add\ Correction      <SID>Add

    The "Plugin" menu is recommended for adding menu items for plugins.  In this
    case only one item is used.  When adding more items, creating a submenu is
    recommended.  For example, "Plugin.CVS" could be used for a plugin that offers
    CVS operations "Plugin.CVS.checkin", "Plugin.CVS.checkout", etc.

    Note that in line 28 ":noremap" is used to avoid that any other mappings cause
    trouble.  Someone may have remapped ":call", for example.  In line 24 we also
    use ":noremap", but we do want "<SID>Add" to be remapped.  This is why
    "<script>" is used here.  This only allows mappings which are local to the
    script. |:map-<script>|  The same is done in line 26 for ":noremenu".
    |:menu-<script>|

    <SID> AND <Plug>					*using-<Plug>*

    Both <SID> and <Plug> are used to avoid that mappings of typed keys interfere
    with mappings that are only to be used from other mappings.  Note the
    difference between using <SID> and <Plug>:

    <Plug>	is visible outside of the script.  It is used for mappings which the
    	user might want to map a key sequence to.  <Plug> is a special code
    	that a typed key will never produce.
    	To make it very unlikely that other plugins use the same sequence of
    	characters, use this structure: <Plug> scriptname mapname
    	In our example the scriptname is "Typecorr" and the mapname is "Add".
    	We add a semicolon as the terminator.  This results in
    	"<Plug>TypecorrAdd;".  Only the first character of scriptname and
    	mapname is uppercase, so that we can see where mapname starts.

    <SID>	is the script ID, a unique identifier for a script.
    	Internally Vim translates <SID> to "<SNR>123_", where "123" can be any
    	number.  Thus a function "<SID>Add()" will have a name "<SNR>11_Add()"
    	in one script, and "<SNR>22_Add()" in another.  You can see this if
    	you use the ":function" command to get a list of functions.  The
    	translation of <SID> in mappings is exactly the same, that's how you
    	can call a script-local function from a mapping.

    USER COMMAND

    Now let's add a user command to add a correction:

     36	if !exists(":Correct")
     37	  command -nargs=1  Correct  :call Add(<q-args>, false)
     38	endif

    The user command is defined only if no command with the same name already
    exists.  Otherwise we would get an error here.  Overriding the existing user
    command with ":command!" is not a good idea, this would probably make the user
    wonder why the command he defined himself doesn't work.  |:command|
    If it did happen you can find out who to blame with:

    	verbose command Correct

    SCRIPT VARIABLES

    When a variable starts with "s:" it is a script variable.  It can only be used
    inside a script.  Outside the script it's not visible.  This avoids trouble
    with using the same variable name in different scripts.  The variables will be
    kept as long as Vim is running.  And the same variables are used when sourcing
    the same script again. |s:var|

    The nice thing about |Vim9| script is that variables are local to the script
    by default.  You can prepend "s:" if you like, but you do not need to.  And
    functions in the script can also use the script variables without a prefix
    (they must be declared before the function for this to work).

    Script-local variables can also be used in functions, autocommands and user
    commands that are defined in the script.  Thus they are the perfect way to
    share information between parts of your plugin, without it leaking out.  In
    our example we can add a few lines to count the number of corrections:

     17	var count = 4
     ...
     28	def Add(from: string, correct: bool)
     ...
     32	  count += 1
     33	  echo "you now have " .. count .. " corrections"
     34	enddef

    "count" is declared and initialized to 4 in the script itself.  When later
    the Add() function is called, it increments "count".  It doesn't matter from
    where the function was called, since it has been defined in the script, it
    will use the local variables from this script.

    THE RESULT

    Here is the resulting complete example:

