More turtles

The Ecstasy type system is called a Turtles Type System, because "it's turtles, the whole way down". This is, in many ways, a revolutionary approach to type systems. Object type systems have traditionally had primitive types (like a "periodic table of elements" for the language) from which all other types are built, but in Ecstasy things are a bit different. For example, an Ecstasy integer is composed of an array of bits, each of which is composed of an integer literal (0 or 1), which is in turn composed of a string, which is composed of an array of characters, each of which is composed of an integer. So we're right back where we started, with an integer -- and you can just recurse infinitely, because the types are all turtles.

Making a type system like this actually work is a challenge, so it didn't appear all at once. Recently, support for recursive type definitions was added, in order to support the JSON parsing project. Consider the following Ecstasy code:

 * JSON primitive types are all JSON values except for arrays and objects.
typedef (Nullable | Boolean | IntLiteral | FPLiteral | String) Primitive;

 * JSON types include primitive types, array types, and map types.
typedef (Primitive | Map<String, Doc> | Array<Doc>) Doc;

Here, in two lines of code (which could even be simplified to a single line, if we didn't want to split out primitive JSON values), we see a complete Ecstasy mapping of the JSON specification. That second typedef, though, is a doozy, because it refers to itself. If you stop and read it carefully it makes a lot of sense: A JSON document is either a primitive value, a map of string keys to JSON values (each of which could be an entire recursive document structure), or an array of JSON values (each of which could be an entire recursive document structure).

To keep it simple, consider the following example:
typedef (Int | List<Manifold>) Manifold;

Manifold m1 = 9;
Manifold m2 = [m1];
Manifold m3 = [m2];

When executed, this code will print:
But the amazing thing isn't that it works at all, but rather that it works with full type safety.

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