[rocqua]

Presumably tye issue lies with mutable collections.

If you have a type Animal, with subtypes Dog and Cat, then covariance means a list of Cat is a subtype of a list of Animal. The problem is that appending a Dog is a legal action for a list of Animals, but not a legal action for a list of Cats.

[btown]

To expand on this, you could be maintaining a list of Cats, call a function which accepts a list of Animals, and that function could append a Dog without causing a type error.

https://github.com/python/mypy/issues/4976 is an interesting discussion on this in Python: should a TypedDict be allowed where a dict argument is accepted? The basic consensus is no, because then one could modify the dict, adding keys that wouldn't be allowed in the specific TypedDict.

But this means that often you have to do cast(dict, my_typed_dict) to be able to interop with third party libraries that (quite naturally) believed that by typing their arguments as accepting a dict, they'd be accepting all dict-like things.

[rocqua]

The solution is to encode mutability. So that you can say:

Const list<cat> is a subtype of Const list<animal>. But that does get complicated because. The difference between a mutable container of immutable elements and an immutable container of mutable elements is hard to get your head around, and verbose to capture in syntad.