Coop is a Neovim plugin that provides an asynchronous operation framework based on native Lua coroutines. If you write Lua code in Neovim, Coop lets you write non-blocking code that looks synchronous. It’s like async/await in some other languages.

Coop was designed to

• Be simple. Coop should be easy to explain.
• Stay close to native Lua coroutines and Lua’s idioms.
• Come with batteries included for Neovim.

Here’s what else you can expect from Coop:

• True parallelism with a rich set of control operators.
• A flexible cancellation mechanism.
• Extensibility — You can turn any callback-based function into a task function. If the callback-based function is non-blocking, so will the task function be.

[!WARNING] Coop.nvim is right now in a preview release.

• The interfaces should be mostly stable.
• Some ports of standard library functions are missing.
• Additional control function are missing.

I want to see if there’s any initial reception before I commit fully to implementing all batteries.

⚡️ Requirements

• Neovim 0.10+

📦 Installation

Install the plugin with your preferred package manager, such as Lazy:

{
  "gregorias/coop.nvim",
}

🚀 Usage

Hello, world

Let’s start with the hello world of asynchronicity: reading a file. Below is an end-to-end code that shows how to concurrently read two files with Coop:

local coop = require("coop")
local uv = require("coop.uv")

--- Reads a file.
---
---@async
---@param path string
---@return string
function readFileAsync(path)
  local err_open, fd = uv.fs_open(path, "r", 438)
  assert(err_open == nil)
  local err_fstat, stat = uv.fs_fstat(fd)
  assert(err_fstat == nil)
  local err_read, data = uv.fs_read(fd, stat.size, 0)
  assert(err_read == nil)
  local err_close = uv.fs_close(fd)
  assert(err_close == nil)
  return data
end

--- Read `foo.txt` and `bar.txt` concurrently.
local foo_task = coop.spawn(readFileAsync, "foo.txt")
local bar_task = coop.spawn(readFileAsync, "bar.txt")

--- Wait 1s for both tasks to finish and print their results.
print(foo_task:await(1000), bar_task:await(1000))

Advanced examples

For more advanced examples, check out lua/coop/examples.lua:

search_for_readme shows filesystem operations. Although search_for_readme is non-blocking, it looks exactly like its synchronous counterpart would look like. One tiny caveat is that you need to spawn it in your main, synchronous thread with coop.spawn(search_for_readme).

sort_with_time shows that Coop achieves true parallelism. It launches parallel timers with coop.spawn and uses a coop.control.as_completed to conveniently capture results as each timer completes.

run_parallel_search is the final example. It shows the flexible cancellation mechanism together with error handling through copcall.

Interface guide

This section introduces the essential interfaces.

In Coop, the two main abstractions are task functions and tasks. Task functions are regular Lua functions that may call task.yield. Task functions are effectively asynchronous functions. Whenever you see a function in Coop that’s annotated with @async, it is a task function. You may nest task functions freely like you would nest regular functions.

The main caveat for task functions is that to run them in your top-level synchronous code, you need to wrap them in a task, which represents a thread. You usually do the wrapping with coop.spawn (see aforementioned examples).

Task

The main abstraction of Coop is a task. A task is an extension of a Lua coroutine with three capabilities:

• Holding results (including errors)
• Awaiting
• Cancellation

A task behaves like a coroutine and comes with its own equivalent functions that behave analogously:

task.create
task.resume

--- Yield throws `error("cancelled")` if in a cancelled task.
task.yield

task.status
task.running

There’s also pyield variant of yield that returns success, results instead of throwing an error.

task.create creates a new thread and accepts task functions. Instead of task.create you should usually use coop.spawn, which creates a new task and resumes it.

Tasks come with two additional functions. A cancel function (which is also a method):

--- Cancels the task.
---
--- The cancelled task will throw `error("cancelled")` in its yield.
---
---@param task Task the task to cancel
function task.cancel(task)
  -- …
end

And an await method that has three variants:

-- Awaits task completion.
function task.await(task, cb_or_timeout, interval)
end

-- task.await() is a task function that waits for the task finish and return a
-- result
result = task.await()

-- task.await(cb) is a callback-based function that calls the callback once the
-- task is finished.
-- It doesn’t wait for the task.
task.await(function(success, result) end)

-- task.await(timeout, interval) is a blocking function that uses vim.wait to implement a busy-waiting loop.
task.await(1000, 100) -- Wait for 1s for the task to finish. Check every 100ms

Tasks implement a call operator that calls await. This allows for a fluent interface where tasks appear as if they were regular task functions:

local get_result_1 = coop.spawn(compute, 100)
local get_result_2 = coop.spawn(compute, 200)
local result = get_result_1()

The essential task-related functions live in coop.task and coop.task-utils modules.

