Kevin Meredith kevinmeredith

Advanced 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

	trait Element[A, B] {
	val input: A
	val filteredValue: Option[B]
	}

	def filterFn[A, B](queryFilterValues: List[B])(implicit ev: A => Element[A, B]): List[A] => List[A] =
	elements => {
	queryFilterValues match {
	case _ :: _ => elements.flatMap { e =>
	ev(e).filteredValue match {

	package net

	import shapeless.{::, DepFn1, HList}

	trait Init[L <: HList] extends DepFn1[L]

	object Init {

	type Aux[L <: HList, Out1] = Init[L] { type Out = Out1}

	scala> import scala.collection.immutable.ListMap
	import scala.collection.immutable.ListMap

	scala> ListMap[Int, String]( (42, "hi"), (66, "world") )
	res1: scala.collection.immutable.ListMap[Int,String] = Map(42 -> hi, 66 -> world)

	scala> val list: List[(Int, String)] = List( (42, "hi"), (66, "world") )
	list: List[(Int, String)] = List((42,hi), (66,world))

	scala> ListMap[Int, String]( list: _* )

	package net

	import shapeless.{::, Generic, HList, HNil, Lazy}

	sealed trait JsonValue
	case class JsonObject(fields: List[(String, JsonValue)]) extends JsonValue
	case class JsonArray(items: List[JsonValue]) extends JsonValue
	case class JsonString(value: String) extends JsonValue
	case class JsonNumber(value: Double) extends JsonValue
	case class JsonBoolean(value: Boolean) extends JsonValue

	scala> :paste
	// Entering paste mode (ctrl-D to finish)

	class A(private [this] val x: Int)
	object A {
	def f(a: A) = a.x
	}

	// Exiting paste mode, now interpreting.

	scala> :paste
	// Entering paste mode (ctrl-D to finish)

	import scala.annotation.tailrec

	@tailrec
	def odd1(n: Int): Boolean = {
	if (n == 0) false
	else even1(n - 1)
	}

	/*

	Overview
	--------

	To run a query using anorm you need to do three things:

	1. Connect to the database (with or without a transaction)
	2. Create an instance of `anorm.SqlQuery` using the `SQL` string interpolator
	3. Call one of the methods on `SqlQuery` to actually run the query

	trait Reads[A]

	case class Service(a: String) {
	implicit val xReads = X.reads(this)
	}

	case class X(y: Int)
	object X {
	def reads(s: Service): Reads[X] = new Reads[X] {}
	}

	sealed trait DoorStatus
	case object Open extends DoorStatus
	case object Closed extends DoorStatus

	object DoorStatus {

	val OpenShowValue = "open"
	val ClosedShowValue = "closed"

	def show(d: DoorStatus): String = d match {