Source code for this article: https://mostafa-asg.github.io/post/writing-internal-dsl-in-scala/

Dfa.scala

class Dfa {

  var states = Seq.empty[State]
  var finalStates = Seq.empty[State]
  var currentState : State = null
  var input:String=""
  val transition = new Transition
  val transitionMap = transition.transitionMap

  def states( block: => Seq[State] ): Dfa = {
    states = block
    this
  }

  def finalStates( block: => Seq[State] ): Dfa = {
    finalStates = block
    this
  }

  def transitions( f: Transition => Unit ): Dfa = {
    f(transition)
    this
  }

  def startFrom(start: State):Dfa = {
    currentState=start
    this
  }

  def withInput(input:String):Dfa = {
    this.input=input
    this
  }

  def run : Boolean = {
    input.foreach { ch =>
      transitionMap.get( (currentState,ch) ) match {
        case Some(state) => currentState=state
        case None => throw new Exception(s"You must provide transition function for input '$ch' when state is ${currentState.toString}")
      }
    }

    finalStates contains currentState
  }

}

class Transition {
  val transitionMap : scala.collection.mutable.HashMap[(State,Char),State] =
    new scala.collection.mutable.HashMap()

  def on(ch : Char) : TransitionFrom = new TransitionFrom(this,ch)
}

class TransitionFrom(val tr: Transition , val ch : Char) {
  def from(s: State) : TransitionTo = new TransitionTo(tr,ch,s)
}

class TransitionTo(val tr:Transition , val ch : Char,val from:State) {
  def to(s : State) : Unit = {
    tr.transitionMap += (from,ch)->s
  }
}

object Dfa {

  def newDfa( f: Dfa => Unit ) : Dfa = {
    val dfa = new Dfa
    f(dfa)
    dfa
  }

}

Main.scala

object Main {

  def main(args: Array[String]): Unit = {

    import Dfa._

    val dfa = newDfa { dfa =>

      dfa states {
        Seq(S0, S1, S2, S3)
      }

      dfa finalStates {
        Seq(S2)
      }

      dfa transitions { transition =>
        transition on '0' from S0 to S1
        transition on '1' from S0 to S3
        transition on '0' from S1 to S2
        transition on '1' from S1 to S1
        transition on '0' from S2 to S2
        transition on '1' from S2 to S1
        transition on '0' from S3 to S3
        transition on '1' from S3 to S3
      }
    } startFrom S0 withInput "010101011110110110000"

    val hasInputAccepted = dfa.run
  }

}

State.scala

sealed trait State

final case object S0 extends State
final case object S1 extends State
final case object S2 extends State
final case object S3 extends State