As indicated on the Project Background Knowledge page, the main purpose of providing visual support is twofold: to provide (1) support to a user to reduce their cognitive load (the amount of information working memory needs to process at any given time) and (2) an indication of the progress on the task that has been made thus far. Visual support will be provided by means of a webpage. This page aims at explaining how you can develop code for your MARBEL agent to create such a webpage.

Using Prolog to build HTML elements in our agent

It is important to be able to synchronize the conversational interaction and the visuals displayed on the webpage shown while talking. The MARBEL agent manages the dialog, and for that reason also is the logical place to locate the control over the visual elements. This agent uses a renderPage(Html) action in the dialog_generation.mod2g module to render a webpage; when performing this action, the Prolog variable Html therefore should be instantiated with HTML webpage code. All we need to know before we can use this action, is how to create this HTML page in Prolog.

The basic idea is that we will create essentially a single string that consists of the HTML webpage code. We will utilize a combination of Prolog, HTML and Bootstrap to generate the code for dynamic webpages. Prolog rules are used to generate our HTML code. In other words, the HTML code represented as a Prolog atom, essentially a string, is manipulated with Prolog. So what does that look like? We will first take a look at how to construct some very basic HTML code, then explain how we can add more complicated HTML code that is also using Bootstrap components, and, finally, how we can piece together these components into a complete webpage.

Generating basic HTML elements with short tags

An HTML page consists of HTML elements. HTML elements consists of HTML tags that are used to organize content on that page. Tags often come in pairs of a start and end tag. Basic HTML tags, for example, are the tags <p>YOUR PARAGRAPH TEXT HERE</p> that defines an HTML paragraph, the <h1>...</h1> defining a large heading, <b>...</b> defining bold text, etc. There are many of these basic tags which, moreover, are also short which facilitates their frequent and easy use. The idea to generate basic HTML code for these elements in Prolog is very simple: Simply use single quotes '...' to generate such an element. The table next lists a few examples:

There are many simple, short tags where it is easiest to simply put them between single quotes to create a Prolog atom for generating the corresponding HTML code. Another class of short tags are the single tags such as '<br>' for breaking a line and '<hr>' for adding a horizontal line. These are also most easily generated by simply quoting them.

Generating more complex HTML elements with attributes

Most HTML tag pairs can be simply generated by using a start and end tag and by adding some text to create the corresponding HTML. For example, <button>Start</Button> can be used to generate a simple button saying Start. However, to unleash the full power of the HTML language, it is often useful to modify the type or style of an HTML element by adding additional attributes in the form of name="value" to the start tag.

Making a button

For example, the following HTML code creates a large, colored button using the class attribute and several button class values made available by Bootstrap:

<button class="btn btn-secondary btn-lg">Start</button>

Here, Start between the start and end tag is the content of the HTML element, button is used for the start and end tags to create a button element, and the class attribute uses the values btn-secondary to style the button and btn-lg to create a larger button.

Of course, we can still create this HTML code in Prolog by simply quoting it. And for a simple button this may still most often be quite OK. This is because the content we typically want to fit inside of a button element is often just some simple text. But it will be useful to allow for a more generic approach for more advanced HTML elements in which we want to embed other more complex HTML code as content. To illustrate our approach to generating HTML elements with attributes in Prolog we therefore begin with introducing a predicate button/3 for generating HTML code for a button.

In the button example above, we illustrated the overall approach and structure we will use for generating HTML code for elements with attributes but did not include the attribute part yet. We will now also add that to complete the button example. For the button example we will only include one attribute class. We thus end up with a button/3 predicate with three arguments button(Content, Class, Html). Predicates for other HTML tags, such as the div tag, for example, where we would also want to add a style attribute, would have 4 arguments. To conclude our button example, to generate the <button class="btn btn-secondary btn-lg">Start</button> code, we can use the Prolog code

button('Start', 'btn btn-secondary btn-lg', Html)

When evaluating this clause, we would end up with Html having the button HTML code as its value.

Changing and receiving button events in a MARBEL agent

If you would inspect the html.pl file and search for the button predicate, you will find a button/2 predicate too. You will also see that both the button/2 and the button/3 predicates actually are defined in terms of a button/4 predicate button(Content, Class, DataValue, Html). This predicate allows to set the value send as a percept to the agent when the button is clicked. The button/3 definition by default sets this value equal to the Content value displayed on the button. That, however, is not that useful if the content itself consists of HTML tags again. For example, button('<b>Start</b>', 'btn btn-secondary btn-lg', Html) would display a bold text Start but would also send the tags as a percept of the form answer('<b>Start</b>') when the button is clicked. To avoid that, we can use button('<b>Start</b>', 'btn btn-secondary btn-lg', 'start', Html) which would send the more plain and easy to process answer('start') to the MARBEL agent.

