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b-studios / handlers.rkt
Last active July 17, 2024 10:49
Effect handlers in Racket
#lang racket
(require racket/control)
(require racket/syntax)
(struct operation (name effect))
(struct effect-call (op args cont))
(define (do op . args)
(control0-at (operation-effect op) k (effect-call op args k)))
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b-studios / 01.TEXT.md
Last active September 1, 2019 14:25
Algebraic Effects and Handlers in under 50 Lines of Ruby

Some people say "effect handlers are the new monads". So I thought, before all these "Effect Handlers are like X" (for X in { Burritos, Containers, ... }) blog posts come into existence, I contribute another point-of-view. This blog post does not really explain the theory behind algebraic effects and handlers. Neither does it explain why you should use them, or how. We just look at how effect handlers work operationally, by implementing them.

This post is a bit longer. If you are in a hurry, I recommend you jump to Step 2.

I do recommend the blog post [Algebraic effects for the rest of us][RestOfUs] which explores another aspect of the topic.

Prelude

I spend most of my awake time doing research on [effect handlers][EffectsResearch] (in particular in [Scala][ScalaEffekt] and [Java][JavaEffekt]). Unsurprisingly, I am happy to see that effect handlers not only catch on in research, but also programmers in industry find them interesting. In particular, some [developers at Facebook][RestOf

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b-studios / 01.README.md
Last active July 15, 2019 11:27
The Existential Unapply Trick

Did you ever run into the situation, where you thought you need higher ranked polymoprhism in Scala? While it is somewhat supported in Scala, once you go down this route, you loose some convenience that eventually disrupts your API. In this post, I show how in some cases we can use our old friend unapply in Scala to recover some of the convenience.

The Problem

Let's assume in your API you want to have users write functions of type:

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b-studios / bug.scala
Last active April 10, 2019 18:07
Bad interaction of type matcher and meta programming?
package examples
import scala.quoted._
object bug extends App {
val toolbox = scala.quoted.Toolbox.make(getClass.getClassLoader)
implied for scala.quoted.Toolbox = toolbox
sealed trait HList
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b-studios / 01.README.md
Last active January 17, 2019 18:27
Even More Lightweight Monadic Regions

Delimiting the lifetime of a resource to a particular scope is a common problem. In this post, I revisit "Lightweight Monadic Regions" (Kiselyov and Shan, [2008][kiselyov2008lightweight]) and show how to generalize the ST-trick to nest monadic regions in a new way.

TLDR; The core idea is to index a monadic type by the set of regions it requires to be "alive". This is not a new idea, but typically the set is represented by some typelevel list. Region subtyping then requires implicit evidence that one typelevel list is a sublist of another one.

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b-studios / implicits.scala
Created December 20, 2017 16:45
Implicits in Unapply
// probably could be used for some cool type state stuff
sealed trait HList
case class HCons[A, R <: HList](head: A, tail: R) extends HList {
def ::[B](b: B): B :+: A :+: R = HCons(b, this)
}
case object HNil extends HList {
def ::[A](a: A): A :+: HNil = HCons(a, this)
}
type HNil = HNil.type
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b-studios / challenge.kk.js
Created November 7, 2017 09:57
Challenge to compile using iterated CPS
effect exc {
raise(n: int): a;
}
val inc = handler {
raise(n) -> raise(n + 1)
}
val get = handler {
raise(n) -> n
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b-studios / effects.js
Created October 26, 2017 07:06
Quick experiments with yielding effects in javascript
function * get() { return yield { label: "get", payload: null }; }
function * put(s) { return yield { label: "put", payload: s }; }
function * state(init, prog) {
var s = init;
var cmd = prog.next();
while (!cmd.done) {
switch (cmd.value.label) {
case "get":
cmd = prog.next(s);
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b-studios / 00-source.scala
Created October 10, 2017 09:02
Result of bytecode instrumentation
trait AmbList[R] extends Amb with Handler[R, List[R]] {
private var coin: Boolean = true
def pure = r => List(r)
@Suspendable
def flip() = {
println("Called with coin state: " + coin)
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b-studios / Example.java
Created May 30, 2017 17:28
Short example demonstrating how to read from a java.nio.ByteBuffer
import java.nio.ByteBuffer;
import java.util.Arrays;
class Header {
private final Short framesize;
public Header(short framesize) {
this(new Short(framesize));
}
public Header(Short framesize) {
this.framesize = framesize;