Maybe null

My colleague Andy posted today about Returning null considered Dishonest and while I agree with his sentiments, I disagree with his solution.

Andy’s premise is that you should never expect or return a null object as “Returning null is dishonest. It requires others to check that we’ve upheld our side of the bargain”.  His solution is to throw meaningful exceptions within the method, so the happy path will return a not-null object, and the unhappy path throws exceptions.

This highlights one practice which I get annoyed with when I see it. Programming by exception is never a good idea. Exceptions should be reserved for exceptional circumstances only and should not contain business logic. If there is a possibility that the system can get into a state which returns null, then it should handle that situation, instead of letting it throw a tantrum and yell at the caller.

A better alternative would be to return an object which may or may not contain the value required. This Maybe object can have two implementations: when it can return the value, it returns a Something which contains the value;  and when it cannot return the value (ie value would be null) it returns a Nothing (perhaps with a message detailing why).

To continue Andy’s example of the Vending Machine which dispenses a drink only if enough money has been inserted and there is enough stock, we can take a look at how Maybe objects can be used.

public Maybe<drink> giveMeADrink()
{
	if (weDontHaveEnoughMoney)
	{
		return new Nothing<drink>(notEnoughMoneyMessage);
	}

	if (weDontHaveEnoughStock)
	{
		new Nothing<drink>(weDontHaveEnoughStockMessage);
	}

	return new Something<drink>(new Drink());
}

// Caller
Maybe[Drink] drink = giveMeADrink()
if (drink.IsPresent)
{
	SlideDownChute(drink.Force())
} else {
	ReportProblem(drink.Force())
}

To me its a much cleaner version, you are coding for the situations that can actually occur, and not relying on try-catch statements around the place. 

My advice? Steer clear of exceptions and return Maybes instead of nulls.