Yet Another Effect System (λÆS)

λÆS is an experimental effect system in Scala inspired by the ideas behind Algebraic Effects. Using Scala 3 context parameters and context functions, it provides a way to define and handle effects in a modular and composable manner.

🎥 Featured Talk

Watch the talk from Scalar 2025 about the main concepts behind the library:

📦 Available Modules

• yaes-core: The main effects of the λÆS library
• yaes-data: Functional data structures that complement the λÆS effects system

λÆS Core

The core module provides a comprehensive set of effects for functional programming:

• IO operations and side effect management
• Structured concurrency with async computations
• Typed error handling and resource management
• Stateful computations with the State effect
• Console I/O, logging, and system integration

λÆS Data

The data module provides functional data structures optimized for use with effects:

• Flow: Cold asynchronous data streams with rich transformation operators
• Future additions: Immutable collections, persistent data structures

🚀 Quick Start

Requirements

• Java 24 or higher is required to run λÆS due to its use of modern Java features like Virtual Threads and Structured Concurrency.

Installation

Add the dependencies to your build.sbt:

libraryDependencies ++= Seq(
  "in.rcard.yaes" %% "yaes-core" % "0.7.0",
  "in.rcard.yaes" %% "yaes-data" % "0.7.0"  // Optional: for Flow and other data structures
)

✨ What’s New in λÆS?

You can choose between monadic style:

import in.rcard.yaes.Random.*
import in.rcard.yaes.Raise.*
import in.rcard.yaes.Yaes.*

def drunkFlip(using Random, Raise[String]): String = for {
  caught <- Random.nextBoolean
  heads  <- if (caught) Random.nextBoolean else Raise.raise("We dropped the coin")
} yield if (heads) "Heads" else "Tails"

Or a more direct style:

import in.rcard.yaes.Random.*
import in.rcard.yaes.Raise.*

def drunkFlip(using Random, Raise[String]): String = {
  val caught = Random.nextBoolean
  if (caught) {
    val heads = Random.nextBoolean
    if (heads) "Heads" else "Tails"
  } else {
    Raise.raise("We dropped the coin")
  }
}

🎯 Core Concepts

In λÆS, types like Random and Raise are Effects:

• A Side Effect is an unpredictable interaction, usually with an external system
• An Effect System manages Side Effects by tracking and wrapping them into Effects
• An Effect describes the type of the Side Effect and the return type of an effectful computation

λÆS uses deferred execution - calling effectful functions returns a value that represents something that can be run but hasn’t yet.

🛠 Effect Management

Effects are managed using Handlers:

import in.rcard.yaes.Random.*
import in.rcard.yaes.Raise.*

val result: String = Raise.run { 
  Random.run { 
    drunkFlip
  }
}

📚 Available Effects

• IO - Side-effecting operations
• Async - Asynchronous computations and fiber management
• Raise - Error handling and propagation
• State - Stateful computations and mutable state management
• Resource - Automatic resource management
• Input - Console input operations
• Output - Console output operations
• Random - Random content generation
• Clock - Time management
• System - System properties and environment variables
• Log - Logging at different levels

🗃 Data Structures

• Flow - Cold asynchronous data streams with rich operators
• More data structures - Additional functional data structures

🔗 Communication Primitives

• Channel - Communication primitive for transferring data between fibers
• More primitives - Additional concurrency primitives

🤝 Contributing

Contributions are welcome! Please check our contributing guidelines to get started.

🙏 Acknowledgments

Special thanks to all the smart engineers who helped with ideas and suggestions, including Daniel Ciocîrlan, Simon Vergauwen, Jon Pretty, Noel Welsh, and Flavio Brasil.