jLox

Following the amazing book by Robert Nystorm, Crafting the Interpreter. This repo closely follows the book, including enhancement proposed as challenge questions.

Required Java >= 13 if one does not want to refactor the code.

1. The Lox Grammar

Start

program → declaration* EOF ;

Declarations

declaration    → classDecl
               | funDecl
               | varDecl
               | statement ;

classDecl      → "class" IDENTIFIER ( "<" IDENTIFIER )?
                 "{" function* "}" ;
funDecl        → "fun" function ;
varDecl        → "var" IDENTIFIER ( "=" expression )? ";" ;

Statements

statement      → exprStmt
               | forStmt
               | ifStmt
               | printStmt
               | returnStmt
               | whileStmt
               | block ;

exprStmt       → expression ";" ;
forStmt        → "for" "(" ( varDecl | exprStmt | ";" )
                           expression? ";"
                           expression? ")" statement ;
ifStmt         → "if" "(" expression ")" statement
                 ( "else" statement )? ;
printStmt      → "print" expression ";" ;
returnStmt     → "return" expression? ";" ;
whileStmt      → "while" "(" expression ")" statement ;
block          → "{" declaration* "}" ;`

Note that block is a statement rule, but is also used as a nonterminal in a couple of other rules for things like function bodies.

Expressions

expression     → assignment ;

assignment     → ( call "." )? IDENTIFIER "=" assignment
               | logic_or ;

logic_or       → logic_and ( "or" logic_and )* ;
logic_and      → equality ( "and" equality )* ;
equality       → comparison ( ( "!=" | "==" ) comparison )* ;
comparison     → term ( ( ">" | ">=" | "<" | "<=" ) term )* ;
term           → factor ( ( "-" | "+" ) factor )* ;
factor         → unary ( ( "/" | "*" ) unary )* ;

unary          → ( "!" | "-" ) unary | call ;
call           → primary ( "(" arguments? ")" | "." IDENTIFIER )* ;
primary        → "true" | "false" | "nil" | "this"
               | NUMBER | STRING | IDENTIFIER | "(" expression ")"
               | "super" "." IDENTIFIER ;

Utility Rules

Keep the other rules cleaner.

function       → IDENTIFIER "(" parameters? ")" block ;
parameters     → IDENTIFIER ( "," IDENTIFIER )* ;
arguments      → expression ( "," expression )* ;

2. Lexical Grammar

NUMBER         → DIGIT+ ( "." DIGIT+ )? ;
STRING         → "\"" <any char except "\"">* "\"" ;
IDENTIFIER     → ALPHA ( ALPHA | DIGIT )* ;
ALPHA          → "a" ... "z" | "A" ... "Z" | "_" ;
DIGIT          → "0" ... "9" ;

3. Tree-Walk Interpreter progress

20230830: Lexer done!
- Added support for C-style /* ... */ block comments (challenge problem!)
20230831: Building AST with Visitor Pattern
20230901 Expression parser done!
- TODO: challenge problems -- comma operator support, Ternary ?: operator support
20230902: Expression evaluator done.
20230903: Take #1 -- statement parsing and evaluation done. Global variable handling is done.
20220904: Take #2 -- variable assignment done. Next TODO: Scoping.
20230908: Take #3 -- completed statement execution with scoping.
- TODO: Fix REPL to execute expressions and statements
- TODO: Fix implicit assignment to nil -- make it an error if a var is not assigned in a scope.
20230908: Completed control flow (if/else, while, for)
20230910: Completed function decls and calls. Starting with closures, resolving and binding."
20230910: Closures work!
20230912: Scope resolution and semantic analysis works.
- TODO: Extend the resolver to report an error if a local variable is never used.
20230912: class syntax works now!
20230912: Try #1 -- Supporting this not working yet!
20230914: Try #2 -- Fixed this.
20230914: Try #3 -- Added init as a special method -- constructor (like python's __init__). Still an error resolving this properly. Will come back to later to resolve the problem after inheritance.
20230914: Try #1 -- Added basic support for inheritance.
20230914: Try #2 -- Done!
TODO:
1. Complete challenges.
2. Remove all errors using LOX scripts from lox-test

mapuna/lox