This is an experiment in defining Koka-style generic algebraic effects and handlers in Scala, using Loom virtual threads. The effects are both stack-safe and colourless.
In a nutshell, algebraic effects are the generic form of control structures like async/await or throw/catch. Just like how generic collections allow users to create their own parametrized collection, algebraic effects provide users with the power to define their own complex control structures.
The Koka documentation describes effect handlers using the following example,
effect yield
ctl yield( i : int ) : bool
fun traverse( xs : list<int> ) : yield ()
match xs
Cons(x,xx) -> if yield(x) then traverse(xx) else ()
Nil -> ()
fun print-elems() : console ()
with ctl yield(i)
println("yielded " ++ i.show)
resume(i<=2)
traverse([1,2,3,4])
This program prints
yielded: 1
yielded: 2
yielded: 3
The equivalent code using this library is,
trait Yield extends Effect1[Int, Boolean]:
def apply(i: Int): Boolean = get(i)
object Yield:
def eval[R](using Handler[Yield])(f: Yield ?=> R): R =
(new FSM(f) with Yield).eval
def traverse(xs: List[Int])(using yields: Yield): Unit =
xs match
case x :: xx => if yields(x) then traverse(xx) else ()
case Nil => ()
@main def printElems(): Unit =
given Handler[Yield] = (_: Yield) => (i) =>
println("yielded " + i.toString)
i <= 2
Yield.eval:
traverse(List(1,2,3,4))This is a bit longer, and it has more keywords, but it's pretty close!