Other tag examples: ul, img, div, span

We introduce a few other tags and discuss how HTML code for these tags can be generated using some of the predefined predicates in the html.pl file. These examples should provide you with a general understanding of how you can generate HTML code for a variety of HTML elements.

ul tag

Displaying lists is a practical feature when you want to show a recipe and its associated ingredient list. The most basic way to do this is to use the unordered list ul and list item li tags. Here’s a simple example of what the HTML code would look like:

<ul class="list-group">
  <li class="list-group-item">1st item</li>
  <li class="list-group-item">2nd item</li>
  <li class="list-group-item">3rd item</li>
</ul>

We have kept the class attribute simple but Bootstrap list groups provides several other values for the class attribute that can be used to change the basic behavior of the ul tag. For example, use list-group-flush for removing the borders and list-group-horizontal for organizing the list items horizontally instead of vertically.

Even though ul is a short tag we still have introduced a corresponding ul/3 predicate ul(Content, Class, Html) because of the embedded content of list items that needs to be supplied and fit in between the start and end tags in this case. Using this predicate we can simply use ul(Content, 'list-group', Html) for generating the unordered list example above. Clearly, a simple quotation approach will not work here as the list of items may easily get very long and unyieldy and upfront we may not even know how many items we need to add to the list (how many ingredients, for example, does a recipe have?). To facilitate the creation of the HTML list item elements, we have provided the predicate itemList(List, Html) that generates the list item elements when provided with a list of the content for each item. For our example, itemList(['1st item', '2nd item', '3rd item'], Html) will generate what we need as content for the ul element.

The final step that remains is to piece the two things together to generate the complete HTML code for our example. For generating HTML code, we will use the fact that Prolog evaluates subqueries in a query in the order they appear. As a general rule of thumb, in this content, we should make sure to generate the HTML elements we need later first. That is, because we need the list item elements as content to complete the unordered list code, we need to create this code first. In other words, we will use the output argument of itemList/2 as the input argument for the content of ul/3. By piecing the separate queries we defined for the list itself and for the list items together and renaming the appropriate argument variables, we get what we need:

itemList(['1st item', '2nd item', '3rd item'], ListItems), ul(ListItems, 'list-group', Html)

This combined query will generate our example HTML code and return it as value in the Html variable.

img tag

div tag

txtHtml('<div class="card-body"><p class="card-text">~a</p></div>').
txtHtml(Txt, Html) :- txtHtml(Template), applyTemplate(Template, Txt, Html).

span tag

The span tag is an inline element that is commonly used to style HTML content. We have added a span/3 predicate span(Content, Style, Html) that just does that and nothing else. An example of a span tag that can be useful is to style the content and change the font family that is used within some HTML content:

<span style="font-family:Roboto;">YOUR HTML CONTENT HERE</span>

A list of system or web-safe fonts (fonts supported on most devices) can be found here.

The span tag has other uses too. We have used it, for example, to set the value of a button in the template we used for defining the button/4 predicate.

Generating an HTML webpage

By means of these and generate with the rule. The HTML code is represented in Bootstrap format, which is also clearly illustrated by examples below or on Bootstrap's documentation website (Bootstrap Documentation).

Prolog rules are used to add a condition to a webpage (i.e. webpage X is shown when Clause Y is true).

You add a condition that checks for a particular pattern ID to a Prolog rule in the html.pl file. Other simple conditions that you can add involve counting the number of recipes that are still available while in the recipe selection phase, etc. The general form of the page lay-out rules would be something like:

myPage(Txt, Button, Html) :-
  <HERE YOUR CONDITION FOR SHOWING THIS PAGE>, 
  % e.g. while recipe is being selected (check for e.g. currently active top level pattern id), we'll display recipe features
  % below code that specifies the page layout for this context
  applyTemplate('<div class="card mx-auto" style="width:67vw">~a</div>, Text, Html),
  …
  .

There are still many options to vary. For example, you can choose the parameters for the myPage predicate that fit your approach best.

Example Prolog Rule for a Page

page(c10, _, Html) :-
	% Condition for when to show this page
	CONDITION X,
	% Constructing HTML page
	atomic_list_concat(['<div class="alert alert-light"><center></br><h1>Hello!</br></br>','I am ~a</h1></br></br></center></div>'], Template),
	% Get the bot's name if it has one; other call it 'your assistant'
	(agentName(Name) -> N = Name ; N = 'your recipe selection assistant'), applyTemplate(Template, N, Body),
	% Create the HTML page
	html(Body, Html).

