/**
 * A structural type for an expression that has a getFirst and a getSecond method
 * which both return a Symbols.Type.
 */
type TwoPartExpression = {
  def getFirst : Symbols.Type
  def getSecond : Symbols.Type
}

/**
 * Determines the type of the given two part expression's operands.
 *
 * If the types are divergent, throws an exception.
 */
def expressionType(expression:TwoPartExpression) : Symbols.Type = {
  val (firstType, secondType) = (expression.getFirst, expression.getSecond)

  if (firstType == secondType)
    firstType
  else
    throw new Exception(...)
}