// DFAs in Scala based on partial functions
import scala.util.Try

// type abbreviation for partial functions
type :=>[A, B] = PartialFunction[A, B]

case class DFA[A, C](start: A,               // starting state
                     delta: (A, C) :=> A,    // transition (partial fun)
                     fins:  A => Boolean) {  // final states

  def deltas(q: A, s: List[C]) : A = s match {
    case Nil => q
    case c::cs => deltas(delta(q, c), cs)
  }

  def accepts(s: List[C]) : Boolean = 
    Try(fins(deltas(start, s))) getOrElse false
}

// the example shown earlier in the handout 
abstract class State
case object Q0 extends State
case object Q1 extends State
case object Q2 extends State
case object Q3 extends State
case object Q4 extends State

val delta : (State, Char) :=> State = 
  { case (Q0, 'a') => Q1
    case (Q0, 'b') => Q2
    case (Q1, 'a') => Q4
    case (Q1, 'b') => Q2
    case (Q2, 'a') => Q3
    case (Q2, 'b') => Q2
    case (Q3, 'a') => Q4
    case (Q3, 'b') => Q0
    case (Q4, 'a') => Q4
    case (Q4, 'b') => Q4 }

val dfa = DFA(Q0, delta, Set[State](Q4))

dfa.accepts("bbabaab".toList)   // true
dfa.accepts("baba".toList)      // false