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jlox

A Lox implementation in Java.

Features

  • tokens and lexing
  • abstract syntax trees
  • recursive descent parsing
  • prefix and infix expressions
  • runtime representation of objects
  • interpreting code using the Visitor pattern
  • lexical scope
  • environment chains for storing variables
  • control flow
  • functions with parameters
  • closures
  • static variable resolution and error detection
  • classes
  • constructors
  • fields
  • methods
  • inheritance

BNF grammar

program        → declaration* EOF ;

declaration    → classDecl
               | funDecl
               | varDecl
               | statement ;

classDecl      → "class" IDENTIFIER ( "<" IDENTIFIER )? "{" function* "}" ;

funDecl        → "fun" function ;
function       → IDENTIFIER "(" parameters? ")" block ;
parameters     → IDENTIFIER ( "," IDENTIFIER )* ;

varDecl        → "var" IDENTIFIER ( "=" expression )? ";" ;

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

block          → "{" declaration* "}" ;

ifStmt         → "if" "(" expression ")" statement
               ( "else" statement )? ;

whileStmt      → "while" "(" expression ")" statement ;

forStmt        → "for" "(" ( varDecl | exprStmt | ";" )
                 expression? ";"
                 expression? ")" statement ;

exprStmt       → expression ";" ;
printStmt      → "print" expression ";" ;
returnStmt     → "return" expression? ";" ;

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 )* ;
arguments      → expression ( "," expression )* ;

primary        → "true" | "false" | "nil"
               | NUMBER | STRING | IDENTIFIER
               | "(" expression ")"
               | "super" "." IDENTIFIER ;

Built-in global functions and constants

  • E: e
  • PI: π
  • input(): read characters from console
  • show(object): print
  • showLine(object): println
  • string(object)
  • bool(object)
  • number(object)
  • randomNumber(upperBound)
  • hasProperty(object, name)