Broken contract programing

[via Digitalmars-d]

On Wednesday, 13 May 2015 at 14:54:05 UTC, Benjamin Thaut wrote:
> On Wednesday, 13 May 2015 at 12:54:52 UTC, Timon Gehr wrote:
>> This will accept the same arguments as Base. Only one 
>> in-contract in the inheritance chain must pass in order for 
>> the call to go through.
>
> But wasn't exactly that the problem about the current 
> implementation? Shouldn't all 'in' contracts in the chain be 
> checked?

The topic is non-trivial. Dart has even deliberately picked an 
unsound inheritance typing-model because it is more useful. 
Consider the following scenario:

You have a superclass Shelter, then you create subclass 
DogShelter. The Shelter has a virtual function add_pet(Animal x) 
. So clearly the DogShelter will have to accept the _type_ Animal 
in its add_pet specialization because of the type system.

That does not mean it is not a bug if you bring a Cat to the 
DogShelter. It merely means that you cannot catch such bugs with 
the type system alone. The DogShelter might sit behind another 
infrastructure that is supposed to prevent non-Dog animals from 
entering.

So it makes sense to not conflate invariants/contracts with the 
type system.