eggtended-js/lib/interp/ast.js

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

// AST class nodes
const xRegExp = require("xregexp");
const utils = require("../utils.js");

const { SpecialForms } = require("./environment.js");

class Value {
  constructor(token) {
    this.type = "value";
    this.value = token.value;
  }

  evaluate() {
    return this.value;
  }
}

class Word {
  constructor(token) {
    this.type = "word";
    this.name = token.value;
  }

  evaluate(env) {
    // Variable
    if (this.name in env) {
      return env[this.name];

      // Object variable referenced without using 'this'
    } else if ("this" in env && this.name in env["this"]) {
      return env["this"][this.name];
    } else {
      throw new ReferenceError(`Undefined variable: ${this.name}`);
    }
  }
}

class Apply {
  constructor(expr) {
    this.type = "apply";
    this.operator = expr;
    this.args = [];
  }

  evaluate(env) {
    // Check if its a specialForm function
    if (this.operator.type === "word" && this.operator.name in SpecialForms) {
      return SpecialForms[this.operator.name].fun(this.args, env);
    }

    // Evaluated operator
    let op = this.operator.evaluate(env);
    if(op.fun !== undefined) {
      op = op.fun;
    }

    // Evaluated arguments
    let evArgs = this.args.map(arg => arg.evaluate(env));

    if (typeof op === "function") {
      // Is a function
      return op(...evArgs);
    }

    if (typeof op !== "undefined") {
      // Is an object, number, string or boolean
      let name = evArgs[0];
      let methodArgs = evArgs.slice(1);

      // Check if the name of the method/property is defined on the object
      if (typeof op[name] !== "undefined") {
        // Execute as function
        if (typeof op[name] === "function") {
          return op[name](...methodArgs);

          // Return as property
        } else {
          return op[name];
        }

        // If the name of the method is not defined on the object...
      } else {
        // Check if its a Map property.
        if (op instanceof Map) {
          if (typeof op.get(name) === "function") {
            return op.get(name)(...methodArgs);
          } else {
            return op.get(name);
          }
        }

        // Try to call the 'missing' method
        if (typeof op["__missing__"] === "function") {
          return op["__missing__"](...methodArgs);

          // As a last resort, throw an Exception
        } else {
          throw new SyntaxError(`The method '${name}' was not found on the
          object '${utils.ins(op)}'`);
        }
      }
    }

    throw new TypeError(`Could not resolve the apply expression`);
  }
}

class Regex {
  constructor(token) {
    this.type = "regex";

    this.body = token.body;
    this.flags = token.flags;
  }

  evaluate() {
    return new xRegExp(this.body, this.flags);
  }
}

module.exports = {
  Value,
  Word,
  Apply,
  Regex
};