Your First Rule

Write a RuleBook with one rule, run it, and read the answer.

Your First Rule

We start with the smallest useful rule: decline anyone with a low credit score.

Create a RuleBook

A RuleBook is a class that extends rulebox.models.RuleBook and adds its rules in a defineRules() method. Save this as models/LoanApproval.bx:

/**
 * Decides a loan application from the applicant's credit score.
 */
class extends="rulebox.models.RuleBook"{

	function defineRules(){
		addRule(
			newRule( "declineLowScores" )
				.withDescription( "Declines a score under 580" )
				.when( ( facts ) => facts.creditScore < 580 )
				.then( ( facts, result ) => result.setValue( "DECLINED" ) )
		)
	}

}

Read it out loud, it says what it does:

• The comment above the class describes the whole rulebook.
• newRule( "declineLowScores" ) makes a rule and names it. Always name your rules. The name shows up when you look at what ran.
• withDescription(...) says in plain words what the rule does. Descriptions do not change how anything runs; they show up in the Visualizer (lesson 9) and in getDescription().
• when(...) is the condition. It receives the facts and returns true or false.
• then(...) is the action. It runs only when the condition is true. It receives the facts and the result to fill in.
• addRule(...) puts the rule in the book.

Run it

Create a handler that runs the rulebook with some facts. Save this as handlers/Loans.bx:

class {

	function decide( event, rc, prc ){
		var decision = getInstance( "LoanApproval" )
			.run( { creditScore: 540 } )
			.getResult()
			.getValue()

		event.renderData( type="text", data=decision )
	}

}

Three calls do the work:

1. run( facts ) hands the facts to the rules and runs them. It returns the RuleBook itself.
2. getResult() gets the Result object.
3. getValue() reads what the rules put in it.

With a score of 540, the answer is DECLINED.

Use getInstance() every time you need a RuleBook. A RuleBook remembers its facts and result, so you want a fresh one for each decision.

Try it

Change 540 to 700 and run it again. The condition is now false, so the then never runs and no value is set. That is not an error, it just means no rule had an opinion. The next lesson shows how to handle that.

Next: facts and results in detail.