Unleashing the Monkey: Building An Interpreter in Go, One Commit at a Time
“The writing of interpreters is, in a way, writing the rules of your own world.”
Setting Out: Why Write An Interpreter?
Some projects begin with a grand vision; others, as a dare to one’s own curiosity. hawkaii/monkey_go started with both—a technical challenge and a learning journey inspired by Thorsten Ball’s excellent Writing An Interpreter In Go. For those who have ever wanted to peek behind the curtain of programming language design, this repository offers a guided tour, one commit at a time.
The Premise: Monkey Language, Go Style
Monkey is a simple, expressive language designed for education. The goal here: reimagine its interpreter in Go, with every line of code a lesson in how languages work under the hood. The codebase is MIT-licensed, approachable, and open for forking, tinkering, and learning.
At the time of writing:
- 47 commits mark the development journey
- 1 star (so far—a hidden gem!)
- 2 releases, the latest v0.0.2 (May 19, 2025)
- Main folders:
lexer,token,repl,ast,evaluator,object,parser
The implementation covers chapters 1–3 of Ball’s book: a working lexer, an interactive REPL, and robust token support. The roadmap promises parsing, ASTs, and evaluation next.
Anatomy of a Learning Interpreter
1. The Lexer: Reading Raw Code
The first milestone was building a lexer—code that scans input and breaks it into tokens. Here’s a snippet from the REPL session:
>> let five = 5;
[LET] [IDENT] [ASSIGN] [INT] [SEMICOLON]This seemingly simple transformation is the foundation of every programming language. The lexer recognizes identifiers, numbers, operators, delimiters, and keywords. Every commit here is a step deeper into the world of language structure.
2. The REPL: Interactive Exploration
With the REPL live, the project transforms from a static library to an interactive playground. Type Monkey code, see tokens, and experience the immediate feedback loop. Error handling was added early—because robust interpreters never leave users wondering what went wrong.
3. Laying the Groundwork: Tokens and Error Handling
Commit messages show a steady progression:
- “Add initial lexer implementation”
- “Support for integer literals and operators”
- “Implement REPL for interactive tokenization”
- “Improve error messages and unknown token handling”
Each push is a log of technical growth and new understanding. Contributors (so far, mostly the repo creator) aren’t afraid to refactor or revisit earlier decisions—a hallmark of educational projects.
The Human Side: Learning by Doing
What makes hawkaii/monkey_go compelling isn’t just the code—it’s the learning process. Every folder represents a chapter in the journey:
- lexer/ – First contact with syntax
- token/ – Defining the language’s vocabulary
- repl/ – Making code come alive interactively
- ast/, parser/, evaluator/ – The next frontiers
Mistakes aren’t just tolerated—they’re expected. The repo is a safe space to experiment, break, and fix. The commit history is honest: incremental improvements, occasional rollbacks, and the slow, deliberate march toward a full interpreter.
Technical Highlights & Insights
- Go idioms: Leveraging Go’s strong typing and simplicity makes interpreter code readable and robust.
- Error handling: Early investment in error reporting pays off—both for users and future contributors.
- Educational structure: Code is organized to mirror the interpreter book’s chapters, lowering the barrier for new learners to join.
- REPL-first: Prioritizing interactivity fosters experimentation and quick feedback.
Roadmap: What’s Next?
The adventure is far from over. Upcoming milestones include:
- Parser: Building syntax trees from tokens monkey_go_parser
- AST (Abstract Syntax Tree): Modeling program structure
- Evaluator: Turning syntax into runtime behavior
- Extending the language: New features, more expressive power, and maybe even a type system or runtime improvements.
The latest release (v0.0.2) marks a stable base for further exploration. The repo welcomes issues, feedback, and pull requests—especially from those learning Go or interested in interpreters.
Reflections & Call to Action
hawkaii/monkey_go is more than a codebase—it’s an invitation. Whether you’re a Go developer, a language nerd, or someone following Thorsten Ball’s book, this project is a perfect launchpad. Read the commits not just for code, but for story. Fork it, run it, break it, improve it.
Curiosity is the only prerequisite.
Ready to write the rules of your own world? Jump in, explore the Monkey interpreter, and join the journey.
Happy hacking!