Proposal: user defined attributes

[F i L]

Jacob Carlborg wrote:
> If D supported passing delegates to a function after the 
> function call several language features could have been 
> implemented in the library instead.
>
> void synchronized (void delegate () dg)
> {
>     try
>     {
>         lock();
>         dg();
>     }
>     finally
>         unlock();
> }
>
> synchronized {
>     // do something
> }
>
> In the same way foreach can be implemented in a library 
> function:
>
> void foreach (T) (T[] arr, void delegate (T) dg)
> {
>     for (size_t i = 0; i < arr.length; i++)
>         dg(arr[i]);
> }
>
> auto arr = [3, 4, 5];
>
> foreach (arr ; e // this is the parameter to the delegate) {
>     writeln(e);
> }

That's pretty slick. D would need delegate inlining + 
@forceInline before foreach could be expressed this way though.