Chorex - Choreographic Programming in Elixir
# Synopsis
**Note:** this documentation is current as of 2024-05-30. The project is evolving rapidly, so this README may occasionally get out-of-sync with what the project can do.
Describe the choreography in a module with the `defchor` macro:
```elixir
defmodule TestChor do
defchor [Buyer, Seller] do
Buyer.get_book_title() ~> Seller.(b)
Seller.get_price(b) ~> Buyer.(p)
Buyer.(p)
end
end
```
Implement the actors:
```elixir
defmodule MyBuyer do
use TestChor.Chorex, :buyer
def get_book_title(), do: "Das Glasperlenspiel"
end
defmodule MySeller do
use TestChor.Chorex, :seller
def get_price(_b), do: 42
end
```
Elsewhere in your program:
```elixir
ps = spawn(MySeller, :init, [])
pb = spawn(MyBuyer, :init, [])
config = %{Seller => ps, Buyer => pb, :super => self()}
send(ps, {:config, config})
send(pb, {:config, config})
receive do
{:choreography_return, Buyer, val} -> IO.puts("Got #{val}")
end
```
The program should print `Got 42` to the terminal.
# Description
Chorex is a library for *choreographic programming* in Elixir. Choreographic programming is a programming paradigm where you specify the interactions between different entities in a concurrent system in one global view, and then *extract implementations* for each of those actors. See [§ Bibliography](#org44e5aee) for references on choreographic programming in general.
## Installation
Chorex is available on Hex.pm. Install by including the following in your `mix.exs` file under the `deps` list:
```elixir
def deps do
[
...,
{:chorex, "~> 0.1.0"},
...
]
end
```
You can install development versions of Chorex directly from GitHub like so:
```elixir
def deps do
[
...,
{:chorex, github: "utahplt/chorex"},
...
]
end
```
Note that this is *experimental software* and stuff *will* break. Please don’t rely on this for anything production-grade. Not yet at least.
## What is a choreography?
A choreography is a birds-eye view of an interaction between nodes in a distributed system. You have some set of *actors/—in Elixir parlance processes—that exchange /messages* while also running some /local computation/—i.e. functions that don’t rely on talking to other nodes in the system.
### Choreography syntax
Chorex introduces some new syntax for choreographies. Here’s a breakdown of how it works:
```elixir
defchor [Actor1, Actor2, ...] do
...choreography body...
end
```
The `defchor` macro wraps a choreography and translates it into core Elixir code. You give `defchor` a list of actors, specified as if they were module names, and then a `do` block wraps the choreography body.
```elixir
Actor1.(var1) ~> Actor2.(var2_a)
Actor1.func_1() ~> Actor2.(var2_b)
Actor1.func_2(var1_a, var1_b) ~> Actor2.(var2_c)
Actor1.(var1_a + var1_b) ~> Actor2.(var2_c)
```
The `~>` indicates sending a message between actors. The left-hand-side must be `Actor1.<something>`, where that `<something>` bit can be one of three things:
1. A variable local to Actor1
2. A function local to Actor1 (with or without arguments, also all local to Actor1)
3. An expression local to Actor1
The right-and-side must be `Actor2.<var_name>`. This means that the left-hand-side will be computed on `Actor1` and send to `Actor2` where it will be stored in variable `<var_name>`.
**ACHTUNG!! `mix format` will rewrite `Actor1.var1` to `Actor1.var1()` which is a function call instead of a variable! Wrap variables in parens like `Actor1.(var1)` if you want to use `mix format`!** This is an unfortunate drawback—suggestions on fixing this would be welcome.
Local functions are not defined as part of the choreography; instead, you implement these in a separate Elixir module. More on that later.
```elixir
if Actor1.make_decision() do
Actor1[L] ~> Actor2
...
else
Actor1[R] ~> Actor2
...
end
```
`if` expressions are supported. Some actor makes a choice of which branch to go down. It is then *crucial* (and, at this point, entirely up to the user) that that deciding actor inform all other actors about the choice of branch with the special `ActorName[L] ~> OtherActorName` syntax. Note the lack of `.` and variable names. Furthermore, the true branch is always `L` (left) and the false branch is always `R` (right).
```elixir
def higher_order_chor(other_chor) do
... other_chor.(...) ...
end
```
Chorex supports higher-order choreographies. These are choreographies that take another choreography as an argument where it can be applied like a function.
```elixir
def some_local_chor(Actor.(var_name)) do
Actor.(var_name) ~> OtherActor.(other_var)
OtherActor.(other_var)
end
```
This creates a choreography that can be passed as an argument to the `higher_order_chor` function. This takes as an argument a variable living at a particular actor, and returns another value on a potentially different node.
You would combine the choreographies like so:
```elixir
defchor [Actor, OtherActor] do
def higher_order_chor(other_chor) do
... other_chor.(...) ...
end
def some_local_chor(Actor.(var_name)) do
Actor.(var_name) ~> OtherActor.(other_var)
OtherActor.(other_var)
end
higher_order_chor(&some_local_chor/1)
end
```
Right now these functions are limited to a single argument.
```elixir
with OtherActor.(other_var) <- other_chor.(Actor.(var)) do
...
end
```
You can use `with` to bind a variable to the result of calling a higher-order choreography. Note that right now you can only have one `<-` in the expression.
## Creating a choreography
To create a choreography, start by making a module, and writing the choreography with the `defchor` macro.
