drstevens

Advanced Functional Programming with Scala - Notes

Copyright © 2017 Fantasyland Institute of Learning. All rights reserved.

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

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trait Writes[T] {
  def writes(o: T): JsValue
}

trait Reads[T] {
  def reads(json: JsValue): T
}

trait Format[T] extends Writes[T] with Reads[T]

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import scalaz._, Scalaz._
 
def setS(i: Int): State[List[Int], Unit] = modify(i :: _)
 
val s = (1 to 10000).foldLeft(state[List[Int], Unit](()).lift[Free.Trampoline]) {
  case (st, i) => st *> setS(i).lift[Free.Trampoline]
}
 
s(Nil).run

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// Doesn't compile. Good.
def futureInt(): Future[Int] = Future.value(Future.value(1))

// Compiles. Not good. Caller cannot wait on completion of nested future.
def futureUnit(): Future[Unit] = Future.value(Future.value(()))

// Also compiles. Nice convenience, but not worth the cost of the issues
// it might cause (as above), right?
def futureUnit(): Future[Unit] = Future.value(1)

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-Dconfig.resource=test.conf -Dlogger.file=test/resources/logback-test.xml

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def allDigits: (String) => ValidationNEL[String, String]
def maxSizeOfTen: (String) => ValidationNEL[String, String] 
def toInt: (String) => ValidationNEL[String, Int]

val validInt: String => ValidationNEL[String, Int] = s =>
  for {
    validStr <- (allDigits(s) |@| maxSizeOfTen(s))((_,x) => x)
    i <- toInt(validStr)