On: Things Go needs more than generics

[_flux]

I wouldn't call it broken duck typing.

The problem is that ShowAbstracts could just modify the array and put an instance of Abstract there that _isn't_ an Impl, thus violating the original constraint of arr containing only instances of Impl.

Once again mutability/invariance raises its ugly head!

[jwalton → _flux]

That is a very good point.