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+*usr_51.txt* For Vim version 8.2. Last change: 2022 May 13
+
+ 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| Writing a plugin that loads quickly
+|51.5| Writing library scripts
+|51.6| Distributing Vim scripts
+
+ Next chapter: |usr_52.txt| Write plugins using Vim9 script
+ 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|.
+
+
+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: >
+
+ 14 iabbrev teh the
+ 15 iabbrev otehr other
+ 16 iabbrev wnat want
+ 17 iabbrev synchronisation
+ 18 \ 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 `vimscript` 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.
+
+
+LINE CONTINUATION AND AVOIDING SIDE EFFECTS *use-cpo-save*
+
+In line 18 above, the line-continuation mechanism is used |line-continuation|.
+Users with 'compatible' set will run into trouble here, they will get an error
+message. We can't just reset 'compatible', because that has a lot of side
+effects. Instead, we will set the 'cpoptions' option to its Vim default
+value and restore it later. That will allow the use of line-continuation and
+make the script work for most people. It is done like this: >
+
+ 11 var save_cpo = &cpo
+ 12 set cpo&vim
+ ..
+ 42 &cpo = save_cpo
+
+We first store the old value of 'cpoptions' in the "save_cpo" variable. At
+the end of the plugin this value is restored.
+
+Notice that "save_cpo" is a script-local variable. A global variable could
+already be in use for something else. Always use script-local variables for
+things that are only used in the script.
+
+
+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: >
+
+ 22 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: >
+
+ 21 if !hasmapto('<Plug>TypecorrAdd;')
+ 22 map <unique> <Leader>a <Plug>TypecorrAdd;
+ 23 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: >
+
+ 30 def Add(from: string, correct: bool)
+ 31 var to = input("type the correction for " .. from .. ": ")
+ 32 exe ":iabbrev " .. from .. " " .. to
+ ..
+ 36 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: >
+
+ 24 noremap <unique> <script> <Plug>TypecorrAdd; <SID>Add
+ ..
+ 28 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: >
+
+ 26 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: >
+
+ 38 if !exists(":Correct")
+ 39 command -nargs=1 Correct :call Add(<q-args>, false)
+ 40 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.
+
+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: >
+
+ 19 var count = 4
+ ..
+ 30 def Add(from: string, correct: bool)
+ ..
+ 34 count += 1
+ 35 echo "you now have " .. count .. " corrections"
+ 36 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 var save_cpo = &cpo
+ 12 set cpo&vim
+ 13
+ 14 iabbrev teh the
+ 15 iabbrev otehr other
+ 16 iabbrev wnat want
+ 17 iabbrev synchronisation
+ 18 \ synchronization
+ 19 var count = 4
+ 20
+ 21 if !hasmapto('<Plug>TypecorrAdd;')
+ 22 map <unique> <Leader>a <Plug>TypecorrAdd;
+ 23 endif
+ 24 noremap <unique> <script> <Plug>TypecorrAdd; <SID>Add
+ 25
+ 26 noremenu <script> Plugin.Add\ Correction <SID>Add
+ 27
+ 28 noremap <SID>Add :call <SID>Add(expand("<cword>"), true)<CR>
+ 29
+ 30 def Add(from: string, correct: bool)
+ 31 var to = input("type the correction for " .. from .. ": ")
+ 32 exe ":iabbrev " .. from .. " " .. to
+ 33 if correct | exe "normal viws\<C-R>\" \b\e" | endif
+ 34 count += 1
+ 35 echo "you now have " .. count .. " corrections"
+ 36 enddef
+ 37
+ 38 if !exists(":Correct")
+ 39 command -nargs=1 Correct call Add(<q-args>, false)
+ 40 endif
+ 41
+ 42 &cpo = save_cpo
+
+Line 33 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.
+
+
+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 *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", but 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
+<
+
+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: >
+
+ let 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.
+
+This does require removing the "C" flag from 'cpoptions' to allow line
+continuation, as mentioned above |use-cpo-save|.
+
+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.
+
+
+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* Writing a plugin that loads quickly *write-plugin-quickload*
+
+A plugin may grow and become quite long. The startup delay may become
+noticeable, while you hardly ever use the plugin. Then it's time for a
+quickload plugin.
+
+The basic idea is that the plugin is loaded twice. The first time user
+commands and mappings are defined that offer the functionality. The second
+time the functions that implement the functionality are defined.
+
+It may sound surprising that quickload means loading a script twice. What we
+mean is that it loads quickly the first time, postponing the bulk of the
+script to the second time, which only happens when you actually use it. When
+you always use the functionality it actually gets slower!
