General problem I'm having in D with the type system

[Vijay Nayar]

On Sunday, 27 May 2018 at 06:00:30 UTC, IntegratedDimensions 
wrote:


The problem description is not very clear, but the catfood 
example gives a bit more to work with.

> animal  ->   food
>   |            |
>   v            v
> cat     ->   catfood
>
>
> Of course, I'm not sure how to avoid the problem in D of
>
>
> animal a = new cat();
>
> a.f = new food()
> auto c = cast(cat)a;

Cast operations are generally not guaranteed to preserve type 
safety and should be avoided when possible.  But if I understand 
your description, you have the following relations and 
transitions:

   animal owns food
   cat    owns catfood
   animal may be treated as a cat (hence the casting)
   food may be treated as a catfood (hence the casting)

It may be that the inheritance relationship is backwards in your 
use case.  If "animal" may be treated as a "cat", then the 
inheritance should be other other way around, and "animal" would 
inherit from "cat".

What specific kinds of relationships are you trying to model 
among what kinds of entities?