Vim documentation: develop

main help file
*develop.txt*          Nvim


                            NVIM REFERENCE MANUAL



Development of Nvim.					*development*

Nvim is open source software.  Everybody is encouraged to contribute.
    https://github.com/neovim/neovim/blob/master/CONTRIBUTING.md

See src/nvim/README.md for an overview of the source code.

                                      Type |gO| to see the table of contents.

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

Design goals						*design-goals*

Most important things come first (roughly).

Note that some items conflict; this is intentional.  A balance must be found.



NVIM IS... IMPROVED					*design-improved*

The IMproved bits of Vim should make it a better Vi, without becoming a
completely different editor.  Extensions are done with a "Vi spirit".
- Use the keyboard as much as feasible.  The mouse requires a third hand,
  which we don't have.  Many terminals don't have a mouse.
- When the mouse is used anyway, avoid the need to switch back to the
  keyboard.  Avoid mixing mouse and keyboard handling.
- Add commands and options in a consistent way.  Otherwise people will have a
  hard time finding and remembering them.  Keep in mind that more commands and
  options will be added later.
- A feature that people do not know about is a useless feature.  Don't add
  obscure features, or at least add hints in documentation that they exist.
- Minimize using CTRL and other modifiers, they are more difficult to type.
- There are many first-time and inexperienced Vim users.  Make it easy for
  them to start using Vim and learn more over time.
- There is no limit to the features that can be added.  Selecting new features
  is based on (1) what users ask for, (2) how much effort it takes to
  implement and (3) someone actually implementing it.



NVIM IS... MULTI PLATFORM				*design-multi-platform*

Vim tries to help as many users on as many platforms as possible.
- Support many kinds of terminals.  The minimal demands are cursor positioning
  and clear-screen.  Commands should only use key strokes that most keyboards
  have.  Support all the keys on the keyboard for mapping.
- Support many platforms.  A condition is that there is someone willing to do
  Vim development on that platform, and it doesn't mean messing up the code.
- Support many compilers and libraries.  Not everybody is able or allowed to
  install another compiler or GUI library.
- People switch from one platform to another, and from GUI to terminal
  version.  Features should be present in all versions.



NVIM IS... WELL DOCUMENTED				*design-documented*

- A feature that isn't documented is a useless feature.  A patch for a new
  feature must include the documentation.
- Documentation should be comprehensive and understandable.  Use examples.
- Don't make the text unnecessarily long.  Less documentation means that an
  item is easier to find.



NVIM IS... HIGH SPEED AND SMALL IN SIZE			*design-speed-size*

Keep Nvim small and fast.
- Computers are becoming faster and bigger each year.  Vim can grow too, but
  no faster than computers are growing.  Keep Vim usable on older systems.
- Many users start Vim from a shell very often.  Startup time must be short.
- Commands must work efficiently.  The time they consume must be as small as
  possible.  Useful commands may take longer.
- Don't forget that some people use Vim over a slow connection.  Minimize the
  communication overhead.
- Vim is a component among other components.  Don't turn it into a massive
  application, but have it work well together with other programs.



NVIM IS... MAINTAINABLE					*design-maintain*

- The source code should not become a mess.  It should be reliable code.
- Use comments in a useful way!  Quoting the function name and argument names
  is NOT useful.  Do explain what they are for.
- Porting to another platform should be made easy, without having to change
  too much platform-independent code.
- Use the object-oriented spirit: Put data and code together.  Minimize the
  knowledge spread to other parts of the code.



NVIM IS... FLEXIBLE					*design-flexible*

Vim should make it easy for users to work in their preferred styles rather
than coercing its users into particular patterns of work.  This can be for
items with a large impact or for details.  The defaults are carefully chosen
such that most users will enjoy using Vim as it is.  Commands and options can
be used to adjust Vim to the desire of the user and its environment.



NVIM IS... NOT						*design-not*

Nvim is not an operating system; instead it should be composed with other
tools or hosted as a component. Marvim once said: "Unlike Emacs, Nvim does not
include the kitchen sink... but it's good for plumbing."


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

Developer guidelines				        *dev*



JARGON	 						*dev-jargon*

API client 
All external UIs and remote plugins (as opposed to regular Vim plugins) are
"clients" in general; but we call something an "API client" if its purpose is
to abstract or wrap the RPC API for the convenience of other applications
(just like a REST client or SDK such as boto3 for AWS: you can speak AWS REST
using an HTTP client like curl, but boto3 wraps that in a convenient python
interface). For example, the Nvim lua-client is an API client:
    https://github.com/neovim/lua-client

Host 
A plugin "host" is both a client (of the Nvim API) and a server (of an
external platform, e.g. python). It is a remote plugin that hosts other
plugins.

Remote plugin 
Arbitrary code registered via |:UpdateRemotePlugins|, that runs in a separate
process and communicates with Nvim via the |api|.

