# ElixirST: Session Types in Elixir
[](https://github.com/gertab/ElixirST/actions/workflows/elixir.yml)
ElixirST (**S**ession **T**ypes in **El**ixir) applies *session types* to a fragment of the Elixir language.
It statically checks that the programs use the correct communication structures (e.g. `send`/`receive`) when dealing with message passing between processes.
<!-- It also ensures that the correct types are being used. -->
<!-- For example, the session type `?Add(number, number).!Result(number).end` expects that two numbers are received (i.e. `?`), then a number is sent (i.e. `!`) and finally the session terminates. -->
The design decisions of ElixirST and its underlying theory are described in the following papers:
- Gerard Tabone and Adrian Francalanza. *[Session Types in Elixir](https://doi.org/10.1145/3486601.3486708)*. AGERE 2021. ([pdf](http://staff.um.edu.mt/afra1/papers/Agere21.pdf))
- Gerard Tabone and Adrian Francalanza. *[Static Checking of Concurrent Programs in Elixir Using Session Types](https://gertab.github.io/ElixirST/archive/UoM%20-%20technical%20report%202022.pdf)*. Technical report.
## Example
To session typecheck modules in Elixir, add `use ElixirST` and include any assertions using the annotations `@session` and `@dual` preceding any public function (`def`). The following is a [`simple example`](/lib/elixirst/examples/small_example.ex), which receives one label (`?Hello()`):
<!-- The `@spec` directives are needed to ensure type correctness for the parameters. -->
```elixir
defmodule Example do
use ElixirST
@session "server = ?Hello().end"
@spec server(pid) :: atom()
def server(_pid) do
receive do
{:Hello} -> :ok
end
end
@dual "server"
@spec client(pid) :: {atom()}
def client(pid) do
send(pid, {:Hello})
end
end
```
ElixirST runs automatically at compile time (`mix compile`) or as a mix task (`mix sessions [module name]`):
```text
$ mix sessions SmallExample
[info] Session typechecking for client/1 terminated successfully
[info] Session typechecking for server/0 terminated successfully
```
If the client sends a different label (e.g. :Hi) instead of the one specified in the session type (i.e. `@session "!Hello()"`), ElixirST will complain:
```text
$ mix sessions SmallExample
[error] Session typechecking for client/1 found an error.
[error] [Line 7] Expected send with label :Hello but found :Hi.
```
## Session Types in Elixir
Session types are used to ensure correct communication between concurrent processes.
The session type operations include the following: `!` refers to a send action, `?` refers to a receive action, `&` refers to a branch (external choice), and `+` refers to an (internal) choice.
Session types accept the following grammar:
```text
S =
!label(types, ...).S (send)
| ?label(types, ...).S (receive)
| &{?label(types, ...).S, ...} (branch)
| +{!label(types, ...).S, ...} (choice)
| rec X.(S) (recurse)
| X (recursion var)
| end (terminate)
types =
atom
| boolean
| number
| atom
| pid
| {types, types, ...} (tuple)
| [types] (list)
```
----------
## Using ElixirST
### Installation
The package can be installed by adding `elixirst` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:elixirst, "~> 0.6.5"}
]
end
```
<!--
```elixir
def deps do
[
{:dep_from_git, git: "https://github.com/gertab/ElixirST.git"}
]
end
```
{:dep_from_git, git: "https://github.com/gertab/ElixirST.git", tag: "0.1.0"}
-->
Documentation can be found at [https://hexdocs.pm/elixirst](https://hexdocs.pm/elixirst/docs.html).
### Use in Elixir modules
To session typecheck a module, link the ElixirST library using this line:
```elixir
use ElixirST
```
Insert any checks using the `@session` attribute followed by a function that should be session typechecked, such as:
```elixir
@session "pinger = !Ping().?Pong()"
def function(), do: ...
```
The `@dual` attribute checks the dual of the specified session type.
```elixir
@dual "pinger"
# Equivalent to: @session "?Ping().!Pong()"
```
Other examples can be found in the [`examples`](/lib/elixirst/examples) folder.
## Acknowledgements
Some code related to Elixir expression typing was adapted from [typelixir](https://github.com/Typelixir/typelixir) by Cassola (MIT [licence](ACK.md)).
This project is licenced under the GPL-3.0 [licence](LICENCE).
