Tree-Walk Interpreter

Goals

• Scanner
• Print Abstract Syntax Tree (AST)
• Parser
• Variable Resolution Pass

Evaluates:

• Expressions & Arithmetic
• Branching & Looping
• Variables, Functions & Function calls
• Parameter binding & Function return statements
• Classes, Inheritance, Instance variables, & Methods

Future Features

• Ability to evaluate string interpolation
• Add support for the C-style conditional or “ternary” operator ?:.

Example Program

class Cake {
  taste() {
    var adjective = "delicious";
    print "The " + this.flavor + " cake is " + adjective + "!";
  }
}

var cake = Cake();
cake.flavor = "German chocolate";
cake.taste(); // Prints "The German chocolate cake is delicious!".

BNF Grammer

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 ;

Great Learning Resources

• Crafting Interpreters by Robert Nystrom
• Building Recursive Descent Parsers by Supriyo Biswas