      1	vim9script noclear
      2	# Vim global plugin for correcting typing mistakes
      3	# Last Change:	2021 Dec 30
      4	# Maintainer:	Bram Moolenaar <Bram@vim.org>
      5	# License:	This file is placed in the public domain.
      6
      7	if exists("g:loaded_typecorrect")
      8	  finish
      9	endif
     10	g:loaded_typecorrect = 1
     11
     12	iabbrev teh the
     13	iabbrev otehr other
     14	iabbrev wnat want
     15	iabbrev synchronisation
     16		\ synchronization
     17	var count = 4
     18
     19	if !hasmapto('<Plug>TypecorrAdd;')
     20	  map <unique> <Leader>a  <Plug>TypecorrAdd;
     21	endif
     22	noremap <unique> <script> <Plug>TypecorrAdd;  <SID>Add
     23
     24	noremenu <script> Plugin.Add\ Correction      <SID>Add
     25
     26	noremap <SID>Add  :call <SID>Add(expand("<cword>"), true)<CR>
     27
     28	def Add(from: string, correct: bool)
     29	  var to = input("type the correction for " .. from .. ": ")
     30	  exe ":iabbrev " .. from .. " " .. to
     31	  if correct | exe "normal viws\<C-R>\" \b\e" | endif
     32	  count += 1
     33	  echo "you now have " .. count .. " corrections"
     34	enddef
     35
     36	if !exists(":Correct")
     37	  command -nargs=1  Correct  call Add(<q-args>, false)
     38	endif

    Line 31 wasn't explained yet.  It applies the new correction to the word under
    the cursor.  The |:normal| command is used to use the new abbreviation.  Note
    that mappings and abbreviations are expanded here, even though the function
    was called from a mapping defined with ":noremap".

    DOCUMENTATION						*write-local-help*

    It's a good idea to also write some documentation for your plugin.  Especially
    when its behavior can be changed by the user.  See |add-local-help| for how
    they are installed.

    Here is a simple example for a plugin help file, called "typecorrect.txt":

      1	*typecorrect.txt*	Plugin for correcting typing mistakes
      2
      3	If you make typing mistakes, this plugin will have them corrected
      4	automatically.
      5
      6	There are currently only a few corrections.  Add your own if you like.
      7
      8	Mappings:
      9	<Leader>a   or   <Plug>TypecorrAdd;
     10		Add a correction for the word under the cursor.
     11
     12	Commands:
     13	:Correct {word}
     14		Add a correction for {word}.
     15
     16							*typecorrect-settings*
     17	This plugin doesn't have any settings.

    The first line is actually the only one for which the format matters.  It will
    be extracted from the help file to be put in the "LOCAL ADDITIONS:" section of
    help.txt |local-additions|.  The first "*" must be in the first column of the
    first line.  After adding your help file do ":help" and check that the entries
    line up nicely.

    You can add more tags inside ** in your help file.  But be careful not to use
    existing help tags.  You would probably use the name of your plugin in most of
    them, like "typecorrect-settings" in the example.

    Using references to other parts of the help in || is recommended.  This makes
    it easy for the user to find associated help.

    SUMMARY							*plugin-special*

    Summary of special things to use in a plugin:

    var name		Variable local to the script.

    <SID>			Script-ID, used for mappings and functions local to
    			the script.

    hasmapto()		Function to test if the user already defined a mapping
    			for functionality the script offers.

    <Leader>		Value of "mapleader", which the user defines as the
    			keys that plugin mappings start with.

    map <unique>		Give a warning if a mapping already exists.

    noremap <script>	Use only mappings local to the script, not global
    			mappings.

    exists(":Cmd")		Check if a user command already exists.

    ==============================================================================

    *51.2*	Writing a filetype plugin	*write-filetype-plugin* *ftplugin*

    A filetype plugin is like a global plugin, except that it sets options and
    defines mappings for the current buffer only.  See |add-filetype-plugin| for
    how this type of plugin is used.

    First read the section on global plugins above |51.1|.  All that is said there
    also applies to filetype plugins.  There are a few extras, which are explained
    here.  The essential thing is that a filetype plugin should only have an
    effect on the current buffer.

    DISABLING

    If you are writing a filetype plugin to be used by many people, they need a
    chance to disable loading it.  Put this at the top of the plugin:

    	# Only do this when not done yet for this buffer
    	if exists("b:did_ftplugin")
    	  finish
    	endif
    	b:did_ftplugin = 1

    This also needs to be used to avoid that the same plugin is executed twice for
    the same buffer (happens when using an ":edit" command without arguments).

    Now users can disable loading the default plugin completely by making a
    filetype plugin with only these lines:

    	vim9script
    	b:did_ftplugin = 1

    This does require that the filetype plugin directory comes before $VIMRUNTIME
    in 'runtimepath'!