```elixir
defmodule Bookstore do
defchor [Actor1, Actor2] do
Actor1.(... some expr ...) ~> Actor2.(some_var)
Actor2.some_computation(some_var) ~> Actor1.(the_result)
...
end
end
```
You will need to make a module for every actor you specify at the beginning of `defchor` and mark which actor you’re implementing like so:
```elixir
defmodule MyFirstActor do
use Bookstore.Chorex, :actor1
...
end
defmodule MySecondActor do
use Bookstore.Chorex, :actor2
def some_computation(val), do: ...
end
```
These modules will need to implement all of the local functions specified in the choreography. Chorex will use Elixir’s behaviour mechanism to warn you if you don’t implement every function needed. In the above example, the `MySecondActor` implements the role of `Actor2` in the choreography, and therefore needs to implement the `some_computation` function.
**Note:** *Actor names do not need to be the same as the modules implementing them!* It is *useful* to do that, but there exist instances where you might want to write one choreography and implement it in different ways.
## Running a choreography
To fire off the choreography, you need to spin up a process for each actor and then tell each actor where to find the other actors in the system. For the above example, you could do this:
```elixir
first_actor = spawn(MyFirstActor, :init, [])
second_actor = spawn(MySecondActor, :init, [])
config = %{Actor1 => first_actor, Actor2 => second_actor, :super => self()}
send(first_actor, config)
send(second_actor, config)
```
Once the actors are done, they will send the last value they computed to `:super` tagged with the actor they were implementing. So, for this example, you could see what `Actor1` computed by awaiting:
```elixir
receive do
{:choreography_return, Actor1, val} -> IO.inspect(val, label: "Actor1's return: ")
end
```
## Using a choreography with the rest of your project
The local functions are free to call any other code you have—they’re just normal Elixir. If that code sends and receives messages not managed by the choreography library, there is no guarantee that this will be deadlock-free.
# Development
## Changelog
We will collect change descriptions here until we come up with a more stable format when changes get bigger.
- v0.1.0; 2024-05-30
Initial release. Lots of rough edges so please, be patient. :)
## High-level internals
The `defchor` macro is implemented in the `Chorex` module.
- The `defchor` macro gathers a list of actors.
- For each actor, call `project` on the body of the choreography. The `project` function keeps track of the current actor as the “label” variable. (This vernacular borrowed from the academic literature.)
- The functions `project` and `project_sequence` are mutually recursive: `project_sequence` gets invoked whenever `project` encounters a block with multiple instructions.
- The `project` function walks the AST, it gathers a list of functions that will need to be implemented by each actor’s implementing module, as well as a list of top-level functions for each projection.
- This gathering is handled by the `WriterMonad` module, which provides the `monadic do ... end` form as well as `return` and `mzero`.
- Finally the macro generates modules for each actor under the `Chorex` module it generates.
So, for example, if you have a simple Choreography like this:
```elixir
defchor [Alice, Bob] do
Alice.pick_modulus() ~> Bob.(m)
Bob.gen_key(m) ~> Alice.(bob_key)
Alice.encrypt(message, bob_key)
end
```
This will get transformed into (roughly) this code:
```elixir
defmodule Chorex do
(
def alice do
quote do
import Alice
@behaviour Alice
def init() do
Alice.init(__MODULE__)
end
end
end
defmodule Alice do
@callback encrypt(any(), any()) :: any()
@callback pick_modulus() :: any()
def init(impl) do
receive do
{:config, config} ->
ret = run_choreography(impl, config)
send(config[:super], {:choreography_return, Alice, ret})
end
end
def run_choreography(impl, config) do
if function_exported?(impl, :run_choreography, 2) do
impl.run_choreography(impl, config)
else
send(config[Bob], impl.pick_modulus())
(
bob_key =
receive do
msg -> msg
end
impl.encrypt(message, bob_key)
)
end
end
end
)
(
def bob do
quote do
import Bob
@behaviour Bob
def init() do
Bob.init(__MODULE__)
end
end
end
defmodule Bob do
@callback gen_key(any()) :: any()
def init(impl) do
receive do
{:config, config} ->
ret = run_choreography(impl, config)
send(config[:super], {:choreography_return, Bob, ret})
end
end
def run_choreography(impl, config) do
if function_exported?(impl, :run_choreography, 2) do
impl.run_choreography(impl, config)
else
m =
receive do
msg -> msg
end
send(config[Alice], impl.gen_key(m))
end
end
end
)
defmacro __using__(which) do
apply(__MODULE__, which, [])
end
end
```
You can see there’s a `Chorex.Alice` module and a `Chorex.Bob` module.
## Testing
Simply clone the repository and run `mix test`.
<a id="org44e5aee"></a>
# Bibliography
- Hirsch & Garg (2022-01-16) *Pirouette: Higher-Order Typed Functional Choreographies*, Proceedings of the ACM on Programming Languages. <https://doi.org/10.1145/3498684>
- Lugović & Montesi (2023-10-15) *Real-World Choreographic Programming: Full-Duplex Asynchrony and Interoperability*, The Art, Science, and Engineering of Programming. <https://doi.org/10.22152/programming-journal.org/2024/8/8>
# Authors
This is a project by the [Utah PLT](https://github.com/utahplt) group. Primary development by [Ashton Wiersdorf](https://lambdaland.org).
