A library for programming with effects and handlers in C#, inspired by the Eff programming language and the implementation of Algebraic Effects in OCaml, Eff Directly in OCaml. Effects are a powerful language feature that can be used to implement dependency injection, exception handling, nondeterministic computation, trace logging and much more.
The Eff library takes advantage of the async method extensibility features available since C# 7.
At its core, the library defines a task-like type, Eff<TResult>, which can be built using async methods:
using Nessos.Effects;
async Eff HelloWorld()
{
Console.WriteLine($"Hello, {await Helper()}!");
async Eff<string> Helper() => "World";
}Note that unlike Task, Eff types have cold semantics and so running
Eff hello = HelloWorld();will have no observable side-effect in stdout.
An Eff instance has to be run explicitly by passing an effect handler:
using Nessos.Effects.Handlers;
hello.Run(new DefaultEffectHandler()); // "Hello, World!"So what is the benefit of using a convoluted version of regular async methods?
A key concept of the Eff library are abstract effects:
public class CoinToss : Effect<bool>
{
}Eff methods are capable of consuming abstract effects:
async Eff TossNCoins(int n)
{
for (int i = 0; i < n; i++)
{
bool result = await new CoinToss();
Console.WriteLine($"Got {(result ? "Heads" : "Tails")}");
}
}So how do we run this method now? The answer is we need write an effect handler that interprets the abstract effect:
public class RandomCoinTossHandler : EffectHandler
{
private readonly Random _random = new Random();
public override async ValueTask Handle<TResult>(EffectAwaiter<TResult> awaiter)
{
switch (awaiter)
{
case EffectAwaiter<bool> { Effect: CoinToss _ } awtr:
awtr.SetResult(_random.NextDouble() < 0.5);
break;
}
}
}We can then execute the method by passing the handler:
TossNCoins(100).Run(new RandomCoinTossHandler()); // prints random sequence of Heads and TailsNote that we can reuse the same method using other interpretations of the effect:
public class BiasedCoinTossHandler : EffectHandler
{
private readonly Random _random = new Random();
public override async ValueTask Handle<TResult>(EffectAwaiter<TResult> awaiter)
{
switch (awaiter)
{
case EffectAwaiter<bool> { Effect: CoinToss _ } awtr:
awtr.SetResult(_random.NextDouble() < 0.01);
break;
}
}
}
TossNCoins(100).Run(new BiasedCoinTossHandler()); // prints sequence of mostly TailsPlease see the samples folder for more examples of Eff applications.