What are the real-world benefits of declarative-UI languages such as XAML and QML?

Question

I\'m currently evaluating QtQuick (Qt User Interface Creation Kit) which will be released as part of Qt 4.7. QML is the JavaScript-based declarative language behind QtQuick.

Answer 1

(Updated)

The misconception with XAML is that it's not compiled. It is indeed compiled down to BAML a binary pre-tokenized XAML. Apparently there was a IL compiled version of XAML too called CAML. The OP pointed me to this good article explaining what XAML/BAML and CAML are.

Anyway, to the question why to use it:

XAML is simply a Serialization Format for C# objects that it is particularly well suited to describe hierarchical object structures, like found in WPF GUIs.

What WPF helps you do is write less boring C# code like this:

var grid = new Grid();
grid.Content.add(new TextBlock() {Text = "Hello"});
grid.Content.add(new TextBlock() {Text = "World"});

and just express it in a more readable way like this:

Since WPF object nesting (putting stuff inside other objects) can get very deep, WPF makes it much easier to read than the resulting C# code.

As for separation of concerns: XAML helps here too since it does only allow you to express objects and their relationships/properties, rather than logic. That forces you to separate logic from UI layout. The MVVM Pattern is very well suited for this task and allows for eay testability and interchangeable Views.

Added complexity in XAML can be also easily dismissed because the same code in C# gets easily more complex than the XAML markup.

I can't give you any insight into QTQuick though. Sorry