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@valtih1978 valtih1978/Expressions.sc
Last active Nov 4, 2015

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Symbolic expressions
trait E {
def e(dict: Map[Const, Int]): Int
def +(another: E): E = (this, another) match {
case (zero, _) => another // Bug! Bug! this unintentionally redefines zero
case (_, zero) => this
case _ => Plus(this, another)
}
def *(another: E): E = {
def par(e: E) = e match {case e @ Plus(a,b) => Par(e); case e => e}
Mult(par(this), par(another))
}
}
case class Const(a: Char) extends E {
override def toString = a toString
def e(dict: Map[Const, Int]): Int = dict.get(this).orElse(Some(a - '0')).get
}
case class Par(e: E) extends E {
override def toString = s"($e)"
def e(dict: Map[Const, Int]): Int = e.e(dict)
}
case class Plus(a: E, b: E) extends E {
override def toString = s"$a+$b"
def e(dict: Map[Const, Int]): Int = a.e(dict) + b.e(dict)
}
case class Mult(a: E, b: E) extends E {
override def toString = s"$a*$b"
def e(dict: Map[Const, Int]): Int = a.e(dict) * b.e(dict)
}
val q = Const('q') ; val p = Const('p') ; val one = Const('5')
//> q : A.Const = q
//| p : A.Const = p
//| one : A.Const = 5
val env = Map(q -> 1, p -> 2) //> env : scala.collection.immutable.Map[A.Const,Int] = Map(q -> 1, p -> 2)
q.e(env) //> res0: Int = 1
Plus(q, p) //> res1: A.Plus = q+p
Plus(p, one).e(env) //> res2: Int = 7
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