In other words instances for forall-types, such as:
instance Foo (forall a. [a]) where ...
It feels obvious to me that there *would* be problems with this, but I'm curious about what, exactly, they are.
Could someone familiar with the matter either elaborate on them, or refer me to an existing explanation, a previous discussion, or something of the sort?
I *don't* have any kind of use case in mind, I'm merely seeking a better understanding of the type-system issues involved.