ReturnThis/@chain

[Lutger]

dsimcha wrote:

> I'm working on the next iteration of my Plot2Kill library, and I **really**
> have fallen in love with property chaining.  It's the greatest thing since
> sliced arrays.  However, I've run into an issue that I had previously
> overlooked:
> 
> class Foo {
>     Type _val;
> 
>     typeof(this) val(Type newVal) {
>         _val = newVal;
>         return this;
>     }
> }
> 
> class Bar : Foo {
>     // More properties.
> }
> 
> void main() {
>     auto bar = new Bar;
>     bar.val(someValue).barProperty(someOtherValue);  // Won't work.
> }
> 
> Given that D2 is mostly finalized, it may be a little late for this, but would
> it be feasible, at least far down the road, to add something like an @chain
> annotation, which would only allow a member function to return this and would
> implicitly downcast it to the subtype that was passed in as the this pointer?
>  For example, assume we attached @chain to Foo.val():
> 
> pragma(msg, typeof(bar.val(someValue)));  // Bar, not Foo.

Why not use covariance? Or is that too much boilerplate? 

this will work:

class Bar : Foo {
    override typeof(this) val(Type newVal) {
        return super(newVal);
    }
}

This could be automated with a mixin.