Design an extensible expense rule evaluator

Problem Statement

You are tasked with building the core of an expense management system that validates expense reports against a set of business rules (e.g., 'Expenses over $500 require manager approval', 'Meals cannot exceed $50 per day', or 'International travel requires VP approval').

Your design must be extensible: adding a new rule (e.g., 'Flag expenses from high-risk vendors') should not require modifying existing core engine code. Assume an expense report object has fields like amount, category, date, employee_level, and vendor_id.

Requirements

1. Define a schema or class structure for a Rule and an ExpenseReport.
2. Implement an Evaluator class that takes a list of rules and an expense report, returning a result indicating if the expense is 'APPROVED', 'REJECTED', or 'NEEDS_REVIEW'.
3. Provide a clear mechanism for registering new rules dynamically.

Interface Example (Python-style)

class ExpenseReport:
    # Define fields: amount, category, date, etc.

class Rule:
    def evaluate(self, report: ExpenseReport) -> str: # e.g., 'PASS', 'FAIL', 'FLAG'
        pass

class Evaluator:
    def add_rule(self, rule: Rule):
        pass
    
    def evaluate_report(self, report: ExpenseReport) -> str:
        pass

Constraints

• Rules may depend on multiple fields.
• An expense might trigger multiple rules; define how to aggregate these results (e.g., if one rule rejects, is the whole report rejected?).
• Consider how to handle rules that involve external data (e.g., checking a currency conversion rate).