neovim/runtime/doc/develop.txt

*develop.txt*          Nvim


                            NVIM REFERENCE MANUAL


Development of Nvim					*development* *dev*

This reference describes design constraints and guidelines, for developing
Nvim applications or Nvim itself.
Architecture and internal concepts are covered in src/nvim/README.md

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

                                      Type |gO| to see the table of contents.

==============================================================================
Design goals						*design-goals*

Most important things come first (roughly).  Some items conflict; this is
intentional.  A balance must be found.


NVIM IS... IMPROVED					*design-improved*

The Neo bits of Nvim should make it a better Vim, without becoming a
completely different editor.
- In matters of taste, prefer Vim/Unix tradition. If there is no relevant
  Vim/Unix tradition, consider the "common case".
- 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.
- 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.
- Backwards compatibility is a feature.  The RPC API in particular should
  never break.


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... 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-guidelines*


PROVIDERS 						*dev-provider*

A primary goal of Nvim is to allow extension of the editor without special
knowledge in the core.  Some core functions are delegated to "providers"
implemented as external scripts.

Examples:

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.

The provider framework invokes VimL from C.  It is composed of two functions
in eval.c:

- eval_call_provider(name, method, arguments, discard): calls
  provider#{name}#Call with the method and arguments. If discard is true, any
  value returned by the provider will be discarded and and empty value be
  returned.
- eval_has_provider(name): Checks the `g:loaded_{name}_provider` variable
  which must be set to 2 by the provider script to indicate that it is
  "enabled and working". Called by |has()| to check if features are available.

For example, the Python provider is implemented by the
"autoload/provider/python.vim" script, which sets `g:loaded_python_provider`
to 2 only if a valid external Python host is found.  Then `has("python")`
reflects whether Python support is working.

							*provider-reload*
Sometimes a GUI or other application may want to force a provider to
"reload".  To reload a provider, undefine its "loaded" flag, then use
|:runtime| to reload it: >

    :unlet g:loaded_clipboard_provider
    :runtime autoload/provider/clipboard.vim


DOCUMENTATION						*dev-doc*

- "Just say it". Avoid mushy, colloquial phrasing in all documentation
  (docstrings, user manual, website materials, newsletters, …). Don't mince
  words. Personality and flavor, used sparingly, are welcome--but in general,
  optimize for the reader's time and energy: be "precise yet concise".
    - Prefer the active voice: "Foo does X", not "X is done by Foo".
- Vim differences:
    - Do not prefix help tags with "nvim-". Use |vim_diff.txt| to catalog
      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|.
- Mention deprecated features in |deprecated.txt| and delete their old doc.
- 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.
- Docstrings: do not start parameter descriptions with "The" or "A" unless it
  is critical to avoid ambiguity.
      GOOD: >
      /// @param dirname Path fragment before `pend`
<      BAD: >
      /// @param dirname The path fragment before `pend`
<

C docstrings ~

Nvim API documentation lives in the source code, as docstrings (Doxygen
comments) on the function definitions.  The |api| :help is generated
from the docstrings defined in src/nvim/api/*.c.

Docstring format:
- Lines start with `///`
- Special tokens start with `@` followed by the token name:
  `@note`, `@param`, `@returns`
- Limited markdown is supported.
  - List-items start with `-` (useful to nest or "indent")
- Use `<pre>`  for code samples.

Example: the help for |nvim_open_win()| is generated from a docstring defined
in src/nvim/api/vim.c like this: >

    /// Opens a new window.
    /// ...
    ///
    /// Example (Lua): window-relative float
    /// <pre>
    ///     vim.api.nvim_open_win(0, false,
    ///       {relative='win', row=3, col=3, width=12, height=3})
    /// </pre>
    ///
    /// @param buffer Buffer to display
    /// @param enter  Enter the window
    /// @param config Map defining the window configuration. Keys:
    ///   - relative: Sets the window layout, relative to:
    ///      - "editor" The global editor grid.
    ///      - "win"    Window given by the `win` field.
    ///      - "cursor" Cursor position in current window.
    /// ...
    /// @param[out] err Error details, if any
    ///
    /// @return Window handle, or 0 on error


Lua docstrings ~
							*dev-lua-doc*
Lua documentation lives in the source code, as docstrings on the function
definitions.  The |lua-vim| :help is generated from the docstrings.

Docstring format:
- Lines in the main description start with `---`
- Special tokens start with `--@` followed by the token name:
  `--@see`, `--@param`, `--@returns`
- Limited markdown is supported.
  - List-items start with `-` (useful to nest or "indent")
- Use `<pre>`  for code samples.

