parser.js

// parseExpression uses three regular expressions
// to spot the three simple (atomic) elements that miniLanguage supports:
// strings, numbers, and words. The parser constructs a 
// different kind of data structure depending on which one matches.
//  If the input does not match one of these three forms, 
//  it is not a valid expression, and the parser throws an error.
// SyntaxError is a standard error object type, which is raised 
// when an attempt is made to run\ an invalid JavaScript program

function parseExpression(program) {
  program = skipSpace(program);
  var match, expr;
  if (match = /^"([^"]*)"/.exec(program))
    expr = {type: "value", value: match[1]};
  else if (match = /^\d+\b/.exec(program))
    expr = {type: "value", value: Number(match[0])};
  else if (match = /^[^\s(),"]+/.exec(program))
    expr = {type: "word", name: match[0]};
  else
    throw new SyntaxError("Unexpected syntax: " + program);  
// We can then cut off the part that we matched from the program string
//  and pass that, along with the object for the expression, 
//  to parseApply, which checks whether the expression is an application.
//  If so, it parses a parenthesized list of arguments.
  return parseApply(expr, program.slice(match[0].length));
}

function skipSpace(string) {
  var first = string.search(/\S/);
  var eol;
  var comment;
  if (first != -1 && first != 0){
    string=string.slice(first);
    return skipSpace(string);
  }else{
    comment = string.search(/#/);
    eol = string.search(/\n/);
    if (comment  != -1 && eol != -1){
        let str1 = string.substr(0,comment);
        let str2 = string.substr(eol+1,string.length);
      string=`${str1}${str2}`
     return skipSpace(string);
    }
  }
  return string;
}

function parseApply(expr, program) {
  program = skipSpace(program);
  if (program[0] != "(")
    return {expr: expr, rest: program};
  program = skipSpace(program.slice(1));
  expr = {type: "apply", operator: expr, args: []};
  while (program[0] != ")") {
    var arg = parseExpression(program);
    expr.args.push(arg.expr);
    program = skipSpace(arg.rest);
    if (program[0] == ",")
      program = skipSpace(program.slice(1));
    else if (program[0] != ")")
      throw new SyntaxError("Expected ',' or ')'");
  }
  return parseApply(expr, program.slice(1));
}

// This is all we need to parse miniLanguage. 
// We wrap it in a convenient parse function 
// that verifies that it has reached the end of the input string after parsing the expression
//  (a miniLanguage program is a single expression),
//   and that gives us the program’s data structure.
function parse(program) {
  var result = parseExpression(program);
  if (skipSpace(result.rest).length > 0)
    throw new SyntaxError("Unexpected text after program");
  return result.expr;
}

function evaluate(expr, env) {
  switch(expr.type) {
    case "value":
      return expr.value;

    case "word":
      if (expr.name in env)
        return env[expr.name];
      else
        throw new ReferenceError("Undefined variable: " +
                                 expr.name);
    case "apply":
      if (expr.operator.type == "word" &&
          expr.operator.name in specialForms)
        return specialForms[expr.operator.name](expr.args,
                                                env);
      var op = evaluate(expr.operator, env);
      if (typeof op != "function")
        throw new TypeError("Applying a non-function.");
      return op.apply(null, expr.args.map(function(arg) {
        return evaluate(arg, env);
      }));
  }
}
var specialForms = Object.create(null);
specialForms["if"] = function(args, env) {
  if (args.length != 3)
    throw new SyntaxError("Bad number of args to if");

  if (evaluate(args[0], env) !== false)
    return evaluate(args[1], env);
  else
    return evaluate(args[2], env);
};

specialForms["while"] = function(args, env) {
  if (args.length != 2)
    throw new SyntaxError("Bad number of args to while");

  while (evaluate(args[0], env) !== false)
    evaluate(args[1], env);

  // Since undefined does not exist in MiniLanguage, we return false,
  // for lack of a meaningful result.
  return false;
};

specialForms["do"] = function(args, env) {
  var value = false;
  args.forEach(function(arg) {
    value = evaluate(arg, env);
  });
  return value;
};

specialForms["define"] = function(args, env) {
  if (args.length != 2 || args[0].type != "word")
    throw new SyntaxError("Bad use of define");
  var value = evaluate(args[1], env);
  env[args[0].name] = value;
  return value;
};

var topEnv = Object.create(null);

topEnv["true"] = true;
topEnv["false"] = false;

["+", "-", "*", "/", "==", "<", ">"].forEach(function(op) {
  topEnv[op] = new Function("a, b", "return a " + op + " b;");
});

topEnv["print"] = function(value) {
  console.log(value);
  return value;
};

function run() {
  var env = Object.create(topEnv);
  var program = Array.prototype.slice
    .call(arguments, 0).join("\n");
  return evaluate(parse(program), env);
}

specialForms["fun"] = function(args, env) {
  if (!args.length)
    throw new SyntaxError("Functions need a body");
  function name(expr) {
    if (expr.type != "word")
      throw new SyntaxError("Arg names must be words");
    return expr.name;
  }
  var argNames = args.slice(0, args.length - 1).map(name);
  var body = args[args.length - 1];

  return function() {
    if (arguments.length != argNames.length)
      throw new TypeError("Wrong number of arguments");
    var localEnv = Object.create(env);
    for (var i = 0; i < arguments.length; i++)
      localEnv[argNames[i]] = arguments[i];
    return evaluate(body, localEnv);
  };
};


topEnv["array"] =function(...args){
  return new Array(...args);
}

topEnv["length"] = function(array){
  return array.length;
}
topEnv["element"] =  function(array,nth){
  return array[nth];
}