Monthly Archives: October 2008

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Software is fiction

It’s hard for me to believe I’ve never run across Richard Gabriel’s essay Lessons from the Science of Nothing at All before.

Suppose you wanted to design and build a gas stove, but no one had ever used gas to cook with before. In fact, assume that before this, all that had ever been used was a wood stove.

Here are some things you wouldn’t know: Will you use the gas flame to heat a piece of metal on which the pots and pans will sit, and that metal will transfer the heat, or will the flame be applied directly to the pots and pans? How will you control the temperature? Will you require that people move their pans from places of intense heat to places of less heat? Will you provide a gas flow controller that will reduce the amount of fuel to the flame? Will you move the flame closer or farther away from the pan or heat transfer metal? Will the heat be a manageable temperature for all kinds of cooking or only for limited use? Will cooks be afraid of using gas? How will you get it into the stove? Will it be stored or continuously supplied? How will you control the temperature of an oven using gas? Will a stove designed this way be affordable? Will the stove explode? Will the gas smell funny? Will the stove need to be inside or outside the house?

There are thousands of questions like this in every software project I’m calling unpredictable, and I’m also claiming that almost all software projects are unpredictable.

(Via Tim Bray.)

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Fun, or rather no fun, with generics (updated)

So, I understand that the following doesn’t work, but why doesn’t it work?

interface Adapter {}

class Adaptulator<I> {
    <e, A extends I & Adapter> void add(Class extl, Class<A> intl) {
    	addAdapterFactory(new AdapterFactory(extl, intl));
    }
}

The add() method gives me a compile error, “Cannot specify any additional bound Adapter when first bound is a type parameter” (in Eclipse), or “Type parameter cannot be followed by other bounds” (in IDEA), take your pick.

Clearly you’re just Not Allowed to use the type parameter I there, before the &, and that’s that. (And before you ask, it doesn’t work if you switch ’em, because there’s no guarantee that I isn’t a concrete class.) But why not? I’ve looked through Angelika Langer’s FAQ and can’t find an answer.

Generally when some generics limitation seems arbitrary, it’s because you’ve created a situation where the type system can’t actually enforce correctness. But I don’t see what case would break what I’m trying to do here. I’d say maybe it has something to do with method dispatch after type erasure, but there’s only one add() method, so it’s not like there’s any ambiguity…

So what’s the problem?

Update: I cross-posted this to stackoverflow.com, and Bruno De Fraine pointed out that the reason multiple bounds were introduced in the first place was to control the erasure. Since I is just going to erase to Object, it doesn’t get you anything there. Unfortunately, all I wanted was the compile-time type safety, so I’ll have to come up with something else…