10. Node States

So far, we’ve seen that Tridash is good at producing an output, which is a function of a given input, and ensuring that it is always synchronized with the input. What we haven’t seen, however, is mapping a previous output to a new output. In-fact, with the tools introduced so far, this is impossible.

To be able to map a previous output to a new output, a binding has to be established in which the source is a function of the target node. As an example, to implement a counter, intuitively we might do the following:

# This does not work!!!
counter + 1 -> counter

The problem here is that a change in the value of counter triggers a change in the value of counter, which triggers a further change in the value of counter ad infinitum. What we require is a way to tell Tridash, that when the value of counter is updated to counter + 1, it should not trigger another update to the value of counter + 1. We also need a way to specify when we would like the value of counter to be updated to counter + 1, as updating it only once is hardly useful.

10.1. Stateful Bindings

Node states allow us to control when the value of a node is updated, beyond the simple rule of whenever the value of one of its dependency nodes changes. The target of a binding can have an explicit state associated with it, using the :: operator. In this case the binding is referred to as a stateful binding.

Explicit state using :: operator. 

a -> b :: state-id

The left-hand side of the :: operator is the node and the right hand side is the state identifier, which is a symbol identifier (similar to a context identifier). The result of this is that the binding a -> b only takes effect when b switches to the state with identifier state-id. The emphasis is on switches as a change in the value of a will not automatically trigger a change in the value of b. Only a change in the state of b will trigger a change in its value. This may seem counter-intuitive at first, however if b was updated on every change in the value of a, and instead of a we have b + 1 we’ll end up with the same problem in the previous section.

[Important]

Stateful bindings declared later in the source take priority over those declared earlier.

A node’s state is determined by the value of the special node /state(node). Binding to it allows us to control a node’s state.

Example: Counter. 

counter + 1 -> counter :: increment

/state(counter) <-
    case(
        should-increment? : '(increment),
        '(default)
    )

[Tip]

The ' operator returns its argument as a literal symbol. '(id) returns the literal symbol id, whereas id on its own is a reference to the value of the node with identifier id. [1]

In this example, a binding counter + 1 -> counter is established which only takes effect when counter switches to the increment state.

The second declaration binds a case expression to the /state(counter) node, thus controlling the state of counter. When should-increment? is true the value of the case expression, and thus the state of counter is increment, otherwise it is default.

The result is that when the value of /state(counter) (the state of counter) is updated to increment, the value of counter is updated to it’s previous value incremented by 1.

10.2. Interfacing with JavaScript

To provide a runnable example, we have to interface with JavaScript in order to hook into its event system. In the next major release of Tridash, this wont be necessary.

This section will go over only the basics of interfacing with JavaScript, which are necessary for completing this tutorial. A full in-depth tutorial on interfacing with JavaScript, will follow.

Tridash nodes are compiled to runtime node objects, which store the node’s value and information about its dependencies, observers, contexts, etc. To be able to reference a runtime node object, from JavaScript, the node has to be given an identifier with which it can be referenced. The public-name attribute, if given, determines the name of this identifier.

Once an identifier is given, it can be accessed as a member of the Tridash.nodes object. Tridash is the object/module storing the Tridash runtime library functions. The nodes member of Tridash is an object storing references to all nodes which have been given a public-name identifier.

[Caution]

Currently Tridash does not check whether multiple nodes are given the same public-name.

Example: Setting public-name identifier. 

/attribute(node, public-name, "aNode")

Example: Referencing the runtime node object in JavaScript. 

var node = Tridash.nodes["aNode"];

// or equivalently if the public-name is a valid JS variable name
var node = Tridash.nodes.aNode;

Once a reference is obtained to the runtime node object, the node’s value can be set using the set_value method, which takes the value as an argument.

Example: Setting Node Value from JavaScript. 

node.set_value(1);

It is important, however, that if set_value will be called on a runtime node object, the node is marked as an input node, by setting its input attribute to true.

/attribute(node, input, 1)
[Note]

Currently an attribute is set to true when it is given the value 1 and set to false when it is given the value 0. In the next release, you’ll be able to use the symbols True and False instead.

[Caution]

Not every Tridash node corresponds to an actual runtime node object, due to some intermediate nodes being optimized out by the compiler. The only nodes, for which it is guaranteed that a runtime node object is created, are input nodes and nodes with no observers, which are assumed to represent output nodes.

10.3. Example: Counter Application

In this section we’ll build a very simple application consisting of a counter which is incremented when a button is pressed.

Let’s start off with the Tridash binding declarations. We’ve in-fact already written the bulk of the code in the previous example, which we can simply copy into our new application.

counter + 1 -> counter :: increment

/state(counter) <-
    case(
        should-increment? : '(increment),
        '(default)
    )

The node counter stores the value of the counter which is incremented when should-increment? is true. We’ll add the attributes which are necessary in order to be able to set the value of should-increment? from JavaScript, namely we need to set a public-name identifier and mark it as an input node.