Library reference

coop.mpsc-queue

The mpsc-queue module provides a multiple-producer single-consumer concurrent queue with an asynchronous pop method.

local MpscQueue = require('coop.mpsc_queue').MpscQueue
local q = MpscQueue.new()

-- Asynchronously print whatever is provided to q.
coop.spawn(function()
  while true do
    vim.print("Read: " .. q:pop())
  end
end)

-- Start two threads that will asynchronously get strings from two sources.
coop.spawn(function()
  while true do
    q:push(read_string_from_user())
  end
end)

coop.spawn(function()
  while true do
    q:push(read_string_from_something_else())
  end
end)

coop.subprocess

The subprocess module provides a way to launch subprocesses and control their I/O with task functions.

local coop = require("coop")
local subprocess = require("coop.subprocess")

---@async
function pass_printf_to_cat()
  local printf = subprocess.spawn("printf", {
    args = { "Hello, world!" },
    stdio = { nil, subprocess.PIPE },
  })
  local cat = subprocess.spawn("cat", {
    stdio = { printf.stdout, subprocess.STREAM },
  })
  vim.print(cat.stdout:read_until_eof())
  printf:await()
  cat:await()
end

coop.spawn(pass_printf_to_cat)

coop.uv

The uv module provides task function versions of asynchronous functions in vim.uv.

coop.uv-utils

The uv-utils module provides the following:

• sleep, a function that puts the running thread to sleep for a number of milliseconds.
• StreamReader and StreamWriter, two wrappers that turn callback-based Libuv streams (uv_stream_t) into objects with asynchronous task functions.

local coop = require("coop")
local StreamReader = require('coop.uv-utils').StreamReader
local StreamWriter = require('coop.uv-utils').StreamWriter

local fds = vim.uv.pipe({ nonblock = true }, { nonblock = true })
local sr = StreamReader.from_fd(fds.read)
local sw = StreamWriter.from_fd(fds.write)

local result = coop.spawn(function()
  sw:write("Hello, world!")
  sw:close()
  local data = sr:read_until_eof()
  sr:close()
  return data
end).await(5000, 1)

assert(result == "Hello, world!")

FAQ

How do I block until an asynchronous function is done in synchronous code?

Asynchronous code doesn’t mix with synchronous functions. If you need to wait in your synchronous code until an asynchronous task is done, Coop implements a busy-waiting mechanism based on vim.wait:

--- This is a synchronous function.
function main()
  local task = coop.spawn(...)
  -- Wait for 5 seconds and poll every 20 milliseconds.
  return task:await(5000, 20)
end

Can I mix coroutine functions and task functions?

No.

You can not use coroutine functions in task functions, because when coroutine yield doesn’t handle cancellation.

You can not use task functions in coroutines, because task.yield requires running in a task.

You may however convert coroutine functions to task functions with coop.task-utils.co_to_tf.

✅ Comparison to similar tools

Nio

Overall, Nio seems like a solid asynchronous framework.

I started Coop before I knew about Nio, and I continued building Coop, because I thought I could make a design and implementation that are clearer and more principled. The litmus test for me was whether I would be able to write a guide into the internals that is easy to follow for someone that knows coroutines.

In terms of features, Coop has a more powerful cancellation mechanism. In Coop, task cancellation causes an error to be thrown in the affected task. This allows the programmer to implement any custom cancellation logic, e.g., cancelling child tasks or unloading resources. As far as I can tell, Nio doesn’t let you safely unload resources upon cancellation as it just makes the task dead.

Plenary Async

Plenary async is effectively broken as it doesn’t support nesting.

🙏 Acknowledgments

The SVG from the logo comes from Uxwing.

🔗 See also

• Coerce — My Neovim plugin for case conversion.
• Toggle — My Neovim plugin for toggling options.
• Coop’s design was presented as an approach structured concurrency in Neovim.
• Reddit thread