    If you do want to use the default plugin, but overrule one of the settings,
    you can write the different setting in a script:

    	setlocal textwidth=70

    Now write this in the "after" directory, so that it gets sourced after the
    distributed "vim.vim" ftplugin |after-directory|.  For Unix this would be
    "~/.vim/after/ftplugin/vim.vim".  Note that the default plugin will have set
    "b:did_ftplugin", it is ignored here.

    OPTIONS

    To make sure the filetype plugin only affects the current buffer use the

    	setlocal

    command to set options.  And only set options which are local to a buffer (see
    the help for the option to check that).  When using `:setlocal` for global
    options or options local to a window, the value will change for many buffers,
    and that is not what a filetype plugin should do.

    When an option has a value that is a list of flags or items, consider using
    "+=" and "-=" to keep the existing value.  Be aware that the user may have
    changed an option value already.  First resetting to the default value and
    then changing it is often a good idea.  Example:

    	setlocal formatoptions& formatoptions+=ro

    MAPPINGS

    To make sure mappings will only work in the current buffer use the

    	map <buffer>

    command.  This needs to be combined with the two-step mapping explained above.
    An example of how to define functionality in a filetype plugin:

    	if !hasmapto('<Plug>JavaImport;')
    	  map <buffer> <unique> <LocalLeader>i <Plug>JavaImport;
    	endif
    	noremap <buffer> <unique> <Plug>JavaImport; oimport ""<Left><Esc>

    |hasmapto()| is used to check if the user has already defined a map to
    <Plug>JavaImport;.  If not, then the filetype plugin defines the default
    mapping.  This starts with |<LocalLeader>|, which allows the user to select
    the key(s) he wants filetype plugin mappings to start with.  The default is a
    backslash.
    "<unique>" is used to give an error message if the mapping already exists or
    overlaps with an existing mapping.
    |:noremap| is used to avoid that any other mappings that the user has defined
    interferes.  You might want to use ":noremap <script>" to allow remapping
    mappings defined in this script that start with <SID>.

    The user must have a chance to disable the mappings in a filetype plugin,
    without disabling everything.  Here is an example of how this is done for a
    plugin for the mail filetype:

    	# Add mappings, unless the user didn't want this.
    	if !exists("g:no_plugin_maps") && !exists("g:no_mail_maps")
    	  # Quote text by inserting "> "
    	  if !hasmapto('<Plug>MailQuote;')
    	    vmap <buffer> <LocalLeader>q <Plug>MailQuote;
    	    nmap <buffer> <LocalLeader>q <Plug>MailQuote;
    	  endif
    	  vnoremap <buffer> <Plug>MailQuote; :s/^/> /<CR>
    	  nnoremap <buffer> <Plug>MailQuote; :.,$s/^/> /<CR>
    	endif

    Two global variables are used:
    |g:no_plugin_maps|	disables mappings for all filetype plugins
    |g:no_mail_maps|	disables mappings for the "mail" filetype

    USER COMMANDS

    To add a user command for a specific file type, so that it can only be used in
    one buffer, use the "-buffer" argument to |:command|.  Example:

    	command -buffer  Make  make %:r.s

    VARIABLES

    A filetype plugin will be sourced for each buffer of the type it's for.  Local
    script variables will be shared between all invocations.  Use local buffer
    variables |b:var| if you want a variable specifically for one buffer.

    FUNCTIONS

    When defining a function, this only needs to be done once.  But the filetype
    plugin will be sourced every time a file with this filetype will be opened.
    This construct makes sure the function is only defined once:

    	if !exists("*Func")
    	  def Func(arg)
    	    ...
    	  enddef
    	endif

    Don't forget to use "noclear" with the `vim9script` command to avoid that the
    function is deleted when the script is sourced a second time.

    UNDO						*undo_indent* *undo_ftplugin*

    When the user does ":setfiletype xyz" the effect of the previous filetype
    should be undone.  Set the b:undo_ftplugin variable to the commands that will
    undo the settings in your filetype plugin.  Example:

    	b:undo_ftplugin = "setlocal fo< com< tw< commentstring<"
    		\ .. "| unlet b:match_ignorecase b:match_words b:match_skip"

    Using ":setlocal" with "<" after the option name resets the option to its
    global value.  That is mostly the best way to reset the option value.

    For undoing the effect of an indent script, the b:undo_indent variable should
    be set accordingly.

    Both these variables use legacy script syntax, not |Vim9| syntax.