/attribute(should-increment?, input, 1)
/attribute(should-increment?, public-name, "should_increment")

We’ll allow the user to set/reset the value of the counter by binding a start node to an input field. We’ll bind start directly to counter and give it an initial value of 0.

start -> counter
0 -> start

Now let’s define the user interface. We need a text input field for entering the initial value for the counter, which will be bound to start and an increment button. The value of the counter will be displayed below the counter.

ui.html

...
<div><label>Start: <input value="<?@ to-int(start) ?>"/></label></div>
<div><button id="increment">Increment</button></div>
<hr/>
<div><strong>Counter: <?@ counter ?></strong></div>
...

[Note]

The HTML boilerplate is not shown.

We’ve given the Increment button the ID increment so that we can attach an event listener to its click event. We’ll do so using the following JavaScript code, in a script tag which should be added below the element where the counter is displayed:

<script>
  var increment = document.getElementById('increment');
  var node_increment = Tridash.nodes.should_increment;

  increment.addEventListener('click', function() {
      node_increment.set_value(true);
      node_increment.set_value(false);
  });
</script>

The first line obtains a reference to the HTML element with ID increment.

The second line obtains a reference to the should-increment? node which was given a public-name of should_increment.

The remainder of the code attaches a listener, for the click event, on the Increment button. In that listener we first set the value of the should-increment? node to true, then immediately afterwards we set it to false again.

Setting the value of should-increment? to true, causes the state of counter to change to increment. Setting it back to false again causes the state to change to default.

[Note]

In a future release, when the HTML library is complete, a subnode clicked?, of the element node, will be available which will automatically be set to true when the button is clicked and to false when it is released. Thus the above JavaScript code wont be necessary.

Build and run the application, and press the increment button a few times:

Start: 0, Counter: 1
Start: 0, Counter: 2

The displayed value, for the counter, is incremented after each press.

Now enter a value in the Start field to reset the counter, and then press the increment button again:

Start: 5, Counter: 5
Start: 5, Counter: 6

The counter is reset to the value entered in the Start field. Pressing increment afterwards increments the new counter value.

10.4. State Transitions

Let’s do a little experiment, comment out the line, in the click event handler, which sets the value of the should-increment? node to false, i.e. comment out the following:

node_increment.set_value(false);

What do you expect to happen? Initially, we might think that since the should-increment? node is not being reset to false, the state of counter is not being reset to default. The state of counter will thus switch to increment once, after which, there are no further state changes. The result is that the counter will only be incremented the first time the increment button is pressed.

Let’s try it. Build and run the application and press the increment button twice.

Start: 0, Counter: 2

What happened? The counter carries on incrementing after the first button press. Why?

The declaration:

counter + 1 -> counter :: increment

states that the binding should only take effect when the state of counter switches to the increment state. Switching from the increment state to the increment is still considered as switching to the increment state, even though the new state is identical to the previous state. This is due to the fact that each time we’re setting the value of the should-increment? node to true, we’re triggering a change in the value of the node /state(counter) and thus the state of counter, even though the new value of should-increment? is identical to its previous value.

The above declaration should thus be thought of as declaring a binding which takes effect whenever counter switches from any state, including increment, to the increment state.

To fix this issue we can specify an explicit from state. When specified, the binding only takes effect when the state of the node changes from the from state to the to state. This is specified using the following syntax:

Node State Binding with Explicit From State. 

a -> b :: previous => next

When the state identifier is a functor of the form previous => next, previous is interpreted as the identifier of the from state and next is interpreted as the identifier of the to state.

To achieve the intuitive behaviour we can limit the binding counter + 1 -> counter to only take effect when the state of counter transitions from default to increment. Replace the binding declaration of counter with the following:

counter + 1 -> counter :: default => increment

This implementation will, however, require two initial button presses before the counter starts incrementing. This is due to the fact that we haven’t given counter an initial state. The first press causes it to change to increment, and then back to default, however since it is not a change from default to increment, the binding does not take effect. We can fix this by giving /state(counter) an initial value of default.

Setting Initial State. 

'(default) -> /state(counter)

Now the counter starts incrementing after the first press.

Let’s repeat the experiment on this new implementation, comment out the line, in the JavaScript script tag, which sets the value of should-increment? to false. The counter should only increment the first time the Increment button is pressed.

This example served to demonstrate the difference between a stateful binding with and without a from state. However, in this example, there is no value in specifying a from state, as the previous example, without a from state, was simpler and performed the same function.

10.5. Example: Alternating Increment Button

In this example, we’ll build a silly application in which the Increment button, the button which actually increments the counter, alternates between two buttons. The goal of this example is to demonstrate what can be done with stateful bindings with a from state that cannot be done with stateful bindings without a from state.

