Question

Given this very simple Variable – Clause model

class Variable(name: String, negated: Boolean = false) {
    def evaluate(b: Boolean): Boolean = negated match {
        case false => b
        case true => !b
    }

    def negate = new Variable(this.name, !this.negated)

    override def toString = negated match {
        case false => name
        case true => "~" + name
    }

    override def hashCode() = name.hashCode()

    override def equals(obj: Any) = name.equals(obj)
}

class Clause(vars: List[Variable]) {
    def evaluate(values: Map[Variable, Boolean]): Boolean = {
        val evaluatedVars = vars.map(v => v.evaluate(values(v)))
        (false /: evaluatedVars)(_ || _)
    }
}

This simple assertion throws an exception

val c = new Variable("c")
val d = new Variable("d")
val c3 = new Clause(List(c.negate, d))
assertTrue( c2.evaluate( Map(c -> false, d -> true) ) )

java.util.NoSuchElementException: key not found: ~c

What am I missing?

Answer 1

Variable is not a String. Your equals is broken.

You want:

override def equals(a: Any) = a match {
  case v: Variable => v == name
  case s: String => name == s
  case _ => false
}

(if you don’t want to expose name even privately, but you want Variable to match random strings).