Lab for Automated Reasoning and Analysis LARA

Tiny Expression Compiler Proved Correct in Leon

/* Expression compiler correctness.
   Viktor Kuncak and Filip Maric
*/
import leon.Utils._
import leon.Annotations._
 
object ExpressionCompiler {
  sealed abstract class Sign
  case class NUM(x : Int) extends Sign
  case object PLUS extends Sign
 
  sealed abstract class List
  case class Cons(head : Sign, tail : List) extends List
  case class Nil() extends List
  def single(x: Sign) = Cons(x, Nil())
 
  sealed abstract class Expr
  case class Num(x: Int) extends Expr
  case class Plus(e1: Expr, e2: Expr) extends Expr
 
  def eval(e: Expr) : Int = {
   e match {
     case Num(x) => x
     case Plus(e1, e2) => eval(e1) + eval(e2)
   }
 }
 
 def postfixAcc(e : Expr, acc : List) : List = {
   e match {
     case Num(x) => Cons(NUM(x),acc)
     case Plus(e1, e2) => postfixAcc(e1, postfixAcc(e2, Cons(PLUS, acc)))
   }
 }
 
 sealed abstract class Stack
 case class Empty extends Stack
 case class Push(i: Int, s: Stack) extends Stack
 
 def run(l:List, s: Stack) : Stack = {
   l match {
     case Nil() => s
     case Cons(NUM(x), ss) => run(ss, Push(x, s))
     case Cons(PLUS, ss) => s match {
       case Push(a, Push(b, s1)) => run(ss, (Push(a + b, s1)))
       case _ => Empty()
     }
   }
 }
 
  // note: no 'holds', it would not work directly
 def run_lemma(e: Expr, ss: List, stack: Stack) = {
   run(postfixAcc(e, ss), stack) == run(ss, Push(eval(e), stack))
 }
 
  // we express induction with generalization using appropriate recursion
 def run_lemma_induct(e: Expr, ss: List, stack: Stack) : Boolean = {
   e match {
     case Num(x) => run_lemma(e, ss, stack)
     case Plus(e1, e2) => run_lemma(e, ss, stack) && 
       run_lemma_induct(e1, postfixAcc(e2, Cons(PLUS, ss)), stack) &&
       run_lemma_induct(e2, Cons(PLUS,ss), Push(eval(e1), stack))
   }
 } holds
 
  // inlining is crucial to knowing that property was proved
  // proof hints are expressed as extra conjuncts
 def test_run_lemma(e: Expr, ss: List, stack: Stack) = {
   run_lemma_induct(e, ss, stack) &&
   run(postfixAcc(e, ss), stack) == run(ss, Push(eval(e), stack))
 } holds
 
  // thanks to first conjunct, this now works
  def thm(e: Expr) = {
    test_run_lemma(e,Nil(),Empty()) &&
    run(postfixAcc(e, Nil()), Empty()) == Push(eval(e), Empty())
  } holds
 
}
 
sav13/expressioncompiler.scala.txt · Last modified: 2013/05/27 13:58 by vkuncak
 
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