Monkey Language Interpreter

A Go implementation of the Monkey programming language interpreter, following the structure from "Writing An Interpreter In Go" by Thorsten Ball.

Table of Contents

• Features
• Components Overview
• Getting Started
• Usage Examples
• Project Structure
• References

Features

• Lexical analysis with tokenization
• Pratt parser for expression parsing
• Abstract Syntax Tree (AST) construction
• Tree-walking evaluation model
• Variable bindings and environment management
• Support for functions, conditionals, and return statements
• Built-in REPL (Read-Eval-Print Loop)

Components Overview

1. Lexer

• Algorithm: Finite-state automaton
• Converts source code input into tokens
• Handles whitespace skipping and token position tracking
• Supports identifiers, literals, and operators

2. Parser

• Algorithm: Pratt parsing (Top-down operator precedence)
• Recursive descent implementation
• Produces an Abstract Syntax Tree (AST)
• Handles operator precedence and associativity

3. AST

• Defines Node interfaces for program structure
• Implements expression and statement nodes
• Supports modification through a Modify() method

4. Evaluator

• Algorithm: Tree-walking interpreter
• Executes the AST through recursive evaluation
• Manages environment and variable bindings
• Handles error propagation and return values

5. Object System

• Defines runtime objects (integers, booleans, strings, etc.)
• Manages environment chaining for scopes
• Implements function objects and error handling

Getting Started

Prerequisites

• Go 1.20+

Installation

git clone https://github.com/sjwhole/my-mk-interpreter.git
cd my-mk-interpreter

Usage Examples

Build monkey executable

go build 

Starting the REPL

./monkey

Running a Monkey Program

Also, you can also run a Monkey program from a file. (like python)

./monkey <path-to-file>

Example Monkey Program

let fibonacci = fn(x) {
    if (x == 0) {
        0
    } else {
        if (x == 1) {
            1
        } else {
            fibonacci(x-1) + fibonacci(x-2)
        }
    }
};
puts(fibonacci(10));

./monkey src/fibo.mk 
# expected output: 55

Project Structure

monkey/
├── ast/          # Abstract Syntax Tree definitions
├── evaluator/    # Execution engine
├── lexer/        # Tokenization
├── object/       # Runtime object system
├── parser/       # Syntax parsing
├── repl/         # Read-Eval-Print Loop
└── token/        # Token definitions

References

• Thorsten Ball's "Writing An Interpreter In Go"
• Pratt Parsing technique
• Monkey language specification