Adding a chatbox to your page layout for testing purposes

The speech interface is fitting to the context of a user in a kitchen, but not always feasible for testing. It may take additional time for the system to process speech input, you may be in a noisy environment, and there may be speech recognition failures. Hereby a pointer to easily enable a textbox to be rendered for input.

In the html.pl file, where you format the pages to render during the conversation, for each of the pages that you may display at any point in the conversation add the following code:

A chatbox can be added for using text instead of speech for input by adding
<div class="text-center"><p class="chatbox mx-auto"></p></div>, for example,
to the footer.

Using and Understanding the code in html.pl

A Quick guide to using Bootstrap components not defined in html.pl

We briefly discuss an example on how to use the code in html.pl yourself with Bootstrap components that you might want to use but have not yet been introduced yet. To that end, suppose that you would like to create a lay-out with three images side-by-side. The first step you could take is to check which components Bootstrap offers to create such a lay-out. It turns out that a card deck might just be what you were looking for. It is advertised as something you would use when you “Need a set of equal width and height cards that aren’t attached to one another? Use card decks.” Bootstrap also provides the HTML template code as an example of how to use it. This is what that looks like:

<div class="card-deck">
  <div class="card">
    <img class="card-img-top" src="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This is the first card.</p>
    </div>
  </div>
  <div class="card">
    <img class="card-img-top" src="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This is the second card.</p>
    </div>
  </div>
  <div class="card">
    <img class="card-img-top" src="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This is the third card.</p>
    </div>
  </div>
</div>

How do we break this code down into pieces that we can (already) manage? We first scan the code for the HTML tags that are used in it. We find that the div tag is often used (with the classes "card-deck", "card", and "card-body"). We also find that the img tag is used as well as some variants of the <h5> and <p> tags with specific card-related classes added. We note that for each of these tags we already have the tools to generate them in Prolog. We can use the div/4 and img/4 predicates for the former tags and can use our strategy to quote basic HTML elements for the h5 and p elements (you can also introduce new predicates for these tags in combination with a class attribute if you like but we prefer to keep it simple here).

Now we have established that all the basic ingredients are available, the next step in our recipe is to create the separate parts of the HTML code by - in a sense - looking from the inside out. We first will generate the elements that are most embedded and then use those to build the elements in which they are directly embedded.

First, we want to render a given image. In the HTML, we see that each of the three images are defined as separate cards.

We will now use Prolog to generate the HTML for such a card, using a given picture URL. The aim is to connect in a single string the static HTML code and the image URL (which differs according to the context of the conversation).

imgCard('<div class="card"><img class="card-img-top" src="~a" alt="Card image cap"></div>').

Note that the HTML is copied, with a replacement of the contents of ‘src’, in the form of ‘~a’ which is a placeholder for the image URL. This is needed in order to dynamically add this information. We will do this with the following rule:

imgCard(Image, Html) :- imgCard(I), format(atom(Html), I, [Image]). 

In this rule, the Image variable is the input and holds the URL of the image, whereas the Html variable is the output HTML code for this card. In the body of the rule, the imgCard/1 is queried to retrieve the HTML code that we already specified. Then, the URL is placed at the location with `~a' in this HTML code (in the Image variable), using the format/3 predicate.

We can now use the imgCard term to render each of the three images. For the complete display, we will combine them in the broader ‘card-deck’ option, like in the example HTML we got from bootstrap. We will do this using a self-defined Prolog term, with as input the text, button text and the three images, and as output the HTML code to render. Within this term, we will start specifying the main template, and then collect the images:

myThreeImagesPage(Txt, Button, Image1, Image2, Image3, Html) :-
  Template='<div class="card-deck">~a</div>',
  imgCard(Image1,I), imgCard(Image2,I2), imgCard(Image1,I3), 
  atom_concat(I,I2,II), atom_concat(II,I3,III), 
  format(atom(Card), Template, [III]), html(Card, Html).

Note that atom_concat is used to append the different HTML snippets for each of the images. Format is then used to plug in the HTML code where the `~a' has been specified in the Template variable (as was done for the imgCard).

The current term will display three given images on the screen. Of course, you need to make sure that the right image URLs can be displayed during the conversation, which you can do using Prolog facts in recipes.pl.

Defining new predicates for generating HTML elements