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is there any way to achieve this behavior without the compilation error?

No. Even if you make `A` of `Reducer` an `in A` your `TodoReducer` still says: "I want a `TodoListAction`. But `Reducer&lt;TodosState, TodoAction&gt;` says "I want a TodoAction". So if that assignment was allowed, your `TodoReducer` could receive something that is a `TodoAction`, but not a `TodoListAction`, and thus the assignment is not valid

I've made a "helper" function in order to allow an exhaustive when...