Peter Mortier kwark

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

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

	object SbtAmazonEcr extends App {
	while(true) {
	println("Hello Docker.")
	Thread.sleep(3000)
	}
	}

	set itermRunning to (application "iTerm" is running)
	set scriptPath to quoted form of POSIX path of ((path to me as text) & "::" & "start.sh")
	set user_shell to do shell script "dscl /Search -read /Users/$USER UserShell \| awk '{print $2}'"

	tell application "iTerm"
	activate
	if not (exists window 1) or (itermRunning = false) then
	reopen
	end if

	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))

	/**
	* Fancy ID generator that creates 20-character string identifiers with the following properties:
	*
	* 1. They're based on timestamp so that they sort after any existing ids.
	* 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
	* 3. They sort lexicographically (so the timestamp is converted to characters that will sort properly).
	* 4. They're monotonically increasing. Even if you generate more than one in the same timestamp, the
	* latter ones will sort after the former ones. We do this by using the previous random bits
	* but "incrementing" them by 1 (only in the case of a timestamp collision).
	*/

	package example.statemachine

	import scalaz.{State, Scalaz}, Scalaz._

	object FSM {
	def apply[I, S](f: PartialFunction[(I, S), S]): FSM[I, S] =
	new FSM((i, s) => f.applyOrElse((i, s), (_: (I, S)) => s))

	private def states[S, O](xs: List[State[S, O]]): State[S, List[O]] =
	xs.sequence[({type λ[α]=State[S, α]})#λ, O]

	import scala.annotation.target.field

	import javax.xml.bind.annotation._
	import javax.xml.bind.annotation.adapters._

	type xmlElement = XmlElement @field
	type xmlTypeAdapter = XmlJavaTypeAdapter @field

	/**
	* NB As with the CustomOptionAdapter we've had to modify the type signature here,

	realm=Sonatype Nexus Repository Manager
	host=nexus.company.com
	user=admin
	password=admin123