Question

The specialForms object is used to define special syntax in Egg. It associates words with functions that evaluate such special forms. It is currently empty. Let’s add some forms.

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);
};

Egg’s if construct expects exactly three arguments. It will evaluate the first, and if the result isn’t the value false , it will evaluate the second. Otherwise, the third gets evaluated. This if form is more similar to JavaScript’s ternary ?: operator than to JavaScript’s if . It is an expression, not a statement, and it produces a value, namely, the result of the second or third argument.

Egg differs from JavaScript in how it handles the condition value to if . It will not treat things like zero or the empty string as false, but only the precise value false .

The reason we need to represent if as a special form, rather than a regular function, is that all arguments to functions are evaluated before the function is called, whereas if should evaluate only either its second or its third argument, depending on the value of the first.

The while form is similar.

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 Egg, we return false,
  // for lack of a meaningful result.
  return false;
};

Another basic building block is do , which executes all its arguments from top to bottom. Its value is the value produced by the last argument.

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

To be able to create variables and give them new values, we also create a form called define . It expects a word as its first argument and an expression producing the value to assign to that word as its second argument. Since define , like everything, is an expression, it must return a value. We’ll make it return the value that was assigned (just like JavaScript’s = operator).

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;
};

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.