An Introduction to the Ecstasy Type System

Ecstasy's type system is designed to be an obvious and predictable type system:

• Ecstasy has a single type system, which is an object type system, and that's it. In other words, Ecstasy only has objects.
• Unlike C# or Java, there is no secondary primitive type system with types like "int" or "boolean" that everything else has to be constructed out of. In Ecstasy, all types are built out of other Ecstasy types. For someone who has used Smalltalk, this concept should be familiar.
• There is a single root called Object. For someone who has used C# or Java, this should seem quite familiar, with one little twist: In Ecstasy, Object is an interface, not a class.
• The Ecstasy type system is called the Turtles Type System, because the entire type system is bootstrapped on itself, and -- lacking primitives -- solely on itself. An Int, for example, is built out of an Array of Bit, and a Bit is built out of an IntLiteral (i.e. 0 or 1), which is built out of a String, which is an Array of Char, and a Char is built out of an Int. Thus, an Int is built out of many Ints. It's turtles, the whole way down.
• The type system is fully generic, and fully reified. This means that a List-of-Int is actually a List-of-Int, and not just a List with some compile-time syntactic sugar.
• The type system is covariant. This means that an Array-of-Int is a List-of-Int is a List-of-Number is a List-of-Object is a List is an Object.
• The type system is module-based, transitively closed, type-checked, and type safe.
• Most type safety checks are performed by the compiler and re-checked by the link-time verifier. Most of the remaining type safety checks are performed by the linker when it transitively closes over the type system. The remaining type safety checks are performed at runtime, for the specific cases in which the types cannot be fully known until runtime.
• A class has a type, but a class is not a type. (A class may have many types, but minimally its existence implies at least four types: structural, private, protected, and public.)
• The type system explicitly supports actual immutability.

In software, making something simple is the hardest thing that we do. The Ecstasy Turtles Type System is proof of that, but the benefits are worth it. Using the type system is incredibly easy, and reading the code is a joy.