Window 
The word "window" is commonly used for several things: A window on the screen,
the xterm window, a window inside Vim to view a buffer.
To avoid confusion, other items that are sometimes called window have been
given another name.  Here is an overview of the related items:

screen		The whole display.
shell		The Vim application.  This can cover the whole screen (e.g.,
		when running in a console) or part of it (xterm or GUI).
window		View on a buffer.  There can be several windows in Vim,
		together with the command line, menubar, toolbar, etc. they
		fit in the shell.
frame		Windows are kept in a tree of frames.  Each frame contains
		a column, row, or window ("leaf" frame).


PROVIDERS 						*dev-provider*

A goal of Nvim is to allow extension of the editor without special knowledge
in the core. But some Vim components are too tightly coupled; in those cases
a "provider" hook is exposed.

Consider two examples of integration with external systems that are
implemented in Vim and are now decoupled from Nvim core as providers:

1. In the Vim source code, clipboard logic accounts for more than 1k lines of
   C source code (ui.c), to perform two tasks that are now accomplished with
   shell commands such as xclip or pbcopy/pbpaste.

2. Python scripting support: Vim has three files dedicated to embedding the
   Python interpreter: if_python.c, if_python3.c and if_py_both.h. Together
   these files sum about 9.5k lines of C source code. In contrast, Nvim Python
   scripting is performed by an external host process implemented in ~2k lines
   of Python.

Ideally we could implement Python and clipboard integration in pure vimscript
and without touching the C code. But this is infeasible without compromising
backwards compatibility with Vim; that's where providers help.

The provider framework helps call vimscript from C. It is composed of two
functions in eval.c:

- eval_call_provider(name, method, arguments): calls provider#(name)#Call
  with the method and arguments.
- eval_has_provider(name): Checks if a provider is implemented. Returns true
  if the provider#(name)#Call function is implemented. Called by |has()|
  (vimscript) to check if features are available.

The provider#(name)#Call function implements integration with an external
system, because shell commands and |RPC| clients are easier to work with in
vimscript.

For example, the Python provider is implemented by the
autoload/provider/python.vim script; the provider#python#Call function is only
defined if a valid external Python host is found. That works well with the
`has('python')` expression (normally used by Python plugins) because if the
Python host isn't installed then the plugin will "think" it is running in
a Vim compiled without the |+python| feature.


DOCUMENTATION						*dev-doc*

- Do not prefix help tags with "nvim-". Use |vim_diff.txt| to document
  differences from Vim; no other distinction is necessary.
- If a Vim feature is removed, delete its help section and move its tag to
  |vim_diff.txt|.
- Move deprecated features to |deprecated.txt|.
- Use consistent language.
    - "terminal" in a help tag always means "the embedded terminal emulator", not
      "the user host terminal".
    - Use "tui-" to prefix help tags related to the host terminal, and "TUI"
      in prose if possible.


API							*dev-api*

Use this pattern to name new API functions:
    nvim_{thing}_{action}_{arbitrary-qualifiers}

If the function acts on an object then {thing} is the name of that object
(e.g. "buf" or "win"). If the function operates in a "global" context then
{thing} is usually omitted (but consider "namespacing" your global operations
with a {thing} that groups functions under a common concept).

Use existing common {action} names if possible:
    add   Append to, or insert into, a collection
    get   Get a thing (or subset of things by some query)
    set   Set a thing
    del   Delete a thing (or group of things)
    list  Get all things

Use consistent names for {thing} in all API functions. E.g. a buffer is called
"buf" everywhere, not "buffer" in some places and "buf" in others.

Example: `nvim_get_current_line` acts on the global editor state; the common
{action} "get" is used but {thing} is omitted.

Example: `nvim_buf_add_highlight` acts on a `Buffer` object (the first
parameter) and uses the common {action} "add".

Example: `nvim_list_bufs` operates in a global context (first parameter is
_not_ a Buffer). The common {action} "list" indicates that it lists all
bufs (plural) in the global context.



API-CLIENT						*dev-api-client*

Package Naming 
API client packages should NOT be named something ambiguous like "neovim" or
"python-client".  Use "nvim" as a prefix/suffix to some other identifier
following ecosystem conventions.

For example, Python packages tend to have "py" in the name, so "pynvim" is
a good name: it's idiomatic and unambiguous. If the package is named "neovim",
it confuses users, and complicates documentation and discussions.

Examples of API-client package names:
        GOOD: nvim-racket
        GOOD: pynvim
        BAD:  python-client
        BAD:  neovim

Implementation 
Consider using libmpack instead of the msgpack.org C/C++ library. libmpack is
small, efficient, and C89-compatible. It can be easily inlined in your
C project source, too.  https://github.com/libmpack/libmpack/



EXTERNAL UI 						*dev-ui*

Compatibility 
External UIs should be aware of the |api-contract|. In particular, future
versions of Nvim may add new items to existing events. The API is strongly
backwards-compatible, but clients must not break if new fields are added to
existing events.

Common Features 
External UIs are expected to implement these common features:
- Cursor style (shape, color) should respond to the 'guicursor' properties
  delivered with the mode_info_set UI event.
- Send the "super" key (Windows key, Apple key) as a |<D-| chord.

Implementation 
- UI-related options ('guifont', 'ambiwidth', …) are published in the
  "option_set" |ui-global| event.  The event is triggered when the UI first
  connects to Nvim and whenever an option is changed by the user or a plugin.


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