A Complex Example

A full home-loan rate calculator, built from real requirements.

A Complex Example

The requirements: MegaBank issues home loans. If an applicant's credit score is less than 600, they must pay 4x the current rate. If a credit score is between 600 and 700, they must pay an additional point on top of their rate. If a credit score is at least 700 and they have at least $25,000 cash on hand, they get a quarter-point reduction. If an applicant is a first-time home buyer, they get a 20% reduction on their calculated rate after credit-score adjustments (first-time buyer discount only applies to a credit score of 600 or greater).

We'll build the rules and track the result using a Result object - see Facts & Results.

The applicant

class{

	property creditScore;
	property cashOnHand;
	property firstTimeHomeBuyer;

	function init( creditScore, cashOnHand, firstTimeHomeBuyer ){
		variables.creditScore        = arguments.creditScore
		variables.cashOnHand         = arguments.cashOnHand
		variables.firstTimeHomeBuyer = arguments.firstTimeHomeBuyer
		return this
	}

}

The rules

/**
 * This rule book determines rules for a home loan rate
 */
class extends="rulebox.models.RuleBook"{

	function defineRules(){
		// credit score under 600 gets a 4x rate increase
		addRule(
			newRule()
				.when( ( facts ) => facts.applicant.getCreditScore() < 600 )
				.then( ( facts, result ) => result.setValue( result.getValue() * 4 ) )
				.stop()
		)

		// credit score between 600 and 700 pays a 1 point increase
		addRule(
			newRule()
				.when( ( facts ) => facts.applicant.getCreditScore() < 700 )
				.then( ( facts, result ) => result.setValue( result.getValue() + 1 ) )
		)

		// credit score is 700+ and they have at least $25,000 cash on hand
		addRule(
			newRule()
				.when( ( facts ) => facts.applicant.getCreditScore() >= 700 && facts.applicant.getCashOnHand() >= 25000 )
				.then( ( facts, result ) => result.setValue( result.getValue() - 0.25 ) )
		)

		// first time homebuyers get 20% off their rate (except if their credit score < 600)
		addRule(
			newRule()
				.when( ( facts ) => facts.applicant.getFirstTimeHomeBuyer() )
				.then( ( facts, result ) => result.setValue( result.getValue() * 0.80 ) )
		)
	}

}

Running it

Create the RuleBook, give it a starting rate and the applicant, run it, and read the result. Here it is from a handler action:

class{

	function rate( event, rc, prc ){
		return getInstance( "HomeLoanRateRuleBook" )
			.withDefaultResult( 4.5 )
			.run( { applicant : new models.Applicant( 650, 20000, true ) } )
			.getResult()
			.getValue()
	}

}

A first-time buyer with a 650 credit score gets 4.4: the starting 4.5 plus one point is 5.5, and 20% off that is 4.4. Without the first-time buyer discount (false), the answer is 5.5.

Testing it

The same rules, checked from a TestBox spec:

describe( "Home Loan Rate Rules", () => {
	it( "gives a first time buyer with a 650 credit score 4.4", () => {
		var rate = getInstance( "HomeLoanRateRuleBook" )
			.withDefaultResult( 4.5 )
			.run( { applicant : new models.Applicant( 650, 20000, true ) } )
			.getResult()
			.getValue()

		expect( rate ).toBe( 4.4 )
	} )

	it( "gives a repeat buyer with a 650 credit score 5.5", () => {
		var rate = getInstance( "HomeLoanRateRuleBook" )
			.withDefaultResult( 4.5 )
			.run( { applicant : new models.Applicant( 650, 20000, false ) } )
			.getResult()
			.getValue()

		expect( rate ).toBe( 5.5 )
	} )
} )

Plain facts instead of an Applicant

You don't need an Applicant class. Named facts work just as well. Only the when conditions change, to read the facts directly. For example, facts.applicant.getCreditScore() < 600 becomes:

newRule()
	.when( ( facts ) => facts.creditScore < 600 )
	.then( ( facts, result ) => result.setValue( result.getValue() * 4 ) )
	.stop()

...and you pass the facts in with run() (here HomeLoanRateRuleBook is the version of the rules that reads plain facts):

getInstance( "HomeLoanRateRuleBook" )
	.withDefaultResult( 4.5 )
	.run( { creditScore : 650, cashOnHand : 20000, firstTimeHomeBuyer : true } )
	.getResult()
	.getValue()