2e:null 2f:["$","title",null,{"children":"JSLox as of the end of Chapter 6 Parsing Expressions | Thomas Ballinger | Observable"}] 34:T8a5,TokenType = { class TokenType { constructor(name) { this.name = name; } toString() { return `TokenType.${this.name}`; } } // Single-character tokens. (TokenType.LEFT_PAREN = new TokenType('LEFT_PAREN')), (TokenType.RIGHT_PAREN = new TokenType('RIGHT_PAREN')), (TokenType.LEFT_BRACE = new TokenType('LEFT_BRACE')), (TokenType.RIGHT_BRACE = new TokenType('RIGHT_BRACE')), (TokenType.COMMA = new TokenType('COMMA')), (TokenType.DOT = new TokenType('DOT')), (TokenType.MINUS = new TokenType('MINUS')), (TokenType.PLUS = new TokenType('PLUS')), (TokenType.SEMICOLON = new TokenType('SEMICOLON')), (TokenType.SLASH = new TokenType('SLASH')), (TokenType.STAR = new TokenType('STAR')), // One or two character tokens. (TokenType.BANG = new TokenType('BANG')), (TokenType.BANG_EQUAL = new TokenType('BANG_EQUAL')), (TokenType.EQUAL = new TokenType('EQUAL')), (TokenType.EQUAL_EQUAL = new TokenType('EQUAL_EQUAL')), (TokenType.GREATER = new TokenType('GREATER')), (TokenType.GREATER_EQUAL = new TokenType('GREATER_EQUAL')), (TokenType.LESS = new TokenType('LESS')), (TokenType.LESS_EQUAL = new TokenType('LESS_EQUAL')), // Literals. (TokenType.IDENTIFIER = new TokenType('IDENTIFIER')), (TokenType.STRING = new TokenType('STRING')), (TokenType.NUMBER = new TokenType('NUMBER')), // Keywords. (TokenType.AND = new TokenType('AND')), (TokenType.CLASS = new TokenType('CLASS')), (TokenType.ELSE = new TokenType('ELSE')), (TokenType.FALSE = new TokenType('FALSE')), (TokenType.FUN = new TokenType('FUN')), (TokenType.FOR = new TokenType('FOR')), (TokenType.IF = new TokenType('IF')), (TokenType.NIL = new TokenType('NIL')), (TokenType.OR = new TokenType('OR')), (TokenType.PRINT = new TokenType('PRINT')), (TokenType.RETURN = new TokenType('RETURN')), (TokenType.SUPER = new TokenType('SUPER')), (TokenType.THIS = new TokenType('THIS')), (TokenType.TRUE = new TokenType('TRUE')), (TokenType.VAR = new TokenType('VAR')), (TokenType.WHILE = new TokenType('WHILE')), (TokenType.EOF = new TokenType('EOF')); return TokenType; }35:Tf9f,scanTokens = source => { // a bunch of stateful helpers here use this state let start = 0; let current = 0; let line = 1; let tokens = []; const isAtEnd = () => current >= source.length; const advance = () => { return source[current++]; }; const addToken = (type, literal = null) => { const text = source.substring(start, current); tokens.push(new Token(type, text, literal, line)); }; const match = expected => { if (isAtEnd()) return false; if (source[current] != expected) return false; current++; return true; }; const peek = () => { if (isAtEnd()) return '\0'; return source.charAt(current); }; const peekNext = () => { if (current + 1 >= source.length) return '\0'; return source.charAt(current + 1); }; const string = () => { while (peek() != '"' && !isAtEnd()) { if (peek() == '\n') line++; advance(); } // Unterminated string. if (isAtEnd()) { reportError(line, "Unterminated string."); return; } // The closing ". advance(); // Trim the surrounding quotes. const value = source.substring(start + 1, current - 1); addToken(TokenType.STRING, value); }; const isDigit = c => { return c >= '0' && c <= '9'; }; const number = () => { while (isDigit(peek())) { advance(); } // Look for a fractional part. if (peek() == '.' && isDigit(peekNext())) { // Consume the "." advance(); while (isDigit(peek())) advance(); } addToken(TokenType.NUMBER, parseFloat(source.substring(start, current))); }; const isAlpha = c => { return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || c == '_'; }; const isAlphaNumeric = c => { return isAlpha(c) || isDigit(c); }; const identifier = () => { while (isAlphaNumeric(peek())) advance(); const text = source.substring(start, current); let type = keywords[text]; if (type === undefined) { type = TokenType.IDENTIFIER; } addToken(type); }; const scanToken = () => { const c = advance(); switch (c) { case '(': addToken(TokenType.LEFT_PAREN); break; case ')': addToken(TokenType.RIGHT_PAREN); break; case '{': addToken(TokenType.LEFT_BRACE); break; case '}': addToken(TokenType.RIGHT_BRACE); break; case ',': addToken(TokenType.COMMA); break; case '.': addToken(TokenType.DOT); break; case '-': addToken(TokenType.MINUS); break; case '+': addToken(TokenType.PLUS); break; case ';': addToken(TokenType.SEMICOLON); break; case '*': addToken(TokenType.STAR); break; case '!': addToken(match('=') ? TokenType.BANG_EQUAL : TokenType.BANG); break; case '=': addToken(match('=') ? TokenType.EQUAL_EQUAL : TokenType.EQUAL); break; case '<': addToken(match('=') ? TokenType.LESS_EQUAL : TokenType.LESS); break; case '>': addToken(match('=') ? TokenType.GREATER_EQUAL : TokenType.GREATER); break; case '/': if (match('/')) { // A comment goes until the end of the line. while (peek() != '\n' && !isAtEnd()) advance(); } else { addToken(TokenType.SLASH); } break; case ' ': case '\r': case '\t': // Ignore whitespace. break; case '\n': line++; break; case '"': string(); break; default: if (isDigit(c)) { number(); } else if (isAlpha(c)) { identifier(); } else { reportError(line, "Unexpected character."); } break; } }; while (!isAtEnd()) { start = current; scanToken(); } tokens.push(new Token(TokenType.EOF, '', '', line)); return tokens; }36:T717,astCode = { const defineClass = (base_class_name, class_name, fields) => `class ${class_name} extends ${base_class_name} { constructor(${fields.join(', ')}) { super(); ${fields.map(field => ` this.${field} = ${field};`).join('\n')} } accept(visitor) { return visitor.visit_${class_name}${base_class_name}(this); } }`; const defineAst = (base_class_name, type_specs) => { const classStrings = []; const visitor_method_strings = []; const classNames = []; for (let type_spec of type_specs) { const class_name = type_spec.split(':')[0].trim(); classNames.push(class_name); const fields = type_spec .split(':')[1] .trim() .split(', '); classStrings.push(defineClass(base_class_name, class_name, fields)); visitor_method_strings.push( ` visit_${class_name}${base_class_name}() { throw Error(${'`visit method '}"visit_${class_name}${base_class_name}" not implemented on ${"${this.constructor.name}`"}); }` ); } return ` ${base_class_name} = { // GENERATED - DO NOT MANUALLY EDIT! // To update it, modify the astCode definition and manually copy/paste its output. class Visitor { ${visitor_method_strings.join('\n')} } class ${base_class_name} { // Call a method on the visitor based on your class name accept(visitor) { throw Error(${"`accept method not implemented on ${this.constructor.name}`"}); } } ${classStrings.join('\n\n')} return {Visitor, ${classNames.join(', ')}}; }`; }; return html`

${defineAst(
    "Expr",
    [
      "Binary   : left, operator, right",
      "Grouping : expression",
      "Literal  : value",
      "Unary    : operator, right"
    ]
  )}

`; }37:T730,Expr = { // GENERATED - DO NOT MANUALLY EDIT! // To update it, modify the astCode definition and manually copy/paste its output. class Visitor { visit_BinaryExpr() { throw Error( `visit method "visit_BinaryExpr" not implemented on ${this.constructor.name}` ); } visit_GroupingExpr() { throw Error( `visit method "visit_GroupingExpr" not implemented on ${this.constructor.name}` ); } visit_LiteralExpr() { throw Error( `visit method "visit_LiteralExpr" not implemented on ${this.constructor.name}` ); } visit_UnaryExpr() { throw Error( `visit method "visit_UnaryExpr" not implemented on ${this.constructor.name}` ); } } class Expr { // Call a method on the visitor based on your class name accept(visitor) { throw Error(`accept method not implemented on ${this.constructor.name}`); } } class Binary extends Expr { constructor(left, operator, right) { super(); this.left = left; this.operator = operator; this.right = right; } accept(visitor) { return visitor.visit_BinaryExpr(this); } } class Grouping extends Expr { constructor(expression) { super(); this.expression = expression; } accept(visitor) { return visitor.visit_GroupingExpr(this); } } class Literal extends Expr { constructor(value) { super(); this.value = value; } accept(visitor) { return visitor.visit_LiteralExpr(this); } } class Unary extends Expr { constructor(operator, right) { super(); this.operator = operator; this.right = right; } accept(visitor) { return visitor.visit_UnaryExpr(this); } } return { Visitor, Binary, Grouping, Literal, Unary }; }38:Tf08,class Parser { constructor(tokens, set_error) { this.tokens = tokens; this.current = 0; this.error_handler = set_error; } /** Returns an Expr instance, or None on parsing error.""" */ parse() { try { return this.expression(); } catch (error) { if (error instanceof ParseError) { return null; } else { throw error; } } } expression() { return this.equality(); } equality() { let expr = this.comparison(); while (this.match(TokenType.BANG_EQUAL, TokenType.EQUAL_EQUAL)) { const operator = this.previous(); const right = this.comparison(); expr = new Expr.Binary(expr, operator, right); } return expr; } comparison() { let expr = this.addition(); while ( this.match( TokenType.GREATER, TokenType.GREATER_EQUAL, TokenType.LESS, TokenType.LESS_EQUAL ) ) { const operator = this.previous(); const right = this.addition(); expr = new Expr.Binary(expr, operator, right); } return expr; } addition() { let expr = this.multiplication(); while (this.match(TokenType.MINUS, TokenType.PLUS)) { const operator = this.previous(); const right = this.multiplication(); expr = new Expr.Binary(expr, operator, right); } return expr; } multiplication() { let expr = this.unary(); while (this.match(TokenType.SLASH, TokenType.STAR)) { const operator = this.previous(); const right = this.unary(); expr = new Expr.Binary(expr, operator, right); } return expr; } unary() { if (this.match(TokenType.BANG, TokenType.MINUS)) { const operator = this.previous(); const right = this.unary(); return new Expr.Unary(operator, right); } return this.primary(); } primary() { if (this.match(TokenType.FALSE)) return new Expr.Literal(false); if (this.match(TokenType.TRUE)) return new Expr.Literal(true); if (this.match(TokenType.NIL)) return new Expr.Literal(null); if (this.match(TokenType.NUMBER, TokenType.STRING)) { return new Expr.Literal(this.previous().literal); } if (this.match(TokenType.LEFT_PAREN)) { const expr = this.expression(); this.consume(TokenType.RIGHT_PAREN, "expected ')' after expression."); return new Expr.Grouping(expr); } console.log('this.peek():', this.peek()); throw this.error(this.peek(), "expected expression"); } match(...token_types) { for (let token_type of token_types) { if (this.check(token_type)) { this.advance(); return true; } } return false; } check(token_type) { if (this.is_at_end()) { return false; } return this.peek().type === token_type; } advance() { if (!this.is_at_end()) { this.current += 1; } return this.previous(); } is_at_end() { return this.peek().type === TokenType.EOF; } peek() { return this.tokens[this.current]; } previous() { return this.tokens[this.current - 1]; } consume(token_type, message) { if (this.check(token_type)) { return this.advance(); } throw this.error(this.peek(), message); } error(token, message) { this.error_handler(token, message); throw ParseError(); } synchronize() { this.advance(); while (!this.is_at_end()) { if (this.previous().type === TokenType.SEMICOLON) { return; } if ( [ TokenType.CLASS, TokenType.FUN, TokenType.VAR, TokenType.FOR, TokenType.IF, TokenType.WHILE, TokenType.PRINT, TokenType.RETURN ].includes(this.peek().type) ) { return; } this.advance(); } } }39:[] 32:{"id":"e1e011cbe368d860","slug":"jslox-as-of-the-end-of-chapter-6-parsing-expressions","title":"JSLox as of the end of Chapter 6 Parsing Expressions","thumbnail":"0dcfd729c299ae481ccf5363d8915457bac6436c5563f1b921ec636219ae8501","access_level":"public","create_time":"$D2019-10-19T16:12:13.444Z","update_time":"$D2019-10-21T05:43:12.631Z","view_time":null,"first_public_time":"$D2019-10-19T20:08:35.317Z","fork":{"id":"4f84432691ac148b","slug":"jslox-as-of-chapter-4-scanning","title":"JSLox as of Chapter 4 Scanning","owner":{"id":"8d59e5c626effee2","login":"ballingt","name":"Thomas Ballinger"}},"stars":0,"starred":false,"owner":{"type":"team","id":"8d59e5c626effee2","login":"ballingt","name":"Thomas Ballinger","avatar_url":"https://avatars-next.observableusercontent.com/fdc16c011489d914c05fa5992a7992413727fc69e4f73671ec38919807f547e7","tier":"starter_2024"},"version":488,"publish_version":488,"role":null,"authored":false,"authors":[{"type":"individual","id":"4c607438c80998ad","login":"ballingt","name":"Thomas Ballinger","avatar_url":"https://avatars-next.observableusercontent.com/fdc16c011489d914c05fa5992a7992413727fc69e4f73671ec38919807f547e7","tier":null}],"members":[{"type":"individual","id":"4c607438c80998ad","login":"ballingt","name":"Thomas Ballinger","avatar_url":"https://avatars-next.observableusercontent.com/fdc16c011489d914c05fa5992a7992413727fc69e4f73671ec38919807f547e7","tier":null,"role":"editor"}],"comments":[],"custom_thumbnail":false,"trashed":false,"lapsed":null,"subscribed":null,"forks":2,"collection":null,"type":"notebook","body":{"title":"JSLox as of the end of Chapter 6 Parsing Expressions","cells":[{"id":0,"value":"md`# JSLox as of the end of Chapter 6 Parsing Expressions`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":173,"value":"main(\"1 + 12 < 23.4\")","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":209,"value":"jsloxAST`12 + 23`","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":212,"value":"md`Try it live!