Ceylon: Algebraic datatypes

	shared interface DecisionTree<out Element, in Argument> of
	Branch<Element,Argument> | Leave<Element,Argument> {
	shared formal Element evaluate(Argument arg);
	}
	shared interface Branch<out Element,in Argument> satisfies DecisionTree<Element,Argument> {
	//the predicate could be represented like this
	//shared formal Callable<Boolean, [Argument]> predicate;
	//but below we see the syntactic sugar declaration
	shared formal Boolean(Argument) predicate;
	shared formal DecisionTree<Element, Argument> left;
	shared formal DecisionTree<Element, Argument> right;
	shared actual Element evaluate(Argument arg) {
	if (predicate(arg)) {
	return left.evaluate(arg);
	} else {
	return right.evaluate(arg);
	}
	}
	}
	shared interface Leave<out Element, in Argument> satisfies DecisionTree<Element,Argument> {
	shared formal Element element;
	}
	class Purchase(isMember, hasCoupon) {
	shared Boolean isMember;
	shared Boolean hasCoupon;
	}
	class PurchaseBranch(left, right, predicate) satisfies Branch<Float, Purchase> {
	shared actual DecisionTree<Float,Purchase> left;
	shared actual Boolean(Purchase) predicate;
	shared actual DecisionTree<Float,Purchase> right;
	}
	class PurchaseLeave(element) satisfies Leave<Float, Purchase> {
	shared actual Float element;
	shared actual Float evaluate(Purchase arg) => element;
	}
	Boolean isMember(Purchase purchase) => purchase.isMember;
	Boolean hasCoupon(Purchase purchase) => purchase.hasCoupon;
	/**
	isMember
	|
	_____________________________
	| |
	true false
	| |
	hasCoupon hasCoupon
	| |
	______________ ________________
	| | | |
	true false true false
	| | | |
	8 10 12 16
	running our program prints
	8.0
	10.0
	12.0
	16.0
	**/
	void algebraic_demo() {
	PurchaseBranch root = PurchaseBranch(
	PurchaseBranch(
	PurchaseLeave(8.0),
	PurchaseLeave(10.0),
	hasCoupon),
	PurchaseBranch(
	PurchaseLeave(12.0),
	PurchaseLeave(16.0),
	hasCoupon),
	isMember
	);
	print(root.evaluate(Purchase(true, true)));
	print(root.evaluate(Purchase(true, false)));
	print(root.evaluate(Purchase(false, true)));
	print(root.evaluate(Purchase(false, false)));
	}

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