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Gavin King

On three-legged elephants

I’ve often argued that good design—of a language, library, or framework—isn’t about packing in as many features as possible into your solution, rather it’s about discovering a small set of interlocking features that act to reinforce each other. That is to say, we want to maximize the power/expressiveness of our solution, while simultaneously minimizing the surface area. I am, furthermore, quite often willing to sacrifice some flexibility for elegance. It’s my view that elegant solutions are easier to learn, more enjoyable to use, and easier to abstract over.

With this in mind, I would like to consider a problem that language designers have been working on for at least two decades: how to combine subtype polymorphism with parametric polymorphism (generics). This is a central problem faced by any object-oriented language with static typing. Recent languages have come progressively closer to a satisfying solution, but I would like to submit, if it doesn’t sound too self-serving, that Ceylon offers the most satisfying solution so far.

Our mascot is Trompon the elephant, because an elephant has four legs and would fall over if one of his legs were missing. Ceylon’s type system is exactly like this! (Yes, this is a baxplanation.)

The four legs of the type system are:

  • declaration site variance
  • ad hoc union and intersection types
  • type inference based on principal typing
  • covariant refinement and principal instantiation inheritance

If we were to take away any one of those four characteristics, all of a sudden stuff that Just Works would simply not work anymore, or, even if we could make it work, it would turn out way more complex, and involve reasoning that is difficult for the programmer to reproduce.

Consider this really simple line of code:

value animals = ArrayList { Cat(), Dog(), Person() };

The inferred type of animals is ArrayList<Cat|Dog|Person>.

  • If we were to take away declaration site covariance, then animals would not be assignable to List<Animal>.
  • If we were to take away union and intersection types, then the process for inferring the type argument would be ambiguous and much more complex. (Ceylon’s type argument inference algorithm is defined in two pages of pseudocode in the language specification, which sounds like a lot, until you realize how problematic and underspecified this algorithm is in other languages, and that the actual implementation of the algorithm is not much more longer.)
  • If we were to take away type inference, or principal typing, we would need to explicitly write down some uninteresting types in this line of code.

Minus any one of these characteristics, we’re left with a three-legged elephant.

Principal instantiation inheritance is a kind-of hidden feature of Ceylon that we don’t talk about much, even though we use it extensively throughout the design of our container types. It lets us say, for example, that a List<Element> is a Ranged<List<Element>>, and that a Sequence<Element> is a List<Element>&Ranged<Sequence<Element>> . Principal instantiation inheritance meshes really nicely with declaration-site covariance, and with ad hoc union/intersection types. Consider the following code:

List<String>|Integer[] ranged = ... ;
value span = ranged.span(0,max); 

Here, the inferred type of span is List<String>|Integer[]. Isn’t that nice? The typechecker has reasoned that the principal supertype instantiation of Ranged for List<String>|Integer[] is the type Ranged<List<String>|Integer[]>, and thereby determined the perfect return type for span().

If we were to take away any one of declaration site covariance, principal instantiation inheritance, or union types, then this reasoning would no longer be sound. The elephant would fall on his ass.

Reference: On three-legged elephants from our JCG partner Gavin King at the Ceylon Team blog blog.

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