How to extend Jatha

How to extend Jatha by adding new functions

The easy way

If you can figure out how to implement it in LISP, do that rather than implement it in Java. It's simpler. Put the definition in a file and load it.

Extending the Java code

Since the new function is a LISP function, you need to define it as a method on the LispValue interface (org.jatha.dynatype.LispValue) and then implement it on one or more of the implementations of LispValue, such as StandardLispSymbol or StandardLispNumber. Put a default method on StandardLispValue that returns a default value or throws an appropriate exception if the argument type(s) are not correct at runtime.

Extending the LISP interpreter

If you just want to call the function from Java, the instructions above are sufficient. However, if you want to make the function available through the LISP interpreter, you also need to do the following:

The LISP compiler (org.jatha.compile.LispCompiler.java) compiles the code into virtual machine code. The virtual machine is based on the one in Kogge's book Symbolic Architectures.
Each compilable function must a subclass of LispPrimitive. See the directory org.jatha.compile for many examples.
The function has to have a compileArgs method and an execute method. For example, look at GethashPrimitive.java. The execute method must push a result on the S register and pop the C (code) register an appropriate number of times.
The easiest thing to do is to copy the Primitive class for a function similar to the one you are adding and modify it to call your new function. This is very easy.
To make the new primitive available to the user via the interpreter, you have to register it. Add a call to Register in org.jatha.compile.LispCompiler.init().