`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":12,"value":"viewof input = html``","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":25,"value":"run(input)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":238,"value":"md`## main`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":3,"value":"function main() {\n const args = [...arguments];\n if (args.length > 1) {\n return 'Pass a string to main';\n } else if (args.length == 1) {\n return run(args[0]);\n } else if (args.length == 0) {\n return 'to run prompt, use input box';\n }\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":33,"value":"run = src => {\n const tokens = scanTokens(src);\n console.log(tokens);\n const tree = parse(tokens);\n return pprint(tree);\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":50,"value":"error = (line, where, message = '') => {\n console.log('running error with', line, where, message);\n return `[line ${line}] Error \"${where}\": ${message}`;\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":241,"value":"md`## scanning`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":85,"value":"scanTokens(input)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":57,"value":"$34","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":75,"value":"class Token {\n constructor(type, lexeme, literal, line) {\n this.type = type;\n this.lexeme = lexeme;\n this.literal = literal;\n this.line = line;\n }\n\n toString() {\n return `${this.type} ${this.lexeme} ${this.literal}`;\n }\n toValue() {\n return this.toString();\n }\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":129,"value":"reportError = (line, msg) => {\n console.error(error(line, '?', msg));\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":158,"value":"keywords = ({\n and: TokenType.AND,\n class: TokenType.CLASS,\n else: TokenType.ELSE,\n false: TokenType.FALSE,\n for: TokenType.FOR,\n fun: TokenType.FUN,\n if: TokenType.IF,\n nil: TokenType.NIL,\n or: TokenType.OR,\n print: TokenType.PRINT,\n return: TokenType.RETURN,\n super: TokenType.SUPER,\n this: TokenType.THIS,\n true: TokenType.TRUE,\n var: TokenType.VAR,\n while: TokenType.WHILE\n})","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":117,"value":"$35","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":199,"value":"jsloxAST = strings => {\n return parse(scanTokens(strings[0]));\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":244,"value":"md`## ast nodes\n\nThis is the metaprogramming bit`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":258,"value":"$36","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":323,"value":"$37","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":346,"value":"md`### visitors`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":471,"value":"pprint(parse(scanTokens(input)))","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":467,"value":"pprint = tree => new ASTPrinter().pprint(tree)","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":366,"value":"class ASTPrinter extends Expr.Visitor {\n pprint(expr) {\n return expr.accept(this);\n }\n visit_BinaryExpr(expr) {\n return `(${expr.operator.lexeme} ${expr.left.accept(\n this\n )} ${expr.right.accept(this)})`;\n }\n visit_GroupingExpr(expr) {\n return `(group ${expr.expression.accept(this)})`;\n }\n visit_LiteralExpr(expr) {\n return expr.value === null ? 'nil' : expr.value;\n }\n visit_UnaryExpr(expr) {\n return `({expr.operator.lexeme} {expr.right.accept(this)})`;\n }\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":389,"value":"md`# Parsing`","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":420,"value":"parse(scanTokens(input))","pinned":true,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":395,"value":"class ParseError extends Error {\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":417,"value":"parse = tokens => {\n const setError = (tok, msg) => {\n throw Error(`${tok}, ${msg}`);\n };\n const p = new Parser(tokens, setError);\n return p.parse(tokens);\n}","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"},{"id":394,"value":"$38","pinned":false,"hidden":false,"mode":"ojs","author":"$undefined","summary":"$undefined","output":"$undefined","database":"$undefined"}],"files":"$W39","stdlib":"1","titling":"auto"}} 33:"$32:body"