Question

I need to define a data structure recursively in Javascript. Here is a simple example of a circular linked list:

// List a very simplified example of what the actual (non list) code does.
function List(f, r) {
    return function(){ return [f, r]; };
}

var first = function (l){ return l()[0]; }
var rest = function (l){ return l()[1]; }

var head = List('a', List('b', List('c', head)));

When this is executed, head in List ‘c’ is resolved to undefined, not List ‘a’ as I need. List is an example function that returns a function (It is not an Javascript list that I can append to).

I tried to wrap the definition of head is a self executing named function, but that blew the stack when head was resolved.

What is the Javascript style solution that I am overlooking?

---

Attempt

Fooling around, I came up with some code that may work:

var f = function(){
    var value;
    return function(v){
        if (value === undefined)
            value = v
        return value.apply(undefined, arguments);
    };
};

var tempHead = f();
var head = List('a', List('b', List('c', tempHead)));
tempHead(head);

first(head); // a
first(rest(head)) // b
first(rest(rest(head))) // c
first(rest(rest(rest(head)))) // a
first(rest(rest(rest(rest(head))))) // b
...

But this is really ugly. Any better solutions?

---

Solution

user1689607 came up with a good solution which I have encapsulated to hide some of the implementation:

var def = function(name, impl) {
    var value;
    return value = impl.apply(Object.defineProperty({}, name, {
       'value': function() { return value.apply(this, arguments); }
    }));
};

function List(f, r) {
    return function(){ return [f, r]; };
}

function first(l){ return l()[0]; }
function rest(l){ return l()[1]; }

var circle = def('head', function() {
    return List('a', List('b', List('c', this.head)));
});

first(circle); // 'a'
first(rest(circle)); // 'b'
first(rest(rest(circle))); // 'c'
first(rest(rest(rest(circle)))); // 'a'
first(rest(rest(rest(rest(circle))))); // 'b'

---

One more update, I ended up going with passing the self reference explicitly instead of changing the scope:

var def = function(impl) {
    var value;
    return (value = impl(function() { return value.apply(this, arguments); }));
};

var circle = def(function(self) {
    return List('a', List('b', List('c', self)));
});

This code is used in parse.js.

Answer 1

This is what you want?

var headCaller = function() { return head.apply(this, arguments); };

var head = List('a', List('b', List('c', headCaller)));

It gives the result you seem to want…

DEMO: http://jsfiddle.net/ruNY3/

var results = [
    first(head), // a
    first(rest(head)), // b
    first(rest(rest(head))), // c
    first(rest(rest(rest(head)))), // a
    first(rest(rest(rest(rest(head))))) // b
];

---

[
    "a",
    "b",
    "c",
    "a",
    "b"
]