Justin du Coeur, AKA Mark Waks jducoeur

What follows are some of my (very) rough thoughts on what we can and should do with respect to CPS transformation in Scala at the language level. I'll try to start with some motivation behind my thinking, as well as some rambling observations on the nature of the problem space, but don't expect too much coherence here. :-)

The Problem

Async programming is hard.

Okay let's actually be more specific than that. High-performance I/O is hard. Signal multiplexing is a powerful technique for achieving high(er) performance I/O, particularly network I/O, but the tradeoff is that, in order to utilize it, the user-space programming model must allow for suspension and resumption of sequential continuations (often called "fibers" or "coroutines"). Achieving this type of programming model without significant tradeoffs in usability is what is exceptionally hard.

If that wasn't bad enough though, these problems are inextricably conflated with another set of problem spaces which are, themselves, very difficult. In

Why you prefer cats instead of zio? TF? It’s looks like zio ecosystem more widely and zio 2.0 has better performance What do you think?

Great question!

Performance-wise, it really depends on what you're doing. The problem with benchmarks (including the ones posted for ZIO and Cats Effect) is that they apply only to abstract situations, which are often nothing like what you see in real applications. A great write-up on this problem by Daniel Spiewak here, he wrote it better than I ever could: https://gist.github.com/djspiewak/f4cfc08e0827088f17032e0e9099d292

Also, this is not an app meant for production - I don't care that much about performance under load because there will be no load. And individual operations will often be bounded by I/O anyway, so the efficiency of the underlying runtime is likely not going to make a noticeable difference in use. Then again, both the client and server are JVM apps, so even the start-up penalty of the client will slow us down than picking even the least efficient e

Cats Anchored

cats-time

Cats Instances for Java Time

cats-scalacheck

Cats Instances for Scalacheck

Monoid

A monoid has two components:

`id` (or `empty`)

"make an instance from nothing"

0 (Unit => Int)
"" (Unit => String)
Nil (Unit => List[_])

Vistors, F-Algebras, and Transducers

So Doug made an interesting PR, which uses the visitor pattern described in Haoyi's post.

As I was trying to understand the vistor pattern, I wanted to see how it compared to using F-algebras.

Haoyi describes the benefits of the vistor pattern to include:

Composability
Not creating intermediate datastructures

Entry point

dotc/Compiler.scala

The entry point to the compiler contains the list of phases and their order.

Phases

Some phases executed independently, but others (miniphases) are grouped for efficiency. See the paper "Miniphases: Compilation using Modular and Efficient Tree Transformation" for details.

Dependency Injection in Functional Programming

There exist several DI frameworks / libraries in the Scala ecosystem. But the more functional code you write the more you'll realize there's no need to use any of them.

A few of the most claimed benefits are the following:

Dependency Injection.
Life cycle management.
Dependency graph rewriting.

Revisiting Tagless Final Interpreters

Tageless Final interpreters are an alternative to the traditional Algebraic Data Type (and generalized ADT) based implementation of the interpreter pattern. This document presents the Tageless Final approach with Scala, and shows how Dotty with it's recently added implicits functions makes the approach even more appealing. All examples are direct translations of their Haskell version presented in the Typed Tagless Final Interpreters: Lecture Notes (section 2).

The interpreter pattern has recently received a lot of attention in the Scala community. A lot of efforts have been invested in trying to address the biggest shortcomings of ADT/GADT based solutions: extensibility. One can first look at cats' Inject typeclass for an implementation of [Data Type à la Carte](http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesA

	// Author: Aly Cerruti
	// Please follow along in Scastie, so you can see the output of statements:
	// https://scastie.scala-lang.org/CLmK5tLuRd6rxXuEnba0rQ

	// A Category
	trait Category[
	// A collection of objects
	Obj <: AnyKind,
	// A constraint on those objects (Scala lacks dependent typing, so i.e. `Val = [A] =>> Monoid[A]` makes the category of Monoids)
	Val[_ <: Obj],

	import akka.actor._
	import scala.concurrent.duration.Duration
	import scala.concurrent.{CanAwait, ExecutionContext, Future, TimeoutException}
	import scala.util.{Failure, Success, Try}

	object LocalExecutor {
	private case class Execute(runnable: Try[Runnable])

	private class LocalExecutionContext(target: ActorRef) extends ExecutionContext {
	override def execute(r: Runnable) = target ! Execute(Try(r))