eggtended-js/lib/parser/parse.js

A extended version of the Egg programming language from the Eloquent Javascript book.

const fs = require("fs");

const { Lexer } = require("./lexer.js");
const { Value, Word, Apply, Regex } = require("../interp/ast.js");

/**
 * Parser class. Iterates through the given string, tokenizes it and returns a AST tree.
 * @access public
 */
class Parser {
  /**
   * Attributes used by the parser, set to default values
   */
  constructor() {
    this.index = 0;
    this.tokens = [];
    this.lookahead = null;
  }

  setProgram(program) {
    this.program = program;

    this.index = 0;
    this.tokens = [];
    this.lookahead = null;

    return this;
  }

  parse(program) {
    this.setProgram(program);

    // Get array of tokens
    this.tokens = Lexer.tokenize(program);

    let tree = null;

    // Parse if the array is not empty
    if (this.tokens.length > 0) {
      this.lookahead = this.__nextToken();
      tree = this.parseExpression();
    }

    if (this.lookahead !== null) {
      throw this.__parserSyntaxError("Unexpected input after the end of file.");
    }

    return tree;
  }

  static parse(program) {
    return new Parser().parse(program);
  }

  parseFromFile(file) {
    const program = fs.readFileSync(file, "utf8");

    return this.parse(program);
  }

  static parseFromFile(file) {
    return new Parser().parseFromFile(file);
  }

  parseExpression() {
    let expr;
    if (this.lookahead.type === "STRING") {
      expr = new Value(this.lookahead);
    } else if (this.lookahead.type === "NUMBER") {
      this.lookahead.value = Number(this.lookahead.value);
      expr = new Value(this.lookahead);
    } else if (this.lookahead.type === "REGEX") {
      const fields = this.lookahead.value.split("/");
      expr = new Regex({ body: fields[1], flags: fields[2] });
    } else if (this.lookahead.type === "WORD") {
      expr = new Word(this.lookahead);
    } else {
      throw this.__parserSyntaxError("Unrecognized token while parsing the expression.");
    }

    return this.parseApply(expr);
  }

  parseApply(expr) {
    // Get next token
    this.lookahead = this.__nextToken();

    // Return if apply is empty (not left parenthesis or null)
    if (!this.lookahead || this.lookahead.type !== "LP") {
      return expr;
    }

    let tree = new Apply(expr);

    this.lookahead = this.__nextToken();
    // Parse all arguments inside the parenthesis
    while (this.lookahead && this.lookahead.type !== "RP") {
      let arg = this.parseExpression();
      tree.args.push(arg);

      if (
        !this.lookahead ||
        (this.lookahead.type !== "COMMA" && this.lookahead.type !== "RP")
      ) {
        throw this.__parserSyntaxError(`Missing , or ) after the expression`);
      }

      // Don't consume RPs as argument tokens
      if (this.lookahead && this.lookahead.type !== "RP") {
        this.lookahead = this.__nextToken();
      }
    }

    return this.parseApply(tree);
  }

  __nextToken() {
    const token = this.tokens[this.index++];

    return token !== undefined ? token : null;
  }

  __parserSyntaxError(message) {
    let start = this.lookahead ? this.lookahead.start : this.lastIndex;

    return new SyntaxError(
      `Parser::Line ${this.line}::Col ${start}\n${message}\n${this.program.slice(
        start - 1,
        start + 10
      )}`
    );
  }
}

module.exports = {
  Parser
};