AsyncWith

The asynchronous version of Elixir's with, resolving the dependency graph and executing the clauses in the most performant way possible!

Installation

Add async_with to your project's dependencies in mix.exs:

def deps do
  [{:async_with, "~> 0.3"}]
end

And fetch your project's dependencies:

$ mix deps.get

Usage

TL;DR: use AsyncWith and just write async in front of with.

async with always executes the right side of each clause inside a new task. Tasks are spawned as soon as all the tasks that it depends on are resolved. In other words, async with resolves the dependency graph and executes all the clauses in the most performant way possible. It also ensures that, if a clause does not match, any running task is shut down.

Let's start with an example:

iex> use AsyncWith
iex>
iex> opts = %{width: 10, height: 15}
iex> async with {:ok, width} <- Map.fetch(opts, :width),
...>            {:ok, height} <- Map.fetch(opts, :height) do
...>   {:ok, width * height}
...> end
{:ok, 150}

As in with/1, if all clauses match, the do block is executed, returning its result. Otherwise the chain is aborted and the non-matched value is returned:

iex> use AsyncWith
iex>
iex> opts = %{width: 10}
iex> async with {:ok, width} <- Map.fetch(opts, :width),
...>            {:ok, height} <- Map.fetch(opts, :height) do
...>  {:ok, width * height}
...> end
:error

In addition, guards can be used in patterns as well:

iex> use AsyncWith
iex>
iex> users = %{"melany" => "guest", "bob" => :admin}
iex> async with {:ok, role} when not is_binary(role) <- Map.fetch(users, "bob") do
...>   :ok
...> end
:ok

Variables bound inside async with won't leak; "bare expressions" may also be inserted between the clauses:

iex> use AsyncWith
iex>
iex> width = nil
iex> opts = %{width: 10, height: 15}
iex> async with {:ok, width} <- Map.fetch(opts, :width),
...>            double_width = width * 2,
...>            {:ok, height} <- Map.fetch(opts, :height) do
...>   {:ok, double_width * height}
...> end
{:ok, 300}
iex> width
nil

An else option can be given to modify what is being returned from async with in the case of a failed match:

iex> use AsyncWith
iex>
iex> opts = %{width: 10}
iex> async with {:ok, width} <- Map.fetch(opts, :width),
...>            {:ok, height} <- Map.fetch(opts, :height) do
...>   {:ok, width * height}
...> else
...>   :error ->
...>     {:error, :wrong_data}
...> end
{:error, :wrong_data}

If an else block is used and there are no matching clauses, an AsyncWith.ClauseError exception is raised.

Order-dependent clauses that do not express their dependency via their used or defined variables could lead to race conditions, as they are executed in separated tasks:

use AsyncWith

async with Agent.update(agent, fn _ -> 1 end),
           Agent.update(agent, fn _ -> 2 end) do
  Agent.get(agent, fn state -> state end) # 1 or 2
end

Check the documentation for more information.

Documentation

Documentation is available at https://hexdocs.pm/async_with

Code formatter

As described in Code.format_string!/2 documentation, Elixir will add parens to all calls except for:

1. calls that have do/end blocks
2. local calls without parens where the name and arity of the local call is also listed under :locals_without_parens

async with expressions should fall under the first category and be kept without parens, because they are similar to with/1 calls.

This is then the recommended .formatter.exs configuration:

[
  # Regular formatter configuration
  # ...

  import_deps: [:async_with]
]

As an alternative, you can add async: 1 and async: 2 directly to the list :locals_without_parens.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/fertapric/async_with. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

Running tests

Clone the repo and fetch its dependencies:

$ git clone https://github.com/fertapric/async_with.git
$ cd async_with
$ mix deps.get
$ mix test

Building docs

$ mix docs

Acknowledgements

I would like to express my gratitude to all the people in the Elixir Core Mailing list who gave ideas and feedback on the early stages of this project. A very special mention to Luke Imhoff (@KronicDeth), Theron Boerner (@hunterboerner), and John Wahba (@johnwahba).

Copyright and License

(c) Copyright 2017-2019 Fernando Tapia Rico

AsyncWith source code is licensed under the MIT License.