Scala implicit composition

// Let's check implicit composition in Scala, e.g.:
new BaseEmailChecker().withLengthMoreThan(10).withDomainCheck("gmail").withEnding(".com")
// Question: how many times the base function will be executed?

trait Checker {
  def execute(s: String): Boolean
}

class BaseEmailChecker extends Checker {
  override def execute(s: String): Boolean = {
    println("CHECKING THE BASE FUNCTION!")
    s.contains("@") && s.contains(".")
  }
}

object CheckerBuilder {
  implicit class RichChecker(base: Checker) {
    def withLengthMoreThan(n: Int) = new Checker {
      override def execute(s: String): Boolean = {
        println("CHECKING LENGTH!")
        base.execute(s) && s.length > n
      }
    }

    def withDomainCheck(domain: String) = new Checker {
      override def execute(s: String): Boolean = {
        println("CHECKING DOMAIN!")
        base.execute(s) && s.contains(domain) // regex would be better than just 'contains'
      }
    }

    def withEnding(ending: String) = new Checker {
      override def execute(s: String): Boolean = {
        println("CHECKING ENDING!")
        base.execute(s) && s.endsWith(ending)
      }
    }
  }
}

import CheckerBuilder._

val checker1 = new BaseEmailChecker()
val checker2 = new BaseEmailChecker().withLengthMoreThan(10).withDomainCheck("gmail").withEnding(".com")



// Come on!
scala> checker1.execute("amitrakov@tala.co")
CHECKING THE BASE FUNCTION!
res0: Boolean = true

scala> checker2.execute("amitrakov@tala.co")
CHECKING ENDING!
CHECKING DOMAIN!
CHECKING LENGTH!
CHECKING THE BASE FUNCTION!
res1: Boolean = false

scala> checker2.execute("amitrakov@gmail.com")
CHECKING ENDING!
CHECKING DOMAIN!
CHECKING LENGTH!
CHECKING THE BASE FUNCTION!
res2: Boolean = true

// summary: the base function is executed only once!