+
+This uses a FuncUndefined autocommand. Since Vim 7 there is an alternative:
+use the |autoload| functionality |51.5|. That will also use |Vim9| script
+instead of legacy script that is used here.
+
+The following example shows how it's done: >
+
+ " Vim global plugin for demonstrating quick loading
+ " Last Change: 2005 Feb 25
+ " Maintainer: Bram Moolenaar <Bram@vim.org>
+ " License: This file is placed in the public domain.
+
+ if !exists("s:did_load")
+ command -nargs=* BNRead call BufNetRead(<f-args>)
+ map <F19> :call BufNetWrite('something')<CR>
+
+ let s:did_load = 1
+ exe 'au FuncUndefined BufNet* source ' .. expand('<sfile>')
+ finish
+ endif
+
+ function BufNetRead(...)
+ echo 'BufNetRead(' .. string(a:000) .. ')'
+ " read functionality here
+ endfunction
+
+ function BufNetWrite(...)
+ echo 'BufNetWrite(' .. string(a:000) .. ')'
+ " write functionality here
+ endfunction
+
+When the script is first loaded "s:did_load" is not set. The commands between
+the "if" and "endif" will be executed. This ends in a |:finish| command, thus
+the rest of the script is not executed.
+
+The second time the script is loaded "s:did_load" exists and the commands
+after the "endif" are executed. This defines the (possible long)
+BufNetRead() and BufNetWrite() functions.
+
+If you drop this script in your plugin directory Vim will execute it on
+startup. This is the sequence of events that happens:
+
+1. The "BNRead" command is defined and the <F19> key is mapped when the script
+ is sourced at startup. A |FuncUndefined| autocommand is defined. The
+ ":finish" command causes the script to terminate early.
+
+2. The user types the BNRead command or presses the <F19> key. The
+ BufNetRead() or BufNetWrite() function will be called.
+
+3. Vim can't find the function and triggers the |FuncUndefined| autocommand
+ event. Since the pattern "BufNet*" matches the invoked function, the
+ command "source fname" will be executed. "fname" will be equal to the name
+ of the script, no matter where it is located, because it comes from
+ expanding "<sfile>" (see |expand()|).
+
+4. The script is sourced again, the "s:did_load" variable exists and the
+ functions are defined.
+
+Notice that the functions that are loaded afterwards match the pattern in the
+|FuncUndefined| autocommand. You must make sure that no other plugin defines
+functions that match this pattern.
+
+==============================================================================
+*51.5* Writing library scripts *write-library-script*
+
+Some functionality will be required in several places. When this becomes more
+than a few lines you will want to put it in one script and use it from many
+scripts. We will call that one script a library script.
+
+Manually loading a library script is possible, so long as you avoid loading it
+when it's already done. You can do this with the |exists()| function.
+Example: >
+
+ if !exists('*MyLibFunction')
+ runtime library/mylibscript.vim
+ endif
+ MyLibFunction(arg)
+
+Here you need to know that MyLibFunction() is defined in a script
+"library/mylibscript.vim" in one of the directories in 'runtimepath'.
+
+To make this a bit simpler Vim offers the autoload mechanism. Then the
+example looks like this: >
+
+ mylib#myfunction(arg)
+
+That's a lot simpler, isn't it? Vim will recognize the function name by the
+embedded "#" character and when it's not defined search for the script
+"autoload/mylib.vim" in 'runtimepath'. That script must define the
+"mylib#myfunction()" function.
+
+You can put many other functions in the mylib.vim script, you are free to
+organize your functions in library scripts. But you must use function names
+where the part before the '#' matches the script name. Otherwise Vim would
+not know what script to load.
+
+If you get really enthusiastic and write lots of library scripts, you may
+want to use subdirectories. Example: >
+
+ netlib#ftp#read('somefile')
+
+For Unix the library script used for this could be:
+
+ ~/.vim/autoload/netlib/ftp.vim
+
+Where the function is defined like this: >
+
+ def netlib#ftp#read(fname: string)
+ # Read the file fname through ftp
+ enddef
+
+Notice that the name the function is defined with is exactly the same as the
+name used for calling the function. And the part before the last '#'
+exactly matches the subdirectory and script name.
+
+You can use the same mechanism for variables: >
+
+ var weekdays = dutch#weekdays
+
+This will load the script "autoload/dutch.vim", which should contain something
+like: >
+
+ var dutch#weekdays = ['zondag', 'maandag', 'dinsdag', 'woensdag',
+ \ 'donderdag', 'vrijdag', 'zaterdag']
+
+Further reading: |autoload|.
+
+==============================================================================
+*51.6* 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 plugins using Vim9 script
+
+Copyright: see |manual-copyright| vim:tw=78:ts=8:noet:ft=help:norl: