Chorex - Choreographic Programming in Elixir
Note: this documentation is current as of 2024-07-22. 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:
defmodule TestChor do
defchor [Buyer, Seller] do
def run(Buyer.(book_title)) do
Buyer.(book_title) ~> Seller.(b)
Seller.get_price(b) ~> Buyer.(p)
Buyer.(p)
end
end
end
Implement the actors:
defmodule MyBuyer do
use TestChor.Chorex, :buyer
end
defmodule MySeller do
use TestChor.Chorex, :seller
def get_price("Das Glasperlenspiel"), do: 42
def get_price("A Tale of Two Cities"), do: 16
end
Elsewhere in your program:
Chorex.start(TestChor.Chorex, %{Seller => MySeller, Buyer => MyBuyer}, ["Das Glasperlenspiel"])
receive do
{:chorex_return, Buyer, val} ->
IO.puts("Got #{val}") # prints "Got 42"
end
Chorex.start(TestChor.Chorex, %{Seller => MySeller, Buyer => MyBuyer}, ["A Tale of Two Cities"])
receive do
{:chorex_return, Buyer, val} ->
IO.puts("Got #{val}") # prints "Got 16"
end
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 for references on choreographic programming in general.
Chorex is available on Hex.pm. Install by including the following in your mix.exs
file under the deps
list:
def deps do
[
...,
{:chorex, "~> 0.4.0"},
...
]
end
You can install development versions of Chorex directly from GitHub like so:
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.
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.
Chorex introduces some new syntax for choreographies. Here's a breakdown of how it works:
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.
The body of the choreography is a set of functions. One function named run
must be present; this will serve as the entry point into the choreography. The arguments to run
come from the third argument to the Chorex.start
function. (More on Chorex.start
and function parameters in a minute.)
defchor [Actor1, Actor2, ...] do
def some_func(...) do
...
end
def run() do
...
end
end
Inside the body of functions you can write message passing expressions. Examples:
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)
Formal syntax:
message_pass ::= $local_exp ~> $actor.($pat)
local_exp ::= $actor.($pat)
| $actor.$func($exp, ...)
| $actor.($exp)
actor ::= Module name (e.g. Actor)
func ::= Function name (e.g. frobnicate(...))
pat ::= Pattern match expr (e.g. a variable like `foo` or tuples `{:ok, bar}` etc.)
exp ::= Elixir expression (e.g. foo + sum([1, 2, 3]))
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:
- A variable local to Actor1
- A function local to Actor1 (with or without arguments, also all local to Actor1)
- An expression local to Actor1
The right-and-side must be Actor2.(<pattern>)
. This means that the left-hand-side will be computed on Actor1
and send to Actor2
where it will be matched against the pattern pattern
.
Local expressions are computations that happen on a single node. These computations are isolated from each other—i.e. every location has its own variables. For example, if I say:
defchor [Holmes, Watson] do
def discombobulate(Holmes.(clue)) do
...
end
end
Then inside the body of that function, I can talk about the variable clue
which is located on the Holmes
node. I can't, for instance, talk about the variable clue
on the Watson
node.
Holmes.(clue + 1) # fine
Watson.(clue * 2) # error: variable `clue` not defined
I can send the value in Holmes' clue
variable to Watson, at which point Watson can do computation with the value:
Holmes.(clue) ~> Watson.(holmes_observes)
if Watson.remember(holmes_observes) do
...
else
...
end
The remember
function here will be defined on the the implementation for the Watson
actor.
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.
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).
defchor [Alice, Bob] do
def run(Alice.(msg)) do
with Bob.({pub, priv}) <- Bob.gen_key() do
Bob.(pub) ~> Alice.(key)
exchange_message(Alice.encrypt(msg <> "\n love, Alice", key), Bob.(priv))
end
end
def exchange_message(Alice.(enc_msg), Bob.(priv)) do
Alice.(enc_msg) ~> Bob.(enc_msg)
Alice.(:letter_sent)
Bob.decrypt(enc_msg, priv)
end
end
Choreographies support functions and function calls—even recursive ones. Function parameters need to be annotated with the actor they live at, and the arguments when calling the function need to match. Calling a function with the wrong actor will result in the parameter getting nil
. E.g. calling exchange_message
above like so will not work properly:
exchange_message(Bob.(msg), Alice.(priv))
(and not just because the variables are wrong—the actor names don't match so the parameters won't get the values they need).
def higher_order_chor(other_chor) do
... other_chor.(...) ...
end
Chorex supports higher-order choreographies. This means you can pass the functions defined inside the defchor
block around as you would with functions. Higher-order choreographic functions don't get an actor prefix and you call them as you would a function bound to a variable, like so:
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
def run() do
higher_order_chor(@some_local_chor/1)
end
end
Note that when referring to the function, you must use the @func_name/3
syntax—the Chorex compiler notices the @
and processes the function reference differently. This is because the functions defined with def
inside the defchor
block have private internal details (when Chorex builds them, they get special implicit arguments added) and Chorex needs to handle references to these functions specially.
with OtherActor.(other_var) <- other_chor.(Actor.(var)) do
...
end
You can bind the result of some expression to a variable/pattern at an actor with with
. In the case of a higher-order choreography (seen above) this is whatever was on node OtherActor
when other_chor
executed. You may also use with
for binding local expressions, as seen in the exchange_message
example under § Function syntax.
To create a choreography, start by making a module, and writing the choreography with the defchor
macro.
defmodule Bookstore do
defchor [Actor1, Actor2] do
def run() do
Actor1.(... some expr ...) ~> Actor2.(some_var)
Actor2.some_computation(some_var) ~> Actor1.(the_result)
...
end
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:
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.
You need three things to fire off a choreography:
- The choreography description
- An implementation for each of the actors
Use the Chorex.start/3
function to start a choreography:
Chorex.start(MyChoreography.Chorex,
%{ Actor1 => MyActor1Impl,
Actor2 => MyActor2Impl },
[args, to, run])
The arguments are as follows:
- The name of the
Chorex
module to use. (Thedefchor
macro creates this module for you; in the above example there is aMyChoreography
module with a top-leveldefchor
declaration that creates theChorex
submodule on expansion.) - A map from actor name to implementation module name.
- A list of arguments to the
run
function in the Choreography. These will automatically get sent to the right nodes.
Once the actors are done, they will send the last value they computed to the current process tagged with the actor they were implementing. So, for this example, you could see what Actor1
computed by awaiting:
receive do
{:chorex_return, Actor1, val} -> IO.inspect(val, label: "Actor1's return: ")
end
Sometimes you might have a choreography where one or more actors need to share some state between different instantiations of the choreography. Returning to our bookseller example, the bookseller might need to keep track of a finite stock of books and ensure that no book gets double-sold.
Chorex can let you share state between different instances of the bookseller actor through a proxy. Details are under the Chorex
module.
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.
Chorex is under active development and things will change and break rapidly.
If you find any bugs or would like to suggest a feature, please open an issue on GitHub.
We will collect change descriptions here until we come up with a more stable format when changes get bigger.
-
v0.4.3; 2024-08-13
Multi-clause
with
blocks work. -
v0.4.2; 2024-08-07
Bugfix: projecting local expressions that call out to an Erlang module.
-
v0.4.1; 2024-08-01
Bugfix: choreographies can now have literal maps in local expressions.
-
v0.4.0; 2024-08-01
Functions can take arbitrary number of arguments from different actors.
-
v0.3.1; 2024-07-30
Fix many problems around local expression projection.
-
v0.3.0; 2024-07-22
Add
Chorex.start
andrun
function as an entry-point into the choreography. -
v0.2.0; 2024-07-03
Add shared-state actors.
-
v0.1.0; 2024-05-30
Initial release. Lots of rough edges so please, be patient. :)
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. Theproject
function keeps track of the current actor as thelabel
variable. (This vernacular borrowed from the academic literature.) - The functions
project
andproject_sequence
are mutually recursive:project_sequence
gets invoked wheneverproject
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 themonadic do ... end
form as well asreturn
andmzero
.
- This gathering is handled by the
- 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:
defchor [Alice, Bob] do
def run() do
Alice.pick_modulus() ~> Bob.(m)
Bob.gen_key(m) ~> Alice.(bob_key)
Alice.encrypt(message, bob_key)
end
end
This will get transformed into (roughly) this code:
defmodule Chorex do
def get_actors() do
[Alice, Bob]
end
def alice do
quote do
import Alice
@behaviour Alice
def init(args) do
Alice.init(__MODULE__, args)
end
end
end
defmodule Alice do
@callback encrypt(any(), any()) :: any()
@callback pick_modulus() :: any()
import Chorex.Proxy, only: [send_proxied: 2]
def init(impl, args) do
receive do
{:config, config} ->
arg = Enum.at(args, 0, nil)
ret = run(impl, config, arg)
send(config[:super], {:chorex_return, Alice, ret})
end
end
def run(impl, config, _) do
send(config[Bob], impl.pick_modulus())
bob_key =
receive do
msg -> msg
end
impl.encrypt(message, bob_key)
end
end
def bob do
quote do
import Bob
@behaviour Bob
def init(args) do
Bob.init(__MODULE__, args)
end
end
end
defmodule Bob do
@callback gen_key(any()) :: any()
import Chorex.Proxy, only: [send_proxied: 2]
def init(impl, args) do
receive do
{:config, config} ->
arg = Enum.at(args, 0, nil)
ret = run(impl, config, arg)
send(config[:super], {:chorex_return, Bob, ret})
end
end
def run(impl, config, _) do
m =
receive do
msg -> msg
end
send(config[Alice], impl.gen_key(m))
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.
Simply clone the repository and run mix test
.
-
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
This is a project by the Utah PLT group. Primary development by Ashton Wiersdorf.