Calling delegate properties without parens

[Jonathan M Davis]

On Saturday, April 14, 2012 20:47:20 Piotr Szturmaj wrote:
> I have following code:
> 
> import std.array, std.range, std.stdio;
> 
> struct CommonInputRange(E)
> {
>      @property bool delegate() empty;
>      @property E delegate() front;
>      void delegate() popFront;
> }
> 
> void main(string[] args)
> {
>      alias CommonInputRange!dchar DCRange;
>      static assert(isInputRange!DCRange);
>      DCRange dc;
>      auto dcr = "abcdefg";
>      auto t = dcr.takeExactly(3);
>      dc.empty = &t.empty;
>      dc.front = &t.front;
>      dc.popFront = &t.popFront;
> 
>      for ( ; !dc.empty(); dc.popFront())
>          writeln(dc.front());
> }
> 
> As you can see in the for loop, range primitives must be called using
> parens (), otherwise they don't work.
> 
> Do you know if there are plans to implement @property for delegates and
> function pointers?

front returns an element in the range. In your case, it's returning a 
delegate, because you have a range of delegates. If front returned a class or 
struct with a member function called foo, which you wanted to call, then you'd 
be doing something like

for(; !dc.empty; dc.popFront())
    writeln(dc.front.foo());

But in your case, it's a delegate, so operating on that delegate means calling 
it, which means using the parens. dc.front just gives you the delegate, and it 
would be horrible if it did anything else. If front also called the delegate, 
the doing something like

auto arr = arr(dc);

would call each and every one of the delegates in the range, because it uses 
front to access the element _without calling it_. If there were some way to 
indicate that a delegate was called with property syntax, then you could no 
longer distinguish between simply returning the delegate and returning and 
call it.

Not to mention, properties are supposed to be an abstraction that mimicks 
member variables. That makes no sense whatsoever with a delegate. And in this 
case, the member variable which _is_ being mimicked (front) happens to be a 
delegate. So, it makes perfect sense that you'd have to use parens on it to 
actually call it.

- Jonathan M Davis