Crash in out contract in interface

[Andrei Alexandrescu]

On 8/24/11 9:45 AM, Timon Gehr wrote:
> On 08/24/2011 04:47 PM, Andrei Alexandrescu wrote:
>> On 8/24/11 6:58 AM, Timon Gehr wrote:
>>> So, basically interfaces are the only place in D?/DMD where you can even
>>> specify contracts without a function body and there they don't work. I
>>> think the bug is not the only problem, actual contracts should /always/
>>> be part of the function declaration and not require an implementation to
>>> work. Everything else degrades contracts to simple syntactic sugar for
>>> assertions.
>>
>> That plus interplay with inheritance.
>>
>> Andrei
>
> Well, yes, that is true. Where one can add contracts, they will be
> inherited. But inheritance is actually where D's current contract
> programming model is weakest:
>
> It should be possible to use inheritance to create classes that move
> gradually from unimplemented, but specified to implemented and extended.
> The fact that unimplemented methods in an abstract class cannot define
> contracts is an unforgivable left-out, because that is where contracts
> are most useful: for the specification of what all implementations of
> the method are allowed to assume and must guarantee.

Indeed. That should be filed as a bug. But you are exaggerating a bit 
because as you yourself note there are obvious workarounds.

> Contracts are declarative:
> "If you guarantee this, I will guarantee that."
>
> Contracts and implementation are two completely orthogonal concepts,
> and there is no reason to couple them in any way. Not allowing
> contracts without implementation is like telling a customer "You are
> not allowed to specify your needs unless we have already written code
> to satisfy them."
>
> Do you agree that this will have to be fixed eventually, when all the
> more important features are fully working?

You should know that we introduced contracts in interfaces in spite of 
considerable implementation difficulties. So yes, we do consider 
implementation-less contracts important.


Andrei