Playing with monads / published by https://github.com/dacr/code-examples-manager #05e12ec0-33a4-4e0e-bed1-bd734d951a5d/abf15f9c2bcad9fdc6b2d3f69f83add9c42db925

monads.sc

// summary : Playing with monads
// keywords : scala, monads, pure-functional, pattern, etw, @testable
// publish : gist
// authors : David Crosson
// license : Apache NON-AI License Version 2.0 (https://raw.githubusercontent.com/non-ai-licenses/non-ai-licenses/main/NON-AI-APACHE2)
// id : 05e12ec0-33a4-4e0e-bed1-bd734d951a5d
// created-on : 2022-01-22T15:13:44+01:00
// managed-by : https://github.com/dacr/code-examples-manager
// run-with : scala-cli $file

// ---------------------
//> using scala "3.4.2"
// ---------------------

// ------------------------------------------------------------
// Inspired from "A Monads Approach for Beginners, in Scala | Rock the JVM"
//   https://www.youtube.com/watch?v=d-dy1x33moA

// ------------------------------------------------------------
// Some definitions :
// - https://www.baeldung.com/scala/monads :
//   Monads are nothing more than a mechanism to sequence computations around values augmented with some additional feature
// - https://en.wikipedia.org/wiki/Monad_(functional_programming) :
//   A monad is a type that wraps another type and gives some form of quality to the underlying type
// -

// ------------------------------------------------------------
// A type is a Monad if it has a unit and bind functions and follow 3 composability laws
// Functions
//   - the unit function : to wrap a value inside the monad
//   - the flatmap (or bind) function : to sequence computations over a value
// PROPERTIES
//   - left identity  : Monad(x).flatMap(f) == f(x)
//   - right identity : Monad(x).flatMap(y => Monad(y)) == Monad(x)
//   - associativity  : Monad(x).flatMap(f).flatMap(g)  == Monad(x).flatMap(y => f(y).flatMap(g)) == f(x).flatMap(g)

// ------------------------------------------------------------
case class SafeValue[+T](private val hiddenValue: T): // THE UNIT FUNCTION (the constructor) (also CALLED THE PURE FUNCTION)
  def get: T = synchronized {
    hiddenValue
  }

  def flatMap[S](transformer: T => SafeValue[S]): SafeValue[S] = // THE FLATMAP FUNCTION (also CALLED THE BIND FUNCTION)
    synchronized {
      transformer(hiddenValue)
    }

// -------------------------------------------------------
// The ETW pattern - Extract/Transform/Wrap - the legacy/classic pattern
val safeMessage            = SafeValue("secret")
val message                = safeMessage.get    // EXTRACT
val capitalizedMessage     = message.capitalize // TRANSFORM
val capitalizedSafeMessage = SafeValue(
  capitalizedMessage // WRAP
)
// -------------------------------------------------------

println(SafeValue(40d).flatMap(v => SafeValue(v + 2)))
println(SafeValue("hello world").flatMap(v => SafeValue(v.capitalize)))

// -------------------------------------------------------
def capitalize(str: String): Option[String] = Some(str.capitalize)
def spacelize(str: String): Option[String]  = Some(str.map(" " + _).mkString)

// -------------------------------------------------------
// MONAD LAW 1 : LEFT IDENTITY
// SYNTAX1 : Monad(x).flatMap(f) == f(x)
// SYNTAX2 : Monad.unit(x).flatMap(f) = f(x)

assert(Option("hello").flatMap(capitalize) == capitalize("hello"))

// -------------------------------------------------------
// MONAD LAW 2 : RIGHT IDENTITY
// SYNTAX1 : Monad(x).flatMap(y => Monad(y)) == Monad(x)
// SYNTAX2 : x.flatMap(y => Monad.unit(y)) = x

assert(Option("hello").flatMap(x => Option(x)) == Option("hello"))
assert(List(1, 2, 3).flatMap(x => List(x)) == List(1, 2, 3))

// -------------------------------------------------------
// MONAD LAW 3 : ASSOCIATIVITY  ( or ETW-ETW )
// SYNTAX1 : Monad(x).flatMap(f).flatMap(g)  ==  Monad(x).flatMap(y => f(y).flatMap(g))  ==  f(x).flatMap(g) (because of LAW1)
// SYNTAX2 : x.flatMap(f).flatMap(g) == x.flatMap(y => f(y).flapMap(g))

assert(Option("hello").flatMap(capitalize).flatMap(spacelize) == Option("hello").flatMap(x => capitalize(x).flatMap(spacelize)))
assert(Option("hello").flatMap(capitalize).flatMap(spacelize) == capitalize("hello").flatMap(spacelize))