anrizal06

package propositional

import scala.annotation.tailrec
sealed abstract class Formula {
	def ∧(q:Formula) = Conjunction(this, q)

	def ∨(q:Formula) = Disjunction(this,q)

	def →(q:Formula) = Implication(this,q)

anrizal06

// To simplify the implementation, foldr_ is introduced. It is the foldr function with a modification in
// combine parameter. Instead of (A, =>B) => B, combine has type (A, B, Stream[A])=>B.
// This makes combine has an easy access to the stream being processed.
// There might be another way to implement the solution, but this one is the one that comes into my head so far.
def foldr_[A, B]( combine: (A, =>B, Stream[A]) => B, base: B)( xs: Stream[A]): B = {
  if (xs.isEmpty) 
     base
  else 
     combine(xs.head, foldr_(combine, base)(xs.tail), xs)
}

anrizal06

class MyA[A, B](val head: A, val tail: Option[MyA[B, A]]) {
     override def toString() =  head + tail.map( "," + _.toString).getOrElse("")
     def ::( x: B) = new MyA(x, Some(this))
}

object MyA {
  def apply[A,B](value: A) : MyA[A, B] = new MyA(value, None)
}
 
// examples: