cross-references within named argument lists

[gavinking]

A feature I have always planned on, that we've recently begin to discuss, including in #744, is the ability to cross reference arguments within a named argument list, for example:

Panel {
    value label = Label { size = Size(50,10); },
    value button = Button {
        label = "Click me";
        size = Size(50,10); 
        onClick() => label.text="Hello!"; 
    };
}

We need at least two things:

1. to be able to assign a name to a listed argument, and
2. to be able to refer to previous arguments by the parameter name or assigned name within the body of subsequent arguments.

We may or may not need the following:

• to be able to assign a local name to an "anonymous" argument,
• to be able to freely declare something in the body of a named argument list that is not an argument (depends on model specification statement as a shortcut for actual declaration #498), and/or
• to be able to refer to later arguments by name.

[gavinking]

• to be able to refer to later arguments by name.

This is by far the most interesting problem here. It raises ordering problems, which we would need to resolve using either:

• the approach we use for toplevels (lazy evaluation), or
• the approach we use for class declaration sections (essentially, restricting them to nested declarations within the argument body).

[oehme]

Referencing later arguments is the most interesting, too. It's hugely common in GUI frameworks where you want to create elements in their visual order, but wire up their listeners at the same time.