Javascript recursive data structure definition

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"
]

Answer 2

When you give head in parameter, it's not yet assigned. You need to use two steps. Also, there is no need to use nested functions.

function List(f, r) {
    return { value: f, next: r};
}

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

In Javascript, there are no pointer, but all Array or Object are assigned by reference.

---

EDIT:

A solution for make it simpler:

function List(f) {
    var list = { value: f, next: null };
    list.next = list;
    return list;
}

function insert(list, elm) {
    list.next = { next: list.next, value: elm };
}

function forward(list, nb) {
    for (var current = list; nb > 0; nb--)
        current = current.next;
    return current;
}

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

console.log(head.value); //a
console.log(head.next.value); //b
console.log(head.value); //c

console.log(forward(head, 3));//a

Answer 3

I am not sure if this is what you want, check it:

function List(f, r) {
    var fr = [f, r];
    var func = function() {
        return fr;
    };
    func.fr = fr; // because that is a reference we can trick it
    return func;
}

var head = List('a', List('b', List('c', null)));
head.fr[1].fr[1].fr[1] = head; // make a loop

Added: a function to do it more easily

// Or this method for lazy people
function createCircularList(f) {
    var head = List(f[f.length - 1], null);
    var firstfr = head.fr;
    for (var i = f.length - 2; i >= 0; i--) {
        head = List(f[i], head);
    }
    firstfr[1] = head;
    return head;
}

var head = createCircularList(['a', 'b', 'c']);

Demo: http://jsfiddle.net/alvinhochun/5nmpR/

In JavaScript, things are interesting. Everything is references, yet the reference itself is a value.

var a = [0, 0];    // `a` is set to the reference of `[0, 0]`
var b = a;         // `b` refers to the same thing as `a`
console.log(b[0]); // shows 0
a[0] = 1;          // it modifies the object that it references to
console.log(b[0]); // shows 1
a = [2, 0];        // a new reference is assigned to `a`
console.log(b[0]); // still shows 1