DIP23 draft: Fixing properties redux

[Jonathan M Davis]

On Monday, February 04, 2013 17:28:24 Andrei Alexandrescu wrote:
> On 2/4/13 2:04 PM, Jonathan M Davis wrote:
> > We could save a lot of boilerplate code if we can simply make it variables
> > and property functions guaranteed to be swappable without breaking code.
> I think this is quite powerful. The way we can do this is by making
> properties emulate a subset of actual variables.

Yes. There are things that propery functions need to emulate from normal 
variables (hence the rewrites with ++ and += and whatnot), and there are of 
course things that they can't emulate (like taking the address of a variable). 
But what we also need is a way to mark a variable as a property and then make 
it so such a variable and property functions have the same capabilities so 
that you're guaranteed to be able to swap between the two without breaking 
code. As soon as one of the two has capabilities that the other doesn't have, 
or there's a major semantic change between them (like taking the address works 
on one but not the other or the type returned by taking the address differs), 
then you can't do that.

Which is why I would argue that we need to be able to mark variables with 
@property (which either makes it so that you can't do anything with them that 
you can't do with a property function or makes it so that it lowers to 
property functions for getting and setting), and we need to disallow taking 
the address of property functions as well as anything else that you could do 
with a variable that we can't emulate with a property function.

- Jonathan M Davis