Swift Sessions

Swift Sessions is a Swift package that implements binary session types, providing a robust framework for ensuring safe and structured communication in concurrent systems.

The library currently supports the following features:

• session type inference (only with closures)
• session types with binary branches
• dynamic linearity checking
• duality constraints on session types
• session initialization with client/server architecture

This library was developed as part of a Bachelor’s degree thesis project at the University of Camerino. The project was completed by Alessio Rubicini under the supervision of professor Luca Padovani.

This library offers two distinct styles for managing session types:

• Continuation with closures: protocol continuations are passed as closures. This approach makes the flow of logic explicit and easy to follow within the closure context. It's particularly useful for straightforward communication sequences.

  await Session.create { e in
      // One side of the communication channel
      await Session.recv(from: e) { num, e in
          await Session.send(num % 2 == 0, on: e) { e in
              await Session.close(e)
          }
      }
  } _: { c in
      // Another side of the communication channel
      await Session.send(42, on: e) { e in
          await Session.recv(from: e) { isEven, e in
              await Session.close(e)
          }
      }   
  }

  • Pros: Complete type inference of the communication protocol.
  • Cons: Nested code structure.

• Continuations with endpoint passing: this style involves returning continuation endpoints from communication primitives. It offers greater flexibility, enabling more modular and reusable code, particularly for complex communication sequences.

  typealias Protocol = Endpoint<Empty, (Int, Endpoint<(Bool, Endpoint<Empty, Empty>), Empty>)>
  
  // One side of the communication channel
  let e = await Session.create { (e: Protocol) in
      let (num, e1) = await Session.recv(from: e)
      let e2 = await Session.send(num % 2 == 0, on: e1)
      await Session.close(e2)
  }
      
  // Another side of the communication channel
  let e1 = await Session.send(42, on: e)
  let (isEven, e2) = await Session.recv(from: e1)
  await Session.close(e2)

  • Pros: Simplicity, particularly for avoiding deep indentation.
  • Cons: Incomplete type inference support.

Each style provides a unique approach to handling session-based binary communication and comes with its own pros and cons. By supporting both styles, SwiftSessions allows you to choose the best approach (or use both in a hybrid way!) according to your needs and coding preferences.

For additional examples, see the Tests folder.

Instead of creating disposable sessions as seen in the previous examples, you can also initialize sessions using a client/server architectural style.

A server is responsible for creating and managing multiple sessions that can handle a specific protocol. Many clients can be spawned and used with the same server to interact dually according to that defined protocol. This allows to define a protocol's side only once, and use it as many times as we want.

// Server side
let server = await Server { e in
    await Session.recv(from: e) { num, e in
        await Session.send(num % 2 == 0, on: e) { e in
            await Session.close(e)
        }
    }
}

// Client side
let c1 = await Client(for: server) { e in
    await Session.send(42, on: e) { e in
        await Session.recv(from: e) { isEven, e in
            await Session.close(e)
        }
    }
}

// You can spawn more clients here...

This architecture is useful for scenarios where multiple clients need to interact with a single server. It's also useful to implement complex protocols that involve loops and recursion.

In upcoming versions of Swift, new features such as non-copyable generics are expected to be introduced, which will significantly enhance the capabilities of this library allowing static linearity checking.

To integrate SwiftSessions into your project, use Swift Package Manager.

1. Add the following dependency to your Package.swift file:

dependencies: [
    .package(url: "https://github.com/alessiorubicini/SwiftSessions.git", .upToNextMajor(from: "1.0.0"))
]

2. Then, add SwiftSessions to your target dependencies:

.target(
    name: "YourTargetName",
    dependencies: [
        .product(name: "SwiftSessions", package: "SwiftSessions")
    ]
)

• Swift 5.5+
• Xcode 13+
• Compatible with iOS 14.0+ / macOS 10.15+ / tvOS 14.0+ / watchOS 6.0+

This project is licensed under the MIT License. See the LICENSE file for more details.