Exploring the Possibilities: Writing a Language on the JVM in Java

In the vast landscape of software development, the JVM (Java Virtual Machine) stands as a versatile platform that can run programs written in a variety of languages. One interesting challenge is to write a language that runs on the JVM but is itself written in Java. This article explores the tools and techniques available for such a task, focusing on the ANTLR framework and its role in language development. Along the way, we will also discuss compilers, transpilers, and interpreters that can be implemented in Java.

ANTLR and Language Development

antlr is a widely used tool for developing language parsers, and it is particularly well-suited for creating parsers for languages that run on the JVM. ANTLR reads linguistic definitions specified in Backus-Naur Form (BNF), a notation designed to describe context-free grammars. This notation has its roots in the work of Noam Chomsky, a significant figure in the field of linguistics and computer science. ANTLR generates skeletons and abstract implementations that validate the correctness of the input code with respect to the specified language, building a syntax tree as a result.

One of the most powerful aspects of ANTLR is its ability to generate visitors for the resulting syntax tree. These visitors can be used to handle the semantic events in any way the developer chooses. This flexibility allows developers to easily emit byte code, machine code, or even generate code for another language—essentially compiling the input language into a JVM-capable binary or another codebase. This class of tool is known as a transpiler.

Compilers and Transpilers

Compilers and transpilers are closely related but have distinct purposes. A compiler takes source code in one language and translates it into an executable format, such as a JVM byte code. In contrast, a transpiler translates source code from one high-level programming language to another, allowing the end result to be compiled into a form executable by a different virtual machine. For example, a Java program could generate C code, which would then be compiled by a C compiler.

Implementing a compiler or transpiler in Java is both feasible and common. For instance, early compiler development often involved using tools like lex and Yacc, which generated C code. However, the modern Java ecosystem offers powerful tools like ANTLR, making these tasks more streamlined and maintainable. With the right tools, a Java program can indeed generate C, Python, or even APL code, based on the requirements.

Interpreters and Direct Execution

Another option for handling code written on the JVM is to generate and execute it directly in a JVM environment. This can be done using interpreters that read and execute code without first compiling it into a more machine-native format. One common pattern is to build a tree of specialized Java objects that reflect the syntax tree of the input code. Upon a successful parse, you can ute the root object, and the execution path follows the tree.

This interpreter approach offers a balance between flexibility and performance. For example, a quick and dirty configuration parser was developed using an ANTLR front end and implemented in the groovy programming language. This allowed for rapid prototyping and refinement of behaviors. While the initial plan was to replace groovy with Java for performance reasons, it turned out that groovy met the requirements, and the project never revisited the initial plan.

Conclusion

The ability to write a language on the JVM in Java opens up a world of possibilities in software development. From the powerful ANTLR framework to the flexibility of transpilers and interpreters, developers have a wide array of tools and techniques at their disposal. This article has explored the key concepts and provided examples to help you understand how to approach such a task. Whether you're building a new language, a compiler, or a transpiler, the modern Java ecosystem provides the necessary tools and libraries to make it happen.