Problem:

A "domain model" is pure code implementing a model of the "business domain" an application operates in.

The problem is what we need to keep the "domain model" separate from platform and language specific details.

Using "domain models" automatically solve the platform problem because the "domain model" doesn't depend on any specific GUI's etc.

However it doesn't solve the language problem:

If we write the "domain model" in C++ or Java then we can only use the domain model on those platforms.

Solution:

The goal is to have a "domain model" language "X" that can be compiled to C++, Java and JavaScript.

The idea is that you write the "domain model" in X ensuring that the domain model can be used on ANY current and future platform.

Problem:

Designing a new language (X) is hard. Also, forcing people to learn a new language dramatically reduces the change of the project actually being used.

Solution:

Use (a subset of) Java as X. It makes it easy for people to understand and use and it is high level enough to be easily compiled to C++, Java and JavaScript.

Problem:

How do we write a compiler for X?

Solution:

Write the compiler in Java and make the compiler compile its own source code.

=> no need to write separate test examples (which is tedious/slow). => compiling a real non-trivial program. => the Java compiler will complain if the source is not Java. => only need to implement what is actually used instead of "handling all of Java".

Problem:

How do we add new non-Java syntax?

Solution:

When the X compiler is working, start using X to write the X compiler.

=> you can now add new language features (however the code will no longer be compileable with Java).