[svermeulen](https://github.com/svermeulen) for fixing [inability to return functions](https://github.com/ms-jpq/lua-async-await/issues/2).

This tutorial assumes that you are familiar with the concept of `async` `await`

You will also need to read through the [first 500 words](https://www.lua.org/pil/9.1.html) of how coroutines work in lua.

Neovim use [libuv](https://github.com/libuv/libuv) for async, the same monster that is the heart of NodeJS.

The `libuv` bindings are exposed through `luv` for lua, this is accessed using `vim.loop`.

Most of the `luv` APIs are similar to that of NodeJS, ie in the form of

Our goal is avoid the dreaded calback hell.

local a = require "async"

local do_thing = a.sync(function (val)

local o = a.wait(async_func())

return o + val

end)

local main = a.sync(function ()

local thing = a.wait(do_thing()) -- composable!

local x = a.wait(async_func())

local y, z = a.wait_all{async_func(), async_func()}

end)

main()

If you don't know how coroutines work, go read the section on generators on [MDN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators).

It is in js, but the idea is identical, and the examples are much better.

Here is an example of coroutines in Lua:

Note that in Lua code `coroutine` is not a coroutine, it is an namespace.

To avoid confusion, I will follow the convention used in the Lua book, and use `thread` to denote coroutines in code.

local co = coroutine

local thread = co.create(function ()

local x, y, z = co.yield(something)

return 12

end)

local cont, ret = co.resume(thread, x, y, z)

Notice the similarities with `async` `await`

In both `async` `await` and `coroutines`, the LHS of the assignment statements receives values from the RHS.

This is how it works in all synchronous assignments. Except, we can defer the transfer of the values from RHS.

The idea is that we will make RHS send values to LHS, when RHS is ready.

To warm up, we will do a synchronous version first, where the RHS is always ready.

Here is how you send values to a coroutine:

co.resume(thread, x, y, z)

The idea is that we will repeat this until the coroutine has been "unrolled"

local pong = function (thread)

local nxt = nil

nxt = function (cont, ...)

if not cont

then return ...

else return nxt(co.resume(thread, ...))

end

end

return nxt(co.resume(thread))

local thread = co.create(function ()

local x = co.yield(1)

print(x)

local y, z = co.yield(2, 3)

print(y)

end)

pong(thread)

We can expect to see `1`, `2 3` printed.

Once you understand how the synchronous `pong` works, we are super close!

But before we make the asynchronous version, we need to learn one more simple concept.

For our purposes a `Thunk` is function whose purpose is to invoke a callback.

i.e. It adds a transformation of `(arg, callback) -> void` to `arg -> (callback -> void) -> void`

local read_fs = function (file)

local thunk = function (callback)

fs.read(file, callback)

end

return thunk

This too, is a process that can be automated:

local wrap = function (func)

local factory = function (...)

local params = {...}

local thunk = function (step)

table.insert(params, step)

return func(unpack(params))

end

return thunk

end

return factory

local thunk = wrap(fs.read)

So why do we need this?

The answer is simple! We will use thunks for our RHS!

With that said, we will still need one more magic trick, and that is to make a `step` function.

The sole job of the `step` funciton is to take the place of the callback to all the thunks.

In essence, on every callback, we take 1 step forward in the coroutine.

local pong = function (func, callback)

assert(type(func) == "function", "type error :: expected func")

local thread = co.create(func)

local step = nil

step = function (...)

local stat, ret = co.resume(thread, ...)

assert(stat, ret)

if co.status(thread) == "dead" then

(callback or function () end)(ret)

else

assert(type(ret) == "function", "type error :: expected func")

ret(step)

end

end

step()

Notice that we also make pong call a callback once it is done.

We can see it in action here:

local echo = function (...)

local args = {...}

local thunk = function (step)

step(unpack(args))

end

return thunk

local thread = co.create(function ()

local x, y, z = co.yield(echo(1, 2, 3))

print(x, y, z)

local k, f, c = co.yield(echo(4, 5, 6))

print(k, f, c)

pong(thread)

We can expect this to print `1 2 3` and `4 5 6`

Note, we are using a synchronous `echo` for illustration purposes. It doesn't matter when the `callback` is invoked. The whole mechanism is agnostic to timing.

You can think of async as the more generalized version of sync.

You can run an asynchronous version in the last section.

One more benefit of thunks, is that we can use them to inject arbitrary computation.

Such as joining together many thunks.

local join = function (thunks)

local len = table.getn(thunks)

local done = 0

local acc = {}

local thunk = function (step)

if len == 0 then

return step()

end

for i, tk in ipairs(thunks) do

local callback = function (...)

acc[i] = {...}

done = done + 1

if done == len then

step(unpack(acc))

end

end

tk(callback)

end

end

return thunk

This way we can perform `await_all` on many thunks as if they are a single one.

All this explicit handling of coroutines are abit ugly. The good thing is that we can completely hide the implementation detail to the point where we don't even need to require the `coroutine` namespace!

Simply wrap the coroutine interface with some friendly helpers

local pong = function (func, callback)

local thread = co.create(func)

...

local await = function (defer)

return co.yield(defer)

local await_all = function (defer)

return co.yield(join(defer))

At this point we are almost there, just one more step!

local sync = wrap(pong)

We `wrap` `pong` into a thunk factory, so that calling it is no different than yielding other thunks. This is how we can compose together our `async` `await`.

It's thunks all the way down.

In Neovim, we have something called `textlock`, which prevents many APIs from being called unless you are in the main event loop.

This will prevent you from essentially modifying any Neovim states once you have invoked a `vim.loop` funciton, which run in a seperate loop.

Here is how you break back to the main loop:

local main_loop = function (f)

vim.schedule(f)

a.sync(function ()

-- do something in other loop

a.wait(main_loop)

-- you are back!

end)()

I have bundle up this tutorial as a vim plugin, you can install it the usual way.

and then call the test functions like so: