Question

Arrays

The easiest way to do this is to represent Egg arrays with JavaScript arrays.

The values added to the top environment must be functions. Array.prototype.slice can be used to convert an arguments array-like object into a regular array.

Closure

Again, we are riding along on a JavaScript mechanism to get the equivalent feature in Egg. Special forms are passed the local environment in which they are evaluated so that they can evaluate their subforms in that environment. The function returned by fun closes over the env argument given to its enclosing function and uses that to create the function’s local environment when it is called.

This means that the prototype of the local environment will be the environment in which the function was created, which makes it possible to access variables in that environment from the function. This is all there is to implementing closure (though to compile it in a way that is actually efficient, you’d need to do some more work).

Comments

Make sure your solution handles multiple comments in a row, with potentially whitespace between or after them.

A regular expression is probably the easiest way to solve this. Write something that matches “whitespace or a comment, zero or more times”. Use the exec or match method and look at the length of the first element in the returned array (the whole match) to find out how many characters to slice off.

Fixing scope

You will have to loop through one scope at a time, using Object.getPrototypeOf to go the next outer scope. For each scope, use hasOwnProperty to find out whether the variable, indicated by the name property of the first argument to set , exists in that scope. If it does, set it to the result of evaluating the second argument to set and then return that value.

If the outermost scope is reached ( Object.getPrototypeOf returns null) and we haven’t found the variable yet, it doesn’t exist, and an error should be thrown.

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.