Question

Arrays

Add support for arrays to Egg by adding the following three functions to the top scope: array(...) to construct an array containing the argument values, length(array) to get an array’s length, and element(array, n) to fetch the n^th element from an array.

Closure

The way we have defined fun allows functions in Egg to “close over” the surrounding environment, allowing the function’s body to use local values that were visible at the time the function was defined, just like JavaScript functions do.

The following program illustrates this: function f returns a function that adds its argument to f 's argument, meaning that it needs access to the local scope inside f to be able to use variable a .

run("do(define(f, fun(a, fun(b, +(a, b)))),",
    "   print(f(4)(5)))");
// → 9

Go back to the definition of the fun form and explain which mechanism causes this to work.

Comments

It would be nice if we could write comments in Egg. For example, whenever we find a hash sign ( # ), we could treat the rest of the line as a comment and ignore it, similar to // in JavaScript.

We do not have to make any big changes to the parser to support this. We can simply change skipSpace to skip comments like they are whitespace so that all the points where skipSpace is called will now also skip comments. Make this change.

Fixing scope

Currently, the only way to assign a variable a value is define . This construct acts as a way both to define new variables and to give existing ones a new value.

This ambiguity causes a problem. When you try to give a nonlocal variable a new value, you will end up defining a local one with the same name instead. (Some languages work like this by design, but I’ve always found it a silly way to handle scope.)

Add a special form set , similar to define , which gives a variable a new value, updating the variable in an outer scope if it doesn’t already exist in the inner scope. If the variable is not defined at all, throw a ReferenceError (which is another standard error type).

The technique of representing scopes as simple objects, which has made things convenient so far, will get in your way a little at this point. You might want to use the Object.getPrototypeOf function, which returns the prototype of an object. Also remember that scopes do not derive from Object.prototype , so if you want to call hasOwnProperty on them, you have to use this clumsy expression:

Object.prototype.hasOwnProperty.call(scope, name);

This fetches the hasOwnProperty method from the Object prototype and then calls it on a scope object.

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.