We’ll start off with a similar interface to the previous counter application, however with two increment buttons.

ui.html

<h1>Counter</h1>
<div><label>Start: <input value="<?@ to-int(start) ?>"/></label></div>
<div>
  <button id="increment1">Increment 1</button>
  <button id="increment2">Increment 2</button>
</div>
<hr/>
<div><strong>Counter: <?@ counter ?></strong></div>

Both button’s have been given ID’s as we’ll need to attach event listeners to both buttons, in JavaScript.

We’ll need two nodes clicked1? and clicked2? which change to true when the first and second buttons are clicked, respectively. The values of these nodes will have to be set via JavaScript, thus we’ll have to mark them as input nodes and set their public-name attributes:

/attribute(clicked1?, input, 1)
/attribute(clicked1?, public-name, "clicked1");

/attribute(clicked2?, input, 1)
/attribute(clicked2?, public-name, "clicked2");

The following JavaScript code attaches event listeners for the clicked events, of both buttons, which simply set the value of the corresponding clicked? node to true and them immediately to false, again.

var increment1 = document.getElementById('increment1');
var increment2 = document.getElementById('increment2');

var clicked1 = Tridash.nodes.clicked1;
var clicked2 = Tridash.nodes.clicked2;

increment1.addEventListener('click', function() {
    clicked1.set_value(true);
    clicked1.set_value(false);
});

increment2.addEventListener('click', function() {
    clicked2.set_value(true);
    clicked2.set_value(false);
});

We’ll need two states for the counter node:

increment1
Corresponds to the first button being clicked last.
increment2
Corresponds to the second button being clicked last.

The value of counter should be incremented only when its state switches from one state to the other, i.e. the button, which was clicked, is different from the previous button to be clicked. This can be achieved using two stateful bindings which take effect during the state transitions increment1 => increment2, increment2 => increment1.

counter + 1 -> counter :: increment1 => increment2
counter + 1 -> counter :: increment2 => increment1
[Tip]

counter + 1 can be refactored into a separate node to avoid having to type it out twice.

counter will retain its previous value during any other state transition.

We need to set counter's state, to increment1 or increment2, based on which button was clicked last. To achieve that we can exploit the fact that each explicit binding to a node, without an explicit context, results in the creation of a new context. When the value of the source node, of the binding, changes, the binding context is activated.

We know that the value of clicked1 changes to true, and then to false, again, when the first button is clicked. The same is true for clicked2 when the second button is pressed. We want the state of counter (/state(counter)) to be bound to the literal symbol increment1, when the value of clicked1 changes regardless of what that value is. For that, we can use the utility !- meta-node, introduced in Section 9.3, “Improved Error Handling in Adding Numbers, which returns the value of its second argument if its first argument does not evaluate to a failure.

The following are the binding declarations which set the state of counter.

clicked1? !- '(increment1) -> /state(counter)
clicked2? !- '(increment2) -> /state(counter)
[Note]

The sole purpose of the !- operator is to force the value of /state(counter) to be updated whenever the value clicked1 or clicked2 changes.

To make it obvious which button should be clicked, we’ll make the button which increments the counter change to green. We’ll bind each button’s background color to "green" if the other button was the last button to be pressed, and "gray" if it was the last button to be pressed. This is achieved with the following:

increment1 <- /state(counter) = '(increment1)
increment2 <- /state(counter) = '(increment2)

case (increment2 : "green", "gray") ->
    self.increment1.style.backgroundColor
case (increment1 : "green", "gray") ->
    self.increment2.style.backgroundColor
[Note]

We’re comparing the state of counter directly to determine which button was pressed last.

Finally, let’s make the initial state of counter, increment2 so that pushing on the first button, increments the counter.

Build and run the application. Initially you should see something similar to the following:

Start: 0, Increment 1 (green), Increment 2, Counter: 0

Click on the green Increment 1 button:

Start: 0, Increment 1, Increment 2 (green), Counter: 1

The counter is incremented by one, and the button changes to grey with the other button changing to green. Clicking on the same, now grey, button will not affect the value of the counter.

Click on the green Increment 2 button:

Start: 0, Increment 1 (green), Increment 2, Counter: 2

The counter is incremented once again, and button Increment 1 changes back to green with button Increment 2 changing back to grey.

Just to make sure everything works properly let’s reset the counter and push the green button to increment it.

Start: 10, Increment 1 (green), Increment 2, Counter: 10
Start: 10, Increment 1, Increment 2 (green), Counter: 11

Everything works as expected. The state of counter does not affect the binding start -> counter as it is a stateless, without an explicit state specified, binding.

10.6. Exercises

Try out the following as an exercise:

  • Change the previous application such that clicking on the grey button, i.e. the button which does not increment the counter, decrements the counter.
  • Implement a toggle button of which the background color, and text, changes when pressed.


[1] This performs a similar function to Lisp’s quote or ' operator