(def queue () (list nil nil 0))
  (def qlen (q) (q 2))

  $def queue; ->
    [[] [] 0
      @{ %len: X -> (X 2) }]

  (def queue ()
    (array nil nil 0
      @(dict %len (fn (x) (x 2)))))

  (w/uniq (len left right)
    (def queue ()
      (dict %len  (fn (x) (x len))
            len   0
            left  nil
            right nil)))

In either case, the object that is returned has a %len property, which is a function that computes the length. The "len" function would then be defined like this:

  (def len (x)
    ((x %len) x))

---

* [1]: You might be wondering what's going on here... well, in Nulan, a "list" is just like a JavaScript array: it's an ordinary object which uses numeric keys and has a custom %len property. In particular, that means that all the tools that work on objects work on arrays too.

The @ syntax is used for splicing. So, for instance, to merge 3 objects together, you'd say { @foo @bar @qux }. And because arrays are objects too, you can splice them.

So what we're doing is, we first create an array of 3 elements, and we then splice in a new object. This new object has a custom %len property, which overrides the %len property of the array.

Alternatively, we could have done it like this:

  (set (array nil nil 0) %len ...)

  { @[[] [] 0]
    %len ... }

  [[] [] 0
    @{ %len ... }]

Anyways, JavaScript gets around this problem by allowing you to define a property on an object that is "non-enumerable". But any kind of system that I add in that lets you define "non-enumerable" properties is going to be big and complicated.

Instead, I found a very simple way to create enumerable objects in a way that is completely customizable, and doesn't even need to be built-in:

  (= %keys (uniq))
  
  (def iterable-object ()
    (let keys []
      { %set  (fn (x k v)
                (pushnew k keys))
        %rem  (fn (x k)
                (pull k v))
        %keys (fn (x) keys) }))

The function "iterable-object" returns a new object that has custom %set, %rem, and %keys properties:

%set is called when assigning a property to the object. It takes the key and pushes it into the "keys" array.

%rem is called when deleting a property from the object. It takes the key and removes it from the "keys" array.

%keys just returns the "keys" array.

Now, these objects are capable of being enumerated, which means they could be passed to "map", for instance. But here's the fun part: you can completely control which properties are enumerated and which aren't.

In this case, the object will only enumerate properties that are added or removed after the object is created. So any properties defined previously are still hidden. At least, depending on how I implement splicing and %set...

---

  $def iterable-object ->
    [ %set  -> x k o n
              [ @x %keys -> ~ (pushnew (keys x) k) ]
      %rem  -> x k o
              [ @x %keys -> ~ (pull (keys x) k) ]
      %keys -> ~ {} ]

1) {} is closer to () than [] is, which is really nice in macros:

  $mac $accessor -> n v
    $uniq %a
      {$def n -> %a
        {{%a v} %a}}

---

By the way, in case you're curious about the Nulan syntax... $ is prefixed to vau/macros, which is why it's "$def" rather than "def"

-> is the function macro, which means (-> x y z ...) is equivalent to (fn (x y z) ...) in Arc

~ is the "wildcard syntax" which matches anything, just like _ in Haskell

[ foo bar ] translates to (dict foo bar), and { 1 2 3 } translates to (array 1 2 3)

@ is for splicing. Which means that [ @foo @bar @qux ] merges three objects into one. If you want to update an object with new properties, it's idiomatic to say [ @foo ... ]

  var iterableObject = function () {
    var a = {};
    a.set = function (x, k, o, n) {
      var a = Object.create(x);
      a.keys = function () {
        return pushnew(keys(x), k)
      }
    };
    a.rem = function (x, k, o) {
      var a = Object.create(x);
      a.keys = function () {
        return pull(keys(x), k)
      }
    };
    a.keys = function () {
      return []
    };
    return a
  }