    FILE NAME

    The filetype must be included in the file name |ftplugin-name|.  Use one of
    these three forms:

    	.../ftplugin/stuff.vim
    	.../ftplugin/stuff_foo.vim
    	.../ftplugin/stuff/bar.vim

    "stuff" is the filetype, "foo" and "bar" are arbitrary names.

    FILETYPE DETECTION					*plugin-filetype*

    If your filetype is not already detected by Vim, you should create a filetype
    detection snippet in a separate file.  It is usually in the form of an
    autocommand that sets the filetype when the file name matches a pattern.
    Example:

    	au BufNewFile,BufRead *.foo		setlocal filetype=foofoo

    Write this single-line file as "ftdetect/foofoo.vim" in the first directory
    that appears in 'runtimepath'.  For Unix that would be
    "~/.vim/ftdetect/foofoo.vim".  The convention is to use the name of the
    filetype for the script name.

    You can make more complicated checks if you like, for example to inspect the
    contents of the file to recognize the language.  Also see |new-filetype|.

    SUMMARY							*ftplugin-special*

    Summary of special things to use in a filetype plugin:

    <LocalLeader>		Value of "maplocalleader", which the user defines as
    			the keys that filetype plugin mappings start with.

    map <buffer>		Define a mapping local to the buffer.

    noremap <script>	Only remap mappings defined in this script that start
    			with <SID>.

    setlocal		Set an option for the current buffer only.

    command -buffer		Define a user command local to the buffer.

    exists("*s:Func")	Check if a function was already defined.

    Also see |plugin-special|, the special things used for all plugins.

    ==============================================================================

    *51.3*	Writing a compiler plugin		*write-compiler-plugin*

    A compiler plugin sets options for use with a specific compiler.  The user can
    load it with the |:compiler| command.  The main use is to set the
    'errorformat' and 'makeprg' options.

    Easiest is to have a look at examples.  This command will edit all the default
    compiler plugins:

    	next $VIMRUNTIME/compiler/*.vim

    Type `:next` to go to the next plugin file.

    There are two special items about these files.  First is a mechanism to allow
    a user to overrule or add to the default file.  The default files start with:

    	vim9script
    	if exists("g:current_compiler")
    	  finish
    	endif
    	g:current_compiler = "mine"

    When you write a compiler file and put it in your personal runtime directory
    (e.g., ~/.vim/compiler for Unix), you set the "current_compiler" variable to
    make the default file skip the settings.

    							*:CompilerSet*
    The second mechanism is to use ":set" for ":compiler!" and ":setlocal" for
    ":compiler".  Vim defines the ":CompilerSet" user command for this.  However,
    older Vim versions don't, thus your plugin should define it then.  This is an
    example:

      if exists(":CompilerSet") != 2
        command -nargs=* CompilerSet setlocal <args>
      endif
      CompilerSet errorformat&		" use the default 'errorformat'
      CompilerSet makeprg=nmake

    When you write a compiler plugin for the Vim distribution or for a system-wide
    runtime directory, use the mechanism mentioned above.  When
    "current_compiler" was already set by a user plugin nothing will be done.

    When you write a compiler plugin to overrule settings from a default plugin,
    don't check "current_compiler".  This plugin is supposed to be loaded
    last, thus it should be in a directory at the end of 'runtimepath'.  For Unix
    that could be ~/.vim/after/compiler.

    ==============================================================================

    *51.4*	Distributing Vim scripts			*distribute-script*

     Vim users will look for scripts on the Vim website:	http://www.vim.org.
    If you made something that is useful for others, share it!

    Another place is github.  But there you need to know where to find it!  The
    advantage is that most plugin managers fetch plugins from github.  You'll have
    to use your favorite search engine to find them.

    Vim scripts can be used on any system.  However, there might not be a tar or
    gzip command.  If you want to pack files together and/or compress them the
    "zip" utility is recommended.

    For utmost portability use Vim itself to pack scripts together.  This can be
    done with the Vimball utility.  See |vimball|.

    It's good if you add a line to allow automatic updating.  See |glvs-plugins|.

    ==============================================================================

    Next chapter: |usr_52.txt|  Write large plugins

    Copyright: see |manual-copyright|  vim:tw=78:ts=8:noet:ft=help:norl:
    top - main help file