neovim/runtime/doc/nvim_provider.txt

*nvim_provider.txt*    For Nvim.					{Nvim}


		 NVIM REFERENCE MANUAL    by Thiago de Arruda


Nvim provider infrastructure			       *nvim-provider*

First of all, this document is meant to be read by developers interested in
contributing to the refactoring effort. If you are a normal user or plugin
developer looking to learn about Nvim |msgpack-rpc| infrastructure for
implementing plugins in other programming languages, see |external-plugin|.
For instructions on how to enable python plugins, see |nvim-python|. For
clipboard, see |nvim-clipboard|.

Instead of doing everything by itself, Nvim aims to simplify it's own
maintenance by delegating as much work as possible to external systems. But
some Vim components are too tightly coupled and in some cases the refactoring
work necessary to swap in-house implementations by code that integrates to
other systems is too great. Nvim provider infrastructure is a facility that
aims to make this task simpler.

To understand why the provider infrastructure is useful, let us consider two
examples of integration with external systems that are implemented in Vim and
are now decoupled from Nvim core as providers:

The first example is clipboard integration: On the original Vim source code,
clipboard functions account for more than 1k lines of C source code(and that
is just on ui.c). All to peform two tasks that are now accomplished with
simple shell commands such as xclip or pbcopy/pbpaste.

The other example is python scripting support: Vim has three files dedicated
to embed 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. On Nvim, python
scripting is performed by an external host process that is running 2k sloc
python program.

In a perfect world, we would implement python and clipboard integration in
pure vimscript and without touching the C code. Unfortunately we can't achieve
these goals without severly compromising backwards compatibility with Vim.
Thats where providers comes to rescue.

In essence, this infrastructure a simple framework that simplifies the task of
calling vimscript from C code, making it simpler to rewrite C functions that
interact with external systems in pure vimscript. It is composed of two
functions in eval.c:

- eval_call_provider(name, method, arguments): Call a provider(name) method
  with arguments
- eval_has_provider(name): Checks if a provider is implemented

What these functions do is simple:

- eval_call_provider will call the provider#(name)#Call function passing in
  the method and arguments.
- eval_has_provider will return true if the provider#(name)#Call function is
  implemented, and is called by the "has" vimscript function to check if
  features are available.

The basic idea is that the provider#(name)#Call function should implement 
integration with an external system, because calling shell commands and
|msgpack-rpc| clients(Nvim only) is easier to do in vimscript.

Now, back to the python example. Instead of modifying vimscript to allow the
definition of lowercase functions and commands(for the |:python|, |:pyfile|
and |:pydo| commands, and the |pyeval()| function), which would break
backwards compatibility with Vim, we implemented the
autoload/provider/python.vim script and the provider#python#Call function
that is only defined if an external python host is started successfully.

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 +python feature.


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