Zach McCoy zmccoy

Thread Pools

Thread pools on the JVM should usually be divided into the following three categories:

CPU-bound
Blocking IO
Non-blocking IO polling

Each of these categories has a different optimal configuration and usage pattern.

Why not `both`?

With the recent announcement of cats-effect, a relevant question from the past resurfaces: why does IO, which is otherwise quite Task-like, not define both or race? To be clear, the type signatures of these functions would be as follows:

object IO {
  def both[A, B](ioa: IO[A], iob: IO[B])(implicit EC: ExecutionContext): IO[(A, B)] = ???
  def race[A, B](ioa: IO[A], iob: IO[B])(implicit EC: ExecutionContext): IO[Either[A, B]] = ???
}

Applied Functional Programming with Scala - Notes

1. Mastering Functions

A function is a mapping from one set, called a domain, to another set, called the codomain. A function associates every element in the domain with exactly one element in the codomain. In Scala, both domain and codomain are types.

val square : Int => Int = x => x * x

Explaining Miles's Magic

Miles Sabin recently opened a pull request fixing the infamous SI-2712. First off, this is remarkable and, if merged, will make everyone's life enormously easier. This is a bug that a lot of people hit often without even realizing it, and they just assume that either they did something wrong or the compiler is broken in some weird way. It is especially common for users of scalaz or cats.

But that's not what I wanted to write about. What I want to write about is the exact semantics of Miles's fix, because it does impose some very specific assumptions about the way that type constructors work, and understanding those assumptions is the key to getting the most of it his fix.

For starters, here is the sort of thing that SI-2712 affects:

def foo[F[_], A](fa: F[A]): String = fa.toString

	import cats._

	// A pure functional, contravariant monoidal string builder
	sealed trait SB[A] {
	final def render(a: A): String = {
	val sb = new StringBuilder
	renderInternal(sb, a)
	sb.toString
	}

	import shapeless._

	@annotation.implicitNotFound("Make sure all cases of ${C} have an instance of ${TC}")
	trait CopTCs[C <: Coproduct, TC[_]] {
	type Out <: HList
	def result: Out
	}
	object CopTCs {
	type Aux[C <: Coproduct, TC[_], O <: HList] = CopTCs[C, TC] { type Out = O }
	def apply[C <: Coproduct, TC[_]](implicit ev: CopTCs[C, TC]) = ev

	package cats.bench

	import cats.data.Xor
	import cats.instances.either._
	import cats.syntax.all._
	import org.openjdk.jmh.annotations.{ Benchmark, Scope, State }

	@State(Scope.Benchmark)
	class EitherBench {
	val ea: Either[String, Int] = Right(1)

	// addCompilerPlugin("com.milessabin" % "si2712fix-plugin" % "1.2.0" cross CrossVersion.full)
	import scalaz._, Scalaz._

	implicit def f2k[F[_], A, B](f: A => F[B]) = Kleisli(f)

	val a: Int => ListT[List, Int] = {
	case 0 => ListT(List(List(0, 1)))
	case 1 => ListT(List(List(0), List(1)))
	}

	val e = Option(List(Right(2), Left("bad")))

	runResult {
	for {
	as <- (e must beSome).opt
	a <- fromList(as)
	i <- (a must beRight).opt
	_ <- (i must be_>(0)).check
	} yield ()
	}