#neovim

###Introduction

Vim is a powerful text editor with a big community that is constantly growing. Even though the editor is over two decades old, people still extend and improve it, mostly using vimscript or one of the supported scripting languages.

###Problem

Over its more than 20 years of life, vim has accumulated about 300k lines of scary C89 code that very few people understand or have the guts to mess with.

Another issue, is that as the only person responsible for maintaing vim's big codebase, Bram Moolenaar has to be extra-careful before accepting patches, because once merged, the new code will be his responsibility.

These problems make it very difficult to have new features and bug fixes merged into the core. Vim just cant keep up with the development speed of its plugin echosystem.

###Solution

Neovim is a vim fork that seeks to aggressively refactor vim in order to achieve the following goals:

• Simplify maintenance to improve the speed that bug fixes and features get merged.
• Split the maintainance work between multiple developers.
• Enable the implementation of new/modern user interfaces without any modifications to the core source.
• Improve the extensibility power with a new plugin architecture based on external processes. Plugins will be written in any programming language without any explicit support from the editor.

A consequence of achieving those goals is that new developers will join the community, consequently improving the editor for all users.

It is important to empathise that this is not a project to rewrite vim from the scratch or transform it into an IDE(though the new features provided will make it possible to build IDE-like distributions of the editor). The changes implemented here should have little impact on vim's editing model or vimscript in general. Most vimscript plugins should continue to work normally.

Each of the following topics will briefly explain the major changes that will be performed in the first iterations:

• Migrate to a cmake-based build
• Legacy support and compile-time features
• Platform-specific code
• New plugin architecture
• New GUI architecture
• Split into many repositories

Migrate to a cmake-based build

The source tree has dozens(if not hundreds) of files dedicated to building vim with on various platforms with different configurations, and many of these files look abandoned or outdated. Most users dont care about selecting individual features and just compile using '--with-features=huge', which still generates an executable that is small enough even for lightweight systems(by today's standards).

All those files will be removed and vim will be built using cmake, a modern build system that generates build scripts for the most relevant platforms.

Legacy support and compile-time features

Vim has a significant amount of code dedicated to supporting legacy systems and compilers. All that code increases the maintainance burden and will be removed.

Most optional features will no longer be optional(see above), with the exception of some broken and useless fetures(eg: netbeans integration, sun workshop) which will be removed permanently. Vi emulation will also be removed(probably leave the 'set nocompatible' command as a no-op).

These changes wont affect most users. Those that only have a C89 compiler installed or use vim on legacy systems such as Amiga, BeOS or MSDOS have two options:

• Upgrade their software
• Continue using vim

Platform-specific code

Most of the platform-specific code will be removed and libuv will be used to handle system differences.

libuv is a modern multi-platform library with functions to perform common system tasks, and supports most unixes and windows, so the vast majority of vim's community will be covered.

New plugin architecture

All code supporting embedded scripting language interpreters will be replaced by a new plugin system that will support extensions written in any programming language.

Compatibility layers will be provided for vim plugins written in some of the currently supported scripting languages such as python or ruby. Most plugins should work on neovim with little modifications, if any.

This is how the new plugin system will work:

• Plugins are long-running programs/jobs that communicate with vim through stdin/stdout using msgpack-rpc or json-rpc.
• Vim will discover and run these programs at startup, keeping two-way communication channels with each plugin.
• Plugins will be able to listen to events and send commands to vim asynchronously.

This system will be built on top of a job control mechanism similar to the one provided by the job control patch

Here's an idea of how a plugin session will work using json-rpc (jsonrpc version omitted):

plugin -> neovim: {"id": 1, "method": "listenEvent", "params": {"eventName": "keyPressed"}}
neovim -> plugin: {"id": 1, "result": true}
neovim -> plugin: {"method": "event", "params": {"name": "keyPressed", "eventArgs": {"keys": ["C"]}}}
neovim -> plugin: {"method": "event", "params": {"name": "keyPressed", "eventArgs": {"keys": ["Ctrl", "Space"]}}}
plugin -> neovim: {"id": 2, "method": "showPopup", "params": {"size": {"width": 10, "height": 2} "position": {"column": 2, "line": 3}, "items": ["Completion1", "Completion2"]}}
plugin -> neovim: {"id": 2, "result": true}}

That shows an hypothetical conversation between neovim and completion plugin that displays completions when the user presses Ctrl+Space. The above scheme gives neovim near limitless extensibility and also improves stability as plugins will automatically be isolated from the main executable.

