Jlox - Java Lox Interpreter

A tree-walk interpreter for the Lox programming language, built in Java following the book Crafting Interpreters by Robert Nystrom.

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Current Progress

Chapter 11: Resolving and Binding The interpreter now includes a semantic analysis pass (Resolver) that resolves variable bindings before execution, fixing closure semantics and enabling static error detection.

Features implemented so far:

• Scanning, parsing, and tree-walk interpretation
• Variables, scopes, and block environments
• Control flow: if, while, for, break, continue
• Functions, closures, and recursion
• Lambda expressions
• Resolver for static variable resolution and closure correctness
• Unused variable detection (reports error for unused local variables)
• Index-based environment lookup for faster variable access

Architecture

The project follows a classic interpreter pipeline:

1. Scanner (Scanner.java)

   • Converts raw source code into a stream of Tokens.
   • Handles keywords, literals, and operators.

2. Parser (Parser.java)

   • Recursive Descent Parser that constructs an Abstract Syntax Tree (AST).
   • Handles operator precedence, error synchronization, and statement parsing.

3. AST (Expr.java, Stmt.java)

   • Represents the code structure (expressions and statements).
   • Uses the Visitor Pattern for operations like printing (AstPrinter) and evaluation (Interpreter).

4. Resolver (Resolver.java)

   • Performs a static analysis pass over the AST before execution.
   • Resolves each variable to its exact scope depth and index.
   • Detects errors such as return outside functions, break/continue outside loops, and unused local variables.

5. Interpreter (Interpreter.java)

   • Traverses the AST recursively to execute code.
   • Uses pre-resolved variable locations for correct and efficient lookup.

6. Environment (Environment.java, GlobalEnvironment.java)

   • Local environments use an index-based List<Object> for fast variable access.
   • Global environment uses a Map<String, Object> for named variable lookup.

Project Structure

src/lox/
├── Lox.java                # Entry point (REPL & Script runner)
├── Scanner.java            # Lexical Analysis
├── Token.java              # Token definition
├── TokenType.java          # Enum of token types
├── Parser.java             # Syntax Analysis
├── Expr.java               # AST Expression Node definitions
├── Stmt.java               # AST Statement Node definitions
├── Resolver.java           # Semantic Analysis & Variable Resolution
├── Interpreter.java        # Evaluation logic
├── Environment.java        # Local scope management (index-based)
├── GlobalEnvironment.java  # Global scope management (name-based)
├── LoxCallable.java        # Callable interface
├── LoxFunction.java        # Function representation
├── BreakException.java     # Break control flow
├── ContinueException.java  # Continue control flow
├── Return.java             # Return control flow
└── AstPrinter.java         # Debug utility to print AST

Usage

Run a script

java lox.Lox script.lox

Interactive REPL

java lox.Lox
> var a = 1;
> var b = 2;
> print a + b;
3

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