deanwampler

trait Transaction(database: DB):
  def begin(query: String): Either[String,Status]
  def rollback(): Either[String,Status]
  def commit(): Either[String,Status] 

trait ConnectionManager(database: DB):
  def createTransaction: Transaction

def doTransaction(query: => String)(
    using cm: => ConnectionManager): Seq[Status] = ...

deanwampler

scala> import scala.language.strictEquality    // Enable the language feature
     
scala> enum Tree[T] derives CanEqual:          // A type hierarchy that derives CanEqual
     |   case Branch(left: Tree[T], right: Tree[T])
     |   case Leaf(elem: T)
// defined class Tree

scala> import Tree.*

scala> val l1 = Leaf("l1")

deanwampler

scala> import scala.annotation.targetName
     
scala> trait Semigroup[T]:             // also used in other examples for Type Classes
     |   extension (t: T)
     |     infix def combine(other: T): T
     |     @targetName("plus") def <+>(other: T): T = t.combine(other)
     |
     | trait Monoid[T] extends Semigroup[T]:
     |   def unit: T
     

deanwampler

case class U1[+T](t: T)
case class U2[+T](t: T)

def f1[T1,T2](name: String)(using u1: U1[T1])(using u2: U2[T2]): String = 
  s"f1: $name: $u1, $u2"
def f2[T1,T2](name: String)(using u1: U1[T1])(u2: U2[T2]): String =       
  s"f2: $name: $u1, $u2"

given u1i: U1[Int] = U1[Int](0)
given u2s: U2[String] = U2[String]("one")

deanwampler

// Compare to scala2-3-MonoidTypeClassNumeric2.scala
// Anonymous givens; just remove the name!
given [T](using num: Numeric[T]): Monoid[T] with
  def unit: T = num.zero
  extension (t: T) def combine(other: T): T = num.plus(t, other)

// or use the _context bound_ syntax:
// given [T: Numeric]: Monoid[T] with
//   def unit: T = summon[Numeric[T]].zero   // Use summon to retrieve the Numeric
//   extension (t: T) def combine(other: T): T = summon[Numeric[T]].plus(t, other)

deanwampler

scala> Service.isAuthenticated
     | Service.authenticate(UserName("Buck Trends"), Password("1234"))
     | Service.isAuthenticated
     |
val res8: Boolean = false
val res9: Boolean = true
val res10: Boolean = true

deanwampler

// Adapted from Programming Scala, Third Edition code repo:
// https://github.com/deanwampler/programming-scala-book-code-examples/blob/master/src/script/scala/progscala3/basicoop/Exports.scala

import java.net.URL

case class UserName(value: String):
  assert(value.length > 0)
case class Password(value: String):
  assert(value.length > 0)

deanwampler

scala> val o1 = Option.Some(1)
val o1: Option[Int] = Some(1)        // Note the inferred types for all four cases.

scala> val o2 = Option.None
val o2: Option[Nothing] = None

scala> import Option._

scala> val o3 = Some(1)
val o3: Some[Int] = Some(1)

deanwampler

enum Option[+T]:
  case Some(x: T)
  case None
  
enum Option2[+T]:
  case Some(x: T) extends Option2[T]
  case None       extends Option2[Nothing]

deanwampler

scala> trait Logger:
     |   def log(message: String): Unit
     
scala> val info = new Logger:
     |   def log(message: String): Unit = println(s"INFO: $message")
val info: Logger = anon$1@16f19cc9

scala> info.log("Hello world!")
INFO: Hello world!