This system can also easily emulate the current scripting languages interfaces to vim. For example, a plugin can emulate the python interface by running python scripts sent by vim in its own context and by exposing a 'vim' module with an API matching the current one. Calls to the API would simply be translated to json-rpc messages sent to vim.

New GUI architecture

Another contributing factor to vim's huge codebase is the explicit support for dozens of widget toolkits for GUI interfaces. Like the legacy code support, gui handling code will be removed from the core.

Neovim will handle GUIs similarly to how it will handle plugins:

• GUIs are separate programs, possibly written in different programming languages.
• Neovim will use its own stdin/stdout to receive input and send updates, again using json-rpc or msgpack-rpc.

The difference between plugins and GUIs is that plugins will be started by neovim, where neovim will be started by programs running the GUI. Here's a sample diagram of the process tree:

GUI program
  |
  ---> Neovim
         |
         ---> Plugin 1
         |
         ---> Plugin 2
         |
         ---> Plugin 3

Hypothetical GUI session:

gui -> vim: {"id": 1, "method": "initClient", "params": {"size": {"rows": 20, "columns": 25}}}
vim -> gui: {"id": 1, "result": {"clientId": 1}}
vim -> gui: {"method": "redraw", "params": {"clientId": 1, "lines": {"5": "   Welcome to neovim!   "}}}
gui -> vim: {"id": 2, "method": "keyPress", "params": {"keys": ["H", "e", "l", "l", "o"]}}
vim -> gui: {"method": "redraw", "params": {"clientId": 1, "lines": {"1": "Hello                   ", "5": "                        "}}}

This new GUI architecture creates many interesting possibilities:

• Modern GUIs written in high-level programming languages that integrate better with the operating system. We can have GUIs written using C#/WPF on Windows or Ruby/Cocoa on Mac, for example.
• Plugins will be able emit custom events that may be handled directly by GUIs. This will enable the implementaton of advanced features such as sublime's minimap.
• A multiplexing daemon could keep neovim instances running in a headless server, while multiple remote GUIs could attach/detach to share editing sessions.
• Simplified headless testing.
• Embedding the editor into other programs.

Here's a diagram that illustrates how a client-server process tree might look like:

Server daemon listening on tcp sockets <------ GUI 1 (attach/detach to running instances using tcp sockets)
  |                                       |
  ---> Neovim                             | 
         |                                GUI 2 (sharing the same session with GUI 1)
         ---> Plugin 1
         |
         ---> Plugin 2
         |
         ---> Plugin 3

Development

Development will happen on the neovim organization, and the code will be split across many repositories. There will be separate repositories for GUIs, plugins, runtime files(official vimscript) and distributions. This will let the editor receive improvements much faster as the patches dont have to go all through a single person for approval.

Travis will also be used for continuous integration, so pull requests will be automatically checked.

###Future

The changes described are relatively simple to integrate and will be part of the first iteration. Here are more possibilities for the future:

• Refactor the way input is read. Heres a great simplification of how vim currently works: while (true) { process_input(getc()); }, we want to remove the while(true) chunks from the core and provide something like this: process_input(char c). This will help extract the editor logic into a library.
• Remove all globals. Basically every function will receive a pointer to a struct representing the editor and containing data currently held by global variables. Helpful if a 'libvim' is implemented in the future.
• Replace the current vimscript C implementation by lua or luajit and compile vimscript into lua, similarly to how coffeescript is compiled into javascript. This will greatly reduce the maintainance burden and give vimscript a real boost in performance.

###Status

Here's a list of things that have been done so far:

• Source tree was cleaned up, leaving only files necessary for compilation/testing of the core.
• Source files were processed with unifdef to remove tons of FEAT_* macros
• Files were processed with uncrustify to normalize source code formatting.
• The autotools build system was replaced by cmake

and of what is being currently worked on:

• Port all IO to libuv

###Dependencies

For Ubuntu 12.04:

sudo apt-get install build-essential cmake libncurses5-dev

TODO: release the Dockerfile which has this in it

TODO: Arch instructions

TODO: OSX instructions

###Building

To generate the Makefiles:

make cmake

To build and run the tests:

make test