[Haskell] Polymorphic types in RHS of type instances

Hi list, I was wondering why GHC doesn't allow usage of polymorphic types in the right-hand side of type instance declarations for type families. The GHC user guide states: "The right-hand side of a type instance must be a monotype (i.e., it may not include foralls) [...]", but it doesn't state the reason. I stumbled upon this limitation when I was trying to generically calculate Johann's and Ghani's interpreter (transformers) for nested data types from their "Initial Algebra Semantics is Enough!" paper. Cheers, Steven