Better workflow for adding single items to the page

[g105b]

I much prefer the terminology data-list when working with elements that represent a list of items that represent an iterable data structure.

However, when I want to add a single item to the page, such as adding an error message to a form, it made more sense calling it data-template... but the workflow for adding the element back to the page was a bit clunky.

As an example, this is what the code looks like to add an error message to a form:

<form>

  <div data-list="error" data-bind:text>Error message goes here</div>

  <label>
    <span>Your email</span>
    <input name="email" />
  <label>
  <label>
    <span>Your password</span>
    <input type="password" name="pass" />
  </label>

  <button name="do" value="login">Log in!</button>
</form>

function do_login(
  Input $input, // obviously required
  ListElementCollection $listElementCollection, // cumbersome name, and overly verbose to use
  DocumentBinder $binder, // we still need to manually bind
  HTMLDocument $document, // AND we need the document to pass in to the list element collection :(
):void {
  if(!isAuthenticated($input->getString("email", "pass")) {
    $el = $listElementCollection->get($document, "error");
    $inserted = $el->insertListItem();
    $binder->bindKeyValue("error", "Invalid authentication details", $inserted);
  }
}

This calls for a new type of templated element, because that code above is stupidly verbose.

Here's my initial idea:

<form>
  <div data-remove-unbound data-bind:text="error">Error message goes here</div>

  <label>
    <span>Your email</span>
    <input name="email" />
  <label>
  <label>
    <span>Your password</span>
    <input type="password" name="pass" />
  </label>

  <button name="do" value="login">Log in!</button>
</form>

The data-remove-unbound attribute will remove the element if it doesn't get anything bound to it; that is, if there isn't a call to $binder->bindKeyValue("error", "something");

So the PHP would look like this:

function do_login(
  Input $input, // obviously required
  DocumentBinder $binder, // we now only need the binder
):void {
  if(!isAuthenticated($input->getString("email", "pass")) {
// only one line required to keep the element in the document:
    $binder->bindKeyValue("error", "Invalid authentication details");
  }
}

[g105b]

Linked PR: #439, but I'm deciding whether to switch to a different attribute name before it's merged.