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Strict Resolution

Dart is an imperative language, where you have the agency to cast values to other types, to execute multiple statements, and ultimately create a program by explicitly declaring every action that should be taken.

HTML, and subseqently, Jael, are declarative markup languages, and thus give you considerably less control over the flow of data and type information. Functionality like type checks, which are manageable in Dart, are both unintuitive and verbose in a markup language.

To compensate, Jael can enable or disable what can be referred to as strict resolution. package:angel_jael by default disables strict resolution, and strictResolution is available as a parameter to both the jael function in Angel, and the Render() constructor in Jael.

Jael's expression parser is not the one from package:analyzer, so the evaluation of expressions at runtime is up to the Renderer class. When strict resolution is on, all referenced identifiers must be present in the scope, and the only values allowed for if, conditionals, and similar expressions are bool.

For example, take the following snippet:

<ul if=user?.name?.isNotEmpty>
  <li>
    Talk to @{{ user.name }}
  </li>
</ul>

If strict resolution is on:

  • If user is not in the scope of values passed to the renderer, an error will be thrown.
  • If the expression user?.name?.isNotEmpty is null, then an error will be thrown.

If strict resolution is off:

  • If user is not in the scope of values, Jael will just substitute it with null.
  • If user?.name is null, Jael will substitute the expression with null.
  • If the expression user?.name?.isNotEmpty does not evaluate to true (that is to say, it can be null!), then the ul will simply not be rendered.

Overall, strict resolution should likely be off for most cases, as type checking is not often that important when writing HTML templates.