Example: the help for |vim.paste()| is generated from a docstring decorating
vim.paste in src/nvim/lua/vim.lua like this: >

    --- Paste handler, invoked by |nvim_paste()| when a conforming UI
    --- (such as the |TUI|) pastes text into the editor.
    ---
    --- Example: To remove ANSI color codes when pasting:
    --- <pre>
    --- vim.paste = (function()
    ---   local overridden = vim.paste
    ---   ...
    --- end)()
    --- </pre>
    ---
    --@see |paste|
    ---
    --@param lines  ...
    --@param phase  ...
    --@returns false if client should cancel the paste.


LUA							*dev-lua*

- Keep the core Lua modules |lua-stdlib| simple. Avoid elaborate OOP or
  pseudo-OOP designs. Plugin authors just want functions to call, they don't
  want to learn a big, fancy inheritance hierarchy. Thus avoid specialized
  objects; tables or values are usually better.


API							*dev-api*

Use this template to name new RPC |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
    del   Delete a thing (or group of things)
    exec  Execute code
    get   Get a thing (or group of things by query)
    list  Get all things
    set   Set a thing (or group of 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.

Use this template to name new API events:
    nvim_{thing}_{event}_event

Example:
    `nvim_buf_changedtick_event`


API-CLIENT						*dev-api-client*

							*api-client*
API clients wrap the Nvim |API| to provide idiomatic "SDKs" for their
respective platforms (see |jargon|). You can build a new API client for your
favorite platform or programming language.

List of API clients:
    https://github.com/neovim/neovim/wiki/Related-projects#api-clients

							*pynvim*
The Python client is the reference implementation for API clients.
    https://github.com/neovim/pynvim

Standard Features ~

- API clients exist to hide msgpack-rpc details. The wrappers can be
  automatically generated by reading the |api-metadata| from Nvim. |api-mapping|
- Clients should call |nvim_set_client_info()| after connecting, so users and
  plugins can detect the client by handling the |ChanInfo| event. This avoids
  the need for special variables or other client hints.
- Clients should handle |nvim_error_event| notifications, which will be sent
  if an async request to nvim was rejected or caused an error.

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

API client implementation guidelines ~

- Separate the transport layer from the rest of the library. |rpc-connecting|
- Use a MessagePack library that implements at least version 5 of the
  MessagePack spec, which supports the BIN and EXT types used by Nvim.
- Use a single-threaded event loop library/pattern.
- Use a fiber/coroutine library for the language being used for implementing
  a client. These greatly simplify concurrency and allow the library to
  expose a blocking API on top of a non-blocking event loop without the
  complexity that comes with preemptive multitasking.
- Don't assume anything about the order of responses to RPC requests.
- Clients should expect requests, which must be handled immediately because
  Nvim is blocked while waiting for the client response.
- Clients should expect notifications, but these can be handled "ASAP" (rather
  than immediately) because they won't block Nvim.
- For C/C++ projects, consider libmpack instead of the msgpack.org library.
    https://github.com/libmpack/libmpack/
  libmpack is small (no dependencies, can inline into your C/C++ project) and
  efficient (no allocations). It also implements msgpack-RPC, the protocol
  required by Nvim.
    https://github.com/msgpack-rpc/msgpack-rpc


EXTERNAL UI 						*dev-ui*

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 (optional) fields are
added to existing events.

Standard Features ~

External UIs are expected to implement these common features:

- Call |nvim_set_client_info()| after connecting, so users and plugins can
  detect the UI by handling the |ChanInfo| event. This avoids the need for
  special variables and UI-specific config files (gvimrc, macvimrc, …).
- Cursor style (shape, color) should conform to the 'guicursor' properties
  delivered with the mode_info_set UI event.
- Send the ALT/META ("Option" on macOS) key as a |<M-| chord.
- Send the "super" key (Windows key, Apple key) as a |<D-| chord.
- Avoid mappings that conflict with the Nvim keymap-space; GUIs have many new
  chords (<C-,> <C-Enter> <C-S-x> <D-x>) and patterns ("shift shift") that do
  not potentially conflict with Nvim defaults, plugins, etc.
- Consider the "option_set" |ui-global| event as a hint for other GUI
  behaviors. Various UI-related options ('guifont', 'ambiwidth', …) are
  published in this event. See also "mouse_on", "mouse_off".


NAMING							*dev-naming*

Naming is important. Consistent naming in the API and UI helps both users and
developers discover and intuitively understand related concepts ("families"),
and reduces cognitive burden. Discoverability encourages code re-use and
likewise avoids redundant, overlapping mechanisms, which reduces code
surface-area, and thereby minimizes bugs...

Naming conventions ~

Use the "on_" prefix to name event handlers and also the interface for
"registering" such handlers (on_key). The dual nature is acceptable to avoid
a confused collection of naming conventions for these related concepts.


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